Complex electrical resistivity measurements on alluvial sediment samples toward sedimentological and petrographic properties estimation

In the last decades, geophysical methods have been increasingly used for groundwater exploration to gain information on geometry and properties of the subsurface. However, the knowledge on petrophysical relationships achieved with the longstanding experience in hydrocarbon exploration and available for reservoirs characterization cannot be applied in a straightforward way for near-surface environments, due to typical lower temperature, pressure and consolidation degree. Moreover hydrogeological studies have less financial support and higher interactions with human activities than hydrocarbon exploration, so that different techniques are used, e.g. surface instead of well-log acquisitions. Essentially, hydrogeophysics is based on the relationships of electrical resistivity and hydraulic conductivity with the porous medium texture. In this work the resistivity-texture relationship is investigated through laboratory measurements of complex electrical resistivity on reworked alluvial sediment samples, texturally ranging from slightly sandy mud to gravelly sand, and saturated with NaCl and CaCl2 solution with electrical conductivity varying between 20 and 1400 microS/cm. Data are collected in the frequency range between 10 mHz and 100 kHz with a spectrometer connected to a sample holder, which allow to investigate a volume of about 2500 cm3. Amplitude and phase spectra are modelled through the Cole-Cole analogue electrical circuit, in order to obtain the best-fitting parameters (direct current resistivity, chargeability, relaxation time and frequency exponent) describing conduction and polarization processes within the samples. The fit of the experimental data is performed not only with a standard least-squares approach, but also with a multiobjective approach, which considers separately the amplitude and the phase spectrum. The latter approach allows to emphasise polarization phenomena primarily linked to the amount and the dispersion degree of the silty-clayey fraction, even if phase values for sediments free from metallic particles are usually smaller than a few mrad. Moreover, the multi-objective approach permits the identification of upper and lower limits for model parameters. However, preliminary results show that the superposition of comparable effects originated from different variables (e.g. both the decrease in the water electrical conductivity and the increase in silt and clay content yield an increase of polarization) prevents one-to-one correlations among the model parameters and the sediments’ litho-textural properties; a multivariate statistical analysis could be a valid support in the interpretation of experimental data.