ESTIMATION OF VAN GENUCHTEN-MUALEM PARAMETERS AND THE SATURATED HYDRAULIC CONDUCTIVITY FROM SIP MEASUREMENTS

Abstract

The estimation of hydrological properties of the subsurface by spectral induced polarization (SIP) surveys is a promising approach to quickly obtain areal information about the soil hydraulic properties. To interpret the measured SIP data correctly, the knowledge of the relationships between the soil hydraulic properties and the parameters resulting from the SIP measurements is essential. Laboratory studies implying both SIP and hydrological measurements can help to find and understand possible relationships. Therefore, we investigated the SIP spectra and soil hydraulic properties of seven natural soils and a medium sand sample in the laboratory. Due to the heterogeneity of the observed complex resistivity spectra, classical models, like the Cole-Cole type models, cannot be fitted to the spectra. Instead, we used the Debye decomposition to derive characteristic parameters describing the measured data. To obtain the soil hydraulic parameters, we fitted the van Genuchten-Mualem model, which is widespread in soil physics, to the hydrological data obtained by Multi-Step Outflow (MSO) experiments and Hyprop measurements, respectively. In this study, we focus on the applicability of the normalized chargeability resulting from the Deybe decomposition to estimate the parameters of the van Genuchten-Mualem model. Compared to the mean relaxation time, which is also determined with the Debye decomposition, the normalized chargeability correlates significantly better with the empirical van Genuchten-Mualem parameter alpha, which is closely related to the inverse of the largest pore radius. On the basis of this relationship, we present two approaches where the normalized chargeability is relevant to the estimation of both, the empirical van Genuchten-Mualem parameter alpha and the saturated hydraulic conductivity.