Prospecting using Magnetic Methods

In magnetic surveying, the geophysicist measures the strength of the earths magnetic field, which will vary locally depending on the amount of magnetic material in the underlying rocks. (Magnetic surveying has also been used, with considerable success, to find buried metal objects like underground fuel tanks.)

Where the rocks have high magnetic susceptibility, the local magnetic field will be strong; here they have low magnetic susceptibility, it will be weaker. This has two applications. Firstly, deposits with magnetic minerals — iron deposits, pyrrhotite-bearing nickel deposits and skarns — can be detected directly using magnetic surveying. Secondly, magnetic surveying can be used as an aid to geological mapping. Units with higher susceptibility will show up as areas of high magnetic field strength.

Magnetic surveys need not be Inc on the ground. An aerial magnetometer is an extremely, sensitive instrument which is either trailed below an airplane or helicopter, or mounted on the aircraft in a so-called “stinger”. By combining readings from this instrument with continuous aerial video photography, a magnetic map of a large can he plotted. Government geological agencies frequently contract for aerial surveys, publishing the results in order to encourage exploration.