Gravity anomaly

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 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 out. 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.

Lateral variations in gravity anomalies are related to anomalous density distributions within the Earth. Gravity measures help us to understand the internal structure of the planet. Synthetic calculations show that the gravity anomaly signature of a thickened crust (for example, in orogenic belts produced by continental collision) is negative and larger in absolute value, relative to a case where thickening affects the entire lithosphere.

The Bouguer anomalies usually are negative in the mountains because they involve reducing out the attraction of the mountain mass, by about 100 milligals per kilometre of mountain height. In large mountain areas, they are even more negative than this because of isostasy: the rock density of the mountain roots is lower, compared with the surrounding earth's mantle, causing a further gravity deficit. Typical anomalies in the Central Alps are −150 milligals (−1.5 mm/s²). Rather local anomalies are used in applied geophysics: if they are positive, this may indicate metallic ores. At scales between entire mountain ranges and ore bodies, Bouguer anomalies may indicate rock types. For example, the northeast-southwest trending high across central New Jersey (see figure) represents a graben of Triassic age largely filled with dense basalts. Salt domes are typically expressed in gravity maps as lows, because salt has a low density compared to the rocks the dome intrudes. Anomalies can help to distinguish sedimentary basins whose fill differs in density from that of the surrounding region - see Gravity Anomalies of Britain and Ireland for example.

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