Gravity anomalies are produced by density variations within the rocks of the Earth's crust and upper mantle. Mapping of these density variations is the primary use of gravity anomalies. Gravity measurements made on the surface of the Earth must be corrected in various ways before they can be made into an anomaly map. The free-air correction adds to the measured gravity the predicted difference in gravity between sea level and the elevation of the observation. The simple-Bouguer correction subtracts the predicted effect of the mass between the observation and sea level, using a uniform slab of constant density to model the mass.
Illumination is from the west.
Because the Bouguer anomaly equals the free-air anomaly at sea level, there is no discontinuity in the gridded data at the shoreline. Isostatic residual gravity anomaly maps are produced by subtracting long-wavelength anomalies, produced by masses deep within the crust or mantle, from the Bouguer anomaly map. Isostatic residual gravity anomaly maps therefore reveal more clearly than Bouguer anomaly maps the density distributions within the upper crust that are of interest in many geologic and tectonic studies. When the gamma-ray system is properly calibrated, the data can be expressed in terms of the estimated concentrations of these radioactive elements. Topographic and bathymetric data are used in the reduction and interpretation of magnetic anomaly and gravity anomaly data and in the construction of geologic and topographic maps.