Airborne Gravity Survey

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Exploration Technique: Airborne 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 the 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): 86.898,689 centUSD
0.0869 kUSD
8.689e-5 MUSD
8.689e-8 TUSD
/ mile
Median Estimate (USD): 274.1727,417 centUSD
0.274 kUSD
2.7417e-4 MUSD
2.7417e-7 TUSD
/ mile
High-End Estimate (USD): 933.2293,322 centUSD
0.933 kUSD
9.3322e-4 MUSD
9.3322e-7 TUSD
/ mile
Time Required
Low-End Estimate: 4 weeks0.0767 years
672 hours
28 days
0.92 months
/ job
Median Estimate: 37.33 weeks0.715 years
6,271.44 hours
261.31 days
8.585 months
/ job
High-End Estimate: 164 weeks3.143 years
27,552.001 hours
1,148 days
37.717 months
/ job
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Airborne Gravity Survey:
Airborne gravity gradiometry (AGG) surveys provide information regarding the mass distribution of the subsurface through the measurement of gravity vector spatial derivatives.
Other definitions:Wikipedia Reegle


 
Introduction
 
Improvements in GPS technology in the 1990’s have expanded the capabilities, and consequently the application, of airborne gravity gradiometry. Gravity gradiometers are preferable in the airborne environment because of the enhanced sensitivity, lower noise levels and spatial resolution in comparison to airborne gravimeters. This technique has been successfully deployed on helicopters, single-engine and multi-engine fixed-wing aircraft. [1]

The advantages of the airborne gravity gradiometry surveys are consistent regional coverage in varied topography and rapid acquisition time relative to a ground gravity survey. The areal coverage, desired resolution, station spacing and topography of the field area are a few of the factors to take into account when deciding between a ground or airborne gravity survey. For instance, an airborne survey may provide better coverage and resolution in areas with steep topography than a ground gravity survey because of difficult ground-based access to the stations. [2] However, a ground gravity survey enables a more detailed characterization and higher resolution of a survey area.
 
Use in Geothermal Exploration
 
Airborne gravity gradiometry is not a technique which has been applied towards geothermal exploration yet. However, the utilization of this technique is advocated because it provides multi-component information to assist in the constraint of the regional geological setting in the vicinity of a geothermal reservoir. [2]


 
Field Procedures
 
The gravity gradiometer is mounted on an aircraft (helicopter, single-engine or multi-engine fixed-wing aircraft) and flown over the survey area to obtain the gravity gradient measurements. The survey is typically flown at an altitude of 80 m or greater with a line spacing dependent on the target of investigation. [3]

Recent developments in airborne gravity gradiometry instrument control technology and processing techniques continue to reduce the error associated with the measurements. From 2004-2011, the gravity gradient error has halved for Fugro’s systems. [1]
 
Environmental Mitigation Measures
 
This is a non-invasive, passive geophysical technique. The primary environmental impact is associated with fuel usage of the aircraft.
 
Physical Properties
 
The physical property of interest is the gravity gradient, which is the rate of change of the gravitational field in a determined direction. For example, the vertical gravity gradient is the change in the gravity vector with depth in the z-direction. The unit of measurement for the gravity gradient is the Eotvos, equivalent to 10-4 mGal/m.

Also, see Ground Gravity Survey.


 
Best Practices
 
The gravity gradient strength decays as 1/r3. Therefore, the signature from buried sources is maximized closer to the earth surface and a low flying altitude is desirable. However, as survey height decreases, the terrain correction error increases and the survey design must include these considerations when determining the survey altitude. [3]









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