Electromagnetic Methods

Electromagnetic, or EM, methods are a useful and rapid way to detect buried conductive bodies. An alternating current is fed into a wire coil held in a prescribed direction, either parallel or perpindecular to the ground surface. This current produces an alternating magnetic field, which induces a current in any nearby electrical conductors. Any Induced current creates its own alternating magnetic field, which is measured by a search coil connected to a sensitive voltage meter.

The method detects conductive bodies, not mineralization. A conductor could be an economic deposit of metal sulphides, but also a barren pyrite body or a zone of conductive graphite.

Traditional EM methods had little ability to “see” more than about 100 metres below surface. Newer, low-frequency EM methods, “pulse” techniques such as

UTEM, and magnetotelluric methods that use the earth’s own EM field have increased the depth penetration of EM prospecting. Another rapid and inexpensive EM technique is the very-low-frequency (VLF) method, which uses the signals from marine-navigation radio stations as a primary field source.

Because EM surveys do not require electrical contact with the ground, they are among the most useful techniques in airborne geophysics. Usually an airborne EM survey is followed up by ground EM work. Elemagnetic surveys can also be done using probes lowered down drill holes. These down-hole surveys are used at more advanced stages of exploration, where some drilling has already been done.