Calibration of Electromagnetic Induction Data based on Geoelectrical Subsurface Models

Abstract:

The use of electromagnetic induction as a tool for mapping soil electrical conductivity is well known and applied for many decades. However, due to calibration issues and signal instability, several limitations exist. Data are restricted for interpretation in a qualitative way which is sufficient if only differences in conductivities are of interest, e.g., for detecting metalliferous lodes or finding archaeological remains. The need for quantitatively evaluable data of apparent soil conductivity (ECa) is obvious where (1) data from different instruments, periods and places has to be combined for Digital Soil Mapping and multiscale investigations, (2) monitoring is required, (3) as prerequisite for depth inversion using data from different coil distances and (4) when applying profile ratio H/V. A calibration approach for such data is outlined by Lavoué et al. (2010) using electrical resistivity tomography (ERT). In our study, we utilize a similar approach as Lavoué et al. (2010) and expand it to in situ measured resistivities in order to overcome issues that might occur due to inversion of ERT results. We applied direct push-based logging of subsurface in situ resistivities (DP-log), ERT and vertical electrical sounding (VES). Data were used as input for a forward model in order to compare simulated ECa and measured ECa values by different EMI instruments.

DP-logs offer the possibility to obtain in situ resistivities and provide a linear correlation function for simulated and measured ECa values, while VES data suffer from a high ambiguity of inversion results.

Lavoue, F., J. van der Kruk, J. Rings, F. Andre, D. Moghadas, J. A. Huisman, S. Lambot, L. Weihermuller, J. Vanderborght, and H. Vereecken (2010), Electromagnetic induction calibration using apparent electrical conductivity modelling based on electrical resistivity tomography, Near Surface Geophysics, 8(6), 553-561.