ISOSTASY AND GRAVITY CORRECTIONS

Isostasy

Gravitational equilibrium between Earth's crust and mantle.

Airy's Theory

Equal density and unequal thickness; based on root and anti-root theory.

Root (in Airy's Theory)

Associated with high elevation, typically mountains.

Anti-root (in Airy's Theory)

Associated with low elevation, typically oceans.

Pratt's Theory

Unequal density at uniform depth; not based on root and anti-root.

Vening Meinesz Theory

Upper layer behaves like an elastic plate overlying a weak fluid; accounts for regional isostatic compensation.

Isostatic Gravity Anomaly

Difference between observed and calculated gravity values.

Overcompensation

Occurs when a land-high is reduced; root shifts upward, leading to a negative isostatic gravity anomaly.

Full Compensation

Computed root is in hydrostatic equilibrium with its root zone; isostatic gravity anomaly is zero.

Undercompensation

Computed root is larger than the real root; leads to a positive isostatic gravity anomaly.

Free Air Correction

Corrects gravity changes due to height; positive if above sea level, negative if below.

Bouguer Correction

Corrects for changes in gravity due to lateral density variation; acts opposite to free air correction.

Terrain Correction

Added correction to nullify terrain feature effects; lowers gravity value.

Latitude Correction

Gravity increases with latitude from the Equator to the Pole, but decreases with radius; varies 57 mgal with 10 degrees latitude.

Tidal Correction

Drift correction for amplitude variation in gravity; typically about 0.3 gal.

Eotvo's Correction

Required in airborne and shipborne gravity surveys.