Remanent Magnetization and Aeromagnetic Interpretation†

Published:January 01, 1967

CiteCitation
Isidore Zietz, G. E. Andreasen, 1967. "Remanent Magnetization and Aeromagnetic Interpretation", 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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Abstract
Quantitative interpretation of aeromagnetic anomalies has been based for the most part on the use of physical models in which the direction of magnetization is parallel to that of the earth’s field, al though laboratory results show that this assumption is invalid for many rock units. Magnetic fields have been calculated for inclinations of magnetization significantly different from that of the earth’s present field. The calculations are for a rectangular flattopped mass with finite and infinite vertical sides and a 75 degree inclination of the earth’s field. Several significant empirical relationships between the physical model and the computed fields have been obtained. For low dips of magnetization, the maximum and minimum points, rather than the points of inflection, mark the edges of the rock masses. The dip of the magnetization vector may be estimated from the ratio between the maximum and minimum anomaly amplitudes. The depth calculation techniques described in Geological Society of America Memoir 47 are based on the assumption of induced polarization, but these same empirical rules can be applied equally well to the total intensity field when remanent magnetization is present. This probably explains the success of the Memoir methods when applied to observed aeromagnetic anomalies over sedimentary basins
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Contents
Mining Geophysics Volume II, Theory
The relative merits of any geophysical method in a given situation can be predicted by careful study of the expected messagetonoise1 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 inducedpolarization 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 inducedpolarization 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 falloff 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 falloff 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.