Applying free air and bouguer corrections to gravity readings

[subopolois]

hello everyone, i was wondering if it would be correct to apply both free air and bouguer corrections to the same gravity reading. would you know under what circumstances we would do this?

 

[JP O'Donnell]

The Free Air anomaly raises theoretical gravity from the reference ellipsoid to the station location for comparison with your gravity reading. However your gravity reading still contains the effects of topographic masses (which you are generally not interested in - you want to isolate density anomalies). You therefore apply the Bouguer correction to remove the effects of topographic masses - that above the geoid.

What remains after subtraction of theoretical gravity are gravity anomalies with the effects of station elevation and 'normal mass' topographic masses removed. These anomalies must therefore be related to anomalous density structure - relative to normal ellipsoidal Earth model.

 

[billiards]

hello everyone, i was wondering if it would be correct to apply both free air and bouguer corrections to the same gravity reading. would you know under what circumstances we would do this?

Yes, just so long as you do it to all your gravity readings and also make the other necessary corrections (i.e. for instrument drift, and latitude). You should do this in almost all circumstances, provided that it is a density anomaly that you are looking to detect.

The free air anomaly corrects your reading to a datum elevation, this elevation is arbitrary but is usually taken to be a surface of gravitational equipotential known as the geoid; the geoid approximates a mathematically defined surface known as the reference ellipsoid. For practical purposes a correction of 3.086 g.u./metre is added to your reading as the station at which the reading was acquired gets further and further away from the centre of the Earth.

The Bouguer correction attempts to correct for the excess mass that exists between your gravity station and the datum. This is achieved via the simplifying assumption that the topography can be represented by a flat plate extending to infinity in all directions. Mathematically, the correction is 2*pi*density*G*h, where G is the gravitational constant = 6.67*10^-11 N m²kg^-2 and h is the plate thickness. The correction needs to be subtracted from your reading.

In areas of high relief, where the Bouguer plate will not suffice to approximate topography, a more rigorous approach to correcting for topography is necessary, these computations are tedious and must be carried out by a computer.