Complex Electrical Measurements on an Undisturbed Soil Core: Evidence for Improved Estimation of Saturation Degree from Imaginary Conductivity

Recent studies have shown that induced polarization (IP) coupled with electrical resistivity surveys can be used for in situ lithological and hydrologic discrimination of the subsurface; yet the driving factors behind the effects of water content dynamics on IP are relatively understudied. We sought to improve the understanding of the relationship of IP on variations in water saturation degree for an undisturbed agricultural soil. Our experiment consisted of collecting IP measurements concurrently with hydraulic data during multistep outflow experiments. We determined the hydraulic properties of the undisturbed soil samples and correlated saturation degree with IP data. Due to an increase in pore fluid conductivity in the column pore water with decreasing saturation degree, we found that imaginary conductivity (σ″) may offer distinct advantages for determining water content over real conductivity (σ′) measurements. Although σ″ exhibits a weaker dependence on saturation degree compared to σ′, the relative insensitivity of σ″ to pore fluid conductivity results in a simpler dependence on saturation change in the presence of varying salinities. As changes in pore fluid conductivity are likely to occur in the field simultaneously with water content variations, we suggest that although IP has mostly been used to discriminate lithology, time lapse IP measurements may additionally provide a robust indicator of changes in saturation degree.