GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 Figure 2. ERT/TDIP full-decay data inversion and permeability prediction: (a) DC conductivity, (b) maximum imaginary conductivity, (c) frequency exponent, (d) intrinsic permeability. The mean model of shear wave velocity resulting from the inversion of MASW data is superimposed with a black solid line. The groundwater level logged in P1 (black rectangle) is marked with a white area. The TDIP model shows three main layers in agreement with the expected stratigraphy: i) an unsaturated shallow layer (thickness around 2 m) showing moderate conductivity ( ~ 20 mS/m, σ 0 resistivity of about 50 Ω m, ~ 0.05-0.15 mS/m). This layer seems quite heterogeneous, with σ '' significant lateral increases (from x=25 to 75 m) of the maximum imaginary conductivity ( σ '' increasing up to 0.1 mS/m), likely due to a different lithology with an increasing fine fraction (peat and clay); ii) an intermediate layer (depth approx. between 2 and 6-7 m from surface) with high conductivity ( ~ 50-100 mS/m, resistivity of about 8-20 Ω m, between 0.1 and 0.3 mS/ σ 0 σ '' m), which reflects both the change in saturation (water table logged in P1 at 2.5 m) and the lithological transition to the sandy silts, characterized by a high surface polarization; iii) a deeper layer (down to the bottom of the model, approx.. 17 m), related to sand with a lower conductivity ( ~ 30-40 mS/m, ~ 0.05 mS/m). The shallow anomalies at x = 80-100 m, which cut the σ 0 σ '' lateral continuity of the layers, are likely due to the presence of utilities linked to the adjacent building. The DC conductivity value of the intermediate layer is compatible with fluid with moderate salinity, as confirmed by the conductivity logged in the well (equal to 130 mS/m, resistivity of 7.5 Ω m). This conductivity can be related to water affected by salinization phenomena but is still fair for human consumption and, even with significant limitations, also for irrigation.