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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-tonoise1 ratio. For example, let us draw or deduce from the subsequent text, the anomaly formulas due to a spherical inhomogeneity in the subsurface:

Each of these anomalies constitutes a message of the form 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 is not readily predicted but probably has a maximum of one half (assume σ2σ1 at one frequency and that σ2σ1 at a second frequency). Thus, except for a factor of two, the magnetic, resistivity, electromagnetic, and induced

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