Magnetic susceptibility and viscosity of soils in a weak time varying field

Abstract

We try in this paper to characterize the magnetic behaviour of soils both in the frequency and time domain. Our aim is to understand the anomalous responses we have observed in many EM surveys. A wide range of soil samples have been analysed with two instruments specially designed for harmonic (80 Hz-10 kHz) and transient (8–100 μs after cut-off) EM low inducing fields (50 μT). The most common spectrum observed could be explained with the well-dispersed single-domain grain theory (SD): a logarithmic decrease of the in-phase susceptibility with frequency and a constant quadrature susceptibility. Moreover, measurements in the frequency domain of the quadrature susceptibility and measurements of the coefficient of viscosity in the time domain have been compared and found to be in good accordance with theory. But other types of magnetic spectrum were encountered and could not be accounted for SD theories. This results in a non-logarithmic dependence of the in-phase susceptibility, a non-constant quadrature susceptibility and in the time domain a coefficient of viscosity S which is time dependent. It is demonstrated that the response due to quadrature magnetic susceptibility in the frequency domain and the magnetic viscosity in the time domain is sufficiently strong to be responsible for the anomalous responses. It is now possible to correct in an EM survey for the soil magnetic effect for a better determination of resistivity or conductivity.