GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 We performed 2 EM surveys: a profile of 180 m and a 2s sampling rate using two different instruments and one ERT line acquired on the same EM transect. The first EM survey was measured by a DUALEM-642S (DUALEM Inc, 2008) instrument with dual-geometry receivers separated at 2, 4, and 6 m from the transmitter and with horizontal co-planar (HCP) and perpendicular (PRP) transmitter-receiver arrays. The DUALEM technology operates at 9 kHz, providing six simultaneous conductivity-sounding depths and a maximum depth of 9 m. The second EM data was acquired by a CMD-Explorer (GF Instruments, 2011), with dual-geometry receivers at separations of 1.5, 2.5, and 4.5 m from the transmitter, or vertical magnetic dipole (VMD) and horizontal magnetic dipole (HMD) transmitter-receivers arrays and 10 kHz frequency; in this case, three simultaneous depths of conductivity sounding are provided for each transmitter-receiver coil orientations and a maximum depth of 6.7 m.  ERT line was acquired using SYSCAL Pro (IRIS Instruments, 2001), with Wenner Alpha and Pole-Dipole configurations and electrode spacing of 2 m with a total length profile of 286 m.  3. RESULTS AND DISCUSSIONS The ERT measurements were inverted with a two-dimensional (2D) model using the ERTLab64 software, and then the inverted model was used for apparent conductivity calculations for the EM calibrations. Fig. 1 shows the inverted resistivity section with a depth of investigation of 28.5 m. From the elevation 7.5 m to 5 m, a resistive medium is observed related to the near-surface heterogeneities and correlated with sand and sandy silt. Two conductive zones (15-40 Ohm.m) are observed between 5 m to 2.5 m and - 5 m to - 21.5 m. A central resistive zone (50 Ohm.m to 120 Ohm.m) stands out between the elevation 2.5 m and - 5 m, where the central resistive core is located at a distance of 110 m and elevation of 0 m. Compared to geotechnical information, this resistive zone is associated with the level of fine sand, and the conductive zones with clay or clayey silt. Geotechnical classification has indicated a natural water content between 21.7% and 26.7%; as expected, our ERT section shows low resistivity values influenced by the clay content and fine sediment associated with moisture content.