GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 represents the only land connection between Moscow city and the northern Arctic territories. The subsoil is characterized by Quaternary glaciomarine sediments lying on Permian coal shale bedrock, with a continuous permafrost layer with thickness ranging between 50 and 100 m (Isaev et al., 2020). Residual thaw layer is occurring annually between the base of the seasonally freezing layer, at 2-3 m depth, and the top of permafrost layer, at 4-5 m depth (Tananaev et al., 2021). Engineering/geocryological surveys (borehole drilling, vegetation analysis and underground temperature measurements), geophysical surveys (electrical resistivity tomography, ERT), landscape microzonation and digital mapping were performed to characterize the unfrozen and frozen ground thickness distribution in the Khanovey field site. The Geoelectric method can accurately distinguish between frozen and unfrozen soil, based on spatial variations of electrical resistivity. Electrical Resistivity Tomography (ERT) has been proven to be the most effective technique for monitoring ice content, since upon freezing, electrolytic conduction is suppressed, reducing ion mobility and implying a marked increase in the resistivity of ground. The Complex Refractive Index Method (CRIM) is also used to integrate and quantitatively validate the results (Rossi et al., 2022), since it is useful to link the underground resistivity values to the petro-physical properties (porosity, clay and ice content) and to enhance integration among the results from geophysical and the other surveys (core lithologies, thermometric data, landscape zonation). The geocryological and geophysical data were mostly acquired between the 1st and the 23rd September 2019, during the international educational course “Vorkuta Engineer-Geocryological Field Work” held by the Lomonosov Moscow State University – MSU in collaboration with Norwegian University of Science and Technology - NTNU. Boreholes were realized since 2012 providing lithological information. Thermometric sensors were installed in 2015 and ground temperatures profiles were reconstructed by using records from 2015 to 2018. Information from three boreholes were used in this study to calibrate geophysical data. Furthermore, previous geoelectric profiles (ERT2018; Fig.1; Fig 2) were combined with ground thermometry and borehole data to get information about the frozen and unfrozen ground distribution closer to the railway. The investigated area was subdivided into two parts: the southern area (Site 1, delimited by light blue line in the map of Fig. 1), main target of this study, and the northern area (Site 2, delimited by red line in the map of Fig. 1), where geocryological data were collected in 2019 for future geophysical investigations. On Site 1, the geological, cryological and geophysical data were collected in 2019 and previous years. The IRIS Syscal Pro electrical resistivity meter, an all-in-one multinode resistivity system, is used for the geoelectric field acquisition. A total of six profiles were acquired (Fig.1; Fig 2): four lines in the Site 1A and two lines in the Site 1B, respectively. The combination of both the dipole-dipole