Long Valley Caldera Geothermal Area

GEOTHERMAL ENERGY

Area Overview

---

Geothermal Area Profile

Location:	California
Exploration Region:	Walker-Lane Transition Zone
GEA Development Phase:	Operational "Operational" is not in the list of possible values (Phase I - Resource Procurement and Identification, Phase II - Resource Exploration and Confirmation, Phase III - Permitting and Initial Development, Phase IV - Resource Production and Power Plant Construction) for this property.
Coordinates:	37.6811°, -118.8356°

Resource Estimate

Mean Reservoir Temp:	240°C 513.15 K 464 °F 923.67 °R
Estimated Reservoir Volume:	34.1 km³ 34,100,000,000 m³ 8.181 mi³ 1,204,230,135,186.1 ft³ 44,601,116,107.9 yd³ 34,100,000,000,000 L
Mean Capacity:	38 MW 38,000 kW 38,000,000 W 38,000,000,000 mW 0.038 GW 3.8e-5 TW
USGS Mean Reservoir Temp:	280°C 553.15 K 536 °F 995.67 °R
USGS Estimated Reservoir Volume:	7 km³
USGS Mean Capacity:	63 MW

• "The Long Valley caldera depression occupies an area approximately 17 by 32 km and is located on the eastern slope of the Sierra Nevada mountains about 483 km north of Los Angeles. The caldera was formed by a catastrophic volcanic eruption about 760 ka, one of many recent eruptions in the Inyo volcanic chain within the last 3.6 million years. This eruption ejected an estimated 625 km^3 of pyroclastic ash. The geothermal field is situated at the southwest edge of a Resurgent Dome, an elevated area located in the west central portion of the caldera. The caldera geology, surface manifestations associated with the present-day hydrothermal system, and relict features associated with past hydrothermal activity were summarized by Sorey (1985), and have been described by numerous studies dating back to 1974.'"`UNIQ--ref-00000006-QINU`"''"`UNIQ--ref-00000007-QINU`"' Existing geothermal surface expressions at Long Valley include a few scattered fumaroles, mudpots, and mineral deposits on the western flanks of the Resurgent Dome, where the land elevation is higher. Hot springs with surface discharge temperatures of 79-93°C occur primarily at Casa Diablo, Hot Creek Gorge, Little Hot Creek, and along the south side of the Resurgent Dome, as exemplified by the Fish Hatchery Springs which discharge a combination of thermal water and cold groundwater at a mean temperature of ~15°C. Additional warm and cold spring discharges occur in the eastern part of the caldera between Hot Creek and Lake Crowley where the surface elevation is relatively low, promoting surface flow of geothermal fluids, as exemplified by warm springs surrounding Big Alkalai Lake.

  The Casa Diablo Geothermal Field supplies the three Mammoth-Pacific LP (MPLP) power plants at Casa Diablo near the junction of Highways 395 and 203 with geothermal fluid for a total of 40 MWe of power capacity. The hybrid-cooled, binary plants use isobutane as a working fluid. The working fluid circulates in a closed loop, while the cooled water exits the power plants and is then injected back into the ground in wells at depths below those of the nearby production wells.'"`UNIQ--ref-00000008-QINU`"' Operational data is transmitted in real time to a control room, which is staffed 24 hours a day.

  The Mammoth complex was fully acquired by Ormat Nevada Inc. in August 2010 through the purchase of Constellation Energy’s 50% stake in the property for $72.5 million.'"`UNIQ--ref-00000009-QINU`"' New developments were begun in 2006 with production from two wells located in the Upper Basalt Canyon area some 4 km west of Casa Diablo and additional production and injection are currently being planned for the Upper Basalt Canyon area." cannot be used as a page name in this wiki.
• The given value was not understood.

The Long Valley caldera depression occupies an area approximately 17 by 32 km and is located on the eastern slope of the Sierra Nevada mountains about 483 km north of Los Angeles. The caldera was formed by a catastrophic volcanic eruption about 760 ka, one of many recent eruptions in the Inyo volcanic chain within the last 3.6 million years. This eruption ejected an estimated 625 km^3 of pyroclastic ash. The geothermal field is situated at the southwest edge of a Resurgent Dome, an elevated area located in the west central portion of the caldera. The caldera geology, surface manifestations associated with the present-day hydrothermal system, and relict features associated with past hydrothermal activity were summarized by Sorey (1985), and have been described by numerous studies dating back to 1974. Existing geothermal surface expressions at Long Valley include a few scattered fumaroles, mudpots, and mineral deposits on the western flanks of the Resurgent Dome, where the land elevation is higher. Hot springs with surface discharge temperatures of 79-93°C occur primarily at Casa Diablo, Hot Creek Gorge, Little Hot Creek, and along the south side of the Resurgent Dome, as exemplified by the Fish Hatchery Springs which discharge a combination of thermal water and cold groundwater at a mean temperature of ~15°C. Additional warm and cold spring discharges occur in the eastern part of the caldera between Hot Creek and Lake Crowley where the surface elevation is relatively low, promoting surface flow of geothermal fluids, as exemplified by warm springs surrounding Big Alkalai Lake.

The Casa Diablo Geothermal Field supplies the three Mammoth-Pacific LP (MPLP) power plants at Casa Diablo near the junction of Highways 395 and 203 with geothermal fluid for a total of 40 MWe of power capacity. The hybrid-cooled, binary plants use isobutane as a working fluid. The working fluid circulates in a closed loop, while the cooled water exits the power plants and is then injected back into the ground in wells at depths below those of the nearby production wells. Operational data is transmitted in real time to a control room, which is staffed 24 hours a day.

The Mammoth complex was fully acquired by Ormat Nevada Inc. in August 2010 through the purchase of Constellation Energy’s 50% stake in the property for $72.5 million. New developments were begun in 2006 with production from two wells located in the Upper Basalt Canyon area some 4 km west of Casa Diablo and additional production and injection are currently being planned for the Upper Basalt Canyon area.

History and Infrastructure

---

Operating Power Plants: 3

Mammoth Pacific I Geothermal Facility Mammoth Pacific II Geothermal Facility PLES 1 Geothermal Facility

Add a new Operating Power Plant

Developing Power Projects: 0

No geothermal projects listed.

Add a new Developing Power Project

Power Production Profile

Gross Production Capacity:	11 MW 11,000 kW 11,000,000 W 11,000,000,000 mW 0.011 GW 1.1e-5 TW
Net Production Capacity:
Owners  :	Ormat
Power Purchasers :
Other Uses:

The given value was not understood.

The complete development history of the Long Valley geothermal resource leading up to the year 2000 is detailed by Campbell (2000). Geothermal exploration in Long Valley started in the late 1950s and early 1960s, when nine wells were drilled and flow tested by Magma Power Company. Flow testing of (Magma) Mammoth-1 and the Endogenous 1-3 wells in 1960, under production conditions, demonstrated the existence of a hot water reservoir at temperatures ranging from 132-181°C at shallow depths of 122-324 m. Total dissolved solids were measured at about 1,500 ppm with carbon dioxide degassing resulting in calcite formation in the wellbore.

In 1971, the U.S. Geological Survey selected Long Valley Caldera for study as a hot water geothermal system type area. Soon afterwards on January 22, 1974, the Federal Government initiated the first Federal Known Geothermal Resource Area (KGRA) Lease Sale, consisting of 7 units within the caldera. Republic Geothermal won the bid for the 1773 acre Unit #3 at $515,767 in conjunction with the City of Burbank, and Federal Lease CA-963 was issued shortly afterwards.

Between 1962 and 1973, 11 temperature-gradient test holes were drilled outside the caldera rim to depths of 113 to 271 m. These drill holes, in addition to some 36 temperature profiles from holes completed to depths ranging from 5 to 325 m within the caldera, were drilled for the purposes of data collection for a number of U.S. Geological Survey heat flow studies. These studies showed that minimal thermal gradients of <55°C/km occur along the caldera rim, which were interpreted as areas of recharge into the caldera groundwater system. Thermal gradients near hot spring occurrences were significantly higher (up to 910°C/km), but decrease with depth and typically reversed within the first hundred meters of the boreholes.

Despite the favorable results from the early production test wells drilled at Casa Diablo, no additional commercial exploration drilling was conducted until 1976, when Republic Geothermal, Inc. drilled the 66-29 well into the southeastern portion of the caldera. This was the first deep exploratory well sited within the caldera, drilled to 2,109 m depth just east of the northern extension of the Hilton Creek fault. Data from well 66-29 are available online through the California Department of Conservation Division of Oil, Gas & Geothermal Resources and have been contributed to studies of the temperature distribution within the caldera conducted by the U.S. Geological Survey. Maximum temperatures of 72°C encountered at the bottom of the hole were too low for commercial power production.

Geothermal exploration at Long Valley during the late 1970s and early 1980s was dominated by several large oil companies seeking to diversify their energy resources, including Unocal Geothermal Division (a subsidiary of Union Oil Company), Phillips, and Santa Fe Geothermal, Inc. (a subsidiary of Oxy/Santa Fe). Federal lease block sales in 1982 and 1985 led these companies to drill several shallow- and intermediate-depth holes to assess the geothermal potential beneath the Resurgent Dome. Of the companies operating at Long Valley during this period, Unocal was the most involved in geothermal exploration. The company conducted much of the deep drilling and held the largest federal land lease within the caldera during this time.

In the early 1980s, nearly all the residential and commercial space heating in the Mammoth Lakes area was provided by electricity from Southern California Edison (SCE) generating facilities. Some of this energy came from local hydroelectric plants, but most was transmitted from the Mojave Desert 322 km to the south. Although the need to augment electricity production in the area with geothermal resources had long been recognized, a few developments led to the necessary effort to make it a reality:

1. Increase in energy demand in the area
2. Passage of the Public Utilities Regulatory Policy Act (PURPA) of 1978 requiring utilities to purchase power from qualifying facilities at a high avoided cost
3. Requirements imposed by the California Energy Commission on utilities to issue Standard Offer Number Four (SO-4) contracts to independent power producers in order to take advantage of the new PURPA regulations
4. Utility interest in alternative energy, such as SCE’s policy to develop 2,100 MW by 1990
5. Tax incentives providing for accelerated depreciation (5 years) and an effective investment tax credit of 25%
6. Development of the concept of small, wellhead generating units, employing a standardized process design with off-the-shelf equipment, allowing construction within a one-year period and ability to relocate to a new site.

This climate incentivized further surface exploration and test drilling during this time period, and led to the construction of the first geothermal binary power plant in 1985, the 10 MW Mammoth Pacific I (MP-I, a.k.a. G1). The original plant was owned by Mammoth-Pacific, a joint venture between Mammoth Binary Power Company and Pacific Energy Resources Inc. (a subsidiary of Pacific Lighting Energy Systems, which is itself a subsidiary of Pacific Lighting Corporation of Los Angeles). The facility was designed by Mammoth Binary Power’s general partner in the venture, Holt Geothermal Company (an affiliate of The Ben Holt Co.). MP-I was the world’s first air-cooled geothermal plant incorporating a closed loop system for the working fluid and a set of injection wells to discharge cooled production fluid back into the ground. Two more binary power plants (MP-II, a.k.a. G2, and PLES-1, a.k.a. G3) came online in 1990, each with 15 MW of generating capacity, bringing the total generation of the geothermal resource to 40 MW. The binary plants use isobutane as a working fluid, contained within a closed loop circulation system. By injecting the cooled geothermal fluid production stream into injection wells somewhat deeper than the production wells, there is no net loss of fluid mass from the geothermal system. Operational data are transmitted in real time to a control room, which is staffed 24 hours a day. Note that all the fluid produced in the wells drilled in the Upper Basalt Canyon area west of Highway 395 is piped to Casa Diablo, adding to the mass flow of geothermal fluid passing through the heat exchanges in the binary power plants and then added to the injected fluid that was originally produced from wells at Casa Diablo.

MPLP was also actively exploring for new geothermal resources in Long Valley during the 1990s following the collapse of the industry, in order to expand the accessible production capacity at the existing power plants at Casa Diablo. The company drilled several intermediate-depth exploration wells in 1992 (wells 66-31 and 38-32) and 2002 (well BC 12-31) to further evaluate the geothermal potential of the caldera’s south moat. The 38-32 core hole was drilled to 353 m depth <1 km south of Casa Diablo. The BC 12-31 core hole was drilled to 600 m depth in Basalt Canyon, approximately 2 km west of the geothermal power plants. The excellent quality of these core holes yielded considerable new information into the stratigraphy beneath the southern moat zone, including evidence supporting the existence of a post-eruption landslide block that might act as an impermeable layer within the hydrothermal flow system. The Basalt Canyon Pipeline was also constructed in 2005 to support the MP-I plant with fluids from wells 57-25 and 66-25, drilled in 2006 approximately 4 km west of Casa Diablo near an area with numerous weak steam vents referred to as the Shady Rest thermal area.

All of the early production in the Mammoth complex came from geothermal leases on private lands and later from public lands originally explored by several developers and subsequently leased during BLM lease sales. Federal lands and are managed by a combination of governmental agencies. The US Forest Service manages surface rights and the Bureau of Land Management (BLM) manages the subsurface mineral rights that control the actual resource. The conditions of these federal leases and the National Geothermal Steam Act require that the leased land be diligently explored for geothermal resources capable of commercial production. These requirements have incentivized numerous exploration efforts by Unocal, Phillips and Oxy/Santa Fe during the mid and late 1980s. With acquisition of the Casa Diablo resource, Ormat funded three geothermal exploration projects in the area leading up to 2002. Previously completed geophysical exploration projects and surface geological mapping by Unocal identified the Basalt Canyon, Upper Basalt Canyon, and Rhyolite Plateau as prime targets for intermediate depth slim holes and eventually full-scale geothermal production wells that could produce the potential geothermal resource. All of the proposed sites were selected to comply with environmental lease stipulations to reduce or avoid potential environmental impacts, while still allowing testing of the geothermal resource. For reference, the Basalt Canyon area lies within a kilometer or so west of Casa Diablo at the faulted southern base of the Resurgent Dome and includes a boiling temperature steam vent (Basalt Fumarole); the adjacent Upper Basalt Canyon area lies west and north of the Basalt Canyon area and borders the Rhyolite Plateau (containing volcanic rocks of the youngest of three moat rhyolite complexes that sit adjacent to the western edge of the Resurgent Dome.

Timeline

Historic: Earliest documented use of the springs at Casa Diablo as a gathering place for local Piute Indians, settlers, and prospectors. The Bodie-Mammoth City stage coach line used the site as a way station during the late 1800s. More recently, the Casa Diablo site was used as an automobile rest stop on Old Highway 395.

1959: Magma Power Company drills and flow tests the (Magma) Mammoth-1 well at Casa Diablo. Additional wells (Endogenous 1-3) are drilled and flow tested at the site the following year.

1961-1962: Magma Power Company drills additional wells at Casa Diablo and Chance Meadow to further evaluate the geothermal potential of the southern moat.

1971: The U.S. Geological Survey initiates several studies of the Long Valley Caldera “hot water geothermal system type.”

1974: The U.S. Federal Government initiates the first KGRA Lease Sale consisting of 7 units within the Long Valley Caldera. Republic Geothermal wins the bid at $515,767 for Federal Lease CA-963 for Unit #3 covering an area of 1773 acres.

1976: Republic Geothermal drills the first deep exploratory well (66-29) within the caldera, sited in the southeast near Lake Crowley.

1978: The Public Utilities Regulatory Policy Act (PURPA) of 1978 is passed, requiring utilities to purchase power from qualifying facilities at a high avoided cost. In response, the California Energy Commission imposes new requirements on utilities to issue Standard Offer Number Four (SO-4) contracts to independent power producers in order to take advantage of the new PURPA regulations.

1979-1985: Federal lease sale opportunities prompt several large oil companies to drill shallow- to intermediate-depth holes to assess the geothermal potential beneath the caldera’s Resurgent Dome. Of the companies involved, Unocal held the largest land lease and was responsible for much of the deep drilling that went on within the caldera, including well 44-16 drilled in the west moat.

1985: The first commercial air-cooled binary 10 MW geothermal power plant (MP-I) is constructed at Casa Diablo under the Mammoth-Pacific joint venture.

1987: The Long Valley Hydrologic Advisory Committee is formed to address water use issues originating from the combined effects of long episodic drought, geothermal operations, recreational development, and usage demands of population growth.

1989: Phase I drilling of the Long Valley Exploratory Well on the Resurgent Dome is completed by Oxy Santa Fe and Unocal in cooperation with Sandia National Laboratories, as part of a DOE-funded project attempting to reach high predicted temperatures up to 500°C at about 6 km depth north of Casa Diablo.

1990: Two more binary power plants (MP-II and PLES-1) are brought online at the Mammoth-Pacific complex, each with 15 MW of generating capacity, bringing the total generation of the facility to 40 MW.

1992: MPLP drills intermediate-depth wells 66-31 and 38-32 south of Casa Diablo to further evaluate the geothermal potential of the caldera’s south moat. These wells confirmed the existence of a metasedimentary landslide block beneath the southern moat that isolate hot geothermal fluid production from the shallow Early Rhyolite reservoir at Casa Diablo from deeper cold injection and recharge waters within the Bishop Tuff.

2002: MPLP drills well BC 12-31 approximately 2 km west of Casa Diablo in Basalt Canyon to assess supplementary production from fluid outflow thought to originate from the caldera’s west moat.

2005: The Basalt Canyon Pipeline is constructed to supply MP-I with additional fluids from wells 57-25 and 66-25, drilled the following year near Shady Rest 4 km to the west of Casa Diablo.

2005-2007: The U.S. Geological Survey conducts several high-precision temperature gradient measurements on and around the Resurgent Dome to better constrain the thermal regime of the caldera.

2010: The Mammoth complex is fully acquired by Ormat Nevada Inc. through the purchase of Constellation Energy’s 50% stake in the property for $72.5 million.

Regulatory and Environmental Issues

---

The given value was not understood.

Demands on the region’s hydrologic resources have greatly increased between the development of the geothermal resource at Long Valley and the continued rapid growth of the resort community of Mammoth Lakes. Increased demand and long episodic droughts have strained the groundwater supply and highlighted the need to balance the water use among competing demands such as geothermal operations, recreational development and the usage demands of population growth. In 1987, the Long Valley Hydrologic Advisory Committee (LVHAC) was formed to address these hydrogeologic issues. The LVHAC is made up of members from both private companies and government agencies and oversees hydrologic monitoring programs designed to enable timely detection of changes in hot springs and streams that may be caused by geothermal use, land development or recreational facilities that may strain the available surface and groundwater resources. In the event that significant hydrologic changes are recognized, the committee informs permitting agencies and recommends mitigation actions to minimize or reverse their impact on the groundwater system. While the LVHAC’s decisions are not legally binding, their work often forms the basis for regulations implemented by the U.S. Forest Service, the BLM, and the Mono County government, among other agencies. The USGS collects and compiles the baseline hydrologic data used in the monitoring program, and Ormat collects daily pressure measurements in a series of static (not produced) monitor wells; results from both monitoring programs are provided to the committee on a quarterly basis.

Various geothermal features at Long Valley fall within the scope of the LVHAC monitoring program, including thermal springs in the central and eastern parts of the caldera. Of these, monitoring of the springs in Hot Creek Gorge and at the Hot Creek State Fish Hatchery is of high priority. The thermal contributions to surface waters are regarded as a recreation asset in high discharge springs in Hot Creek Gorge and in minor springs that contribute a small 5% fraction of thermal water to the local Hot Creek Fish Hatchery. The USGS provides hydrologic data on flow rate, temperature, and chemical composition for these springs on a continuous or periodic basis. The monitoring program also incorporates pressure (i.e., water level), temperature, and water chemistry measurements from numerous wells in the region, and equivalent data from geothermal production and injections wells that are collected and compiled by the power plant operator. These data are used to evaluate the hydrologic effects of current geothermal developments and provide the committee with a means to assess whether new projects are economically feasible, environmentally sound, and acceptable to all involved interest groups. The LVHAC plays a crucial role in this realm by providing a forum for discussion on the key issues and concerns in the community, and often facilitates successful and informed compromises so development can continue unimpeded.

Working fluid leaks occur very rarely at the Casa Diablo binary power plant. Any leakage is repaired quickly to prevent loss of the high-cost isobutane working fluid and to maintain the efficiency of the system, which requires complete integrity of the heat exchanger. The isobutane fluid is a low-solubility non-toxic gas that partitions strongly to the vapor phase. Short-term working fluid leaks into the Long Valley geothermal system have been used as an inadvertent tracer for testing subsurface fluid flow within the caldera. Rare leaks through the heat exchanger have resulted in the addition of trace amounts of unreactive and immiscible isobutane to the spent geothermal fluid that is re-injected into the Bishop Tuff formation deep beneath Casa Diablo. Between 1993 and 2004, trace amounts of isobutane were detected in dissolved gas samples taken from wells, hot springs, and fumaroles throughout the caldera. These sample locations include (with distances relative to Casa Diablo):

• Well 66-25 (5 km west)
• Basalt Fumarole (2 km west)
• Hot Bubbling Pool (5 km east)
• Well 28-34 (3 km east)
• Well CW-3 (5 km east)
• Fumarole Valley (4 km east)
• Hot Creek gorge springs (10 km east)
• Well CH10B (10 km east)

Based on the combination of isobutane migration and transmission of reservoir pressure in the production and injection zones near Casa Diablo, it can be inferred that a high degree of lateral continuity exists within the relatively shallow hydrothermal system beneath the caldera’s south moat.

Future Plans

---

The given value was not understood.

The Mammoth complex was fully acquired by Ormat Nevada Inc. in August 2010, through the purchase of Constellation Energy’s 50% stake in the property for $72.5 million. Ormat plans to double the generating capacity at Casa Diablo using the same strategies exploited at the Steamboat complex, which include modernization of equipment and the addition of new equipment. The improvements to the efficiency of the equipment, in addition to subsurface injection of all produced fluid should allow for increased electricity production with very little risk of overproducing the thermal resource. Ormat has also proposed plans to add (1) an additional power plant at the Case Diablo site, (2) westward additions to the Basalt Canyon Pipeline that would connect the power plants to existing and planned wells west of Highway 395, (3) construction of a new pipeline to transmit some of the injection fluid from Casa Diablo westward to injection wells to be drilled in the Upper Basalt Canyon area, and (4) addition of a smaller pipeline providing access to planned wells 55-32 and 65-32 just east of the existing MP-II and PLES-1 power plants. Existing wells to be used in the project plan include:

• 14-25
• 12-25
• 57-25
• 66-25
• 12-31

Permitted wells to be drilled as part of the project plan in the Basalt Canyon area include:

• 15-25
• 25-25
• 34-25
• 56-25
• 38-25
• 50-25
• 81-36
• 77-25
• 26-30
• 12A-31
• 23-31
• 35-31
• 55-31

Depending on the performance of individual wells and modeling of the geothermal reservoir, up to 50% of the produced fluid will be re-injected in the west at several locations in or around Basalt Canyon.

Exploration History

---

First Discovery Well

Completion Date:	1959/01/01
Well Name:	(Magma) Mammoth No. 1
Location:	37.6478°, -118.9067°
Depth:	366 m 0.366 km 0.227 mi 1,200.787 ft 400.261 yd
Initial Flow Rate:	59 kg/s
Flow Test Comment:
Initial Temperature:	132°C 405.15 K 269.6 °F 729.27 °R

• "<br>

  Early Exploration

  Geothermal exploration in Long Valley Caldera began in the late 1950s and early 1960s, when nine wells were drilled and flow tested by Magma Power Company.'"`UNIQ--ref-00000041-QINU`"' These wells included:

  • The (Magma) Mammoth No. 1 well drilled in 1959 to approximately 366 m depth (148°C)
  • The Endogenous No. 1-3 wells drilled west of Highway 395 in 1960 in association with Natural Steam Corporation (formerly Endogenous Power Co.) to depths of 192-, 247-, 174-m and temperatures of 178, 174, 172°C, respectively
  • The Endogenous No. 4 and Chance No. 1 (C-1) wells drilled in 1961 to the east of Highway 395 and at Casa Diablo Hot Pool, respectively
  • The Endogenous No. 5-7 wells drilled in 1962 at Casa Diablo to approximately 123-, 230-, 204-m depth, respectively.

  Temperature measurements reported above represent equilibrated temperatures, taken after the wells had remained under static conditions for periods ranging from several weeks to six months. Flow testing of (Magma) Mammoth No. 1 and the Endogenous 1-3 wells in 1960, under production conditions, demonstrated the existence of a hot water reservoir at temperatures ranging from 132-181°C at shallow depths of 122-324 m. Total dissolved solids were measured at about 1,500 ppm and with carbon dioxide degassing resulting in calcite formation in the wellbore. At first, the tests involved free-flowing the wells for periods varying from a few days to weeks. Later a shaft-driven, downhole pump was used to keep the fluid in single-phase flow, eliminating calcite formation in the wells and surface equipment. The downhole pumps also keep the brine at the downhole temperatures, rather than allowing it to cool by flashing.

  Eleven temperature-gradient test holes were drilled to depths of 113 to 271 m outside the caldera rim between 1962 and 1973 as a part of several U.S. Geological Survey heat flow studies of the Long Valley area. These drill holes, in addition to some 36 temperature profiles from holes completed to depths ranging from 5 to 325 m within the caldera, included the DC well and the CH1-7 core holes in the eastern part of the caldera, among others.'"`UNIQ--ref-00000042-QINU`"''"`UNIQ--ref-00000043-QINU`"' The maximum encountered temperature reported for these holes was approximately 110°C in CH7. Temperature reversals were detected in wells CH3, CH5, and CH7, and suggest that warm water flows laterally through the groundwater system to the east from Hot Creek Gorge toward Lake Crowley. These studies showed that minimal thermal gradients of <55°C/km occur along the caldera rim, which were interpreted as areas of recharge into the caldera groundwater system. Thermal gradients near hot spring occurrences were significantly higher (up to 910°C/km), but decrease with depth and typically reversed within the first hundred meters of the boreholes.

  The caldera geology, distribution of active hydrothermal surface manifestations, and relict features associated with past hydrothermal activity were summarized by Sorey (1985) and have been described by numerous studies dating back to 1974.'"`UNIQ--ref-00000044-QINU`"''"`UNIQ--ref-00000045-QINU`"''"`UNIQ--ref-00000046-QINU`"' '"`UNIQ--ref-00000047-QINU`"' Existing geothermal surface expressions at Long Valley include fumaroles, mudpots, and mineral deposits in the western caldera moat and on the flanks of the Resurgent Dome, where the elevation is higher. Hot springs with surface discharge temperatures of 79-93°C occur primarily at Casa Diablo, Hot Creek Gorge, Little Hot Creek, and along the south side of the Resurgent Dome. Additional warm and cold spring discharges occur in the eastern part of the caldera between Hot Creek and Lake Crowley where the surface elevation is relatively low, promoting surface flow of geothermal fluids, as exemplified at Fish Hatchery Springs. The distribution, quantity, and age of borate minerals in Searles Lake, located some 200 km south of Long Valley, indicate that hydrothermal activity associated with the caldera has persisted for the past 300 ka, with the establishment of the present hydrothermal system within the past 40 ka.'"`UNIQ--ref-00000048-QINU`"' These results are in good agreement with previous estimates of the occurrence of peak hydrothermal activity at approximately 300 ka based on pervasive hydrothermal alteration in lacustrine deposits southeast of the Resurgent Dome, which are interbedded with Hot Creek moat rhyolite, erupted at about 280 ka.'"`UNIQ--ref-00000049-QINU`"' More exact age constraints on recent hydrothermal activity at Long Valley were obtained through age determinations of silica and carbonate hot spring deposits using the 230Th/234U disequilibrium'"`UNIQ--ref-0000004A-QINU`"''"`UNIQ--ref-0000004B-QINU`"' and 230Th/232Th versus 234U/232Th ratio least-squares linear regression methods;'"`UNIQ--ref-0000004C-QINU`"' these yielded an age range of < 6.4 ka to > 310 ka.'"`UNIQ--ref-0000004D-QINU`"' Similar geochemical dating techniques applied to hydrothermal minerals recovered from the major fracture zone intersected in the LVEW deep well on the Resurgent Dome (at 2,600 m depth within the pre-caldera basement rocks) yielded ages close to 300 ka, in agreement with previously noted evidence of mineral alteration in rocks of the 280 ka Hot Creek moat rhyolite. This correspondence suggests that the location and magnitude of the caldera’s geothermal systems reached a peak that was perhaps associated with renewed magmatic eruptive activity approximately ~300 ka years ago.'"`UNIQ--ref-0000004E-QINU`"'

  Exploration efforts continued in 1976, when Republic Geothermal, Inc. drilled the 2,109-m-deep 66-29 well into the southeastern portion of the caldera. '"`UNIQ--ref-0000004F-QINU`"' '"`UNIQ--ref-00000050-QINU`"' This was the first deep exploratory well sited within the caldera, drilled to 2,109 m depth. Data from well 66-29 are available online'"`UNIQ--ref-00000051-QINU`"' through the California Department of Conservation Division of Oil, Gas & Geothermal Resources and have been contributed to studies of the temperature distribution within the caldera conducted by the U.S. Geological Survey.'"`UNIQ--ref-00000052-QINU`"' Temperatures measured in the bottom of well 66-29 were deemed too low for commercial power production at <75°C.'"`UNIQ--ref-00000053-QINU`"'

  Additional drilling activities continued between 1976 and 1985. Several of these holes were full-size wells, including:

  • The (Unocal) Mammoth-1 well, drilled by Union Oil Company in 1979 at Casa Diablo to 1605 m depth, with maximum encountered temperature of 152.7°C at 121m but a temperature of 102.2°C at TD
  • The Clay Pit 1 (CP-1) well also drilled by Union Oil Company in 1979 into the Resurgent Dome to 1799 m depth, with maximum encountered temperatures of 147.8°C'"`UNIQ--ref-00000054-QINU`"'
  • Wells 68-28 and 13-26, drilled by Santa Fe Geothermal Inc. on the Resurgent Dome after 1980 (temperature, lithology, structural data, and specific completion dates for these wells were unavailable for the purposes of this review).'"`UNIQ--ref-00000055-QINU`"'

  The (Unocal) Mammoth-1 well was the first well drilled at Casa Diablo to test the deep production potential from the intracaldera Bishop Tuff and caldera basement. It was also the first well to intersect the metasedimentary landslide block at 466 m depth beneath the caldera’s southern moat, a tumultuous mix of metapelite and granite that separates the Early Rhyolite reservoir from the underlying Bishop Tuff.'"`UNIQ--ref-00000056-QINU`"''"`UNIQ--ref-00000057-QINU`"'

  Several core holes were also drilled in the caldera’s west moat by Phillips Petroleum Company in 1982, including:

  • PLV-1, drilled to approximately 711 m depth
  • PLV-2, drilled to approximately 640 m depth.

  Thermal gradient drilling also continued during this period, consisting of several holes including:

  • The CH8-10 thermal-gradient holes drilled by the U.S. Geological Survey prior to 1978 to relatively shallow depths ranging from about 55 to 290 m, with maximum encountered temperatures ranging from about 5 to 104°C'"`UNIQ--ref-00000058-QINU`"'
  • The LVFU 13-21 thermal-gradient hole drilled on the Resurgent Dome by Union Oil Company, approximately ¾ km south and east of the future site of the Long Valley Exploratory Well (LVEW)
  • Thermal-gradient holes 28-29 and 34-29, and the 48-29 observation well drilled by Oxy Santa Fe in December 1985 under the CA-11667 lease.

  The PLV-1 and PLV-2 wells represent some of the first intermediate-depth holes drilled into the caldera’s west moat and provide temperature, lithologic, and geochemical data for this area that were previously unavailable.'"`UNIQ--ref-00000059-QINU`"' The maximum temperature encountered in PLV-1 was about 125°C and about 50°C in PLV-2.'"`UNIQ--ref-0000005A-QINU`"''"`UNIQ--ref-0000005B-QINU`"' Temperature gradients in both wells were favorable; however, the shallow depths of these wells prohibit accurate prediction of reservoir temperatures with increased depth below the west moat. Assuming no temperature reversal were to occur beneath the west moat and gradients remain consistent with depth, reservoir temperatures of 218-288°C in the Bishop Tuff or early rhyolite were predicted for PLV-1, consistent with chemical geothermometer temperature estimates for the hole.'"`UNIQ--ref-0000005C-QINU`"' In the caldera’s east moat, just south of Hot Creek gorge, a temperature reversal was detected in the CH10 well, providing additional evidence supporting the eastward flow of thermal water through the caldera groundwater system.

  Post-Development Exploration and Resource Expansion

  Several additional wells were drilled in the caldera after 1983 to the east and north of Highway 395. Among these wells were exploration and monitoring wells drilled near the Fish Hatchery Springs in preparation for the siting of a second binary geothermal power plant, which included the CW-2 and the MPLP CW-3 (a.k.a. Chance 3) wells along the northern edge of the Chance Meadow.'"`UNIQ--ref-0000005D-QINU`"' These two wells are located within 30 m of a significant thermal spring denoted as the Hot Bubbling Pool. The two wells exhibited temperature profiles similar to that encountered in the upper 300 m of (Unocal) Mammoth-1, with maximum temperatures of 132°C encountered in CW-2.

  After several temperature-gradient holes were drilled in 1982 to the west of the Resurgent Dome and a geothermal lease acquisition in the western caldera by Unocal in 1983, two test wells were drilled in the west moat that encountered reservoir temperatures of > 200°C. These wells included the Research Drilling Office #8 (RDO-8) core hole drilled to 715 m depth at Shady Rest in June 1986 as a part of the DOE Continental Scientific Drilling Program'"`UNIQ--ref-0000005E-QINU`"''"`UNIQ--ref-0000005F-QINU`"' '"`UNIQ--ref-00000060-QINU`"' and the 44-16 well rotary drilled by Union Oil Company to 1,800 m depth in 1985.'"`UNIQ--ref-00000061-QINU`"''"`UNIQ--ref-00000062-QINU`"' A temperature reversal was measured in RDO-8 at approximately 351 m depth within the Early Rhyolite after the well reached about 202°C, measured on June 20, 1986.'"`UNIQ--ref-00000063-QINU`"' Well 44-16 encountered two temperature reversals, the first occurring at the base of the Bishop Tuff after temperatures of 218°C were reached at 1.1 km depth, and the second occurring within Tertiary Dacites and Andesites below the Bishop Tuff after temperatures of 192°C were reached at approximately 1.5 km depth.'"`UNIQ--ref-00000064-QINU`"' A bottom hole temperature of 181°C encountered in 44-16 represents the hottest temperature measured in metamorphic basement within the caldera at the time.

  Single-phase liquid samples were collected at Casa Diablo in 1985 and 1986 from the production wellhead using a cooling coil to prevent flashing and from a mini-separator that allowed flashing of the fluid to liquid and gas under known conditions.'"`UNIQ--ref-00000065-QINU`"' '"`UNIQ--ref-00000066-QINU`"' Samples were also collected from wells RDO-8 and 44-16, which were drilled in the west moat zone in 1986.'"`UNIQ--ref-00000067-QINU`"' Samples from RDO-8 were taken approximately five months after the well was completed and about one month after perforation of the well casing in October 1986. The authors note that some loss of steam and noncondensible gas occurred prior to sampling of well 44-16, and that the sample contained relatively high levels of Na and SO4 due to drilling mud contamination.'"`UNIQ--ref-00000068-QINU`"' '"`UNIQ--ref-00000069-QINU`"' Interpretations of the isotopic data obtained through analysis of the sampled waters was useful in a variety of reservoir characterization goals,'"`UNIQ--ref-0000006A-QINU`"' and are discussed in the geochemistry section of this review.

  Three additional core holes were drilled in the caldera’s west moat in 1987-1988, including:

  • The INYO-4 (a.k.a. RDO-10) 22° slant hole drilled to 854 m depth (610 m vertical depth) approximately 216 m southwest of the Inyo Craters in July 1987 (max temperature of 80°C in a thick early rhyolite flow at 490 m vertical depth)'"`UNIQ--ref-0000006B-QINU`"'
  • The MLGRAP-1 and -2 temperature-gradient holes drilled between November 1987 and January 1988 to depths of about 468 and 491 m near the town of Mammoth Lakes, respectively (max temperatures of 75°C encountered in the bottom of each hole).'"`UNIQ--ref-0000006C-QINU`"'

  The wells were drilled as part of scientific investigations (in the case of INYO-4) and to investigate the feasibility of using thermal waters beneath Mammoth Lakes for direct use (MLGRAP-1 and -2). Data from these wells have been used to constrain the extent of hydrothermal circulation beneath the caldera’s west moat presented in more recent conceptual models of the larger hydrothermal system.'"`UNIQ--ref-0000006D-QINU`"'

  Unocal released the results from 158 time-domain electromagnetic (TDEM) sounding surveys in 1986, in addition to results from 77 magnetotelluric (MT) stations collected in conjunction with Chevron Resources. Data from these surveys have been published by Nordquist (1987)'"`UNIQ--ref-0000006E-QINU`"' and reinterpreted by Park and Torres-Verdin (1988).'"`UNIQ--ref-0000006F-QINU`"' These geophysical surveys were designed to assess the Long Valley hydrothermal system and to identify possible deep geothermal drilling targets beneath the western portion of the caldera. Public-domain MT surveys published in 1988 and 1991 contribute additional information to the already extensive electrical data set for the Long Valley region.'"`UNIQ--ref-00000070-QINU`"''"`UNIQ--ref-00000071-QINU`"' The Unocal MT data outline a broad region with < 50 ohm-m invariant apparent resistivity at 1 Hz, and four regions with < 20 ohm-m resistivity within the caldera.'"`UNIQ--ref-00000072-QINU`"' Lithologic logs from wells drilled into these lower resistivity regions revealed strong correlations between intervals of extensive hydrothermal clay alteration and intervals of low resistivity in the wellbores. Reinterpretation of the Unocal and Chevron data sets have identified similar regions that exhibit low resistivity at shallow depth.'"`UNIQ--ref-00000073-QINU`"' Regions of low resistivity were also identified during the 1988 public-domain MT investigations.'"`UNIQ--ref-00000074-QINU`"' Two-dimensional MT modeling of both the Unocal and public-domain data sets across the caldera’s west moat revealed a U-shaped low resistivity region with limbs elongated to the northeast and northwest of the (Unocal) Mammoth-1 well at Casa Diablo. This low resistivity region is unusually deep, extending into the pre-caldera basement to the northwest; it is roughly aligned with the projected location of the Laurel-Convict fault within the caldera. The intervals of well RDO-8 that host high temperatures and hydrothermal alteration also correlate with this deep region of low resistivity, reinforcing the association between low resistivity, extensive clay alteration, and active hydrothermal circulation. Intervals of low resistivity near well 44-16 are confined to geothermal reservoirs within the early rhyolite sequence and Bishop Tuff. Resistivity in the vicinity of Mammoth Mountain appears to be similar to the resistivity structure encountered around well 44-16, but this interpretation is less certain due to sparse data coverage in the southwestern portion of the caldera. The MT data, together with data from wells drilled into and adjacent to the northwestern limb of the low resistivity region, support the interpretation that thermal waters ascend along deep basement faults and migrate into volcanic caldera-fill rock units beneath the caldera’s west moat. However, the resistivity data obtained through the MT surveys do not eliminate Mammoth Mountain as a possible upflow zone within the present-day hydrothermal system.

  Phase I drilling of the LVEW (formerly known as the LVF 51-20 core hole) was completed on the Resurgent Dome in the summer of 1989 by Oxy Santa Fe and Unocal in cooperation with Sandia National Laboratories, as part of a DOE-funded project attempting to reach high predicted temperatures up to 500°C at about 6 km depth north of Casa Diablo.'"`UNIQ--ref-00000075-QINU`"''"`UNIQ--ref-00000076-QINU`"' Temperature gradients measured in LVF 51-20 were 50°C /km following completion of the well, and indicate that recharge of cool meteoric water occurs within the upper 750 m of the Resurgent Dome. This well also provided high-quality lithologic and structural data within the Resurgent Dome, which were previously unavailable.'"`UNIQ--ref-00000077-QINU`"' The measured thermal conductivity of core from well LVF 51-20 of 2.3 W/m°C was applied to a thermal gradient of 40°C /km in CP-1 to extrapolate a conductive heat flow estimate of 90 mW/m^2 for the Resurgent Dome.'"`UNIQ--ref-00000078-QINU`"' This heat flow estimate is similar to the 120 mW/m^2 measured in the cored section of LVF 51-20.'"`UNIQ--ref-00000079-QINU`"' These conductive heat flow measurements are somewhat lower than the values predicted to exist beneath the Resurgent Dome. At the time, a heat flow value of 250 mW/m^2 was expected under the hypotheses that a steady-state magma body exists beneath the center of the caldera at 5-8 km depth or that periodic reinjection of new magma into this magma chamber sustains the heat flow through the caldera. These results suggest either that a shallow magma body no longer exists below the caldera’s Resurgent Dome, as was previously predicted, or that deep non-thermal groundwater circulation along extensional faults influences measured heat flow values in the overlying formations.

  The Oh Well-1 temperature-gradient hole was drilled to 664 m depth in December 1991 near the town of Mammoth Lakes in the western caldera, and encountered maximum temperatures of < 85°C. Although the measured temperature gradient was slightly higher than typical background gradients within the caldera, impermeable Early Rhyolite units penetrated by the borehole lacked thermal fluid input and were considered unproductive.'"`UNIQ--ref-0000007A-QINU`"''"`UNIQ--ref-0000007B-QINU`"'

  Recent Investigations and Refinement of Conceptual Model

  Sorey et al. (1991) integrated information from previous scientific and private industry investigations with new data obtained from fluid sampling, test drilling, and geological and geophysical studies conducted between 1985 and 1988 into a comprehensive conceptual model of the present-day hydrothermal flow system at Long Valley caldera.'"`UNIQ--ref-0000007C-QINU`"' Lithology and temperature-gradient data from wells drilled prior to 1988 are summarized in detail in the compilation, which includes information from numerous wells described in previous studies (discussed above). Data from many of the wells are also available online through the USGS.'"`UNIQ--ref-0000007D-QINU`"' Thermal conductivity, XRD, and isotopic analyses of core cuttings from several of the wells discussed have been completed in several studies and seem to prove useful in most cases.'"`UNIQ--ref-0000007E-QINU`"''"`UNIQ--ref-0000007F-QINU`"''"`UNIQ--ref-00000080-QINU`"' Results from these studies are also summarized in Sorey et al. (1991).'"`UNIQ--ref-00000081-QINU`"' Relevant data from chemical and isotopic studies published during the same year are also considered in the review.'"`UNIQ--ref-00000082-QINU`"''"`UNIQ--ref-00000083-QINU`"''"`UNIQ--ref-00000084-QINU`"'

  Farrar et al. (2003) provide a comprehensive conceptual model of the different stages of hydrothermal activity, flow, and recharge in the Long Valley caldera groundwater system based on detailed integration of results from pump tests, fluid level monitoring, temperature logging, and fluid sampling/analysis of the LVEW with information obtained from other wells drilled on or near the Resurgent Dome.'"`UNIQ--ref-00000085-QINU`"' Temperature data for five wells drilled on the Resurgent Dome were considered, including new data from the recently completed LVEW borehole. Data from these wells were compared against temperature data from five wells drilled into the hydrothermal system in the west and south moat zones. Temperature profiles in several wells showed peak temperatures between about 120-220°C at relatively shallow depths above ~1000 m depth that decline to the east, associated with eastward and southward lateral flow of thermal water through the system. These occurred in:

  • The wells located on the Resurgent Dome east and southeast of LVEW (wells 13-21 and 13-26)
  • The west moat (wells RDO-8 and 44-16)
  • The south moat (Unocal Mammoth-1 well at Casa Diablo).

  This thermal regime differs significantly from the dominantly conductive temperature gradients averaging about 35 °C/km, observed in the Bishop Tuff in the upper interval of LVEW, and throughout the Clay Pit-1 well. This contrasts sharply with the temperature profile in the underlying metamorphic basement, in which temperatures in LVEW become isothermal below 2000 m depth. This isothermal temperature interval is inferred to be related to a major permeable fracture zone intersected by LVEW at 2600 m depth that contains a 103°C fluid, which must be flowing more-or-less vertically outside the wellbore to produce constant temperatures observed across the ~600 m depth interval. The authors of the original study postulate that this fracture zone penetrated by LVEW may be the Eastern Graben Fault that bounds the eastern side of the caldera’s medial graben.'"`UNIQ--ref-00000086-QINU`"' Other, shallower wells drilled on or around the Resurgent Dome (wells Unocal Mammoth-1, 13-21, and 13-26) also exhibit isothermal intervals in the Bishop Tuff at about 100°C approaching the bottom of the wellbores. Of these wells, only (Unocal) Mammoth-1 penetrates metamorphic basement, in which the isothermal interval continues to the bottom of the well, similar to LVEW.

  Geothermal exploration was ongoing in the Long Valley caldera during the period that the LVEW was being drilled and tested. MPLP drilled several intermediate-depth exploration wells to further evaluate the geothermal potential of the caldera’s south moat.'"`UNIQ--ref-00000087-QINU`"' These wells included:

  • Well 66-31 in 1992
  • The 38-32 core hole, drilled to 353 m depth <1 km south of Casa Diablo in 1992
  • The BC 12-31 core hole, drilled to 600 m depth ~ 2 km west of the geothermal power plants and Highway 395 in 2002.

  The excellent quality of these core holes yielded considerable new information into the stratigraphy beneath the southern moat zone, including evidence supporting the existence of a post-eruption landslide block that separates the Bishop Tuff from the overlying Early Rhyolite units (see Figure 3). The landslide block acts as an impermeable layer within the hydrothermal flow system, isolating shallow thermal fluid outflow from deeper cool injection fluid and natural meteoric recharge that might otherwise quench the system.'"`UNIQ--ref-00000088-QINU`"'

  Integrated modeling of deformation, microgravity, and seismic data was also conducted in 2003 to investigate the cause of recent uplift of the Resurgent Dome.'"`UNIQ--ref-00000089-QINU`"''"`UNIQ--ref-0000008A-QINU`"''"`UNIQ--ref-0000008B-QINU`"''"`UNIQ--ref-0000008C-QINU`"''"`UNIQ--ref-0000008D-QINU`"' Modeling of deformation and microgravity data suggests that there are two sources of subsurface inflation beneath the caldera—one between 7-10 km depth below the Resurgent Dome and a deeper source ~15 km below the caldera's south moat.'"`UNIQ--ref-0000008E-QINU`"''"`UNIQ--ref-0000008F-QINU`"' These data suggest that the shallower source is of intermediate density between magma and aqueous fluid; therefore, uplift of the Resurgent Dome may relate to pressure buildup associated with magmatic brine or gas sourced from deep beneath the caldera’s south moat.'"`UNIQ--ref-00000090-QINU`"''"`UNIQ--ref-00000091-QINU`"' These ascending fluids would pool at or below the transition between brittle and ductile deformation conditions, inferred to occur at 6-7 km below the Resurgent Dome based on the depth of the apparent bottom of the seismogenic zone.'"`UNIQ--ref-00000092-QINU`"'

  Between 2005 and 2007, several high-precision temperature profiles were measured in existing thermal gradient wells in order to assess the thermal regime of the Long Valley Resurgent Dome. The specific goals of this investigation were to understand the extent of advective (lateral) heat transport through the Resurgent Dome to guide future geothermal exploration efforts within the caldera, to provide constraints for assessing the presence or absence of new magma injected below the Resurgent Dome, and to supply a baseline dataset for measuring changes in the thermal regime of the caldera in response to volcanic activity, large earthquakes, and/or geothermal production.'"`UNIQ--ref-00000093-QINU`"' These USGS temperature measurements, in addition to past temperature measurements contributed by private industry (taken with various methods and levels of accuracy) and by Sandia National Laboratory, were compiled in a publically accessible database by Farrar et al. (2010).'"`UNIQ--ref-00000094-QINU`"' These high-precision temperature profiles were measured between 2005 and 2006 in boreholes 13-21, 13-26, 68-28, and 46-28 using the USGS's heat flow logging truck. Temperatures were measured in the cased wells in June 2005. Boreholes 13-21, 13-26, and 68-28 were perforated in September 2005 to allow sampling of formation waters/gases and establish hydraulic connectivity between the wellbores and the surrounding rocks. Temperatures were re-measured in wells 13-21 (perforated at a depth of 670 m) and 46-28 (un-perforated) in July 2006. The USGS's heat flow logging truck encountered difficulties when attempting to access borehole 35-28 in Fumarole Valley, and so a portable device was used to measure temperatures in the well at 6.10 m intervals in July 2006 and at 3.05 m intervals in May 2007.'"`UNIQ--ref-00000095-QINU`"' '"`UNIQ--ref-00000096-QINU`"'

  A shallow high-temperature interval encountered in the Fumarole Valley boreholes (35-28, 46-28, and 68-28) exists at the same elevation as the upper high-temperature interval of the (Unocal) Mammoth-1 production well at Casa Diablo.'"`UNIQ--ref-00000097-QINU`"' Temperatures also appear to decrease across the geothermal field within the shallow depth interval between 2,220-2,300 m elevation to the south and east, from 170°C at Casa Diablo to 128°C in boreholes 35-28 and 46-28.'"`UNIQ--ref-00000098-QINU`"' Temperature gradients at the bottom of all wells were <50 °C/km, with temperature reversals and gradient decreases measured below zones of high temperature that occur either at the contact between volcanic flow and tuff units or in fractured zones of the volcanic flow units within the Early Rhyolite sequence. Intervals showing negative temperature gradients also coincided with zones that exhibited the largest temperature difference from 2005 to 2006 in well 46-28 and from 2006 to 2007 in well 35-28. Together, these results indicate that lateral flow of hydrothermal waters occurs at relatively shallow depths in the Early Rhyolite sequence and that at least some of the flow occurs through the southern part of the Resurgent Dome.'"`UNIQ--ref-00000099-QINU`"' Data also suggest that cooling of the active hydrothermal system occurs along its flow path and at much lower rates over time. Taken with the occurrence of abundant seismic activity in the (hotter) southern part of the Resurgent Dome compared to the relatively quiet (and cooler) northern part of the Resurgent Dome, these temperature data suggest that a recently emplaced magma body is absent beneath much of the Resurgent Dome.

  A study was published in 2011 that investigated the hypothesis that hydrothermal fluids upwell beneath the west moat area from a source reservoir with estimated temperatures ranging from 240 to 273°C, then flow laterally to the east and mix with cool groundwater that infiltrates and recharges the system along ring fractures and faults (an interpretation supported by previous researchers).'"`UNIQ--ref-0000009A-QINU`"''"`UNIQ--ref-0000009B-QINU`"''"`UNIQ--ref-0000009C-QINU`"''"`UNIQ--ref-0000009D-QINU`"' Details of the modeling parameters, inputs, and results are discussed in the geochemistry section of this page.

  Data from the deep (Unocal) Mammoth-1 and other test wells suggest that producing wells at Casa Diablo only tap into a shallow (<200m) thermal resource restricted to permeable Early Rhyolite flow units.'"`UNIQ--ref-0000009E-QINU`"' Results from deep drilling further suggest that the heat source sustaining the current hydrothermal system is not located directly beneath the Resurgent Dome.'"`UNIQ--ref-0000009F-QINU`"' Age dating of hydrothermal alteration minerals in and around the Resurgent Dome also provides evidence for past geothermal activity at Long Valley from 300 to 130 ka,'"`UNIQ--ref-000000A0-QINU`"' while well and borehole data point to a decline in magmatic activity beneath the caldera’s Resurgent Dome over the past 100 ka. Drilling results also establish that increased magmatic activity in the western caldera sustains the present-day hydrothermal system, in which the shallow production wells at Casa Diablo tap into a 170°C outflow originating from upflow from an active geothermal system to the west. Spent geothermal fluid at Casa Diablo is returned to the system via injection into deeper (750m) permeable zones in the Bishop Tuff that underlies the shallow Early Rhyolite reservoir. The production and injection zones are stratigraphically separated by an impermeable landslide block that restricts the vertical migration of subsurface fluids in the caldera’s southern moat. The landslide block is a critical feature that enables sustained production and injection at Casa Diablo, as it effectively isolates hot shallow outflow fluids from deeper cold injection and natural recharge waters that might otherwise quench the system.'"`UNIQ--ref-000000A1-QINU`"'" cannot be used as a page name in this wiki.
• The given value was not understood.

Early Exploration

Geothermal exploration in Long Valley Caldera began in the late 1950s and early 1960s, when nine wells were drilled and flow tested by Magma Power Company. These wells included:

• The (Magma) Mammoth No. 1 well drilled in 1959 to approximately 366 m depth (148°C)
• The Endogenous No. 1-3 wells drilled west of Highway 395 in 1960 in association with Natural Steam Corporation (formerly Endogenous Power Co.) to depths of 192-, 247-, 174-m and temperatures of 178, 174, 172°C, respectively
• The Endogenous No. 4 and Chance No. 1 (C-1) wells drilled in 1961 to the east of Highway 395 and at Casa Diablo Hot Pool, respectively
• The Endogenous No. 5-7 wells drilled in 1962 at Casa Diablo to approximately 123-, 230-, 204-m depth, respectively.

Temperature measurements reported above represent equilibrated temperatures, taken after the wells had remained under static conditions for periods ranging from several weeks to six months. Flow testing of (Magma) Mammoth No. 1 and the Endogenous 1-3 wells in 1960, under production conditions, demonstrated the existence of a hot water reservoir at temperatures ranging from 132-181°C at shallow depths of 122-324 m. Total dissolved solids were measured at about 1,500 ppm and with carbon dioxide degassing resulting in calcite formation in the wellbore. At first, the tests involved free-flowing the wells for periods varying from a few days to weeks. Later a shaft-driven, downhole pump was used to keep the fluid in single-phase flow, eliminating calcite formation in the wells and surface equipment. The downhole pumps also keep the brine at the downhole temperatures, rather than allowing it to cool by flashing.

Eleven temperature-gradient test holes were drilled to depths of 113 to 271 m outside the caldera rim between 1962 and 1973 as a part of several U.S. Geological Survey heat flow studies of the Long Valley area. These drill holes, in addition to some 36 temperature profiles from holes completed to depths ranging from 5 to 325 m within the caldera, included the DC well and the CH1-7 core holes in the eastern part of the caldera, among others. The maximum encountered temperature reported for these holes was approximately 110°C in CH7. Temperature reversals were detected in wells CH3, CH5, and CH7, and suggest that warm water flows laterally through the groundwater system to the east from Hot Creek Gorge toward Lake Crowley. These studies showed that minimal thermal gradients of <55°C/km occur along the caldera rim, which were interpreted as areas of recharge into the caldera groundwater system. Thermal gradients near hot spring occurrences were significantly higher (up to 910°C/km), but decrease with depth and typically reversed within the first hundred meters of the boreholes.

The caldera geology, distribution of active hydrothermal surface manifestations, and relict features associated with past hydrothermal activity were summarized by Sorey (1985) and have been described by numerous studies dating back to 1974. Existing geothermal surface expressions at Long Valley include fumaroles, mudpots, and mineral deposits in the western caldera moat and on the flanks of the Resurgent Dome, where the elevation is higher. Hot springs with surface discharge temperatures of 79-93°C occur primarily at Casa Diablo, Hot Creek Gorge, Little Hot Creek, and along the south side of the Resurgent Dome. Additional warm and cold spring discharges occur in the eastern part of the caldera between Hot Creek and Lake Crowley where the surface elevation is relatively low, promoting surface flow of geothermal fluids, as exemplified at Fish Hatchery Springs. The distribution, quantity, and age of borate minerals in Searles Lake, located some 200 km south of Long Valley, indicate that hydrothermal activity associated with the caldera has persisted for the past 300 ka, with the establishment of the present hydrothermal system within the past 40 ka. These results are in good agreement with previous estimates of the occurrence of peak hydrothermal activity at approximately 300 ka based on pervasive hydrothermal alteration in lacustrine deposits southeast of the Resurgent Dome, which are interbedded with Hot Creek moat rhyolite, erupted at about 280 ka. More exact age constraints on recent hydrothermal activity at Long Valley were obtained through age determinations of silica and carbonate hot spring deposits using the 230Th/234U disequilibrium and 230Th/232Th versus 234U/232Th ratio least-squares linear regression methods; these yielded an age range of < 6.4 ka to > 310 ka. Similar geochemical dating techniques applied to hydrothermal minerals recovered from the major fracture zone intersected in the LVEW deep well on the Resurgent Dome (at 2,600 m depth within the pre-caldera basement rocks) yielded ages close to 300 ka, in agreement with previously noted evidence of mineral alteration in rocks of the 280 ka Hot Creek moat rhyolite. This correspondence suggests that the location and magnitude of the caldera’s geothermal systems reached a peak that was perhaps associated with renewed magmatic eruptive activity approximately ~300 ka years ago.

Exploration efforts continued in 1976, when Republic Geothermal, Inc. drilled the 2,109-m-deep 66-29 well into the southeastern portion of the caldera. This was the first deep exploratory well sited within the caldera, drilled to 2,109 m depth. Data from well 66-29 are available online through the California Department of Conservation Division of Oil, Gas & Geothermal Resources and have been contributed to studies of the temperature distribution within the caldera conducted by the U.S. Geological Survey. Temperatures measured in the bottom of well 66-29 were deemed too low for commercial power production at <75°C.

Additional drilling activities continued between 1976 and 1985. Several of these holes were full-size wells, including:

• The (Unocal) Mammoth-1 well, drilled by Union Oil Company in 1979 at Casa Diablo to 1605 m depth, with maximum encountered temperature of 152.7°C at 121m but a temperature of 102.2°C at TD
• The Clay Pit 1 (CP-1) well also drilled by Union Oil Company in 1979 into the Resurgent Dome to 1799 m depth, with maximum encountered temperatures of 147.8°C
• Wells 68-28 and 13-26, drilled by Santa Fe Geothermal Inc. on the Resurgent Dome after 1980 (temperature, lithology, structural data, and specific completion dates for these wells were unavailable for the purposes of this review).

The (Unocal) Mammoth-1 well was the first well drilled at Casa Diablo to test the deep production potential from the intracaldera Bishop Tuff and caldera basement. It was also the first well to intersect the metasedimentary landslide block at 466 m depth beneath the caldera’s southern moat, a tumultuous mix of metapelite and granite that separates the Early Rhyolite reservoir from the underlying Bishop Tuff.

Several core holes were also drilled in the caldera’s west moat by Phillips Petroleum Company in 1982, including:

• PLV-1, drilled to approximately 711 m depth
• PLV-2, drilled to approximately 640 m depth.

Thermal gradient drilling also continued during this period, consisting of several holes including:

• The CH8-10 thermal-gradient holes drilled by the U.S. Geological Survey prior to 1978 to relatively shallow depths ranging from about 55 to 290 m, with maximum encountered temperatures ranging from about 5 to 104°C
• The LVFU 13-21 thermal-gradient hole drilled on the Resurgent Dome by Union Oil Company, approximately ¾ km south and east of the future site of the Long Valley Exploratory Well (LVEW)
• Thermal-gradient holes 28-29 and 34-29, and the 48-29 observation well drilled by Oxy Santa Fe in December 1985 under the CA-11667 lease.

The PLV-1 and PLV-2 wells represent some of the first intermediate-depth holes drilled into the caldera’s west moat and provide temperature, lithologic, and geochemical data for this area that were previously unavailable. The maximum temperature encountered in PLV-1 was about 125°C and about 50°C in PLV-2. Temperature gradients in both wells were favorable; however, the shallow depths of these wells prohibit accurate prediction of reservoir temperatures with increased depth below the west moat. Assuming no temperature reversal were to occur beneath the west moat and gradients remain consistent with depth, reservoir temperatures of 218-288°C in the Bishop Tuff or early rhyolite were predicted for PLV-1, consistent with chemical geothermometer temperature estimates for the hole. In the caldera’s east moat, just south of Hot Creek gorge, a temperature reversal was detected in the CH10 well, providing additional evidence supporting the eastward flow of thermal water through the caldera groundwater system.

Post-Development Exploration and Resource Expansion

Several additional wells were drilled in the caldera after 1983 to the east and north of Highway 395. Among these wells were exploration and monitoring wells drilled near the Fish Hatchery Springs in preparation for the siting of a second binary geothermal power plant, which included the CW-2 and the MPLP CW-3 (a.k.a. Chance 3) wells along the northern edge of the Chance Meadow. These two wells are located within 30 m of a significant thermal spring denoted as the Hot Bubbling Pool. The two wells exhibited temperature profiles similar to that encountered in the upper 300 m of (Unocal) Mammoth-1, with maximum temperatures of 132°C encountered in CW-2.

After several temperature-gradient holes were drilled in 1982 to the west of the Resurgent Dome and a geothermal lease acquisition in the western caldera by Unocal in 1983, two test wells were drilled in the west moat that encountered reservoir temperatures of > 200°C. These wells included the Research Drilling Office #8 (RDO-8) core hole drilled to 715 m depth at Shady Rest in June 1986 as a part of the DOE Continental Scientific Drilling Program and the 44-16 well rotary drilled by Union Oil Company to 1,800 m depth in 1985. A temperature reversal was measured in RDO-8 at approximately 351 m depth within the Early Rhyolite after the well reached about 202°C, measured on June 20, 1986. Well 44-16 encountered two temperature reversals, the first occurring at the base of the Bishop Tuff after temperatures of 218°C were reached at 1.1 km depth, and the second occurring within Tertiary Dacites and Andesites below the Bishop Tuff after temperatures of 192°C were reached at approximately 1.5 km depth. A bottom hole temperature of 181°C encountered in 44-16 represents the hottest temperature measured in metamorphic basement within the caldera at the time.

Single-phase liquid samples were collected at Casa Diablo in 1985 and 1986 from the production wellhead using a cooling coil to prevent flashing and from a mini-separator that allowed flashing of the fluid to liquid and gas under known conditions. Samples were also collected from wells RDO-8 and 44-16, which were drilled in the west moat zone in 1986. Samples from RDO-8 were taken approximately five months after the well was completed and about one month after perforation of the well casing in October 1986. The authors note that some loss of steam and noncondensible gas occurred prior to sampling of well 44-16, and that the sample contained relatively high levels of Na and SO4 due to drilling mud contamination. Interpretations of the isotopic data obtained through analysis of the sampled waters was useful in a variety of reservoir characterization goals, and are discussed in the geochemistry section of this review.

Three additional core holes were drilled in the caldera’s west moat in 1987-1988, including:

• The INYO-4 (a.k.a. RDO-10) 22° slant hole drilled to 854 m depth (610 m vertical depth) approximately 216 m southwest of the Inyo Craters in July 1987 (max temperature of 80°C in a thick early rhyolite flow at 490 m vertical depth)
• The MLGRAP-1 and -2 temperature-gradient holes drilled between November 1987 and January 1988 to depths of about 468 and 491 m near the town of Mammoth Lakes, respectively (max temperatures of 75°C encountered in the bottom of each hole).

The wells were drilled as part of scientific investigations (in the case of INYO-4) and to investigate the feasibility of using thermal waters beneath Mammoth Lakes for direct use (MLGRAP-1 and -2). Data from these wells have been used to constrain the extent of hydrothermal circulation beneath the caldera’s west moat presented in more recent conceptual models of the larger hydrothermal system.

Unocal released the results from 158 time-domain electromagnetic (TDEM) sounding surveys in 1986, in addition to results from 77 magnetotelluric (MT) stations collected in conjunction with Chevron Resources. Data from these surveys have been published by Nordquist (1987) and reinterpreted by Park and Torres-Verdin (1988). These geophysical surveys were designed to assess the Long Valley hydrothermal system and to identify possible deep geothermal drilling targets beneath the western portion of the caldera. Public-domain MT surveys published in 1988 and 1991 contribute additional information to the already extensive electrical data set for the Long Valley region. The Unocal MT data outline a broad region with < 50 ohm-m invariant apparent resistivity at 1 Hz, and four regions with < 20 ohm-m resistivity within the caldera. Lithologic logs from wells drilled into these lower resistivity regions revealed strong correlations between intervals of extensive hydrothermal clay alteration and intervals of low resistivity in the wellbores. Reinterpretation of the Unocal and Chevron data sets have identified similar regions that exhibit low resistivity at shallow depth. Regions of low resistivity were also identified during the 1988 public-domain MT investigations. Two-dimensional MT modeling of both the Unocal and public-domain data sets across the caldera’s west moat revealed a U-shaped low resistivity region with limbs elongated to the northeast and northwest of the (Unocal) Mammoth-1 well at Casa Diablo. This low resistivity region is unusually deep, extending into the pre-caldera basement to the northwest; it is roughly aligned with the projected location of the Laurel-Convict fault within the caldera. The intervals of well RDO-8 that host high temperatures and hydrothermal alteration also correlate with this deep region of low resistivity, reinforcing the association between low resistivity, extensive clay alteration, and active hydrothermal circulation. Intervals of low resistivity near well 44-16 are confined to geothermal reservoirs within the early rhyolite sequence and Bishop Tuff. Resistivity in the vicinity of Mammoth Mountain appears to be similar to the resistivity structure encountered around well 44-16, but this interpretation is less certain due to sparse data coverage in the southwestern portion of the caldera. The MT data, together with data from wells drilled into and adjacent to the northwestern limb of the low resistivity region, support the interpretation that thermal waters ascend along deep basement faults and migrate into volcanic caldera-fill rock units beneath the caldera’s west moat. However, the resistivity data obtained through the MT surveys do not eliminate Mammoth Mountain as a possible upflow zone within the present-day hydrothermal system.

Phase I drilling of the LVEW (formerly known as the LVF 51-20 core hole) was completed on the Resurgent Dome in the summer of 1989 by Oxy Santa Fe and Unocal in cooperation with Sandia National Laboratories, as part of a DOE-funded project attempting to reach high predicted temperatures up to 500°C at about 6 km depth north of Casa Diablo. Temperature gradients measured in LVF 51-20 were 50°C /km following completion of the well, and indicate that recharge of cool meteoric water occurs within the upper 750 m of the Resurgent Dome. This well also provided high-quality lithologic and structural data within the Resurgent Dome, which were previously unavailable. The measured thermal conductivity of core from well LVF 51-20 of 2.3 W/m°C was applied to a thermal gradient of 40°C /km in CP-1 to extrapolate a conductive heat flow estimate of 90 mW/m^2 for the Resurgent Dome. This heat flow estimate is similar to the 120 mW/m^2 measured in the cored section of LVF 51-20. These conductive heat flow measurements are somewhat lower than the values predicted to exist beneath the Resurgent Dome. At the time, a heat flow value of 250 mW/m^2 was expected under the hypotheses that a steady-state magma body exists beneath the center of the caldera at 5-8 km depth or that periodic reinjection of new magma into this magma chamber sustains the heat flow through the caldera. These results suggest either that a shallow magma body no longer exists below the caldera’s Resurgent Dome, as was previously predicted, or that deep non-thermal groundwater circulation along extensional faults influences measured heat flow values in the overlying formations.

The Oh Well-1 temperature-gradient hole was drilled to 664 m depth in December 1991 near the town of Mammoth Lakes in the western caldera, and encountered maximum temperatures of < 85°C. Although the measured temperature gradient was slightly higher than typical background gradients within the caldera, impermeable Early Rhyolite units penetrated by the borehole lacked thermal fluid input and were considered unproductive.

Recent Investigations and Refinement of Conceptual Model

Sorey et al. (1991) integrated information from previous scientific and private industry investigations with new data obtained from fluid sampling, test drilling, and geological and geophysical studies conducted between 1985 and 1988 into a comprehensive conceptual model of the present-day hydrothermal flow system at Long Valley caldera. Lithology and temperature-gradient data from wells drilled prior to 1988 are summarized in detail in the compilation, which includes information from numerous wells described in previous studies (discussed above). Data from many of the wells are also available online through the USGS. Thermal conductivity, XRD, and isotopic analyses of core cuttings from several of the wells discussed have been completed in several studies and seem to prove useful in most cases. Results from these studies are also summarized in Sorey et al. (1991). Relevant data from chemical and isotopic studies published during the same year are also considered in the review.

Farrar et al. (2003) provide a comprehensive conceptual model of the different stages of hydrothermal activity, flow, and recharge in the Long Valley caldera groundwater system based on detailed integration of results from pump tests, fluid level monitoring, temperature logging, and fluid sampling/analysis of the LVEW with information obtained from other wells drilled on or near the Resurgent Dome. Temperature data for five wells drilled on the Resurgent Dome were considered, including new data from the recently completed LVEW borehole. Data from these wells were compared against temperature data from five wells drilled into the hydrothermal system in the west and south moat zones. Temperature profiles in several wells showed peak temperatures between about 120-220°C at relatively shallow depths above ~1000 m depth that decline to the east, associated with eastward and southward lateral flow of thermal water through the system. These occurred in:

• The wells located on the Resurgent Dome east and southeast of LVEW (wells 13-21 and 13-26)
• The west moat (wells RDO-8 and 44-16)
• The south moat (Unocal Mammoth-1 well at Casa Diablo).

This thermal regime differs significantly from the dominantly conductive temperature gradients averaging about 35 °C/km, observed in the Bishop Tuff in the upper interval of LVEW, and throughout the Clay Pit-1 well. This contrasts sharply with the temperature profile in the underlying metamorphic basement, in which temperatures in LVEW become isothermal below 2000 m depth. This isothermal temperature interval is inferred to be related to a major permeable fracture zone intersected by LVEW at 2600 m depth that contains a 103°C fluid, which must be flowing more-or-less vertically outside the wellbore to produce constant temperatures observed across the ~600 m depth interval. The authors of the original study postulate that this fracture zone penetrated by LVEW may be the Eastern Graben Fault that bounds the eastern side of the caldera’s medial graben. Other, shallower wells drilled on or around the Resurgent Dome (wells Unocal Mammoth-1, 13-21, and 13-26) also exhibit isothermal intervals in the Bishop Tuff at about 100°C approaching the bottom of the wellbores. Of these wells, only (Unocal) Mammoth-1 penetrates metamorphic basement, in which the isothermal interval continues to the bottom of the well, similar to LVEW.

Geothermal exploration was ongoing in the Long Valley caldera during the period that the LVEW was being drilled and tested. MPLP drilled several intermediate-depth exploration wells to further evaluate the geothermal potential of the caldera’s south moat. These wells included:

• Well 66-31 in 1992
• The 38-32 core hole, drilled to 353 m depth <1 km south of Casa Diablo in 1992
• The BC 12-31 core hole, drilled to 600 m depth ~ 2 km west of the geothermal power plants and Highway 395 in 2002.

The excellent quality of these core holes yielded considerable new information into the stratigraphy beneath the southern moat zone, including evidence supporting the existence of a post-eruption landslide block that separates the Bishop Tuff from the overlying Early Rhyolite units (see Figure 3). The landslide block acts as an impermeable layer within the hydrothermal flow system, isolating shallow thermal fluid outflow from deeper cool injection fluid and natural meteoric recharge that might otherwise quench the system.

Integrated modeling of deformation, microgravity, and seismic data was also conducted in 2003 to investigate the cause of recent uplift of the Resurgent Dome. Modeling of deformation and microgravity data suggests that there are two sources of subsurface inflation beneath the caldera—one between 7-10 km depth below the Resurgent Dome and a deeper source ~15 km below the caldera's south moat. These data suggest that the shallower source is of intermediate density between magma and aqueous fluid; therefore, uplift of the Resurgent Dome may relate to pressure buildup associated with magmatic brine or gas sourced from deep beneath the caldera’s south moat. These ascending fluids would pool at or below the transition between brittle and ductile deformation conditions, inferred to occur at 6-7 km below the Resurgent Dome based on the depth of the apparent bottom of the seismogenic zone.

Between 2005 and 2007, several high-precision temperature profiles were measured in existing thermal gradient wells in order to assess the thermal regime of the Long Valley Resurgent Dome. The specific goals of this investigation were to understand the extent of advective (lateral) heat transport through the Resurgent Dome to guide future geothermal exploration efforts within the caldera, to provide constraints for assessing the presence or absence of new magma injected below the Resurgent Dome, and to supply a baseline dataset for measuring changes in the thermal regime of the caldera in response to volcanic activity, large earthquakes, and/or geothermal production. These USGS temperature measurements, in addition to past temperature measurements contributed by private industry (taken with various methods and levels of accuracy) and by Sandia National Laboratory, were compiled in a publically accessible database by Farrar et al. (2010). These high-precision temperature profiles were measured between 2005 and 2006 in boreholes 13-21, 13-26, 68-28, and 46-28 using the USGS's heat flow logging truck. Temperatures were measured in the cased wells in June 2005. Boreholes 13-21, 13-26, and 68-28 were perforated in September 2005 to allow sampling of formation waters/gases and establish hydraulic connectivity between the wellbores and the surrounding rocks. Temperatures were re-measured in wells 13-21 (perforated at a depth of 670 m) and 46-28 (un-perforated) in July 2006. The USGS's heat flow logging truck encountered difficulties when attempting to access borehole 35-28 in Fumarole Valley, and so a portable device was used to measure temperatures in the well at 6.10 m intervals in July 2006 and at 3.05 m intervals in May 2007.

A shallow high-temperature interval encountered in the Fumarole Valley boreholes (35-28, 46-28, and 68-28) exists at the same elevation as the upper high-temperature interval of the (Unocal) Mammoth-1 production well at Casa Diablo. Temperatures also appear to decrease across the geothermal field within the shallow depth interval between 2,220-2,300 m elevation to the south and east, from 170°C at Casa Diablo to 128°C in boreholes 35-28 and 46-28. Temperature gradients at the bottom of all wells were <50 °C/km, with temperature reversals and gradient decreases measured below zones of high temperature that occur either at the contact between volcanic flow and tuff units or in fractured zones of the volcanic flow units within the Early Rhyolite sequence. Intervals showing negative temperature gradients also coincided with zones that exhibited the largest temperature difference from 2005 to 2006 in well 46-28 and from 2006 to 2007 in well 35-28. Together, these results indicate that lateral flow of hydrothermal waters occurs at relatively shallow depths in the Early Rhyolite sequence and that at least some of the flow occurs through the southern part of the Resurgent Dome. Data also suggest that cooling of the active hydrothermal system occurs along its flow path and at much lower rates over time. Taken with the occurrence of abundant seismic activity in the (hotter) southern part of the Resurgent Dome compared to the relatively quiet (and cooler) northern part of the Resurgent Dome, these temperature data suggest that a recently emplaced magma body is absent beneath much of the Resurgent Dome.

A study was published in 2011 that investigated the hypothesis that hydrothermal fluids upwell beneath the west moat area from a source reservoir with estimated temperatures ranging from 240 to 273°C, then flow laterally to the east and mix with cool groundwater that infiltrates and recharges the system along ring fractures and faults (an interpretation supported by previous researchers). Details of the modeling parameters, inputs, and results are discussed in the geochemistry section of this page.

Data from the deep (Unocal) Mammoth-1 and other test wells suggest that producing wells at Casa Diablo only tap into a shallow (<200m) thermal resource restricted to permeable Early Rhyolite flow units. Results from deep drilling further suggest that the heat source sustaining the current hydrothermal system is not located directly beneath the Resurgent Dome. Age dating of hydrothermal alteration minerals in and around the Resurgent Dome also provides evidence for past geothermal activity at Long Valley from 300 to 130 ka, while well and borehole data point to a decline in magmatic activity beneath the caldera’s Resurgent Dome over the past 100 ka. Drilling results also establish that increased magmatic activity in the western caldera sustains the present-day hydrothermal system, in which the shallow production wells at Casa Diablo tap into a 170°C outflow originating from upflow from an active geothermal system to the west. Spent geothermal fluid at Casa Diablo is returned to the system via injection into deeper (750m) permeable zones in the Bishop Tuff that underlies the shallow Early Rhyolite reservoir. The production and injection zones are stratigraphically separated by an impermeable landslide block that restricts the vertical migration of subsurface fluids in the caldera’s southern moat. The landslide block is a critical feature that enables sustained production and injection at Casa Diablo, as it effectively isolates hot shallow outflow fluids from deeper cold injection and natural recharge waters that might otherwise quench the system.

Well Field Description

---

Well Field Information

Development Area:
Number of Production Wells:	12
Number of Injection Wells:	8
Number of Replacement Wells:
Average Temperature of Geofluid:	170°C 443.15 K 338 °F 797.67 °R
Sanyal Classification (Wellhead):	Low Temperature
Reservoir Temp (Geothermometry):	232°C 505.15 K 449.6 °F 909.27 °R
Reservoir Temp (Measured):	218°C 491.15 K 424.4 °F 884.07 °R
Sanyal Classification (Reservoir):	High Temperature
Depth to Top of Reservoir:	150 m 0.15 km 0.0932 mi 492.126 ft 164.042 yd
Depth to Bottom of Reservoir:	200 m 0.2 km 0.124 mi 656.168 ft 218.722 yd
Average Depth to Reservoir:	175 m 0.175 km 0.109 mi 574.147 ft 191.382 yd

• "File:longvalleywells.png|thumb|500px|right|Figure 1. Map showing the locations of select exploration and temperature-gradient wells drilled in the Long Valley caldera. An interactive version of this map can be accessed online through the USGS website, complete with links to temperature data, depth, and completion dates for most wells.'"`UNIQ--ref-000000A9-QINU`"']]

  The following is a summary of the monitoring well and development drilling activities that were not discussed in detail in the previous section. It includes well names, total depths, and completion dates of the majority of the drill holes that comprise the Long Valley caldera well field, in addition to the injection/flow testing dates of select wells. The locations of many of these wells are shown in map view in Figure 1; an interactive version of this map can be accessed online through the U.S. Geological Survey website.'"`UNIQ--ref-000000AA-QINU`"' Additional links to maps and well data are also available on the California Department of Conservation Division of Oil, Gas & Geothermal Resources website.'"`UNIQ--ref-000000AB-QINU`"'

  After 1983, several full sized production and injection wells were drilled within the caldera to the east of Highway 395. These included two injection wells (IW-1, IW-2) and four production wells (MBP-1, 2, 3, and 5) drilled at Casa Diablo to support the MP-I geothermal power plant, a joint venture between Mammoth Binary Power Company and Pacific Energy Resources Inc.'"`UNIQ--ref-000000AC-QINU`"' '"`UNIQ--ref-000000AD-QINU`"' '"`UNIQ--ref-000000AE-QINU`"' These wells exhibited temperature profiles similar to that encountered in the upper 300 m of (Unocal) Mammoth-1. Successful construction of the MP-I power plant in 1985 by Kennebec Construction Company (a subsidiary of The Ben Holt Co., who designed the facility) marked the completion of the world’s first air-cooled binary cycle geothermal power plant.'"`UNIQ--ref-000000AF-QINU`"'

  Two more binary power plants (MP-II and PLES-1) came online in 1990, each with 15 MW of generating capacity, bringing the total generation of the Long Valley geothermal resource to 40 MW. The developments included the drilling of eight additional production wells (24-32, 24A-32, 24C-32, 24D, 32, 24-32, 25A-32, 35-32, and 35A-32) and four injection wells (43-32, 43A-32, 44-32, and 44A-32) to support the two new power plants.'"`UNIQ--ref-000000B0-QINU`"' The production wells at the Casa Diablo field are relatively shallow - about 137 m deep. Pumps are used to move water flowing in the western portion of the fields to the power plants. The average production rate of geothermal fluid between the Mammoth-Pacific facility’s three power plants initially ranged from 12,700 to 14,000 gpm to produce a total of 40 MWe (gross).'"`UNIQ--ref-000000B1-QINU`"' '"`UNIQ--ref-000000B2-QINU`"' '"`UNIQ--ref-000000B3-QINU`"' After being utilized for power production, the cooled fluid is injected back into the reservoir to about 610 m depth in a deep fault zone on the east side of the field.

  An injection test was run following completion of Phase II drilling of LVEW after the well was cased to 2313 m depth, leaving a 215 m uncased interval of metasedimentary rock at the bottom of the wellbore.'"`UNIQ--ref-000000B4-QINU`"' The results of this injection test are difficult to interpret due to possible flow of injected water into the annulus between the well casings; however, an upper permeability limit of 0.001 millidarcy for the open-hole section of the well is considered accurate.

  A second injection test was run in October 1999, approximately one year after Phase III drilling of LVEW was completed, to a final depth of 2997 m.'"`UNIQ--ref-000000B5-QINU`"' Approximately 80,000 L of fresh water was used during the test, supplied from the adjacent 270-m-deep Santa Fe well. Cold water was injected at a rate of 3 to 5 L/s over a two-day period, yielding an average injectivity value of 2 L/s/MPa.

  Initial pumping tests of the LVEW were conducted in May and July 2000 to evaluate the pumping and fluid disposal systems.'"`UNIQ--ref-000000B6-QINU`"' Flow rates measured during the final test, performed September 9-12, 2000, were between 0.66 and 0.74 L/s. Total measured drawdown over the course of the test was 0.32 MPa, with a brief period of flattening and slight pressure rise at 24 hours related in part to declining flow rates. The pressure data obtained during this flow test are best matched using modeled solutions for a flow system consisting of a rock matrix with finite hydraulic conductivity cut by a steeply dipping fracture with infinite hydraulic conductivity. For this model to match the pressure data, the horizontal extent of the fracture zone was assumed to be a few hundred meters and a constant head boundary was assigned at a distance of 100 to 400 m from the wellbore. The results of the flow testing, together with analysis of water level changes across the Resurgent Dome, suggest the fracture zone in LVEW can transmit pressure changes over hundreds to thousands of meters over short time periods (i.e., days). Continuous pressure monitoring of the LVEW could therefore be useful in detecting pressure changes associated with inputs of new magma or hydrothermal fluid into the shallow crust beneath the Resurgent Dome.

  Declines in the production temperature and flow rate of the Casa Diablo field between 1993 and 1995 prompted the construction of the Basalt Canyon Pipeline later in 2005 to support the MP-I plant with additional fluids from wells 57-22 and 66-25 near Shady Rest to the east (completed in 2006 to approx. 500 m depth).'"`UNIQ--ref-000000B7-QINU`"' Higher temperature (180°C) fluids from these wells supply the Mammoth-Pacific facility with approximately 4,000 gpm of supplementary production flow for a total sustained flow rate of 12,000 gpm to the geothermal power plants.'"`UNIQ--ref-000000B8-QINU`"'" cannot be used as a page name in this wiki.
• The given value was not understood.

Figure 1. Map showing the locations of select exploration and temperature-gradient wells drilled in the Long Valley caldera. An interactive version of this map can be accessed online through the USGS website, complete with links to temperature data, depth, and completion dates for most wells.

The following is a summary of the monitoring well and development drilling activities that were not discussed in detail in the previous section. It includes well names, total depths, and completion dates of the majority of the drill holes that comprise the Long Valley caldera well field, in addition to the injection/flow testing dates of select wells. The locations of many of these wells are shown in map view in Figure 1; an interactive version of this map can be accessed online through the U.S. Geological Survey website. Additional links to maps and well data are also available on the California Department of Conservation Division of Oil, Gas & Geothermal Resources website.

After 1983, several full sized production and injection wells were drilled within the caldera to the east of Highway 395. These included two injection wells (IW-1, IW-2) and four production wells (MBP-1, 2, 3, and 5) drilled at Casa Diablo to support the MP-I geothermal power plant, a joint venture between Mammoth Binary Power Company and Pacific Energy Resources Inc. These wells exhibited temperature profiles similar to that encountered in the upper 300 m of (Unocal) Mammoth-1. Successful construction of the MP-I power plant in 1985 by Kennebec Construction Company (a subsidiary of The Ben Holt Co., who designed the facility) marked the completion of the world’s first air-cooled binary cycle geothermal power plant.

Two more binary power plants (MP-II and PLES-1) came online in 1990, each with 15 MW of generating capacity, bringing the total generation of the Long Valley geothermal resource to 40 MW. The developments included the drilling of eight additional production wells (24-32, 24A-32, 24C-32, 24D, 32, 24-32, 25A-32, 35-32, and 35A-32) and four injection wells (43-32, 43A-32, 44-32, and 44A-32) to support the two new power plants. The production wells at the Casa Diablo field are relatively shallow - about 137 m deep. Pumps are used to move water flowing in the western portion of the fields to the power plants. The average production rate of geothermal fluid between the Mammoth-Pacific facility’s three power plants initially ranged from 12,700 to 14,000 gpm to produce a total of 40 MWe (gross). After being utilized for power production, the cooled fluid is injected back into the reservoir to about 610 m depth in a deep fault zone on the east side of the field.

An injection test was run following completion of Phase II drilling of LVEW after the well was cased to 2313 m depth, leaving a 215 m uncased interval of metasedimentary rock at the bottom of the wellbore. The results of this injection test are difficult to interpret due to possible flow of injected water into the annulus between the well casings; however, an upper permeability limit of 0.001 millidarcy for the open-hole section of the well is considered accurate.

A second injection test was run in October 1999, approximately one year after Phase III drilling of LVEW was completed, to a final depth of 2997 m. Approximately 80,000 L of fresh water was used during the test, supplied from the adjacent 270-m-deep Santa Fe well. Cold water was injected at a rate of 3 to 5 L/s over a two-day period, yielding an average injectivity value of 2 L/s/MPa.

Initial pumping tests of the LVEW were conducted in May and July 2000 to evaluate the pumping and fluid disposal systems. Flow rates measured during the final test, performed September 9-12, 2000, were between 0.66 and 0.74 L/s. Total measured drawdown over the course of the test was 0.32 MPa, with a brief period of flattening and slight pressure rise at 24 hours related in part to declining flow rates. The pressure data obtained during this flow test are best matched using modeled solutions for a flow system consisting of a rock matrix with finite hydraulic conductivity cut by a steeply dipping fracture with infinite hydraulic conductivity. For this model to match the pressure data, the horizontal extent of the fracture zone was assumed to be a few hundred meters and a constant head boundary was assigned at a distance of 100 to 400 m from the wellbore. The results of the flow testing, together with analysis of water level changes across the Resurgent Dome, suggest the fracture zone in LVEW can transmit pressure changes over hundreds to thousands of meters over short time periods (i.e., days). Continuous pressure monitoring of the LVEW could therefore be useful in detecting pressure changes associated with inputs of new magma or hydrothermal fluid into the shallow crust beneath the Resurgent Dome.

Declines in the production temperature and flow rate of the Casa Diablo field between 1993 and 1995 prompted the construction of the Basalt Canyon Pipeline later in 2005 to support the MP-I plant with additional fluids from wells 57-22 and 66-25 near Shady Rest to the east (completed in 2006 to approx. 500 m depth). Higher temperature (180°C) fluids from these wells supply the Mammoth-Pacific facility with approximately 4,000 gpm of supplementary production flow for a total sustained flow rate of 12,000 gpm to the geothermal power plants.

Research and Development Activities

---

The given value was not understood.

The Mammoth-Pacific facility at Long Valley has been a test site for a number of technologies that have led to important advances in geothermal heat extraction. Various research projects have been conducted regularly at the facilities in cooperation with agencies such as the U.S. Department of Energy (DOE), the National Renewable Energy Lab (NREL), Idaho National Laboratory, Lawrence Livermore National Laboratory (LLNL), Brookhaven National Laboratory, Duke University, and the California Energy Commission. The projects evaluate innovative technologies such as high temperature polymer production pump bearings, the use of reclaimed water for cooling purposes, the use of geothermal water for cooling purposes, mineral recovery, supersaturated vapor expansions in turbines, membrane-based non-condensable gas removal systems, micro-earthquake analysis, and corrosion resistant heat exchanger coatings--all with the purpose of improving operations and efficiency.

Although the silica content of the geothermal water at the Mammoth-Pacific power plant complex is low compared to typical geothermal water, the co-produced silica is of high, and marketable quality represents a large potential source of revenue. The low content makes conventional methods for recovery less effective than average geothermal facilities, inspiring a DOE-funded collaborative effort in 2002 between power plant managers and LLNL to develop new techniques for removing dissolved silica from the geothermal water. The geothermal water undergoes reverse osmosis, concentrating the silica. Then the fluid flows through a stirred reactor where salts or polyelectrolytes (synthetic chemicals used to promote clumping of solid materials) are added to induce silica precipitation. The simple silica molecules bond together to form colloids--silica particles about 10 to 100 nanometers in size--which continue to cluster to form particles big enough to be removed by filters downstream from the reactor. A 2006 study suggests that silica recovery at Mammoth Lakes could reduce the cost of geothermal electricity production by 1.0¢/kWh, producing an added estimated profit of $11 million per year. LLNL is also considering using reverse osmosis to separate lithium, cesium, rubidium, and tungsten. However, these activities have not yet been pursued.

The Mammoth-Pacific power plants use a hybrid air-water cooling system to save water while still retaining a high output in the summertime, rewarded by their Power Purchase Agreement with SCE. Although water-assisted air cooling is expensive, the adequate supply of tertiary treated sewage water from the Mammoth Community Water District (MCWD) also helped make the project more appealing. A 2.5-mile pipeline to bring MCWD wastewater to the complex was built in 2001. The water-assisted cooling project was originally limited to the facility, with smaller air condensers for small-scale testing. After large-scale testing and design improvements were implemented, developers successfully tested geothermal fluid as a water supply for evaporative cooling in 2003. They improved their summer output by 20% and increased electricity production by an average 10-15%. The hybrid cooling system using fiberglass mats is still working at Mammoth, but has yet to be installed at other geothermal facilities.

Geology of the Area

---

Geologic Setting

Tectonic Setting:	Extensional Tectonics
Controlling Structure:	Displacement Transfer Zone
Topographic Features:	Cinder Cone, Horst and Graben
Brophy Model:	Type C: Caldera Resource
Moeck-Beardsmore Play Type:	CV-1b: Magmatic - Intrusive

Geologic Features

Modern Geothermal Features:	Geysers, Hot Springs, Mudpots, Mud Pools, or Mud Volcanoes
Relict Geothermal Features:	Hydrothermal Alteration, Hydrothermal Deposition
Volcanic Age:	Quaternary
Host Rock Age:	Quaternary
Host Rock Lithology:	Bishop Tuff, Metamorphic Basement
Cap Rock Age:	Quaternary
Cap Rock Lithology:	Hydrothermally Altered Early Rhyolite

The given value was not understood.

Regional Setting

Figure 2. Regional map showing the location of the Long Valley Caldera and associated topographic features. The location of select wells and springs in the caldera are also shown. Modified from Figure 1 in Tempel, Sturmer, and Schilling (2011).

Long Valley is a large silicic caldera located in east-central California along the frontal fault of the Sierra Nevada mountain range. The area is bordered by the Transitional Walker Lane structural belt, and is tectonically affected by the extension of the Basin and Range province. The caldera crater occupies an approximately 17 by 32 km area, formed 760,000 years ago during a large rhyolitic eruption (600 km3) that deposited the Bishop Tuff; later eruptions produced the Early Rhyolite and the Moat Rhyolites. More recent activity has included basalt flows, viscous rhyolite flows, and phreatic/magmatophreatic eruptions represented by the Moat Basalts and Inyo Volcanic Chain, respectively. Increased earthquake activity and uplift of a Resurgent Dome in the 1980s and early 1990s are thought to have resulted from the injection of additional magma into the magma chamber and/or to the intrusion of shallow dikes beneath the caldera.

Stratigraphy

The lowermost unit of the caldera-fill stratigraphic sequence consists of a 1,180-m-thick sequence unwelded to densely welded Bishop Tuff (Qbt), which is underlain by Paleozoic metamorphic basement rocks below 1,830 m depth beneath the Resurgent Dome. The Resurgent Dome at the center of the caldera consists of a complex 650-m sequence of Early Rhyolite (Qer) extrusive flows and tuffs, with minor zones of obsidian and perlite that erupted following the deposition of the Bishop Tuff. The Early Rhyolite units are overlain by a sequence mafic and silicic units in the western moat, erupted between 190- and 160 ka. Recent drilling has defined a metasedimentary landslide block (Pzms) that exists between the Bishop Tuff and overlying rhyolite flow units. The subsurface landslide block is present in the south moat at Casa Diablo and extends beneath the Resurgent Dome. It is overlain by a 3-m-thick layer of thinly bedded lacustrine clays, and sits on top of a thin layer of intensely clay-altered ash froth that caps the Bishop Tuff. Trachydacitic lavas that formed Mammoth Mountain erupted ca. 68 ka on the caldera’s southwestern rim during the Mammoth eruptive sequence from 120- to 58 ka. Quartz latite tuff that erupted from Mammoth Mountain (Qqm) overlies the post-caldera Early Rhyolite eruptive units and extends at least 7 km to the east beneath the south moat. The caldera’s south moat sequence includes an approximately 200 m interval of moat basalt (Qmb) that is interbedded with a thick alluvial fill sequence (Qal). More recent volcanic sequences were deposited between 41- and 29 ka in the northwest caldera and from 9 ka to present along the Inyo chain. Silicic volcanism was accompanied by venting of mafic lavas during all of these volcanic episodes, suggesting that the silicic eruptions may have been driven by heat from alkali basalt intrusions into the shallow crust. Phreatic eruptions are thought to have persisted in the area around Mammoth Mountain up to 0.7 ka. Figure 3 shows the caldera-fill stratigraphy in cross section beneath the south moat.

Figure 3. N-S cross section showing the structure and stratigraphy beneath the south moat of Long Valley caldera. The landslide block defined by recent exploration drilling in the south moat is also shown. Figure 5 from Suemnicht et al (2006).

Structure

Current models of the Long Valley hydrothermal system assert that post-caldera magmatic activity and hydrothermal fluid upflow/discharge are structurally controlled, whereas lateral fluid flow to the east through the system is largely controlled by the intracaldera stratigraphy. A major ring fault bounds the structural caldera, which lies well within the caldera’s topographic rim at the surface. An additional important structural feature in the system is represented by the Resurgent Dome, a topographic high in the west-central portion of the caldera that stands approximately 500 m above the valley floor. The majority of the uplift of the Resurgent Dome occurred within 150 ky after the caldera-forming eruption that occurred at about 760 ka during the expulsion of the Bishop Tuff. The dome is cut by numerous north–northwest striking normal faults, including a central graben 4 km in length. These faults are commonly associated with zones of extensive fracturing in the upper sequence of Early Rhyolites and host pervasive argillic and kaolinitic alteration.

Interpretations of resistivity data, temperature profiles in several deep wells, geochemical isotope studies, and stratigraphic displacements encountered in wells drilled into the west moat have provided considerable insight into the association of structures with fluid migration in the Long Valley hydrothermal system. The alignment of low resistivity zones established from Unocal and public-domain MT data with intracaldera projections of fault intersections between the Laurel-Convict fault, the NE-trending Discovery Fault Zone, and the westernmost faults of the Resurgent Dome provide likely candidates for areas hosting enhanced vertical permeability. Such areas would facilitate geothermal fluid upflow beneath the rhyolite plateau from the source reservoir located in deep metamorphic basement rocks. Thermal fluids are then inferred to flow laterally at shallower depths through the volcanic caldera-fill sequence, as indicated by temperature reversals measured in virtually every thermal well drilled in the caldera leading up to 1988, to the west toward well 44-16, and to the southeast in the direction of well RDO-8 and Casa Diablo. Resistivity data are less complete beneath Mammoth Mountain; however, subsurface intersections between north-south and northeast-trending faults within the ring fracture zone in this area cannot be ruled out as a potential zone of upflow in the caldera’s southwest corner. The isothermal temperature interval intersected by LVEW at 2600-m depth is also interpreted to represent a major permeable fracture zone associated with the Eastern Graben Fault that bounds the eastern side of the caldera’s medial graben. The Eastern Graben Fault is a steeply dipping normal fault thought to represent an extension of the northwest-trending Hilton Creek Fault, a major frontal fault of the Sierra Nevada Mountains to the south.

Hydrothermal System

---

• "The hydrothermal system is inferred to be driven by heat from magma bodies that have intruded the shallow crust beneath Mammoth Mountain during the last 200–300 ka.'"`UNIQ--ref-000000DA-QINU`"''"`UNIQ--ref-000000DB-QINU`"''"`UNIQ--ref-000000DC-QINU`"' Rain and snow melt from the mountains enter the caldera ring fractures to depths greater than 1,067 m, where they are heated by contact with basement rocks to about 232°C. Hydrothermal fluids ascend from the basement along faults beneath the west and south moats from this deep source reservoir, and then flow laterally to the southeast where they discharge in the area around Hot Creek.'"`UNIQ--ref-000000DD-QINU`"''"`UNIQ--ref-000000DE-QINU`"''"`UNIQ--ref-000000DF-QINU`"' The landslide block beneath the south moat is thought to act as an impermeable layer that isolates the shallow lateral flow system from deeper flow, helping to maintain moderate-temperature outflow in lacustrine and Early Rhyolite units east of Shady Rest.'"`UNIQ--ref-000000E0-QINU`"' Surface discharges are scattered throughout the caldera, and occur in three predominant groups. Hot spring discharges are encountered around the south and east sides of the Resurgent Dome and within the east moat zone of the caldera. A second group of discharges includes the hot springs that occur at Casa Diablo, Hot Creek Gorge, and Little Hot Creek and at other scattered sites in the south moat.'"`UNIQ--ref-000000E1-QINU`"' Steam vents are also present along northwest–southeast trending faults a few kilometers to the west of Casa Diablo, and also issue from the north side of Mammoth Mountain.'"`UNIQ--ref-000000E2-QINU`"''"`UNIQ--ref-000000E3-QINU`"''"`UNIQ--ref-000000E4-QINU`"' Geothermal well data indicate that deeper hydrothermal waters have temperatures exceeding 200°C beneath the west moat zone, while hydrothermal fluids from more shallow reservoirs within the Bishop Tuff and overlying Early Rhyolite tuffs on the east side of the caldera have a maximum temperature of approximately 170°C.'"`UNIQ--ref-000000E5-QINU`"''"`UNIQ--ref-000000E6-QINU`"''"`UNIQ--ref-000000E7-QINU`"''"`UNIQ--ref-000000E8-QINU`"'" cannot be used as a page name in this wiki.
• The given value was not understood.

The hydrothermal system is inferred to be driven by heat from magma bodies that have intruded the shallow crust beneath Mammoth Mountain during the last 200–300 ka. Rain and snow melt from the mountains enter the caldera ring fractures to depths greater than 1,067 m, where they are heated by contact with basement rocks to about 232°C. Hydrothermal fluids ascend from the basement along faults beneath the west and south moats from this deep source reservoir, and then flow laterally to the southeast where they discharge in the area around Hot Creek. The landslide block beneath the south moat is thought to act as an impermeable layer that isolates the shallow lateral flow system from deeper flow, helping to maintain moderate-temperature outflow in lacustrine and Early Rhyolite units east of Shady Rest.

Surface discharges are scattered throughout the caldera, and occur in three predominant groups. Hot spring discharges are encountered around the south and east sides of the Resurgent Dome and within the east moat zone of the caldera. A second group of discharges includes the hot springs that occur at Casa Diablo, Hot Creek Gorge, and Little Hot Creek and at other scattered sites in the south moat. Steam vents are also present along northwest–southeast trending faults a few kilometers to the west of Casa Diablo, and also issue from the north side of Mammoth Mountain. Geothermal well data indicate that deeper hydrothermal waters have temperatures exceeding 200°C beneath the west moat zone, while hydrothermal fluids from more shallow reservoirs within the Bishop Tuff and overlying Early Rhyolite tuffs on the east side of the caldera have a maximum temperature of approximately 170°C.

Heat Source

---

• "A strong correlation exists between increased seismic activity and rising helium isotope ratios in fumaroles at Mammoth Mountain measured in 1989.'"`UNIQ--ref-000000E9-QINU`"' Similar seismic unrest along the caldera’s western margin coupled with high temperature gradients suggest that recent magmatic intrusions beneath the western moat contribute to the heat input of the present hydrothermal system. Completion of deep drilling of the LVEW in 1998 confirmed that such a magma body was not present beneath the Resurgent Dome, as was previously expected. Subsurface intersections between north-south and northeast-trending faults within the ring fracture zone along the caldera’s west-southwest margin also represent potential conduits that allow magma and hydrothermal fluids to infiltrate the shallow crust." cannot be used as a page name in this wiki.
• The given value was not understood.

A strong correlation exists between increased seismic activity and rising helium isotope ratios in fumaroles at Mammoth Mountain measured in 1989. Similar seismic unrest along the caldera’s western margin coupled with high temperature gradients suggest that recent magmatic intrusions beneath the western moat contribute to the heat input of the present hydrothermal system. Completion of deep drilling of the LVEW in 1998 confirmed that such a magma body was not present beneath the Resurgent Dome, as was previously expected. Subsurface intersections between north-south and northeast-trending faults within the ring fracture zone along the caldera’s west-southwest margin also represent potential conduits that allow magma and hydrothermal fluids to infiltrate the shallow crust.

Geofluid Geochemistry

---

Geochemistry

Salinity (low):	1260
Salinity (high):	1430
Salinity (average):	1345
Brine Constituents:
Water Resistivity:

The given value was not understood.

Hydrologic and geochemical data from exploration wells and hot springs examined by numerous published studies have been used to estimate reservoir temperatures in the Long Valley hydrothermal system, and to investigate the extent to which re-equilibration and mixing affect thermal fluid chemistry along the eastward flow path. Prior to 1976, Gerald A. Waring and Lawrence M. Willey conducted early sampling and chemical-isotopic analysis of geothermal well and hot spring waters at Long Valley. Some of these early samples were also reanalyzed later to investigate corrections to the sulfate geothermometer to account for the effects of subsurface boiling and dilution; this was done in order to increase the effective temperature range of the geothermometer from 100-200°C to 140-350°C. This range encompasses the temperature range of greatest interest in geothermal exploration/production. Analysis and geothermometry were described as applied to the Yellowstone, WY, Long Valley, CA, and Raft River, ID geothermal systems. Waters analyzed from the Casa Diablo Magma-Ritchie No. 5 geothermal well, Hot Creek Spring, and unnamed hot spring GT 31 were apparently of mixed composition between thermal water and dilute near-surface meteoric water, and yielded minimum reservoir temperature estimates from 184 to 246°C through use of the sulfate isotope geothermometer. Of these locations, the Magma-Ritchie No. 5 well returned the highest temperature estimate.

Additional investigations of the chemical-isotopic characteristics of fluids across the caldera demonstrated that the deuterium versus O18 ratios for select hydrologic features show progressive depletions in the isotopic content of cold meteoric water, from isotopically heavy waters along the caldera’s western rim to isotopically lighter waters approaching the eastern rim. Data from thermal wells at Casa Diablo (MBP) and Chance Meadow (CW-2) in the south moat, wells RDO-8 and 44-16 in the west moat, and cold-water wells (WD) and springs (VR) on the southeast flank of Mammoth Mountain show a similar distribution with respect to their isotopic contents. Spring and well water samples follow a general mixing trend, with thermal waters sampled in the west and south moat zones occupying one end and cold meteoric waters sampled from the south and east moat zones occupying the other. Dilution of thermal waters with meteoric groundwater is indicated by progressively lighter isotopic values with distance eastward from Casa Diablo. In the vicinity of Casa Diablo, near-surface boiling causes an observable shift in the deuterium versus O18 isotopic ratio between the average value for the single-phase liquid samples collected from the MBP wells and the spring samples from Casa Diablo Geyser and Colton Spring. Tritium concentrations in the thermal wells generally show an inverse correlation with the chloride contents measured in the sampled waters.

Temperature estimates of the geothermal reservoir were also calculated for the RDO-8 and Casa Diablo (MBP-1 and MBP-3) well samples using five different geothermometers, which produced a range of temperatures from 181-248°C. The Na/K and Na/K/Ca geothermometers yielded an average temperature of 218°C, in good agreement with the maximum temperature of 214°C measured in the reservoir at that time. Silica-geothermometer temperature estimates for the well samples ranged from 196-202°C. This temperature range is lower than the cation-geothermometer temperature estimates for the same samples, indicating loss of silica in association with declining reservoir temperatures or with dilution by waters of comparatively lower silica content. Sulfate-water isotope geothermometer temperature estimates for the two Casa Diablo wells were 222 and 232°C, whereas the anhydrite-solubility geothermometer temperature estimates for the RDO-8 and Casa Diablo MBP-3 wells were 231 and 248°C, respectively. Given the lack of information on the existence of anhydrite in the reservoir rock, the sulfate-water isotope temperature estimate is considered the more reliable of the estimates provided by the two sulfate-dependent geothermometers. While the temperature estimates calculated using the various geothermometers are fairly consistent, they greatly exceed the reservoir fluid temperatures encountered in wells drilled within the caldera to date.

These geochemical data, integrated with data from drill holes, rock analysis, geologic mapping, and geophysical investigations, indicate that the roots of the present-day hydrothermal system at Long Valley exist beneath the caldera’s west moat. Under this interpretation, the roots of the system include the heat source, the source reservoir consisting of the deepest and hottest temperature reservoir within the system, and the principal zone(s) of fluid upflow that transmits thermal waters into the shallower outflow zones that exist within the caldera-fill volcanic rocks. Reservoir temperatures between 202-214°C encountered in volcanic fill penetrated by wells drilled in the west moat are among the highest measured at Long Valley prior to 1991; however, chemical geothermometers suggest that maximum reservoir temperatures as high as 248°C exist within the hydrothermal system. Fluids from the wells were isotopically similar to those sampled from springs and wells at Casa Diablo, suggesting a common source reservoir. Isotope data, as well as geophysical, lithology- and temperature-log data (discussed in the structure section of this review), suggest that the source reservoir is located in metamorphic basement rocks beneath the west moat, from which thermal fluids likely ascend along steeply dipping structures. Recharge of the hydrothermal system is thought to occur along the western rim of the caldera (including Mammoth Mountain), as indicated by stable water isotope data.

Gas sampling and analysis have also proved to be valuable tools for understanding the complex magmatic-hydrothermal system at Long Valley caldera. Fumarolic CO2 sampled at Casa Diablo contained deltaC13 values of -5.6 to -5.7. DeltaC13 values become systematically more negative to the east of Casa Diablo, suggesting that dissolution of isotopically light carbonate minerals in the host rock occurs as reservoir temperatures decline in response to progressive mixing with meteoric waters.

Hot spring waters and steam were also analyzed for their 3He/4He isotopic contents, which reveal values that ranged from 4.5 x Ra (west moat and at Casa Diablo) to 6.0-6.5 x Ra (Hot Creek Gorge and at Little Hot Creek). Significant changes in 3He/4He content were also measured in a fumarole on the north side of Mammoth Mountain from 1989-1990, during which time helium isotope values increased from 3.6 x Ra to 5.5 x Ra from July to October 1989, and then stabilized at 6 x Ra from January to November 1990. This shift exceeds the measured seasonal variation in 3He/4He content, and is suggestive of the addition of magmatic helium to the hydrothermal system. The increase in x Ra coincides with an intense period of seismic activity beneath Mammoth Mountain from May to October 1989 that was accompanied by a magmatic intrusion, as indicated by earthquake hypocenters and deformation data. These data suggest that recent magmatic activity may contribute to the thermal high beneath the caldera’s western margin. Gas isotope data from these and previous studies have been tabulated by Winnett and Janik and Sorey et al., and are discussed in comparison with gas isotope data from the Yellowstone, WY and Valles, NM calderas by Goff and Janik.

Fluids sampled from LVEW between May 2000 and September 2001 were also used to identify magmatic gases present in the well fluids. Isotopic ratios of helium and CO2 determined for gases sampled at LVEW were 3.66 Ra (3He/4He ratio) and -6.4 %o (delta C13 in dissolved inorganic carbon). These values resemble those of thermal water equilibrated with a magmatic gas input, and the relatively heavy delta C13 value is similar to those measured in other thermal waters in the caldera, although the dissolved carbon could also be sourced from metamorphic basement rocks beneath the Resurgent Dome.

Data from previous hydrologic and geochemical studies of exploration wells and hot springs have been integrated into a comprehensive simulation that evaluates the hypothesis that hydrothermal fluids upwell beneath the west moat area from a source reservoir with estimated temperatures ranging from 240 to 273°C, then flow laterally to the east and mix with cool groundwater that infiltrates and recharges the system along ring fractures and faults. The simulation also considers previous petrographic studies in order to provide a realistic assessment of aquifer mineral reactions along the flow path. The extent of water-rock interactions were constrained by mass balance calculations and thermodynamic modeling based on these data. These considerations allow for a more quantitative examination of hydrologic and geochemical processes that occur in the shallow geothermal aquifer between Casa Diablo and Shady Rest. The degree of fluid mixing required to produce a geothermal fluid with a similar composition to those sampled in geothermal well MBP-4 at Casa Diablo was modeled using the fluid properties of samples from Shady Rest and Laurel Spring as shallow thermal and cold groundwater endmember fluids. Simulated mixing and reaction of the thermal fluids with aquifer minerals along their flow path around the southern side of the Resurgent Dome toward Casa Diablo yielded a fluid with a composition similar to that measured in well MBP-4. The simulation showed that: (1) secondary mineral assemblages encountered within the shallow Long Valley caldera reservoirs are best represented in the model under open system conditions, (2) the chemical composition of the Casa Diablo well fluids is most closely reproduced through conservative mixing of 82% Shady Rest and 18% Laurel Spring waters, and (3) saturation of chemical components that leads to the precipitation of quartz, pyrite, smectite, and hematite in the system appears to relate strongly to water-rock interactions affecting the composition of the fluid phase. These modeling results are consistent with previous studies that inferred that high flow rates of 100–200 m/yr in the Long Valley system are indicative of an open system. Similarities between the water compositions produced by the reaction path model of water–rock interactions, conservative water mixing model, and actual water compositions measured in the Casa Diablo well suggests that dissolution of feldspars (the dominant reactive mineral phase in the system) provides the chemical components needed for the conservative formation of clay minerals and calcite in the reservoir.

NEPA-Related Analyses (4)

---

Below is a list of NEPA-related analyses that have been conducted in the area - and logged on OpenEI. To add an additional NEPA-related analysis, see the NEPA Database.

CSV

Document #	Analysis Type	Applicant	Application Date	Decision Date	Lead Agency	Development Phase(s)	Techniques
DOI-BLM-CA-017-05-051	EA	Mammoth Pacific			Bureau of Land Management	Geothermal/Well Field
DOI-BLM-CA-017-P006-60	EIS	Pacific Energy		9 June 1989	Bureau of Land Management	Geothermal/Well Field Geothermal/Power Plant	Development Drilling
DOI-BLM-CA-170-02-15	EA	Mammoth Pacific		21 February 2002	Bureau of Land Management	Geothermal/Exploration	Drilling Methods Exploration Drilling Exploratory Well Slim Holes
DOI-BLM-CA-ES-2013-002+1793-EIS	EIS	ORNI 50 LLC	17 February 2010	12 August 2013	Bureau of Land Management	Geothermal/Power Plant

Exploration Activities (84)

---

Below is a list of Exploration that have been conducted in the area - and cataloged on OpenEI. Add a new Exploration Activity

Page	Technique	Activity Start Date	Activity End Date	Reference Material
Analytical Modeling At Long Valley Caldera Geothermal Area (White & Peterson, 1991)	Analytical Modeling	1985	1988	Chemical equilibrium and mass balance relationships associated with the Long Valley hydrothermal system, California, U.S.A.
Compound and Elemental Analysis At Long Valley Caldera Geothermal Area (Bergfeld, Et Al., 2006)	Compound and Elemental Analysis		2006	Carbon Dioxide Emissions From Vegetation-Kill Zones Around The Resurgent Dome Of Long Valley Caldera, Eastern California, USA
Compound and Elemental Analysis At Long Valley Caldera Geothermal Area (Evans, Et Al., 2002)	Compound and Elemental Analysis		2002	Tracing And Quantifying Magmatic Carbon Discharge In Cold Groundwaters- Lessons Learned From Mammoth Mountain, USA
Compound and Elemental Analysis At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Compound and Elemental Analysis	2000	2001	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Compound and Elemental Analysis At Long Valley Caldera Geothermal Area (McKenzie & Truesdell, 1977)	Compound and Elemental Analysis	1976	1977	Geothermal Reservoir Temperatures Estimated from the Oxygen Isotope Compositions of Dissolved Sulfate and Water from Hot Springs and Shallow Drillholes Geochemistry of Thermal Waters in Long Valley, Mono County, California
Conceptual Model At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Conceptual Model	2003	2003	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Conceptual Model At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)	Conceptual Model	1985	1988	New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Core Analysis At Long Valley Caldera Geothermal Area (Pribnow, Et Al., 2003)	Core Analysis		2003	Fluid Flow In The Resurgent Dome Of Long Valley Caldera- Implications From Thermal Data And Deep Electrical Sounding
Core Analysis At Long Valley Caldera Geothermal Area (Smith & Suemnicht, 1991)	Core Analysis	1985	1988	Oxygen Isotope Evidence For Past And Present Hydrothermal Regimes Of Long Valley Caldera, California
Core Analysis At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)	Core Analysis	1985	1988	New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits Hydrothermal Alteration and Past and Present Thermal Regimes in the Western Moat of Long Valley Caldera A Sr-Isotopic Comparison Between Thermal Waters, Rocks, And Hydrothermal Calcites, Long Valley Caldera, California Oxygen Isotope Evidence For Past And Present Hydrothermal Regimes Of Long Valley Caldera, California
Core Holes At Long Valley Caldera Geothermal Area (Benoit, 1984)	Core Holes	1982	1982	Initial Results from Drillholes PLV-1 and PLV-2 in the Western Moat of the Long Valley Caldera Present State of the Hydrothermal System in Long Valley Caldera, California
Core Holes At Long Valley Caldera Geothermal Area (Chu, Et Al., 1990)	Core Holes		1989	The Magma Energy Program Deep Drilling to the Magmatic Environment in Long Valley Caldera The Magma Energy Exploratory Well Summary- Long Valley Exploratory Well
Core Holes At Long Valley Caldera Geothermal Area (Eichelberger, Et Al., 1988)	Core Holes	1987	1988	Structure and Stratigraphy Beneath a Young Phreatic Vent: South Inyo Crater, Long Valley Caldera, California New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Core Holes At Long Valley Caldera Geothermal Area (Lachenbruch, Et Al., 1976)	Core Holes	1962	1973	The Near-Surface Hydrothermal Regime of Long Valley Caldera The Hydrothermal System of Long Valley Caldera, California
Core Holes At Long Valley Caldera Geothermal Area (Urban, Et Al., 1987)	Core Holes	1986	1986	Hydrothermal Regime of the Southwest Moat of the Long Valley Caldera, Mono County, California, and Its Relation to Seismicity--New Evidence from the Shady Rest Borehole (RD08) Temperatures and Natural Gamma-Ray Logs Obtained in 1986 from Shady Rest Drill Hole RD08, Mammoth Lakes, Mono County, California A Core Hole in the Southwestern Moat of the Long Valley Caldera: Early Results New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Cuttings Analysis At Long Valley Caldera Geothermal Area (Pribnow, Et Al., 2003)	Cuttings Analysis		2003	Fluid Flow In The Resurgent Dome Of Long Valley Caldera- Implications From Thermal Data And Deep Electrical Sounding
Cuttings Analysis At Long Valley Caldera Geothermal Area (Smith & Suemnicht, 1991)	Cuttings Analysis	1985	1988	Oxygen Isotope Evidence For Past And Present Hydrothermal Regimes Of Long Valley Caldera, California
Development Wells At Long Valley Caldera Geothermal Area (Associates, 1987)	Development Wells	1990	1990	Mammoth Pacific Geothermal Development Projects: Units II and III Technical Geologic Overview of Long Valley Caldera for the Casa Diablo IV Geothermal Development Project Hydrology of the Geothermal System in Long Valley Caldera, California Hydrologic Monitoring Summary Long Valley Caldera, California
Development Wells At Long Valley Caldera Geothermal Area (Holt & Campbell, 1984)	Development Wells	1983	1985	Mammoth Geothermal Project Mammoth Pacific Geothermal Development Projects: Units II and III Hydrologic and Geochemical Monitoring in Long Valley Caldera, Mono County, California, 1982-1984 Technical Geologic Overview of Long Valley Caldera for the Casa Diablo IV Geothermal Development Project
Development Wells At Long Valley Caldera Geothermal Area (Suemnicht, Et Al., 2007)	Development Wells	2005	2006	The Shallow Hydrothermal System of Long Valley Caldera, California Technical Geologic Overview of Long Valley Caldera for the Casa Diablo IV Geothermal Development Project
Direct-Current Resistivity Survey At Long Valley Caldera Geothermal Area (Pribnow, Et Al., 2003)	Direct-Current Resistivity Survey		2003	Fluid Flow In The Resurgent Dome Of Long Valley Caldera- Implications From Thermal Data And Deep Electrical Sounding
Exploratory Boreholes At Long Valley Caldera Geothermal Area (Suemnicht, Et Al., 2007)	Exploratory Boreholes	1992	2002	The Shallow Hydrothermal System of Long Valley Caldera, California
Exploratory Well At Long Valley Caldera Geothermal Area (McNitt, 1963)	Exploratory Well	1959	1962	Exploration and Development of Geothermal Power in California The Hydrothermal System of Long Valley Caldera, California New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Exploratory Well At Long Valley Caldera Geothermal Area (Smith & Rex, 1977)	Exploratory Well	1976	1976	Drilling results from eastern Long Valley Caldera The Hydrothermal System of Long Valley Caldera, California
Exploratory Well At Long Valley Caldera Geothermal Area (Sorey, 1985)	Exploratory Well	1976	1985	Present State of the Hydrothermal System in Long Valley Caldera, California New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits Results of Deep Drilling in the Western Moat of Long Valley, California Long Valley Caldera Field Trip Log
Exploratory Well At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)	Exploratory Well	1983	1983	New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits Hydrologic and Geochemical Monitoring in Long Valley Caldera, Mono County, California, 1982-1984
Exploratory Well At Long Valley Caldera Geothermal Area (Suemnicht, 1987)	Exploratory Well	1985	1985	Results of Deep Drilling in the Western Moat of Long Valley, California Basement Structure and Implications for Hydrothermal Circulation Patterns in the Western Moat of Long Valley Caldera, California New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Field Mapping At Long Valley Caldera Geothermal Area (Sorey & Farrar, 1998)	Field Mapping	1985	1997	Changes in Surficial Features Associated with Geothermal Development in Long Valley Caldera, California, 1985-1997
Field Mapping At Long Valley Caldera Geothermal Area (Sorey, 1985)	Field Mapping	1974	1985	Present State of the Hydrothermal System in Long Valley Caldera, California Volcanism, Structure, and Geochronology of Long Valley Caldera, Mono County, California Uranium Series Disequilibrium: Applications to Environmental Problems Hydrothermal Alteration Mineral Studies in Long Valley, In- Proceedings of the Second Workshop on Hydrologic and Geochemical Monitoring in the Long Valley Caldera
Flow Test At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Flow Test	2000	2001	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Gas Flux Sampling At Long Valley Caldera Geothermal Area (Bergfeld, Et Al., 2006)	Gas Flux Sampling		2006	Carbon Dioxide Emissions From Vegetation-Kill Zones Around The Resurgent Dome Of Long Valley Caldera, Eastern California, USA
Gas Flux Sampling At Long Valley Caldera Geothermal Area (Lewicki, Et Al., 2008)	Gas Flux Sampling		2008	Six-Week Time Series Of Eddy Covariance CO2 Flux At Mammoth Mountain, California- Performance Evaluation And Role Of Meteorological Forcing
Geodetic Survey At Long Valley Caldera Geothermal Area (Newman, Et Al., 2006)	Geodetic Survey	1995	2000	A Four-Dimensional Viscoelastic Deformation Model For Long Valley Caldera, California, Between 1995 And 2000
Geothermal Literature Review At Long Valley Caldera Geothermal Area (Goldstein & Flexser, 1984)	Geothermal Literature Review	1984	1984	Melt Zones Beneath Five Volcanic Complexes in California: An Assessment of Shallow Magma Occurrences
Geothermal Literature Review At Long Valley Caldera Geothermal Area (Sorey, Et Al., 2003)	Geothermal Literature Review		2003	Summary Of Recent Research In Long Valley Caldera, California
Geothermometry At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Geothermometry	2000	2001	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Geothermometry At Long Valley Caldera Geothermal Area (Mariner & Willey, 1976)	Geothermometry		1976	Geochemistry of Thermal Waters in Long Valley, Mono County, California Chemical and Isotopic Prediction of Aquifer Temperatures in the Geothermal System at Long Valley, California New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Geothermometry At Long Valley Caldera Geothermal Area (McKenzie & Truesdell, 1977)	Geothermometry	1976	1977	Geothermal Reservoir Temperatures Estimated from the Oxygen Isotope Compositions of Dissolved Sulfate and Water from Hot Springs and Shallow Drillholes Geochemistry of Thermal Waters in Long Valley, Mono County, California
Geothermometry At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)	Geothermometry	1985	1988	New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Ground Gravity Survey At Long Valley Caldera Geothermal Area (Battaglia, Et Al., 2003)	Ground Gravity Survey		2003	The Mechanics of Unrest at Long Valley Caldera, California: 1. Modeling the Geometry of the Source Using Gps, Leveling and Two-Color EDM Data The Mechanics of Unrest at Long Valley Caldera, California. 2. Constraining the Nature of the Source Using Geodetic and Micro-Gravity Data Deformation of the Long Valley Caldera, California: Inferences from Measurements from 1988 to 2001 Relations Between Seismicity and Deformation During Unrest in Long Valley Caldera, California, from 1995 Through 1999 Temperatures at the Base of the Seismogenic Crust Beneath Long Valley Caldera, California, and the Phlegrean Fields Caldera, Italy. In- Volcanic Seismology Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Ground Gravity Survey At Long Valley Caldera Geothermal Area (Laney, 2005)	Ground Gravity Survey		2004	Federal Geothermal Research Program Update - Fiscal Year 2004
Hyperspectral Imaging At Long Valley Caldera Geothermal Area (Martini, Et Al., 2004)	Hyperspectral Imaging	1999	1999	Hyperspectral Mineral Mapping In Support Of Geothermal Exploration- Examples From Long Valley Caldera, CA and Dixie Valley, NV, USA
Injectivity Test At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Injectivity Test	1999	1999	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Injectivity Test At Long Valley Caldera Geothermal Area (Morin, Et Al., 1993)	Injectivity Test		1992	Results of the Flowmeter-Injection Test in the Long Valley Exploratory Well (Phase II), Long Valley, California Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Isotopic Analysis At Long Valley Caldera Geothermal Area (Evans, Et Al., 2002)	Isotopic Analysis- Fluid		2002	Tracing And Quantifying Magmatic Carbon Discharge In Cold Groundwaters- Lessons Learned From Mammoth Mountain, USA
Isotopic Analysis At Long Valley Caldera Geothermal Area (Goff, Et Al., 1991)	Isotopic Analysis- Fluid	1985	1986	A Sr-Isotopic Comparison Between Thermal Waters, Rocks, And Hydrothermal Calcites, Long Valley Caldera, California
Isotopic Analysis At Long Valley Caldera Geothermal Area (Smith & Suemnicht, 1991)	Isotopic Analysis- Fluid	1985	1988	Oxygen Isotope Evidence For Past And Present Hydrothermal Regimes Of Long Valley Caldera, California
Isotopic Analysis- Fluid At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Isotopic Analysis- Fluid	2000	2001	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Isotopic Analysis- Fluid At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)	Isotopic Analysis- Fluid	1985	1988	New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Isotopic Analysis- Fluid At Long Valley Caldera Geothermal Area (Taylor & Gerlach, 1983)	Isotopic Analysis- Fluid	1983	1986	Chemical and Isotopic Composition of Casa Diablo Hot Spring: Magmatic CO2 near Mammoth Lakes, CA Isotopic Composition of Carbon in Fluids from the Long Valley Geothermal System, California, In- Proceedings of the Second Workshop on Hydrologic and Geochemical Monitoring in the Long Valley Caldera New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Isotopic Analysis- Gas At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Isotopic Analysis- Fluid	2000	2001	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling Carbon Dioxide and Helium Emissions from a Reservoir of Magmatic Gas Beneath Mammoth Mountain, California Tracing And Quantifying Magmatic Carbon Discharge In Cold Groundwaters- Lessons Learned From Mammoth Mountain, USA
Isotopic Analysis- Gas At Long Valley Caldera Geothermal Area (Welhan, Et Al., 1988)	Isotopic Analysis- Fluid	1988	1990	Helium Isotopes in Geothermal and Volcanic Gases of the Western United States, II. Long Valley Caldera Increases in 3He/4He in Fumarolic Gas Associated with the 1989 Earthquake Swarm Beneath Mammoth Mountain, California The 1989 Earthquake Swarm Beneath Mammoth Mountain, California: An Initial Look at the 4 May Through 30 September Activity Isotopic Composition of Carbon in Fluids from the Long Valley Geothermal System, California, In- Proceedings of the Second Workshop on Hydrologic and Geochemical Monitoring in the Long Valley Caldera Gas Geochemistry Of The Valles Caldera Region, New Mexico And Comparisons With Gases At Yellowstone, Long Valley And Other Geothermal Systems
Isotopic Analysis-Fluid At Long Valley Caldera Geothermal Area (1977)	Isotopic Analysis- Fluid	1977	1977	Geothermal reservoir temperatures estimated from the oxygen isotope compositions of dissolved sulfate and water from hot springs and shallow drillholes
Isotopic Analysis-Fluid At Long Valley Caldera Geothermal Area (McKenzie & Truesdell, 1977)	Isotopic Analysis- Fluid	1976	1977	Geothermal Reservoir Temperatures Estimated from the Oxygen Isotope Compositions of Dissolved Sulfate and Water from Hot Springs and Shallow Drillholes Geochemistry of Thermal Waters in Long Valley, Mono County, California
Magnetotellurics At Long Valley Caldera Geothermal Area (Hermance, Et Al., 1988)	Magnetotellurics	1988	1991	The Long Valley/Mono Basin Volcanic Complex: A Preliminary Magnetotelluric and Magnetic Variation Interpretation Magnetotelluric Transect of Long Valley Caldera: Resistivity Cross Section, Structural Implications, and the Limits of a 2-D Analysis New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Magnetotellurics At Long Valley Caldera Geothermal Area (Nordquist, 1987)	Magnetotellurics		1986	Mapping the Hydrothermal System Beneath the Western Moat of Long Valley Caldera Using Magnetotelluric and Time-domain Electromagnetic Measurements A Systematic Approach to the Interpretation of Magnetotelluric Data in Volcanic Environments with Applications to the Quest for Magma in Long Valley, California New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Mercury Vapor At Long Valley Caldera Geothermal Area (Klusman & Landress, 1979)	Mercury Vapor		1979	Mercury In Soils Of The Long Valley, California, Geothermal System
Micro-Earthquake At Long Valley Caldera Geothermal Area (Foulger, Et Al., 2004)	Micro-Earthquake		2004	Non-Double-Couple Microearthquakes At Long Valley Caldera, California, Provide Evidence For Hydraulic Fracturing
Micro-Earthquake At Long Valley Caldera Geothermal Area (Stroujkova & Malin, 2001)	Micro-Earthquake		2001	Multiple Ruptures For Long Valley Microearthquakes- A Link To Volcanic Tremor(Question)
Modeling-Computer Simulations At Long Valley Caldera Geothermal Area (Battaglia, Et Al., 2003)	Modeling-Computer Simulations		2003	The Mechanics of Unrest at Long Valley Caldera, California: 1. Modeling the Geometry of the Source Using Gps, Leveling and Two-Color EDM Data The Mechanics of Unrest at Long Valley Caldera, California. 2. Constraining the Nature of the Source Using Geodetic and Micro-Gravity Data Deformation of the Long Valley Caldera, California: Inferences from Measurements from 1988 to 2001 Relations Between Seismicity and Deformation During Unrest in Long Valley Caldera, California, from 1995 Through 1999 Temperatures at the Base of the Seismogenic Crust Beneath Long Valley Caldera, California, and the Phlegrean Fields Caldera, Italy. In- Volcanic Seismology Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Modeling-Computer Simulations At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Modeling-Computer Simulations	2001	2003	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Modeling-Computer Simulations At Long Valley Caldera Geothermal Area (Newman, Et Al., 2006)	Modeling-Computer Simulations	1995	2000	A Four-Dimensional Viscoelastic Deformation Model For Long Valley Caldera, California, Between 1995 And 2000
Modeling-Computer Simulations At Long Valley Caldera Geothermal Area (Pribnow, Et Al., 2003)	Modeling-Computer Simulations		2003	Fluid Flow In The Resurgent Dome Of Long Valley Caldera- Implications From Thermal Data And Deep Electrical Sounding
Modeling-Computer Simulations At Long Valley Caldera Geothermal Area (Tempel, Et Al., 2011)	Modeling-Computer Simulations		2011	Geochemical Modeling of the Near-Surface Hydrothermal System Beneath the Southern Moat of Long Valley Caldera, California
Multispectral Imaging At Long Valley Caldera Geothermal Area (Pickles, Et Al., 2001)	Multispectral Imaging		2001	Geobotanical Remote Sensing For Geothermal Exploration
Resistivity Log At Long Valley Caldera Geothermal Area (Nordquist, 1987)	Single-Well and Cross-Well Resistivity		1986	Mapping the Hydrothermal System Beneath the Western Moat of Long Valley Caldera Using Magnetotelluric and Time-domain Electromagnetic Measurements The Long Valley/Mono Basin Volcanic Complex: A Preliminary Magnetotelluric and Magnetic Variation Interpretation Magnetotelluric Transect of Long Valley Caldera: Resistivity Cross Section, Structural Implications, and the Limits of a 2-D Analysis New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Rock Sampling At Long Valley Caldera Geothermal Area (Goff, Et Al., 1991)	Rock Sampling	1985	1986	A Sr-Isotopic Comparison Between Thermal Waters, Rocks, And Hydrothermal Calcites, Long Valley Caldera, California
Soil Sampling At Long Valley Caldera Geothermal Area (Klusman & Landress, 1979)	Soil Sampling		1979	Mercury In Soils Of The Long Valley, California, Geothermal System
Static Temperature Survey At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Static Temperature Survey	1998	2002	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Static Temperature Survey At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2010)	Static Temperature Survey	2005	2007	Temperature Data From Wells in Long Valley Caldera, California The Thermal Regime In The Resurgent Dome Of Long Valley Caldera, California- Inferences From Precision Temperature Logs In Deep Wells
Teleseismic-Seismic Monitoring At Long Valley Caldera Geothermal Area (Newman, Et Al., 2006)	Teleseismic-Seismic Monitoring	1995	2000	A Four-Dimensional Viscoelastic Deformation Model For Long Valley Caldera, California, Between 1995 And 2000
Thermal Gradient Holes At Long Valley Caldera Geothermal Area (Conservation, 2009)	Thermal Gradient Holes	1991	1991	Casa Diablo/Long Valley Caldera Area, Mono County Technical Geologic Overview of Long Valley Caldera for the Casa Diablo IV Geothermal Development Project
Thermal Gradient Holes At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Thermal Gradient Holes	1998	2002	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
Thermal Gradient Holes At Long Valley Caldera Geothermal Area (Lachenbruch, Et Al., 1976)	Thermal Gradient Holes	1962	1973	The Near-Surface Hydrothermal Regime of Long Valley Caldera The Hydrothermal System of Long Valley Caldera, California
Thermal Gradient Holes At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1978)	Thermal Gradient Holes	1978	1985	The Hydrothermal System of Long Valley Caldera, California
Thermal Gradient Holes At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)	Thermal Gradient Holes	1985	1988	New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Time-Domain Electromagnetics At Long Valley Caldera Geothermal Area (Nordquist, 1987)	Time-Domain Electromagnetics		1986	Mapping the Hydrothermal System Beneath the Western Moat of Long Valley Caldera Using Magnetotelluric and Time-domain Electromagnetic Measurements New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits
Trace Element Analysis At Long Valley Caldera Geothermal Area (Klusman & Landress, 1979)	Trace Element Analysis		1979	Mercury In Soils Of The Long Valley, California, Geothermal System
Water Sampling At Long Valley Caldera Geothermal Area (Evans, Et Al., 2002)	Water Sampling		2002	Tracing And Quantifying Magmatic Carbon Discharge In Cold Groundwaters- Lessons Learned From Mammoth Mountain, USA
Water Sampling At Long Valley Caldera Geothermal Area (Goff, Et Al., 1991)	Water Sampling	1985	1986	A Sr-Isotopic Comparison Between Thermal Waters, Rocks, And Hydrothermal Calcites, Long Valley Caldera, California
Water Sampling At Long Valley Caldera Geothermal Area (McKenzie & Truesdell, 1977)	Water Sampling	1976	1976	Geothermal Reservoir Temperatures Estimated from the Oxygen Isotope Compositions of Dissolved Sulfate and Water from Hot Springs and Shallow Drillholes Geochemistry of Thermal Waters in Long Valley, Mono County, California
Water Sampling At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)	Water Sampling	1985	1986	New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits Hydrologic and Geochemical Monitoring in Long Valley Caldera, Mono County, California, 1985 Hydrologic and Geochemical Monitoring in Long Valley Caldera, Mono County, California, 1986 Temperature Data From Wells in Long Valley Caldera, California
Water-Gas Samples At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)	Water-Gas Samples	2000	2001	Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling
X-Ray Diffraction (XRD) At Long Valley Caldera Geothermal Area (Flexser, 1991)	X-Ray Diffraction (XRD)	1985	1988	Hydrothermal Alteration and Past and Present Thermal Regimes in the Western Moat of Long Valley Caldera

References

---

---

List of existing Geothermal Resource Areas.

Some of the content on this page was part of a case study conducted by: NREL Interns

{{}}

The U.S. Geological Survey has compiled a bibliography of over 1000 works pertaining to the Long Valley Caldera. The list can be accessed online through the USGS website.