Calibration of multi-frequency EMI data: example at a test site in Rouen (France)

Summary Electromagnetic induction (EMI) is an efficient method, widely used to characterize the ground a large range of applications from hydrogeology, archeology to pedology. It is particularly interesting as it is not only sensitive to electrical properties of the ground but also magnetic and in certain condition dielectric as well. However, to quantitatively interpret EMI measurements, the data needs to be calibrated. The calibration process usually requires additional measurements, often a DC resistivity sounding or profile. Multi-frequency EMI devices, like the GEM-2 (Geophex Ltd) do not allow depth soundings like multiple offsets devices but multi-parameters interpretation is possible more often as the contribution of the different parameters to the measurement varies with frequency. Using the consistency of the measurements at different frequencies, should allow the calibration of the out-of-phase part of the data without additional measurements. In particular the interpretation of the out-of-part of the lowest frequency data should theoretically give a very similar apparent conductivity. An example from a well characterized test-site in Rouen is presented here, showing the apparent resistivity calculated from the data before and after this self-calibration process. The confrontation with an inverted DC resistivity profile, show a good agreement between the methods after the calibration.