Comments on magnetic petrophysics

Abstract

Measurements of the magnetic properties of rocks constrain magnetic interpretation and resolve much of the ambiguity which afflicts the magnetic method. Magnetic petrophysical studies invariably indicate the importance of remanence as a source of magnetic anomalies. The susceptibility, remanent intensity and Koenigsberger ratio exhibited by a rock containing magnetic mineral grains is not only a function of the volume fraction of magnetic material with a given composition, but is also sensitively dependent on the domain state of the magnetic grains. Superparamagnetic, stable single domain and multidomain size ranges are given for magnetite, titanomagnetite, maghaemite, haematite and monoclinic pyrrhotite. The susceptibilities, remanent intensities and Koenigsberger ratios of superparamagnetic, single domain and multidomain grains of these minerals are plotted. Charts of typically observed magnetic parameters for various rock types are presented. Rocks often bear a multicomponent remanent magnetisation. The various components are often carried by grains with different coercivity or blocking temperature spectra and can be resolved using palaeomagnetic cleaning techniques. Surface samples typically contain palaeomagnetic noise which must be identified and removed if representative remanence values for the rock unit are to be determined. Under favourable conditions probable remanence directions of rock units can be inferred from a knowledge of the age of the rock and its locality and of apparent polar wander with respect to the crustal block within which the rock unit is located. Formulae are given for inversion of palaeomagnetic data to obtain magnetisation directions, and a table of primary remanence directions throughout the Phanerozoic is given for Australia. Apart from providing input to magnetic interpretation, many other applications of magnetic petrophysics are apparent. Palaeomagnetic dating of mineralisation, magnetic fabric studies for structural interpretation, magnetostratigraphy, detection of redox chemical remanent magnetisation effects and magnetic techniques of mineral identification and quantitative analysis, are some of the promising applications in the mineral exploration and processing industries.