Ground Gravity Survey
Exploration Technique: Ground Gravity Survey
|Exploration Technique Information|
|Exploration Group:||Geophysical Techniques|
|Exploration Sub Group:||Gravity Techniques|
|Parent Exploration Technique:||Gravity Techniques|
|Information Provided by Technique|
|Lithology:||Distribution of density in the subsurface enables inference of rock type.|
|Stratigraphic/Structural:||Delineation of steeply dipping formations, geological discontinuities and faults, intrusions and large-scale deposition of silicates due to hydrothermal activity.|
|Hydrological:||Density of sedimentary rocks are strongly influenced by fluid contained within pore space. Dry bulk density refers to the rock with no moisture, while the wet bulk density accounts for water saturation; fluid content may alter density by up to 30%.(Sharma, 1997)|
|Thermal:||Determination of potential heat source of the system related to the low density signature of molten intrusions. (Bruhn, 2010)|
|Low-End Estimate (USD):|| 35.00|
3,500 centUSD/ station
|Median Estimate (USD):|| 68.31|
6,831 centUSD/ station
|High-End Estimate (USD):|| 300.00|
30,000 centUSD/ station
|Low-End Estimate:|| 0.12 days|
3.285421e-4 years/ 10 stn
|Median Estimate:|| 0.25 days|
6.844627e-4 years/ 10 stn
|High-End Estimate:|| 0.67 days|
0.00183 years/ 10 stn
|Cost/Time Dependency:||Location, Size, Resolution, Terrain, Weather, Permitting Restrictions|
The survey design is based on the anticipated depth of investigation, density contrast and structure of the geological feature. Additional parameters to take into account for the survey design are the accuracy of the gravimeter, accuracy of the measurement station in location and elevation, the accuracy of the topography in the vicinity of the station, and the frequency of the re-occupation of the base station.
In a gravity survey, measurements must be taken in a closed loop or series of closed loops throughout the measurement period. There is a primary base station to be visited and measured at the beginning and end of each field acquisition day, as well as intermediate base station loops established in the field area. This is because the instrument can experience drift or experience tares due to rough handling. Tidal effects due to the position of the sun and moon and Earth's revolution can be accounted for by repeat occupation of base stations at periods of 2 to 3 hours. High resolution gravity surveys are best conducted in conjunction with differential GPS measurements although conventional survey methods can be used. GPS is fast and can be done by the simultaneously by the gravimeter operator or by one field assistant. Conventional equipment require significantly more time and personnel.
Field notes should include the station name, gravimeter reading, time of measurement, station location (latitude, longitude, elevation). Terrain data for corrections can be acquired from digital terrain models from the NGDC web site. 
There are corrections to the data which are intrinsic to the gravitational method and involve the removal of the regional field to obtain the residual Bouguer anomaly map.
"Corrections included in the calculations of the Bouguer anomaly are as follows:
1. Latitude correction. Here we correct for a calculated normal field. The correction takes into account the Earth's rotation, as well as the fact that the distance to the centre of the Earth's mass varies with latitude.
2. Free-air correction. A correction is made for the reduction in the gravity field from sea level to the altitude of the measuring site.
3. Bouguer correction. Here we subtract the effect of the rock mass between the measuring site and sea level.
4. Terrain correction. Account is taken of the topography in the vicinity of the measuring station. Here it is necessary to have a good map (preferably digital) of the area around the observation site."
• The line direction should be positioned perpendicular to the dominant geologic strike direction.
• Measurement spacing should be designed to include at least five magnetic measurements per anomaly.
• Line spacing and station interval need to be spaced finely enough to characterize spatial distribution of anticipated anomalies.
- Baltazor Hot Springs Area
- Blue Mountain Geothermal Area
- Chocolate Mountains Area
- Clear Lake Area
- Coso Geothermal Area
- Cove Fort Geothermal Area
- Crump's Hot Springs Geothermal Area
- Dixie Valley Geothermal Area
- Dixie Valley Geothermal Field Area
- Fort Bliss Area
- Glass Buttes Area
- Hot Pot Area
- Kilauea East Rift Geothermal Area
- Lake City Hot Springs Area
- Lightning Dock Area
- Lightning Dock Geothermal Area
- Long Valley Caldera Geothermal Area
- Marysville Mt Area
- Maui Area
- Mcgee Mountain Area
- Mokapu Penninsula Area
- Mt Princeton Hot Springs Geothermal Area
- Neal Hot Springs Geothermal Area
- New River Area
- Newberry Caldera Area
- North Brawley Geothermal Area
- Raft River Geothermal Area
- Rio Grande Rift Geothermal Region
- Roosevelt Hot Springs Geothermal Area
- Salt Wells Geothermal Area
- San Emidio Desert Area
- San Francisco Volcanic Field Area
- Snake River Plain Geothermal Region
- Truckhaven Area
- U.S. West Region
- Under Steamboat Springs Area
- Valles Caldera - Redondo Geothermal Area
- Valles Caldera - Sulphur Springs Geothermal Area
- Walker Lake Valley Area
- Waunita Hot Springs Geothermal Area