Summary of Gravity Types

We have now described the host of corrections* that must be applied to our observations of gravitational acceleration to isolate the effects caused by geologic structure. The wide variety of corrections applied can be a bit intimidating at first and has led to a wide variety of names used in conjunction with gravity observations corrected to various degrees. Let's recap all of the corrections commonly applied to gravity observations collected for exploration geophysical surveys, specify the order in which they are applied, and list the names by which the resulting gravity values go.

• Observed Gravity (gobs) - Gravity readings observed at each gravity station after corrections have been applied for instrument drift and tides.
• Latitude Correction (gn) - Correction subtracted from gobs that accounts for the earth's elliptical shape and rotation. The gravity value that would be observed if the earth were a perfect (no geologic or topographic complexities), rotating ellipsoid is referred to as the normal gravity.
• Free Air Corrected Gravity (gfa) - The Free-Air correction accounts for gravity variations caused by elevation differences in the observation locations. The form of the Free-Air gravity anomaly, gfa, is given by;

  gfa = gobs - gn + 0.3086 h (mgal)

  where h is the elevation at which the gravity station is above the elevation datum chosen for the survey (this is usually sea level).

• Bouguer Slab Corrected Gravity (gb) - The Bouguer correction is a first-order correction to account for the excess mass underlying observation points located at elevations higher than the elevation datum. Conversely, it accounts for a mass deficiency at observations points located below the elevation datum. The form of the Bouguer gravity anomaly, GB, is given by;

  GB = gobs - gn + 0.3086 h - 0.04193 r h (mgal)

  where r is the average density of the rocks underlying the survey area.

• Terrain Corrected Bouguer Gravity (gt) - The Terrain correction accounts for variations in the observed gravitational acceleration caused by variations in topography near each observation point. The terrain correction is positive regardless of whether the local topography consists of a mountain or a valley. The form of the Terrain corrected, Bouguer gravity anomaly, gt, is given by;

  gt = gobs - gn + 0.3086 h - 0.04193 r h + TC (mgal)

  where TC is the value of the computed Terrain correction.

Assuming these corrections have accurately accounted for the variations in gravitational acceleration they were intended to account for, any remaining variations in the gravitational acceleration associated with the Terrain Corrected Bouguer Gravity, gt, can now be assumed to be caused by geologic structure.

Another way of summarising the situation now is that the corrections discussed here are removing the effects which we already know about, and the resulting Terrain Corrected Bouguer Gravity is the effect of the unknown component of the geology.

Finally (here) it is important to remember that the observation stations have not moved. We have removed the effect of topography, and the mass making it up, but the gravity anomaly which results is the gravitational acceleration caused by density anomalies in the subsurface, measured at the observation points. This can be an important point in detailed modelling of anomalies.

*"corrections" might better be called "reductions", because, except for the drift correction, they imply no deficiency in the data observed and would be applied with equal validity to perfect data collected with a perfect instrument - and perfect observers.