Editing Gravity anomaly (section)

{{Short description|Difference between ideal and observed gravitational acceleration at a location}}
{{Distinguish|Gravitational anomaly}}
{{for|the formulas|Gravity of Earth#Mathematical models}}
{{Missing information|gravity anomalies on other celestial objects, particularly the Moon, Mars, Vesta, and Pluto|date=June 2024}}
The '''gravity anomaly''' at a location on the Earth's surface is the [[Anomaly (natural sciences)|difference]] between the observed value of [[gravity]] and the value predicted by a theoretical model. If the Earth were an ideal [[oblate spheroid]] of uniform density, then the gravity measured at every point on its surface would be given precisely by a simple algebraic expression. However, the Earth has a rugged surface and non-uniform composition, which distorts its gravitational field. The theoretical value of gravity can be corrected for altitude and the effects of nearby terrain, but it usually still differs slightly from the measured value. This gravity anomaly can reveal the presence of subsurface structures of unusual density. For example, a mass of dense ore below the surface will give a positive anomaly due to the increased gravitational attraction of the ore.

Different theoretical models will predict different values of gravity, and so a gravity anomaly is always specified with reference to a particular model. The [[Bouguer anomaly|Bouguer]], [[Free-air gravity anomaly|free-air]], and isostatic gravity anomalies are each based on different theoretical corrections to the value of gravity.

A gravity survey is conducted by measuring the gravity anomaly at many locations in a region of interest, using a portable instrument called a [[gravimeter]]. Careful analysis of the gravity data allows geologists to make inferences about the subsurface geology.