Free-air gravity anomaly

[1][2] Studies of the subsurface structure and composition of the Earth's crust and mantle employ surveys using gravimeters to measure the departure of observed gravity from a theoretical gravity value to identify anomalies due to geologic features below the measurement locations.

The theoretical gravity value at a location is computed by representing the Earth as an ellipsoid that approximates the more complex shape of the geoid.

For studies of subsurface structure, the free-air anomaly is further adjusted by a correction for the mass below the measurement point and above the reference of mean sea level or a local datum elevation.

The equation for this approach is simply rearranging terms in the first equation of this section so that reference gravity is adjusted and not the observed gravity: Gravitational acceleration decreases as an inverse square law with the distance at which the measurement is made from the mass.

The free air correction is calculated from Newton's Law, as a rate of change of gravity with distance:[5] At 45° latitude,

to correct it to the reference level: Here we have assumed that measurements are made relatively close to the surface so that R does not vary significantly.

Over the ocean where gravity is measured from ships near sea level, there is no or little free-air correction.

In marine gravity surveys, it was observed that the free-air anomaly is positive but very small over the Mid-Ocean Ridges in spite of the fact that these features rise several kilometers above the surrounding seafloor.

[6] The small anomaly is explained by the lower density crust and mantle below the ridges resulting from seafloor spreading.