- Copyright © 2001 Society of Exploration Geophysicists
Can gravity really detect a 200-m throw fault at a depth of 3000 m?
Such a question may be frequently asked when we interpret gravity data. At present, with gravity gradiometry available and the exploration activity in areas such as the Gulf of Mexico, another parallel question is raised.
Can gravity gradient data resolve a salt layer 200 m thick at 3000 m depth?
It is well known that qualitatively, gravity gradiometry is more sensitive than conventional gravimetry to shallow anomalous masses. However, in practice, we often need a more quantitative figure.
Down to what depth is gravity gradiometry better than gravimetry? Is it 1000 or 3000 or 5000 m?
An answer to these questions depends on two main factors: the accuracy of gravimetry and gravity gradiometry and the geometric shape of targets. The work presented here attempts to derive such characteristic depths and to generate a common understanding about the absolute and relative vertical resolution of gravity and vertical gravity gradient, by using three simple and representative geometrical models: a sphere, a vertical cylinder, and a vertical fault. In practice, an ore body may be approximated by a sphere and a salt dome by a vertical cylinder. Fault detection is of interest to both mineral and petroleum exploration.
In the comparison, 0.1, 0.2, 0.5, and 1.0 mGal are chosen for gravity data accuracy and 1, 2, and 5 Eötvös for vertical gravity gradient data accuracy due to the following two considerations. First, 0.1 mGal represents a good land gravity data accuracy after routine corrections/reductions are applied. Claims have been made that under excellent conditions, the airborne gravimeter can achieve an accuracy of 1 mGal. Similarly, 0.2 and 0.5 mGal may be regarded as representative accuracies of marine gravity data. Hopefully, 1, 2, and 5 Eötvös represent the accuracy of, …