Bouguer plate
A Bouguer plate is an infinite, flat plate, used as a model in
Gravity outside the plate is perpendicular to the plate, towards it, with magnitude 2πG times the mass per unit area, where G = 6.67 × 10−11 N m² kg-2, hence we have 4.191 × 10−10 N m² kg-2 times the mass per unit area.
Using 1 Gal = 0.01 m/s² we get 4.191 × 10−5 mGal m² kg-1 times the mass per unit area.
For mean rock density (2.67 g/cm³) this gives 0.1119 mGal/m.
This is independent of the distance to the plate.
Bouguer gradient
On Earth the effect on gravity of elevation is 0.3086 mGal/m decrease when going up, minus the gravity of the Bouguer plate, giving the Bouguer gradient of 0.1967 mGal/m.
General
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.
In particular, a combination of two equal parallel infinite plates does not produce any gravity inside.
See also
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