Mapping of quadrature magnetic susceptibility/magnetic viscosity of soils by using multi-frequency EMI

Abstract

Measuring magnetic viscosity significantly improves the information brought by the magnetic susceptibility about the history of soils. In the field its mapping can be achieved by TDEM measurement. Here we study the applicability of multi-frequency FDEM viscosity measurement in the low frequency range using a commercial EMI instrument. The dependence of the in-phase and quadrature out-of-phase components of the ratio of secondary magnetic field to primary magnetic field, electrical conductivity, magnetic susceptibility and magnetic viscosity is first described. The procedure allowing the determination of the three apparent properties is then proposed. It delivers first the conductivity using the differences between the quadrature responses at two different frequencies. Then, after removing the conductivity effects both in the in-phase and quadrature components, it provides the values of the magnetic susceptibility and viscosity. This procedure is tested on 1D and 3D synthetic cases to assess any arising uncertainty. The application of the method is attested in two archaeological case histories in Thessaly in conductive and magnetic soil contexts. The apparent magnetic viscosity maps are significantly different from magnetic susceptibility and conductivity maps thus bringing new information into the game.

1D sequential inversion of portable multi‐configuration electromagnetic induction data

“…; Simon et al . ). Recently, much effort has been made to invert the out‐of‐phase data in order to retrieve a 1D layered model of electrical conductivity (Saey et al .…”

Reservoir assessment using non-invasive geophysical techniques

“…The more conductive the ground, the shallower the GEM-2 can measure; for very conductive ground conditions the depth at which the lower frequencies sense is greater than the depth to which the EM radiation can penetrate, and the lower frequencies are effectively blinded (Reynolds 2011). The GEM-2 can exhibit various results at various frequencies if the soil contains magnetic materials, and then the device is sensitive to magnetic susceptibility and viscosity (Simon et al 2015); this is not the case in the present study.…”

Slingram EMI Devices for Characterizing Resistive Features Using Apparent Conductivity Measurements: check of the DualEM‐421S Instrument and Field Tests

“…The Vieil‐Evreux roman site remains (mainly walls) are situated in a sedimentary (clay) geological context where the conductivity response is significantly high; consequently the two major limitations of EMI cannot be considered in the present study but they merit to be recalled in this conclusion because they, at least for a part, explain the restricted use of the technique. They are: (1) metallic objects and features disturb measurements and practically disqualify its application specially in urban contexts and (2) in highly resistive soil contexts the conductivity response is too small and difficult to separate from the quadrature magnetic susceptibility in the quadrature response (Tabbagh, ) in spite of recent research works aiming to overcome this problem (Simon et al ., ).…”