Gravimetric methods allow to determine the distribution of densities beneath the surface. There is a wide range in density among rock types, and therefore knowing the density distribution beneath the surface it is possible to make inferences about the structure of the ground. The intensity of the gravity force due to a buried mass difference (positive or negative) is superimposed on the larger force of gravity due to the total mass of the Earth (and to a lesser extent, the Moon and the Sun). Thus, several components of gravitational forces are measured at the Earth’s surface. Measuring very precisely the gravity field and doing a very careful correction accounting for variations in the longer wavelength component (due the whole Earth, Moon and Sun), a gravity survey is able to detect density variations associated to natural or man-made voids, variations in the depth to bedrock, and geologic structures of engineering interest.
Measurements are done at points (stations) separated typically in the range of 1 to 50 m, depending on the target of the survey. Raw data needs to be corrected for latitude, Free Air (change of gravity for the station elevation above sea level), Bouguer correction (attraction of the rock mass between a datum level and the station) and finally, topographic corrections accounting for all the masses higher than the gravity stations.
Once the basic latitude, free-air, Bouguer and terrain corrections are made it is necessary to make the regional-residual separation.
The residual gravity values along a profile or traverse are used as an input to construct synthetic density models. The geophysicist constructs a subsurface polygonal-shaped model and assigns a density contrast to it. When the body has been placed, the gravity effect of the trial body is calculated and shown in a graphical display against the observed data. The geophysicist can then begin changing parameters in order to match both curves (the observed and the synthetic) closer together.
Bouguer residual anomaly around the Añana diapir in North Spain