D. A. Hansen, R. E. MacDougall, 1967. "Introduction", 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 magnetic method is one of the ffrst geophysical techniques to be applied to mineral exploration. It was used in the location of iron ore bodies as early as 1640. Today, the magnetic method accounts for a major portion of the mining geophysical effort
This chapter covers two major aspects of the method. The first concerns the characteristics of the earth’s magnetic field and the magnetic properties of the rocks and minerals which reside in that field. The second section deals with methods of treatment and interpretation of data obtained in magnetic surveys.
A number of the papers to follow appear in print for the first time; others have been published elsewhere and, because of their significance, have been selected for republication in this volume. A Selected Bibliography is included as the final contribution to this chapter. The entries have been selected as the most significant contributions in the field of magnetics.
Figures & Tables
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.