## How is free air correction calculated?

In gravity surveying, a correction of 0.3086 mGal/m [0.09406 mGal/ft] added to a measurement to compensate for the change in the gravitational field with height above sea level, assuming there is only air between the measurement station and sea level.

**What is the difference between Bouguer and free air gravity anomalies?**

In geodesy and geophysics, the Bouguer anomaly (named after Pierre Bouguer) 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 gravity anomaly. Otherwise the Bouguer reduction doesn’t work properly.

### What does Bouguer correction account for?

Terrain Corrected Bouguer Gravity (gtc) The Terrain correction accounts for variations in the observed gravitational acceleration caused by variations in topography near each observation point.

**What is the purpose of Bouguer reduction?**

A refined (or complete) Bouguer reduction removes the effects of terrain more precisely. The difference between the two is called the terrain effect (or terrain correction) and is due to the differential gravitational effect of the unevenness of the terrain; it is always negative.

## What is the purpose of free-air correction?

The free-air correction adjusts measurements of gravity to what would have been measured at mean sea level, that is, on the geoid. The gravitational attraction of earth below the measurement point and above mean sea level is ignored and it is imagined that the observed gravity is measured in air, hence the name.

**What is free-air correction?**

[ frē′âr′ ] A compensation factor used in gravitational surveys that takes into account the decrease in the force of gravity with increasing altitude, assuming only air intervenes between the observer and sea level. It is equal to -0.3086 mGal per meter above sea level.

### What does a negative Bouguer anomaly mean?

The Bouguer anomaly is positive over ocean basins and negative over high continental areas. This shows that the low elevation of ocean basins and high elevation of continents is compensated by the thickness of the crust at depth.

**What is a negative Bouguer anomaly?**

The Bouguer anomaly includes only the negative correction for the high terrain and so is strongly negative. More generally, the Airy isostatic anomaly is zero over regions where there is complete isostatic compensation.

## What is Bouguer gravity anomaly?

[ bōō-zhâr′ ] The difference between the expected value of gravity at a given location (taking into account factors such as latitude, longitude, altitude, and the rotation of the Earth) and its actual value.

**What is free-air reduction?**

### What is a positive gravity anomaly?

The gravity anomaly at a location on the Earth’s surface is the difference between the observed value of gravity and the value predicted by a theoretical model. For example, a mass of dense ore below the surface will give a positive anomaly due to the increased gravitational attraction of the ore.