Bouguer anomaly


In geodesy and geophysics, the Bouguer anomaly is a gravity anomaly, corrected for the height at which it is measured and the attraction of terrain. The height correction alone gives a free-air gravity anomaly.
Image:Nj cboug.jpg|thumb|right|Bouguer anomaly map of the state of New Jersey

Definition

The Bouguer anomaly defined as:
Here,
The free-air anomaly, in its turn, is related to the observed gravity as follows:
where:
A Bouguer reduction is called simple if the terrain is approximated by an infinite flat plate called the Bouguer plate. A refined Bouguer reduction removes the effects of terrain more precisely. The difference between the two is called the terrain effect and is due to the differential gravitational effect of the unevenness of the terrain; it is always negative.

Simple reduction

The gravitational acceleration outside a Bouguer plate is perpendicular to the plate and towards it, with magnitude 2πG times the mass per unit area, where is the gravitational constant. It is independent of the distance to the plate. The value of is, so is times the mass per unit area. Using = we get times the mass per unit area. For mean rock density this gives
The Bouguer reduction for a Bouguer plate of thickness is
where is the density of the material and is the constant of gravitation. On Earth the effect on gravity of elevation is 0.3086 mGal m−1 decrease when going up, minus the gravity of the Bouguer plate, giving the Bouguer gradient of 0.1967 mGal m−1.
More generally, for a mass distribution with the density depending on one Cartesian coordinate z only, gravity for any z is 2πG times the difference in mass per unit area on either side of this z value. A combination of two parallel infinite if equal mass per unit area plates does not produce any gravity between them.