Gravity Techniques

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Exploration Technique: Gravity Techniques

Exploration Technique Information
Exploration Group: Geophysical Techniques
Exploration Sub Group: Gravity Techniques
Parent Exploration Technique: Geophysical 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)
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Gravity Techniques:
The gravitational method is the study of the distribution of mass in the subsurface.
Other definitions:Wikipedia Reegle


 
Introduction
The gravity technique provides information regarding the density distribution in the subsurface and can identify anomalous geological features (of varying density) in order to detect structural or lithological contrasts in the subsurface. Gravity measurements can be recorded from the earth surface, from an airborne platform, aboard a marine vessel, or in a borehole.
 
Use in Geothermal Exploration
Gravimetric studies may provide a constraint on the structure and extent of the geothermal reservoir, to a depth of ~2km. Fault location, dip and offset, as well as depth to basement, are commonly interpreted from a gravity survey. Changes in density may also be related to zones of hydrothermal alteration, intrusions, highly fractured rock or deposition of silicates in the vicinity of hydrothermal activity.[1]

Additionally, examining components of the gravity field can be useful in geothermal exploration. For instance, the horizontal gravity gradient enables identification of regions with the greatest contrast in density, such as at fault contacts. [2]

Gravity techniques are also applied towards reservoir monitoring for subsidence and mass gain or loss within a geothermal reservoir using the microgravity technique.[3]

Residual Bouguer anomalies from the Taupo Volcanic Zone, New Zealand from the GNS database. [4]



 
Physical Properties
The fundamental physical laws defining the behavior of the gravitational field are Newton's Law of Attraction and Newton's Second Law. The gravitational field at a point is measured in milligals (mGal). The total gravity field and directional gradients of the gravity field can be measured.

1 Gal=1cm/s^2=1000 mGal

Density is the physical property of interest for a gravity survey. Density is an intrinsic property of a material and is measured in mass per unit volume (kg/m^3).



 
Potential Pitfalls
The gravity method produces an ambiguous, non-unique solution for the subsurface structure. The density distribution is the product of the mass and the volume of the body in the subsurface. Therefore, many combinations of mass and volume may result in the same anomaly as portrayed in the gravity data. Additional geophysical techniques or geological evidence may be required to reduce the ambiguity and further constrain the gravity model.




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