S. Parker Gay, Jr., 1967. "Standard Curves for Interpretation of Magnetic Anomalies Over Long Tabular Bodies", 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 anomalies in Z, HTV for the thin infinite dike are shown to belong to a single mathematical family of curves for all values of dip and strike of the dike and all values of inclination of the magnetising field. The complete family of standard curves has been constructed and is incorporated into an interpretational scheme based on superposition with observed magnetic profiles. This technique should give more reliable interpretations than methods based on only a few isolated points of a profile curve.
By integration of the general thin dike response, ageneral expression of similar form has been derived for thick dikesten sheets of curves for dikes of varying width indices have been constructed. By employing the method of subtraction of curves, these serve for constructinganomaly profilesover bodies of finite depth extent. Additionally, for thin dikes the much-neglected demagnetization corrections have been incorporated in the interpretational method following verification by model studies. One important disclosure of this work is that the depth and location of the apex of an infinite tabular body may be determined without knowing the intensity or direction of magnetization within the body, assuming only that these quantities are constant throughout.
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.