Free-air anomaly
Encyclopedia
In geophysics
, the free-air gravity anomaly, often simply called the free-air anomaly, is the measured gravity anomaly
after a free-air correction is applied to correct for the elevation at which a measurement is made. The free-air correction does so by adjusting these measurements of gravity to what would have been measured at sea level.
The free-air gravity anomaly is given by the equation:
Here,
is the free-air gravity anomaly, 
is observed gravity, 
is the correction for latitude (because the Earth is not a perfect sphere), and 
is the free-air correction.
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:
The difference between gravity measurements at sea level and at an altitude of
above sea level is:
.
Here we have assumed that measurements are made relatively close to the earth's surface so that R doesn't vary significantly. Also, there is an assumption that no mass exists between the observation point and sea level. The Bouguer anomaly
and terrain correction are used to account for this.
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
, the free-air gravity anomaly, often simply called the free-air anomaly, is the measured gravity anomaly
Gravity anomaly
A gravity anomaly is the difference between the observed acceleration of Earth's gravity and a value predicted from a model.-Geodesy and geophysics:...
after a free-air correction is applied to correct for the elevation at which a measurement is made. The free-air correction does so by adjusting these measurements of gravity to what would have been measured at sea level.
The free-air gravity anomaly is given by the equation:
Here,
is the free-air gravity anomaly, is observed gravity, is the correction for latitude (because the Earth is not a perfect sphere), and is the free-air correction.Gravitational acceleration
Gravitational acceleration
In physics, gravitational acceleration is the acceleration on an object caused by gravity. Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate relative to the center of mass....
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:
The difference between gravity measurements at sea level and at an altitude of
above sea level is:.Here we have assumed that measurements are made relatively close to the earth's surface so that R doesn't vary significantly. Also, there is an assumption that no mass exists between the observation point and sea level. The Bouguer anomaly
Bouguer anomaly
In geodesy and geophysics, the Bouguer anomaly 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 anomaly.-Bouguer reduction:...
and terrain correction are used to account for this.