Bouguer anomaly

In geodesy and geophysics, the Bouguer anomaly (named after Pierre Bouguer) 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.

The Bouguer anomaly is related to the observed gravity ${\displaystyle g_{obs}}$ as follows:

Here,

A Bouguer reduction is called simple or incomplete if the terrain is approximated by an infinite flat plate called the Bouguer plate. A refined or complete Bouguer reduction removes the effects of terrain precisely. The difference between the two, the differential gravitational effect of the unevenness of the terrain, is called the terrain effect. It is always negative.

The gravitational acceleration ${\displaystyle g}$ outside a Bouguer plate is perpendicular to the plate and towards it, with magnitude 2πG times the mass per unit area, where ${\displaystyle G}$ is the gravitational constant. It is independent of the distance to the plate (as can be proven most simply with Gauss's law for gravity, but can also be proven directly with Newton's law of gravity). The value of ${\displaystyle G}$ is 6.67 × 10⁻¹¹ N m² kg⁻², so ${\displaystyle g}$ is 4.191 × 10⁻¹⁰ N m² kg⁻² times the mass per unit area. Using 1 Gal = 0.01 m s⁻² (1 cm s⁻²) we get 4.191 × 10⁻⁵ mGal m² kg⁻¹ times the mass per unit area. For mean rock density (2.67 g cm⁻³) this gives 0.1119 mGal m⁻¹.

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