Spectral induced polarization imaging to monitor seasonal and annual dynamics of frozen ground at a mountain permafrost site in the Italian Alps

Abstract

Abstract. We investigate the application of spectral induced polarization (SIP) monitoring to understand seasonal and annual variations in the freeze–thaw processes in permafrost by examining the frequency dependence of subsurface electrical properties. We installed a permanent SIP monitoring profile at a high-mountain permafrost site in the Italian Alps in 2019 and collected SIP data in the frequency range between 0.1–75 Hz over 3 years. The SIP imaging results were interpreted in conjunction with complementary seismic and borehole data sets. In particular, we investigated the phase frequency effect (ϕFE), i.e., the change in the resistivity phase with frequency. We observe that this parameter (ϕFE) is strongly sensitive to temperature changes and might be used as a proxy to delineate spatial and temporal changes in the ice content in the subsurface, providing information not accessible through electrical resistivity tomography (ERT) or single-frequency IP measurements. Temporal changes in ϕFE are validated through laboratory SIP measurements on samples from the site in controlled freeze–thaw experiments. We demonstrate that SIP is capable of resolving temporal changes in the thermal state and the ice / water ratio associated with seasonal freeze–thaw processes. We investigate the consistency between the ϕFE observed in field data and groundwater and ice content estimates derived from petrophysical modeling of ERT and seismic data.