Accounting for Elevation Variations: The Free-Air Correction

To account for variations in the observed gravitational acceleration that are related to elevation variations, we incorporate another correction to our data known as the Free-Air Correction. In applying this correction, we mathematically convert our observed gravity values to ones that look like they were all recorded at the same elevation, thus further isolating the geological component of the gravitational field.

To a first-order approximation, the gravitational acceleration observed on the surface of the earth varies at about -0.3086 mgal per meter in elevation difference. The minus sign indicates that as the elevation increases, the observed gravitational acceleration decreases. The magnitude of the number says that if two gravity readings are made at the same location, but one is done a meter above the other, the reading taken at the higher elevation will be 0.3086 mgal less than the lower. Compared to size of the gravity anomaly computed from the simple model of an ore body, 0.025 mgal, the elevation effect is huge!

To apply an elevation correction to our observed gravity, we need to know the elevation of every gravity station. If this is known, we can correct all of the observed gravity readings to a common elevation* (usually chosen to be sea level) by adding -0.3086 times the elevation of the station in meters to each reading. Given the relatively large size of the expected corrections, how accurately do we actually need to know the station elevations?

If we require a precision of 0.01 mgals, then relative station elevations need to be known to about 3 cm. To get such a precision requires very careful location surveying to be done. In fact, one of the primary costs of a high-precision gravity survey is in obtaining the relative elevations needed to compute the Free-Air correction.

*This common elevation to which all of the observations are corrected to is usually referred to as the datum elevation.

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