see
https://www.sciencedirect.com/topics/engineering/gravity-data
Dear Shanker:
There is a keyword in your question: "anomaly".
If you are talking about an anomaly, in Geophysical Prospecting it means that the observed values have been corrected and brought to a reference level: Geoid or Ellipsoid Model. The anomaly is independent of the observed place, below or above MSL.
A key initial step in such analysis is the proper reduction of a gravity measurement made on or near the earth´s surface to an anomaly value that reflects density variations (tectonic / structural geology) in the crust and upper mantle. This involves a series of operations or reductions, , and, if they are successful, will leave us with a gravity anomaly that reflects density variations in the crust and upper mantle. The most important: Latitude correction, Free-air correction (elevation), Topography or terrain correction, Earth-tides, Eötvös correction.
A better way to describe the series of corrections is to consider them each as contributors to observed gravity. The subsequent sum shows the various components to observed gravity with the name of the corrections shown in parentheses (Potential Theory in Gravity and Magnetic Application by Richard J. Blakely, Cambridge University Press, 1996, ISBN 0-521-41508-X, p. 136 - 151):
observed gravity attraction of the reference ellipsoid
+ effect of elevation above sea level (free air)
+ effect of "normal" mass above sea level (Bouguer and terrain)
+ time-dependent variations (tidal)
+ effect of moving platform (Eötvös)
+ effect of masses that support topographic loads
+ effect of crust and upper mantle density variations ("geology").
Therefore, the gravity anomaly value spreading (maps) from the point of view of Geophysical Prospecting interpretations can be triggered by density differences—i.e., differences in density between Earth and the theoretical model—that can lie anywhere either above or below the reference level. Our goal is to understand the gravity anomaly distribution, the mass distribution, and also it is important to say you that the anomaly value represent the density contrast of the anomalous masses with respect to normal density, rather than the their total densities.
Final interpretations would normally be made from the Bouguer anomaly, but also note the word "anomaly" illustrates another geological scenarios:
Free-air anomaly: are strongly correlated with terrain.
Bouguer anomaly: over continental areas are strongly negative, because the Bouguer correction has removed the effects of normal crust above se level but has left the effects of deeper masses that isostatically support that crust.
Isostatic residual anomaly: represent the lateral variations in density of the middle and upper crust.
Best regards
Mario E. Sigismondi
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