Ground Gravity Survey

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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)
Cost Information
Low-End Estimate (USD): 35.003,500 centUSD
0.035 kUSD
3.5e-5 MUSD
3.5e-8 TUSD
/ station
Median Estimate (USD): 68.316,831 centUSD
0.0683 kUSD
6.831e-5 MUSD
6.831e-8 TUSD
/ station
High-End Estimate (USD): 300.0030,000 centUSD
0.3 kUSD
3.0e-4 MUSD
3.0e-7 TUSD
/ station
Time Required
Low-End Estimate: 0.12 days3.285421e-4 years
2.88 hours
0.0171 weeks
0.00394 months
/ 10 stn
Median Estimate: 0.25 days6.844627e-4 years
6 hours
0.0357 weeks
0.00821 months
/ 10 stn
High-End Estimate: 0.67 days0.00183 years
16.08 hours
0.0957 weeks
0.022 months
/ 10 stn
Additional Info
Cost/Time Dependency: Location, Size, Resolution, Terrain, Weather, Permitting Restrictions
Dictionary.png
Ground Gravity Survey:
The ground gravitational method is the study of the distribution of mass in the subsurface with the observation point at the earth's surface.
Other definitions:Wikipedia Reegle



 
Use in Geothermal Exploration





 
Field Procedures
A ground gravity survey is a passive, low impact, non-invasive geophysical technique. An instrument called a gravimeter is used for the measurement. A relative gravimeter, commonly used for exploration, is transportable by one person with a backpack and weighs roughly 8kg (17.5 lb).[1] The instrument is carried to the measurement station, placed on the ground surface, and leveled. The gravity measurement takes a few minutes (depending on the type of gravimeter), and then the gravimeter can be picked up and transported to the next station. There is negligible surface disturbance.

Scintrex CG-5 gravimeter[1]

 
Environmental Mitigation Measures
The means of access to the gravity stations (truck, all-terrain vehicle, helicopter, boat, hiking) is the only potential environmental disturbance associated with a gravity survey. The measurement itself is non-invasive and there is negligible disturbance to the topsoil.
 
Physical Properties
 
Data Access and Acquisition
There are two types of gravity instruments: an absolute and a relative gravimeter. An absolute gravimeter is a highly precise tool applied to reservoir monitoring and subsidence studies (See Microgravity). [2] The commonly used gravimeter in geophysical exploration applications is the relative gravimeter.[1] The LaCoste and Romberg and Scintrex instruments are the most commonly used gravimeters.

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. [3]

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.[4][5]

"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."[6]

 
Best Practices
In the survey design planning:

• 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.
 
Potential Pitfalls









Page Area Activity Start Date Activity End Date Reference Material
Ground Gravity Survey (Nannini, 1986) Unspecified


Ground Gravity Survey At Baltazor Hot Springs Area (Isherwood & Mabey, 1978) Baltazor Hot Springs Area


Ground Gravity Survey At Blue Mountain Geothermal Area (Fairbank Engineering Ltd, 2003) Blue Mountain Geothermal Area


Ground Gravity Survey At Blue Mountain Geothermal Area (U.S. Geological Survey, 2012) Blue Mountain Geothermal Area 2008 2009


Ground Gravity Survey At Chocolate Mountains Area (Alm, Et Al., 2010) Chocolate Mountains Area


Ground Gravity Survey At Clear Lake Area (Skokan, 1993) Clear Lake Area


Ground Gravity Survey At Coso Geothermal Area (1980) Coso Geothermal Area 1980 1980


Ground Gravity Survey At Coso Geothermal Area (1990) Coso Geothermal Area 1990 1990


Ground Gravity Survey At Cove Fort Area (Toksoz, Et Al, 2010) Cove Fort Geothermal Area


Ground Gravity Survey At Cove Fort Area (Warpinski, Et Al., 2002) Cove Fort Geothermal Area


Ground Gravity Survey At Cove Fort Area (Warpinski, Et Al., 2004) Cove Fort Geothermal Area


Ground Gravity Survey At Cove Fort Area - Vapor (Warpinski, Et Al., 2002) Cove Fort Geothermal Area


Ground Gravity Survey At Cove Fort Area - Vapor (Warpinski, Et Al., 2004) Cove Fort Geothermal Area


Ground Gravity Survey At Crump's Hot Springs Area (DOE GTP) Crump's Hot Springs Geothermal Area


Ground Gravity Survey At Dixie Valley Geothermal Area (Allis, Et Al., 2000) Dixie Valley Geothermal Area 1999 2000


Ground Gravity Survey At Dixie Valley Geothermal Area (Blackwell, Et Al., 2003) Dixie Valley Geothermal Area 1979 2000


Ground Gravity Survey At Dixie Valley Geothermal Area (Iovenitti, Et Al., 2013) Dixie Valley Geothermal Area 2012 2012


Ground Gravity Survey At Dixie Valley Geothermal Area (Schaefer, 1983) Dixie Valley Geothermal Area 1983 1983


Ground Gravity Survey At Dixie Valley Geothermal Field Area (Blackwell, Et Al., 2009) Dixie Valley Geothermal Field Area


Ground Gravity Survey At Fort Bliss Area (DOE GTP) Fort Bliss Area


Ground Gravity Survey At Glass Buttes Area (DOE GTP) Glass Buttes Area


Ground Gravity Survey At Hot Pot Area (DOE GTP) Hot Pot Area


Ground Gravity Survey At Kilauea East Rift Geothermal Area (Broyles, Et Al., 1979) Kilauea East Rift Geothermal Area 1975 1975


Ground Gravity Survey At Kilauea East Rift Geothermal Area (FURUMOTO, 1976) Kilauea East Rift Geothermal Area 1974 1974


Ground Gravity Survey At Kilauea East Rift Geothermal Area (Leslie, Et Al., 2004) Kilauea East Rift Geothermal Area 1998 1998


Ground Gravity Survey At Kilauea East Rift Geothermal Area (Thomas, 1986) Kilauea East Rift Geothermal Area 1978 1987


Ground Gravity Survey At Lake City Hot Springs Area (Warpinski, Et Al., 2004) Lake City Hot Springs Area


Ground Gravity Survey At Lightning Dock Area (Cunniff & Bowers, 2005) Lightning Dock Area


Ground Gravity Survey At Lightning Dock Area (Warpinski, Et Al., 2002) Lightning Dock Area


Ground Gravity Survey At Lightning Dock Geothermal Area (Swanberg, 1976) Lightning Dock Geothermal Area


Ground Gravity Survey At Long Valley Caldera Geothermal Area (Battaglia, Et Al., 2003) Long Valley Caldera Geothermal Area 2003


Ground Gravity Survey At Long Valley Caldera Geothermal Area (Laney, 2005) Long Valley Caldera Geothermal Area 2004


Ground Gravity Survey At Marysville Mt Area (Blackwell) Marysville Mt Area


Ground Gravity Survey At Maui Area (DOE GTP) Maui Area


Ground Gravity Survey At Mcgee Mountain Area (DOE GTP) Mcgee Mountain Area


Ground Gravity Survey At Mokapu Penninsula Area (Thomas, 1986) Mokapu Penninsula Area


Ground Gravity Survey At Mt Princeton Hot Springs Geothermal Area (Case, Et Al., 1984) Mt Princeton Hot Springs Geothermal Area 1984 1984


Ground Gravity Survey At Neal Hot Springs Geothermal Area (Colwell, Et Al., 2012) Neal Hot Springs Geothermal Area 2011 2011


Ground Gravity Survey At Neal Hot Springs Geothermal Area (U.S. Geothermal Inc., 2007) Neal Hot Springs Geothermal Area 2006 2006


Ground Gravity Survey At New River Area (DOE GTP) New River Area


Ground Gravity Survey At Newberry Caldera Area (DOE GTP) Newberry Caldera Area


Ground Gravity Survey At North Brawley Geothermal Area (Biehler, 1964) North Brawley Geothermal Area 1961 1964


Ground Gravity Survey At North Brawley Geothermal Area (Department, 1979) North Brawley Geothermal Area 1971 1971


Ground Gravity Survey At Raft River Geothermal Area (1957-1961) Raft River Geothermal Area 1957 1961


Ground Gravity Survey At Raft River Geothermal Area (1978) Raft River Geothermal Area 1978 1978


Ground Gravity Survey At Rio Grande Rift Region (Aiken & Ander, 1981) Rio Grande Rift Geothermal Region


Ground Gravity Survey At Roosevelt Hot Springs Geothermal Area (Faulder, 1991) Roosevelt Hot Springs Geothermal Area 1985 1985


Ground Gravity Survey At Roosevelt Hot Springs Geothermal Area (Ward, Et Al., 1978) Roosevelt Hot Springs Geothermal Area 1978 1978


Ground Gravity Survey At Salt Wells Area (Bureau of Land Management, 2009) Salt Wells Geothermal Area 2008 2008


Ground Gravity Survey At San Emidio Desert Area (DOE GTP) San Emidio Desert Area


Ground Gravity Survey At San Francisco Volcanic Field Area (Warpinski, Et Al., 2004) San Francisco Volcanic Field Area


Ground Gravity Survey At Snake River Plain Region (DOE GTP) Snake River Plain Geothermal Region


Ground Gravity Survey At Truckhaven Area (Layman Energy Associates, 2009) Truckhaven Area


Ground Gravity Survey At Truckhaven Area (Warpinski, Et Al., 2004) Truckhaven Area


Ground Gravity Survey At U.S. West Region (Aiken & Ander, 1981) U.S. West Region


Ground Gravity Survey At Under Steamboat Springs Area (Warpinski, Et Al., 2002) Under Steamboat Springs Area


Ground Gravity Survey At Under Steamboat Springs Area (Warpinski, Et Al., 2004) Under Steamboat Springs Area


Ground Gravity Survey At Valles Caldera - Redondo Geothermal Area (Wilt & Haar, 1986) Valles Caldera - Redondo Geothermal Area 1986


Ground Gravity Survey At Valles Caldera - Sulphur Springs Geothermal Area (Wilt & Haar, 1986) Valles Caldera - Sulphur Springs Geothermal Area 1986


Ground Gravity Survey At Walker Lake Valley Area (Shoffner, Et Al., 2010) Walker Lake Valley Area


Ground Gravity Survey At Waunita Hot Springs Geothermal Area (Lange, 1981) Waunita Hot Springs Geothermal Area


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