Measured electric responses of unconsolidated layered and brine-saturated sand and sand-clay packs under continuous fluid flow conditions

We have investigated the effects of pore solution concentration on the complex electric response of two different unconsolidated samples, layered sand and sand-clay in the frequency range from 30 kHz to 3 MHz. The electric parameters that describe the electric response of the samples—real part of permittivity, conductivity amplitude and phase—are obtained through two-electrode electric measurements. Plots of the conductivity amplitude and phase as a function of frequency show large variations with water saturation and NaCl concentrations. This sensitivity may be useful for the characterization of the vadose zone. Under continuous fluid flow conditions, first drainage and secondary imbibition cycles were conducted for the two three-layered samples saturated with saline water in three different NaCl solution concentrations at atmospheric pressure and temperatures between 21 °C and 22 °C. Electrode polarization distorted the measurements, particularly in the kHz range. The distortion becomes negligible above a limiting lower frequency, which depends on NaCl solution concentration and the kind of sample. To obtain the intrinsic behaviour of the samples, the electric permittivities were analyzed above the limiting frequencies. Analysis of the real part of electric permittivity versus saturation indicates that, with increasing salinity concentration, the real part of the electric permittivity increases. Also, the hysteretic effect, the difference between first drainage and second imbibition, becomes more pronounced and remains present at higher frequencies. For the two samples, we observed a different correlation between conductivity amplitude/phase spectra and pore fluid concentration for seven saturation levels, suggesting that conductivity amplitude/phase spectra contain information about water saturation, salt solution concentration, and geotechnical properties (e.g., fines content) of unconsolidated near-surface soils. A simplified five-parameter double Cole–Cole model could fit the experimental data.

► We measured the electric response of layered sand and sand-clay. ► Increasing the salt concentration increases permittivity and the hysteretic effect. ► A clay fraction reduces the hysteretic effect significantly. ► It also causes the imbibition curves to lie below the drainage curves. ► The extended Cole-Cole model fits the observed permittivity versus saturation.