|Abstract from DBPedia||
A gravity anomaly is the difference between the observed acceleration of free fall, or gravity, on a planet's surface, and the corresponding value predicted from a model of the planet's gravity field. Typically the model is based on simplifying assumptions, such as that, under its self-gravitation and rotational motion, the planet assumes the figure of an ellipsoid of revolution. Gravity on the surface of this reference ellipsoid is then given by a simple formula which only contains the latitude, and subtraction from observed gravity in the same location will yield the gravity anomaly. Anomaly values are typically much smaller than the values of gravity itself, as the bulk contributions of the total mass of the planet, of its rotation and of its associated flattening, have been subtracted. As such, gravity anomalies describe the local variations of the gravity field around the model field. A location with a positive anomaly exhibits more gravity than predicted by the model—suggesting the presence of a sub-surface positive mass anomaly, while a negative anomaly exhibits a lower value than predicted—suggestive of a sub-surface mass deficit. These anomalies are thus of substantial geophysical and geological interest. When gravity measurements have been made on the topography above sea level, a careful reduction process, also involving the effect of local topographic masses, must be carried out to obtain geophysically useful gravity anomalies, of which there are several different types. Cleanly extracting the response to the local sub-surface geology is the typical goal of applied geophysics.