The Bouguer anomaly
is a minor variation in gravitational
force equal to ± 300 milligals. (One milligal is equal to .001 galileos
, a unit of acceleration
equal to 1 cm/sec2
). These variations in gravity occurr due to differences in the density of the rock that is underneath a certain location. This was first discovered by Pierre Bouguer
from research conducted in the Andes
The anomaly in gravity is calculated with the following formula:
Bouguer Anomaly = Gravity measured + Free Air Correction - Bouguer Correction - Gravity theoretical
Gravity measured is a value that is measured with a gravitometer.
Free Air Correction is equal to ((2) (go) (he)) / R
Where go is equal to 980.000 gals
And he is the height above mean sea level at which you are taking measurements
And R is equal to the distance from the center of the earth to mean sea level. (6.37 x 10 6 meters)
Bouguer Correction is equal to ((2) (π) (G) (ρ) (h rock))
Where G is Cavendigh's Constant, 6.67 x 10 -8 (dyne cm2) / g2
And ρ is the density of the rocks below the area being measured
And h rock is the distance from the base of the rock that makes up the area (the root of a mountain, for example) to the top of the rock that makes up the area.
Gravity theoretical is calculated with a different formula, based on lattitude (represented by φ. This formula is:
978.049 ( 1 + .005288 sin2φ - .0000059 sin22φ) galileos.
It is worth noting that there are some descrepancies that can be found in calculating this. I have also seen different numbers for various parts of the formula. I have seen the free air constant listed as 0.3086 mgal/m, and the Bouguer correction listed as 0.4185 <ρ> h, where h is = to he from above, in the free air correction. I have also seen a slightly different formula for the theoretical gravity, in which .0000059 sin4φ. If someone knows for certain exactly which ones of these things are correct, please /msg me, and I will change it.