Gravity Technique:

Gravity techniques measure minute variations in the earth's gravity field. Based on these variations, subsurface density and thereby composition can be inferred.

Gravity techniques are used in groundwater investigations to map bedrock and in geotechnical investigations to map bedrock and detect voids.

Gravity surveys begin by taking measurements of the earthsgravity field across the survey area. Because the variations in gravity, caused by the shallow subsurface features being investigated, are very small, the gravity measurements must be corrected for numerous small, but potentially confounding, influences. Examples of such influences include: earth tides (the gravitational influences of the sun and moon), instrumental drift, elevation, terrain, and the centrifugal force of the earths rotation. The data is corrected for these factors and then gridded and displayed in a gravity map of the area.

The gravity map is used to create a model of the subsurface. Since gravity is caused by mass, denser rocks create stronger gravitational fields than less dense rock of the same size. Since rocks of different composition generally have different densities, rock composition can be inferred from gravity data. Thus, by using gravity data, a geophysicist can create a model of the subsurface. The hypothetical gravity field that this model would create is then calculated and the model is refined until its hypothetical gravity field fits the measured gravity data.

Gravity methods have distinct advantages in certain situations. They are cost effective for large areas and are not influenced by manmade electromagnetic fields or metal objects. However, gravity interpretations are non-unique, meaning that multiple interpretations can fit the same gravity data. For this reason, it is very useful to use other techniques in conjunction with gravity such as magnetics, seismic or simply a knowledge of the geology of the area being surveyed.