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The geophysical analysis of magnetic anomalies Hall, Donald Herbert

Abstract

Traditional methods of interpretation of the results of magnetic surveys neglect effects due to permanent magnetization. Recent geomagnetic research on the remanent magnetization of rocks has shown this to be unjustified. Moreover techniques now being employed provide better measurements of magnetic field variations than have ordinarily been available in the past. In order to take advantage of these new developments, equations for the magnetic field over a point dipole, a horizontal line of dipoles, a thin dipping sheet, a thick dipping sheet and a sloping step are derived in the cases when both the directions of measurement and polarization are arbitrary. It is found that these directions combine with other properties of the bodies to form parameters, which determine various features of the magnetic anomalies over the bodies. In terms of these combined parameters, it is possible to give expressions for the higher derivatives of the fields over these bodies, and to develop methods of determining the unknown parameters of the bodies when magnetic profiles over them are given. Further, it is shown that the field over four of these bodies treated can be obtained by successive differentiation of a single function. This fact is used in drawing charts for computing values of the fields and their derivatives at points along profiles over any of these bodies. Tables of the higher derivatives are given, as well as graphs showing the position of special points such as peaks and inflection points on the profiles for any direction of polarization and measurement. It is shown how these more general methods may be applied to the interpretation of aeromagnetic surveys, and examples are given, of their use in the analysis of magnetic survey data over the La-Plonge area, Saskatchewan, and Texada Island, British Columbia. In the latter area, the general question of what geological information may be obtained from magnetic data is considered and a comparison is made of aeromagnetic anomalies with structural data obtained from aerial photographs.

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