GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 Fig. 1 - Inverted section of resistivity. The resistive zone from 55 Ohm.m to 120 Ohm.m is associated with sand intrusions. The root mean square (RMS) is 1.47%. The black strokes rectangle corresponds to the selected data for building the EM model. From ERTLab software, we exported the resistivity inverted section in the XYZ format containing the resistivities of each measured level. This format is read by commercial EM4Soil software (EMTOMO 2013), which can build an EM model from the ERT model. We selected the ERT data from a distance of 59 m to 120 m and from elevation 7.5 m to -2.5 m (as shown in Fig. 1), avoiding the portioning extrapolation of the model at the edges of the investigated area, where fewer measured points were recorded. Fig 2 shows the measured and calculated data from DUALEM-642 and CMD-Explorer. A qualitative analysis of Fig. 2 shows consistency in the shapes of the calculated and measured curves. Considering a baseline for all curves, the analysis suggests a shift towards lower conductivity values or higher resistivity values. The shallow vertical heterogeneities have caused more perturbations in the signal of the HCP mode than in the VMD mode. Fig. 2 – Comparison of the apparent conductivities calculated from the EM model and the apparent conductivities measured by DUALEM-642 and CMD-Explorer instrumentations. The suffixes c and m on the modes are the calculated and measured apparent conductivities. Similarly, shallow lateral heterogeneities have caused more perturbations in the signal of the HMD mode than in the PRP mode. However, in this second case, it is essential to consider that the HMD mode of the CMD-Explorer cannot be compared directly with the PRP mode of the DUALEM-642, as they are oriented differently. Tab. 1 expresses how the measured EM data explain the calculated EM data. A loss of resolution is observed as increases the coil spacing on the sensors HCP, PRP, and VMD. Tab. 1 – Coefficients correlation between measured and calculated EM data. DUALEM 246 CMD-Explorer