Field-dependent susceptibility of rocks and ores - implications for magnetic petrophysics and magnetic modelling

Abstract

It is usually assumed that the initial magnetisation curve for a rock, soil or ore sample is linear in the applied field, for fields much less than the coercivity of the magnetic minerals in the sample. This implies that the measured susceptibility, defined as the induced magnetisation divided by the inducing applied field, is independent of the field H that is used in the measurement and that the induced magnetisation of the rock unit in situ can be calculated, irrespective of the field used by the measuring instrument, by multiplying the measured susceptibility by the Earth’s field at the location of the rock unit. A better approximation for many materials that contain ferromagnetic ( sensu lato) minerals is a quadratic dependence of the weak-field magnetisation on the applied field, given by Rayleigh’s Law, which yields a linear dependence of susceptibility on applied field. This field-dependent susceptibility is associated with hysteresis and a phase lag of magnetisation behind the applied field for AC measurements, which can masquerade as a phase lag produced by magnetic viscosity. Field-dependence of susceptibility is strongly affected by self-demagnetisation, so measurements of the Rayleigh coefficient η of strongly magnetic samples, as well as the initial susceptibility χ, must be corrected for self-demagnetisation in order to calculate intrinsic properties of the rock unit. Self-demagnetisation also largely explains why rocks containing low-Ti magnetite grains, which have high intrinsic susceptibility, exhibit only weak field-dependence of susceptibility, whereas rocks bearing titaniferous magnetite, monoclinic pyrrhotite or multidomain hematite exhibit relatively pronounced field-dependence of susceptibility. Under the conditions of the Neel approximation (η H « χ), the Rayleigh laws are still obeyed even when self-demagnetisation is considered. However, considerable departures from the Rayleigh relations occur when η H > χ. This paper examines implications of field-dependent susceptibility for measurements of susceptibility and its anisotropy, and methods for correcting calculations of induced magnetisation.