Out-of-phase magnetic susceptibility of rocks and soils: a rapid tool for magnetic granulometry

The magnetic susceptibility measured in alternating field can be resolved into a component that is in-phase with applied field and a component that is out-of-phase. While the former component is widely used for solving various geophysical, geological and environmental problems, the latter component is paid only minor attention. The theoretical relationship between the frequency-dependent in-phase susceptibility and the out-of-phase susceptibility is described by the π/2 law valid for materials in which the latter is due to presence of magnetic particles on transition between superparamagnetic and stable single-domain states possessing sufficiently wide distribution of relaxation times. To advantageously use the out-of-phase susceptibility, which is measured simultaneously with the in-phase susceptibility during one measuring process, in magnetic granulometry, new parameters, X ON and X OD, are proposed approximately converting the out-of-phase susceptibility into the X FN and X FD parameters of the frequency-dependent susceptibility. The validity of the new parameters was tested through mathematical modelling and through investigating samples of various sediments. The correlations found seem to be acceptable from the practical point of view. In addition, simple test is proposed for checking that the out-of-phase susceptibility is solely due to the viscous phenomena and not due to electrical eddy currents or weak field hysteresis. As the out-of-phase susceptibility is measured automatically along the in-phase-susceptibility with some instruments and can be directly interpreted in magnetic granulometry terms, it is to recommend to be routinely investigated in solving various problems of environmental magnetism.