GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 3.2 436 The L2 model (NW zone) shows resistivity values well below 5 � m (Fig. 2a) related to the saline intrusion inland, where local resistive layers ( z = 5-10 and > 35 m b.s.l.) are likely due to an increase of clay/silt content with a consequent permeability decrease. This evidence is also confirmed by the expected increase of MN (Fig. 2d), which is highly sensitive to clay content (Slater and Lesmes, 2002). Moving toward the SE zone (L4 and L5 models), we observe higher resistivity values due to an increased distance from the sea (L4, Fig. 2b and 2e) and to a reduced influence of the saline intrusion with a less groundwater circulation (L5, Fig. 2c and 2f). Saltwater seems to affect only partly the L5 line ( x < 110 m in Fig. 2c and 2f), being the distance from the sea approximately the same, when compared to the L2. Therefore, the MN anomalies due to a lithologic control (increase of clay content) are here associated with relatively conductive layers, though located at the same depths seen on the L2. The most significant IP effect (MN > 10 mS/m), seen on the left-bottom of the L5 line (Fig. 2f), is likely due to an increase of both clay content and pore-fluid conductivity. Therefore, we focus on this line performing a spectral Cole-Cole inversion (Fig. 3), by selecting only 24 electrodes (electrode spacing=10 m) for decreasing the computational effort required for the Jacobian calculation. Fig. 3 - Spectral TDIP inversion of the L5 line. (a) DC resistivity, (b) chargeability, (c) relaxation time, (d) frequency exponent.