Investigating IP imaging measurements in frozen rocks for a better understanding of electrical signatures in alpine permafrost investigations

In mountain permafrost areas, frozen rocks are thawing due to the rise in air temperatures and, thus, ground ice content decreases, which in turn does not only lead to changes in subsurface water storage but also affects slope stability in solid rock walls. Monitoring changes of the electrical conductivity in the subsurface has emerged as a suitable technique to differentiate between non-frozen and frozen areas because of the much lower conductivity of frozen than of unfrozen media. However, the direct estimation of ice content directly from conductivity measurements is challenging because this property is also dependent on the temperature and the geological media, i.e., porosity, saturation, and fluid conductivity of the pore-water and the surface conductivity taking place at the interface between water and grains or ice. For a proper discrimination between frozen and unfrozen areas, the induced polarization (IP) has emerged as a suitable method, as it measures not only the conductivity but also the electrical capacitive properties (polarization) in the low-frequency range (mHz - kHz). Previous studies have revealed an increase in the IP effect with decreasing temperature, arguing that such response is due to the polarization either from charges in the ice (at the kHz range) or at the interface between ice and water (around 100 Hz). In this study, we investigated the IP response from small rocks in an imaging framework under well-controlled freezing conditions in the laboratory. First, we aimed to understand the role of surface conductivity in frozen rocks by a multi-salinity analysis (in the range between 0.1 and 10 S/m), which also permits to estimate the porosity of the rocks. Second, we investigate the polarization response of rocks in presence of features with high ice content in multi-electrode imaging configurations. The rocks have been collected at different sites in the European Alps to evaluate the effect in the data due to changing lithology. IP imaging measurements were conducted over a broad range of frequencies (0.1 Hz - 30 kHz) using to-date approaches to reduce capacitive coupling arising from changes in galvanic contact of the electrodes with the rocks at frequencies above 100 Hz. The data were inverted in ResIPy, which solves for the conductivity magnitude and phase angle by using complex calculus. The salinity experiments result in porosities around 2-4% and a linear relation between the surface conductivity and the polarization (quadrature conductivity) with a slope around 0.01, which reveals the importance of surface conductivity, even at low frequencies and positive temperatures. For measurements on rocks with ice features, inversion results show that the IP imaging method is able to delineate ice-saturated holes due to a contrast in polarization. Based on our results, we evaluate existing petrophysical relationships linking the frequency-dependence of the IP results with porosity, ice content and temperature.