GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.2 GNGTS 2024 Fig. 2 – Apparent conductvity map down to a nominal depth of 2.5 m on Lake n.16. Red doted line marks the path of the FDEM survey, while the blue triangle the locaton of borehole BC1 (taken from Azzaro et al., 2021 ). Results and discussion FIG.2 shows the mean apparent conductvity map of the Lake n.16 down to a maximum depth of about 2.5 m. It is characterized by a very low conductvity close to 0 mS/m, typical of the ice, however, a high conductvity spot is apparent, reaching a maximum value of more than 12 mS/m. In Azzaro et al., 2021 , the borehole named BC1 whose locaton is marked in FIG.2 revealed the saltest brines pocket of this lake at a depth of about 2.5 m; it is very close to the high conductvity spot highlighted by FDEM. It is interestng to note the accurate correlaton between the informaton provided by the borehole and by the FDEM survey, both indicatng the target depth at about 2.5 m, even considering the temporal interval of 8 years between the two surveys (2014 for the borehole and 2022 for the FDEM). GPR profles in FIG.3A-3B cross close to the high conductvity spot and the borehole BC1 of Azzaro et al., 2021. On the two almost perpendicular GPR sectons, it can be recognized the base of the lake as a high amplitude refecton at the botom of the mainly transparent facies of the lake ice. However, the contnuity of such horizon is interrupted near the BC1 borehole locaton, where the amplitude of the EM signal is strongly atenuated. Such extremely high atenuaton is most likely produced by the high conductvity of the brines pocket, which limited the propagaton of the EM signal. Such peculiar setng was found also on the GPR dataset of the neighbour CALM grid. FIG.3C shows the same high EM signal atenuaton with the horizon interrupton deepening, as described for Lake n.16. The spectral behaviour of the EM signal reveals a rapid decrease of frequency content, moving towards lower frequencies, corresponding to the transparent and high atenuated facies (red colour in FIG.3C’). In additon, the texture-related atributes highlight a more chaotc facies (FIG.3C’’), possibly