Investigation of electrical relaxation mechanisms in soil through broadband standoff EMI

Abstract:

For nearly forty years, the use of electromagnetic induction (EMI) techniques for soil investigations has been well established in the geophysical community. Invariably the operating frequencies of non-invasively sensing commercial field systems reside well within the quasi-magneto-static regime, usually in the one to tens of kilohertz frequency range, and limited to investigating conductivity and magnetic properties through analysis of in-phase and quadrature components of the electromagnetic field. By expanding the range of the frequency-domain measurements from 1kHz through 10MHz, frequency dependent spectral properties of the soils can begin to be explored, while straddling the boundaries of the quasi-magneto-static and wave regimes. In the quasi-magneto-static regime, signals related to the conductivity (i.e. eddy currents) of the soil dominate the quadrature response; however, in the wave regime, the signals related to the dielectric permittivity (i.e. charge displacement/polarization) become the dominant contribution to the quadrature response. Spectral analysis of soils using field-scale EMI data becomes a possibility with this advancement, which in turn allows for measurement and observation of charge relaxation effects, historically limited to laboratory testing. Broadband relaxation effects of soils have been linked to various physical charge transfer mechanisms from macro- through nano-scales, with a proportional dependence of the scale on the transmitted wavelength. Here we evaluate the use of a newly developed, novel, standoff, high-frequency EMI system to measure electromagnetic properties of soil with varying fractions of pyrite and clay in an effort to measure quadrature based broadband relaxation information, and demonstrate greater soil information through spectral analysis and interpretation, including induced polarization and dielectric properties.