GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 Results The ERT and GPR profiles along the piezometer #1 here presented were acquired with 1 m of electrode spacing and 400 MHz of frequency. The electric data inversion was carried out using the software Res2dinvx64 Version 4.0 (Geotomo Software, Loke and Barker, 1996). The inversion results including topography gave a misfit of 3.9%; the resistivity model is plotted in Fig. 2a. The dashed line in figure 2a shows the interface between the active layer, which is electrically less resistive (around 400 Ohm m), and the underlying layers that can be associated to the permafrost ( ρ > 1.0 kOhm m). A more detailed reconstruction of the transition between the active layer and the permafrost is retrieved by the GPR results, of the B-scan acquired along the same profile (Fig. 2b). In the radargram, the continuity of the main reflector, located at a depth of approximately 2 meters below the ground surface, clearly marks the top of the permafrost (black dashed line in Fig. 2b), allowing to delineate its morphology. The reliability of the response of ERT and GPR methods and their complete integrability is testified by the good agreement of the results. Overlapping the ERT and the GPR profiles in Fig. 2, the top of the permafrost (black dashed line in figure 2a exactly falls in the transition zone between the less resistive layer (active layer) and the more resistive one (permafrost). The resistivity distribution and radargrams of the other ERT and GPR lines, which investigated deeper subsoil structures, showed a more complex framework with strong local heterogeneities, which can be explained by hydrological evidence suggested by the piezometers monitoring in terms of very different water quantity and physical-chemical features (electrical conductivity and temperature) in space and time. These results will be further interpreted along with hydrological and geochemical data, with additional geological/geomorphological information provided by the project partners. Conclusion This novel characterization of the subsurface properties of a vast area in Ny-Ålesund provides many geophysical models useful to better understand the hydrogeology and hydrological processes of the area. Thanks to the 1m spaced ERT measurements and 400 MHz GPR, the active layer and its local discontinuities are correlated to the time series of piezometers. With the other (10m spaced) ERT surveys, deeper frozen permafrost areas are distinguished from unfrozen permafrost zones, allowing to map supra- and sub-permafrost aquifers. The preliminary interpretation of the ERT section obtained by commercial software will be updated by using the open-source software ResIPy for a more quantitative interpretation of the petrophysics of the active layer and permafrost. Correlations with climate will be developed in the piezometers area, and future time-lapse geophysical measurements will map yearly or seasonal changes in the permafrost in Ny-Ålesund.