There may be times when a G&G interpreter, somewhat unfamiliar with gravity or magnetic interpretation, will look at a gravity or magnetic contour map of a frontier area and wonder whether the anomalies represent shallow or deep source bodies. The interpreter understands that anomaly amplitudes are related to source body density or magnetization, not depth, and that short-wavelength anomalies have shallower sources than long-wavelength anomalies with deep sources. But how shallow is shallow, and how deep is deep? The answers may reveal whether the contour map represents a prospective area and whether the map provides some insight into the area’s geology. Before deciding on whether to spend the money for a detailed quantitative gravity or magnetic interpretation, the interpreter can quickly make his/her own rough “rule-of-thumb” depth estimates from time-honored techniques. Then, if the preliminary estimates were favorable, a more sophisticated interpretation would be well-justified.
After completing this module, you will know how to:
- Select key anomalies from a gravity or magnetic contour map
- Determine the characteristic anomaly signature of 9 types of geologic bodies or structures
- How to extract appropriate profiles across those mapped anomalies
- Determine diagnostic points along the profiles
- Apply appropriate formulae to obtain “rule-of-thumb” depth estimates to the source body. Most of the formulae used below are appropriate for gravity interpretation where the initial estimate is to the center of mass of the source body. Magnetic depth estimates are normally estimates to the depth of a magnetized surface, which is usually equivalent to depth to the top of the source body. Some basic techniques for magnetic depth estimation can be found in Mod#26 Depth Estimation Techniques.