The classic salt dome anomaly shows a sharp, intense gravity low, reflecting the typically large density contrast between salt and surrounding sedimentary rocks. In the Gulf Coast, shallow cap rock may produce an even sharper gravity high at or near the center of the low, as seen in the illustration on the left. This is an indication of high-density materials such as anhydrite or limestone. Many examples of such anomalies in real data can be found in the older literature.
A similar effect occurs when the salt is shallower than the density crossover depth, i.e., when the salt is more dense than the sediments. These alternatives are resolvable when integrated with the top of salt from seismic, density control from nearby wells, and/or densities calculated from sedimentary velocities.
Cap-rock anomalies are nothing to be ignored. Unocal has used carefully analyzed gravity to interpret hydrocarbonfilled cap-rock reservoirs.