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 p s after cut-off) EM low inducing fields (50 pT).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.

Evaluating the Effects of Magnetic Susceptibility in UXO Discrimination Problems

“…This relationship has also been derived without the use of equation (1) [Mullins and Tite, 1974], and has been observed in complex susceptibility measurements by Dabas et al [Dabas et al, 1992]. Therefore, by determining the slope of the inphase component from the susceptibility measurements, we can immediately estimate the quadrature component.…”

“…The Cole-Cole model has also been used to represent the magnetic susceptibility (for example [Olhoeft and Strangway, 1974] and [Dabas et al, 1992]). The parameter is related to the distribution of relaxation times.…”

Magnetic Viscosity for Cyclostratigraphic Logging of Argillaceous Sediiment

“…This relationship has been experimentally verified (Bloemendal et al, 1985;Dabas et al, 1992) in the presence of hyperfine grains. The role of hyperfine grains, at the transition between the superparamagnetic (SP) and stable single domain (SSD) states, has been confirmed (Dearing et al, 1996;Worm, 1998) and has been demonstrated to be very significant in soils with grain sizes below 30 nm as an indicator of the presence of secondary ferrimagnetic minerals (SFM).…”

UXO Target Detection and Discrimination with Electromagnetic Differential Illumination

“…Within a certain range of frequencies determined by the end-member time constants, the in-phase susceptibility will vary linearly with the logarithm of the frequency while the quadrature susceptibility is constant with frequency. Furthermore, for τ 2 >> τ 1 the slope of the in-phase and value of the quadrature susceptibility are related by (Dabas et al, 1992),…”