Distinguishing water salinity, water saturation and clay content using spectral induced polarization: a tool for monitoring arid soil conditions

In arid and semi-arid climates, it is critical to assess the state of the soil in terms of water content and water salinity. The use of geophysics, and geoelectrical methods in particular, to this end faces the challenge of discriminating between the effects of water content and pore water salinity on soil electrical resistivity, since these two factors are both inversely related to resistivity. The heterogeneity of soils, with its possible varying clay content, makes the interpretation even more complex. We have investigated the combined effect of water saturation, pore water salinity and clay content using spectral induced polarization (SIP) on controlled laboratory samples. The experiments contribute to the development and application of the SIP method in the area of small-scale data acquisition and processing for hydrogeophysical and environmental purposes. In our experimental setup the above-mentioned three variables were gradually modified under controlled conditions. Ca-montmorillonite and very fine to coarse sand were mixed during multiple dry-wet mixing cycles in order to create artificial soil samples that mimic natural soils. In total, four samples were used with clay content varying from 2 to 8 mass per cent of clay. The other two variables were changed as well: water saturation ranging from 100 per cent to 10 per cent in 9–15 steps, and electrical conductivity of the pore water ranging from 0.05 to 0.7 S m⁻¹ in four steps. The statistical analysis of the results indicates that there is a significant positive correlation between quadrature conductivity and the three variables. The obtained data were fitted using a double Cole–Cole model. The analysis of the obtained Cole–Cole parameters along the three variables shows promising results for the separation between the effects of pore water salinity and water content, thus paving the way for fruitful field applications in arid and semi-arid environments.