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

International audienceThis article addresses the characterization of resistive archaeological targets and near surface structures by electromagnetic induction (EMI). It presents tests achieved with the DualEM-421S instrument (Dualem Inc., Milton, Canada) in order to be able to quantitatively compare these measurements to the standard technique of direct-current (d.c.) resistivity. The test was done over the Gallo-roman site of Vieil-Evreux in Normandy, France and one-dimensional (1D) and three-dimensional (3D) inversions were applied to the data set obtained. We have first investigated the signal-to-noise ratio of each of the six DualEM receiver coils both in a static mode and for a quad-pulled system. The dependence on the roll angle was also measured and it is shown that rotation of DualEM must be taken into account if the roll angle is more than ±10°. Absolute calibration and in-phase/quadrature (out of phase) component discrimination was checked by measuring the response of a small conductive and non-magnetic sphere. Several electromagnetic soundings by measuring the instrument response at different heights were done in order to check the quadrature (out-of-phase) response of the instrument. Inversions of these electromagnetic soundings were compared to d.c. vertical electric soundings (VESs) over four locations and found in accordance. Several maps using different coil configurations (HCP, VCP, PERP) and different heights were performed and inverted, both for a wide mesh (5 m) and for a finer one (0.5 m). The wide mesh allows a global and rapid description of the surface geology context (continuous d.c. measurements cannot deliver equivalent depth of investigation). The fine mesh conductivity maps clearly show the walls of a fanum (temple) as well as other structures and prove that the DualEM-421S is able to map correctly archaeological resistive targets. These maps and their interpretations were compared to previous results obtained by d.c. technique