Calibration of a novel impedance sensor for water content measurement in rocks

The water content (q) of the subsoil is an important parameter affecting all the processes that occur in the vadose zone, playing a key role in the infiltration, aquifer recharge, and flow and transport of water and substances, as well as groundwater storage. Although the vadose zone is usually considered constituted of soil, it may also be comprised of rock. In the latter case, q cannot yet be operationally measured despite its unquestionable importance. The aim of the present work was therefore to investigate the potential of a novel electrical impedance sensor for q measurement in rocks and to evaluate the effects of frequency and salinity on the calibration of this device. In this study, only the resistance (R), i.e., the real part of the measured impedance, was investigated because of its direct correlation with q. Calcarenite, a sedimentary porous rock, was used for the calibration in the laboratory via the installation of penetration-type probes. The independence of the measured R from the different frequencies set on the device was checked using a statistical approach. The q-R calibration functions obtained for the different saturation solutions have a power-law dependence with a good degree of correlation. The results highlight both the strong reproducibility of the experimental data (by using the penetration-type probes) and the suitability of the device for q measurement in calcarenite. The method could be advantageous for field applications that involve rocks.