Gravity, data to anomalies

From a geophysical perspective, a gravity anomaly is best thought of as the difference between measured gravity and a model value, at the same point, that accounts for large-scale, non-geological effects that mask anomalies related to subsurface geology. The model gravity value is derived by correcting normal gravity for the effects of height above the reference ellipsoid (free-air correction) and the mass of rock between the reference ellipsoid and the measurement point (Bouguer correction). Additional corrections (terrain and atmospheric corrections, indirect effect) improve on the relatively straightforward free-air and Bouguer corrections. Since the earliest use of gravity data, our ability to compute the corrections to normal gravity has improved to the point where simplistic application of the corrections is giving way to procedures that consider global influences. Depending on the application, long-wavelength field components remaining in the Bouguer anomaly may also be removed (isostatic correction or regional–residual field separation). Gravity anomalies can then be used to constrain geological interpretations of relatively shallow crustal features (e.g., basins, ore bodies, or even archeological sites), or be used to examine crustal structure and isostatic state.