Hydraulic conductivity estimation from induced polarisation data at the field scale — the Krauthausen case history

Recently, encouraging results have been obtained to estimate hydraulic conductivity in unconsolidated sediments from induced polarisation (IP) measurements. The work focussed on laboratory experiments in order to establish semi-empirical relationships between complex electrical conductivity and hydraulic parameters. Although field studies are certainly important to evaluate the feasibility of the method in practice, only a limited number of case histories have been reported. We carried out an IP survey at the Krauthausen hydrogeological test site in Germany. Multifrequency data were recorded along two profiles with a dipole–dipole configuration and 2 m electrode spacing. The data were inverted with a 2-D inversion code to obtain images of conductivity magnitude and phase for each frequency. We used a novel focussing regularisation scheme that does not suppress sharp boundaries and yields realistic images of the subsurface. The gross structural features compare well with the known lithology, which consists of an approx. 9 m thick sand/gravel aquifer over fine sands and a thin clay layer which constitutes the base at approx. 11 m depth. From the complex electrical conductivity distribution we calculate hydraulic conductivity estimates using the empirical equations suggested by Börner et al. [Börner, F. D., Schopper J. R., Weller, A., 1996. Evaluation of transport and storage properties in the soil and groundwater zone from induced polarisation measurements. Geophys. Prosp. 44, 583–601] and by Slater and Lesmes [Slater, L. D., Lesmes, D. P., 2002a. Electric hydraulic relationships observed for unconsolidated sediments. Water Res. Research, 38 (10), doi:10.1029/2001WR001075] and compare the results with values obtained from grain size data at two boreholes. The Börner model yields hydraulic conductivities in the same order of magnitude as the grain size data, whereas the Slater and Lesmes model substantially underestimates hydraulic conductivities. We also compare the specific inner surface estimated from our IP data with values obtained from BET measurements at the two borehole locations. In this case, we observe a reasonable agreement with both the Börner and the Slater and Lesmes equations. Our results constitute an encouraging example where reasonable estimates of hydraulic parameters are obtained at the field scale.