GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 HCP -2 High positive correlation (0.82) HMD -1.5 Moderate positive correlation (0.65) PRP- 2 Very high positive correlation (0.9) VMD- 1.5 High positive correlation (0.83) HCP -4 Low positive correlation (0.45) HMD -2.5 Moderate positive correlation (0.68) PRP- 4 Very high positive correlation (0.89) VMD- 2.5 Moderate positive correlation (0.68) HCP -6 Negligible correlation (0.11) HMD -4.5 Moderate positive correlation (0.6) PRP- 6 Very high positive correlation (0.79) VMD- 4.5 Negligible correlation (0.24) The better result of the PRP sensor over the HCP occurs because the HCP mode is more sensitive to depth structures but returns values that reflect cumulative electrical conductivity distribution over a large depth range. Generally, VMD dipoles showed better results than HMD dipoles: shallow lateral heterogeneities have caused more perturbations on the HMD signal. This behavior was the opposite for the DUALEM modes. Finally, we shifted the measured data curves for all DUALEM-642 and CMD-Explorer modes using EM4Soil software. The baselines HCP 2,4 and 6 were multiplied by 0.89, 0.98, 0.85; the same procedure was then applied to PRP 2, 4, and 6, multiplying them by 0.83, 0.93, 0.88. The VMD 1.5, 2.5, and 4.5 baselines were multiplied by 1.18, 1.21, 1.13, and HMD 1.5, 2.5, and 4.5 baselines by 0.72, 0.90, and 1.06, respectively. Shift values greater than 1 indicate a displacement based on increased conductivity, while values less than 1 indicate a displacement based on decreased conductivity. Fig. 3 shows the final results with a depth of investigation around 6 m (Fig. 3a, 3c) and 9 m (Fig. 3b,3d). Conductive zones are associated with silt loam, silty clay, and clay, while resistive zones are associated with sand and loamy sand. The intrusion of sand and loamy sand inside the levee is more notorious after the data calibration. A discontinuity of the conductive zone is observed at distances 50 and 60 m. Geotechnical tests showed an increase in grain size as increasing depth, combined with a loss of pressure between the river and the center of the levee. It can be inferred that the observed discontinuity is a preferential path responsible for the input of sandy material on the shallow surface.