Stanley. H. Ward, 1967. "Part C: The Electromagnetic Method", Mining Geophysics Volume II, Theory, Don A. Hansen, Walter E. Heinrichs, Jr., Ralph C. Holmer, Robert E. MacDougall, George R. Rogers, John S. Sumner, Stanley H. Ward
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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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The relative merits of any geophysical method in a given situation can be predicted by careful study of the expected message-to-noise1 ratio. For example, let us draw or deduce from the subsequent text, the anomaly formulas due to a spherical inhomogeneity in the subsurface and the symbols in each formula are explained in the text. The gravity, magnetic, resistivity, and induced-polarization surveys all are volume dependent, whereas the electromagnetic method is dependent only upon the area of the inhomogeneity, normal to the inducing field. Thus, a thin disk can give nearly the same electromagnetic anomaly as a sphere of the same radius.
If we can make a reasonable estimate of the physical property contrast anticipated to exist between ore and host, we can then predict the anomaly magnitude expected from the sphere, when buried at any given depth, via the geometric factor. Note that from this viewpoint, given the maximum or saturation value of unity for the physical property factor, the magnetic and resistivity methods theoretically give the same percent anomaly due to a sphere. The physical property function M–iN for the electromagnetic method has a maximum value of one half for a sphere while the change with frequency of the electrical resistivity contrast.
Thus, except for a factor of two, the magnetic, resistivity, electromagnetic, and induced-polarization methods should give the same maximum anomaly. Note that the geometry of the anomalous fields for each of these methods is an induced dipole with a resultant fall-off of peak anomaly proportional to the inverse cube of the depth to the center of the sphere below the measuring plane. In contrast, the gravity method exhibits an inverse second power fall-off due to an induced monopole. The density contrast between ore and host sometimes exhibits a maximum value of two. Thus f r om a maximum message viewpoint, one would be inclined to rate the methods in the order given above. However, we need to counter this bias by considering expected values of the physical property factor and the noise for any given geologic situation.
Let us look, then, at iron ore, massive sulfides, and disseminated sulfides, items treated i n Volume I. We should expect the following physical property ranges: A very wide range of properties is evident and hence the prediction of an anomaly magnitude looks hopeless.