Joint measurement setup for determining spectral induced polarization and soil hydraulic properties

The prediction capabilities of unsaturated flow and transport models are often limited by insufficient knowledge about the structural and textural heterogeneity of the soil. To obtain more information about soil structure, texture, and heterogeneity, as well as hydraulic properties, noninvasive spectral induced polarization (SIP) measurements have shown promise. However, there clearly is a need for more detailed investigations on the relation between SIP and hydraulic properties, in particular for unsaturated soil. To address this need, we developed a novel experimental setup that allows simultaneous determination of hydraulic and electrical properties. Our setup consists of multistep outflow equipment for hydraulic measurements and an electrical impedance spectrometer for measurements of the complex electrical conductivity. Two different measurement cells were constructed. The first measurement cell can only be used for nonshrinking soil, while the other one also allows measurements on mildly shrinking soil. Test measurements showed that the measurement accuracy is very high for the complex electrical conductivity. Nonpolarizable point electrodes are used for voltage measurements, ensuring a good contact to the sample for a wide range of water saturations. An experiment performed on unconsolidated sand investigated the dependence of the complex electrical conductivity and fitted Cole-Cole parameters on water saturation. The phase values and chargeability increased with decreasing saturation. The relaxation time associated with a phase peak was independent of saturation and corresponded reasonably to the average grain size of the sand. Therefore, no relationship between relaxation time and unsaturated hydraulic conductivity was found for this well-sorted sand.