Electromagnetic Methods

Electromagnetic methods are a quick and useful way of detecting conductive bodies which are buried. An alternating current is fed into a wire coil which is held in a prescribed direction, it can either be perpendicular or parallel to the surface of the ground. This current produces an alternating magnetic field, which induces a current in any electrical conductors that are nearby. Any induced current creates its own magnetic field that is alternating, which is measured by a search coil that is connected to a sensitive meter of voltage.

Conductive bodies are detected by this method, but not mineralization. A conductor could be an economic deposit of metal sulphides, but also a zone of conductive graphite or a barren pyrite body.

The traditional electromagnetic methods had little possibility of seeing more than about 100 meters under the surface. Nowadays there are newer, low-frequency Electromagnetic methods, techniques of pulse such as UTEM, and magnetotellurgic methods that use the earth’s own electromagnetic field have increased the penetration in depth of electromagnetic prospecting. Another inexpensive and quick electromagnetic technique is the very-low-frequency method, this method uses signals from marine-navigation radio stations as a primary field source.

Due to the fact that electromagnetic surveys do not require electrical ground contact, they are among the most useful techniques in airborne geophysics. In most cases, an airborne electromagnetic survey is followed up by ground electromagnetic work. Electromagnetic surveys can also be performed by using probes which are lowered down through drill holes. These down-hole surveys are used at more advanced stages of exploration, where some drilling has already been performed.