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

• We do a correction of electrical conductivity by using in-phase measurement.
• We determine magnetic viscosity by using multi-frequency measurement.
• We test the procedure on 1D and 3D synthetic cases.
• We attest this method in two archaeological case histories.

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.