Static Temperature Survey At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2010)

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Exploration Activity: Static Temperature Survey At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2010)

Exploration Activity Details
Location Long Valley Caldera Geothermal Area
Exploration Technique Static Temperature Survey
Activity Date 2005 - 2007
Usefulness useful
DOE-funding Unknown

Exploration Basis
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 U.S. Geological Survey 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).
High-precision temperature profiles were measured between 2005 and 2006 in boreholes 13-21, 13-26, 68-28, and 46-28 using the U.S. Geological Survey's heat flow logging truck. Temperature measurements were taken every 30.48 cm (1 foot) using a precision Platinum Resistance Temperature Device (Pt-RTD) that was lowered on a calibrated four-conductor cable at a constant rate of 6 m/min. The Pt-RTD was calibrated using a NIST-traceable Platinum Resistance Thermometer to a lab accuracy of better than 0.01°C. Temperatures were initially measured in June 2005 through the well casings in boreholes that varied in diameter from 3.81 cm to 7.32 cm. Then in September 2005, boreholes 13-21, 13-26, and 68-28 were perforated to allow sampling of formation waters/gases and establish hydraulic connectivity between the wellbores and the surrounding rocks. The casing in wells 13-26 and 68-28 were damaged during this process, prohibiting access to deeper parts of these wells below the perforation depth. Temperatures were re-measured in wells 13-21 (perforated at a depth of 670 m) and 46-28 (un-perforated) in July 2006.

Bottom hole temperature measurements ranged from 99.4°C in 13-21 to a maximum of 129.5°C in 68-28. The shallow high-temperature interval encountered in the Fumarole Valley boreholes (35-28, 46-28, and 68-28) also 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 the resurgent dome.


Additional References