GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 and pole-dipole configurations ensures a high sensitivity to lateral and vertical resistivity variations, as well as a good signal to noise ratio and penetration. The geoelectric data were processed and inverted by adopting ERTLab ™ —ViewLab3D ™ (http://www.geostudiastier.it/) package. Four parallel 240 m long profiles (ERT1, ERT2, ERT3, ERT4) were recorded in the Site 1A, using a 5 m electrode spacing. Two parallel 48 m long profiles (ERT1S and ERT2S) were acquired in the Site 1B, using 1 m electrode spacing. The lines are oriented in the NE-SE direction in Site 1A and in the N-S direction is Site 1B. In the latter, only the dipole-dipole configuration was used for both lines. The resulting ERT sections were then interpreted using the Kingdom ™ (IHS Markit) and ArcGIS software products, to produce 2D ALT maps. According to the geocryological and landscape data the area was subdivided into zones (Fig.2) as a function of: size relief, geomorphological characteristic, slope exposition, vegetation cover, drainage condition, ALT direct estimates, lithology and cryogenic processes (e.g., thermokarst, frost heaving, frost boil and solifluction). Borehole data and landscape microzonation results indicate a massive prevalence of clay and silty clay at shallow depths in this area, while the riverbank and the northern sector of Site 2 is covered by sand and silty sand deposits (Fig.2). Comparing the borehole data with the resistivity values of ERT2018 section, it is possible to correlate the active layer base and the permafrost table to resistivity values of approximately 100 Ω m and 200 Ω m. Based on this assumption, the active layer shows a thickness ranging from 0.5 m to 7 m (Fig.3) and the average ALT is about 4 m. Moreover, in the active layer there is a coexistence of frozen and unfrozen unconsolidated sediments showing resistivity values from about 30 Ω m to 100 Ω m. Here, the ice content estimated using CRIM ranges between about 0.3-0.4 to 0.9. The transition zone between the active layer base and the permafrost table, whose resistivity values ranged from 100 to 200 Ω m for this kind of sediments, shows higher ice contents ranging from 0.9 to 1.0. The underlying permafrost shows resistivity values higher than 200 Ω m. Very high-resistivity values exceeding 600 Ω m and up to 10 K Ω m are typical of frozen sediments with massive cryostructure clay content lower than 30%. If the underground lithology does not vary significantly with depth and clay is predominant, the average thickness of the permafrost layer should be at least 50 m. This thickness gradually reduces approaching the riverbank. Open, closed and deep taliks with resistivity ranging between about 3 Ω m and 70 Ω m, are scattered in the study area (Fig.2; Fig.3). Taliks, often located in bushy and swamp depressions with birch, shrub and grass, are generally characterized by warmer unfrozen ground. Generally, shrubby hillslopes exhibit the higher ALT values, while the lowest one is typical of lichens covered moundy hills. Thermokarst is a permafrost degradation process, initially caused by a disruption of the ground thermal equilibrium, that locally increases the ALT (e.g., Rossi et al., 2022). This phenomenon is dangerous because it can lead to the development of taliks, which represent a serious hazard for railway embankment stability. In the Khanovey area, thermokarst processes and taliks develop mainly along the railroad area, as a consequence of the thermal regime variation effect due to