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Abstract

The conventional, or artificial field, electromagnetic method of geophysical prospecting is based on the measurement of alternating magnetic fields associated with currents artificially maintained in the sub-surface. If the subsurface currents are induced by a primary alternating field, the name inductive electromagnetic method is applied. In contrast, if the subsurface currents are applied through grounded electrodes, the name given is the conductive electromagnetic method.

Inductive techniques are more common and typically involve a magnetic dipole source, or transmitter, consisting of a number of turns of wire through which an alternating current is caused to ffow. With the conductive techniques, a long wire is laid out on the surface of the earth and grounded at each end. A generator in series with the long wire provides current which flows through the subsurface via the in the subsurface by the alternating current flowing in the long electrodes. Also, some currents are induced wire.

Numerous techniques have arisen in application of the electromagnetic method, many of which will be described subsequently. Natural electromagnetic fields serve as sources of signal for solid earth studies on the one hand, or serve as sources of noise for artificial field methods on the other hand. Of the four natural field methods-telluric, magnetotel-luric, magnetic variationAfmag-only Afmag will be described in detail. Each of these four methods depends upon the electromagnetic induction of currents in conductivity. Most frequently, the target sought is a massive sulfide ore body (ref. Chapter 3, Volume I) although the method has been

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