Demonstrating the contribution of dielectric permittivity to the in‐phase EMI response of soils: example from an archaeological site in Bahrain

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ABSTRACTElectromagnetic induction instruments (also called loop–loop, dipole–dipole, or Slingram) are now commonly used in archaeological prospecting. They are truly light instruments, which are able to measure both the apparent electrical conductivity and the apparent magnetic susceptibility of the ground. During a field test in Bahrain Island where the soil has a high clay content and a high salt content, surprisingly high values of in‐phase response were obtained at all inter‐coil spacings using CMD “Mini‐Explorer” (GF Instruments Ltd., Brno, Czech Republic) at 30 kHz, in both horizontal coplanar and vertical coplanar configurations, and the horizontal coplanar and vertical coplanar susceptibility variations were in total opposition. This apparent discrepancy is explained by considering the in‐phase responses to be dominated by the relative dielectric permittivity. Using the raw and in‐phase vertical coplanar and horizontal coplanar data, it is possible to determine and map the apparent permittivity and apparent magnetic susceptibility. For this case of slated soils with high clay content, the relative permittivity is strong but in agreement with both experimental data at lower frequencies and theoretical models reported in the literature.