Spectral Induced Polarization Measurements on Sand-clay Mixtures in Dependence of Water Saturation

Abstract

Spectral induced polarization (SIP) measurements are promising for the in-situ determination of soil hydraulic properties. Here, we investigate the influence of water saturation on SIP measurements. Combined electrical and hydraulic measurements were conducted on sand and sand-clay mixtures using a multi-step outflow setup to desaturate the samples with several pressure steps. Our experimental results show that the ohmic conduction decreases with decreasing water content in accordance with Archie’s Law. For the sand-clay mixtures, the phase angle as well as the chargeability increases down to a critical water content and then decreases again in accordance with the short-narrow-pore model. In this model, the chargeability first increases with decreasing water content because the variation in the effective pore radii and the variation in the ion transparency becomes larger. When the saturation decreases further, the chargeability decreases due to a reduction of the thickness of grain-contact water rings resulting in a decrease of effective pore diameters and ion transparency. It is also observed that the phase peak shifts to higher frequencies, i.e. smaller relaxation times, when the saturation decreases, suggesting that electrical relaxation is controlled by the saturated part of the pore space.