177 5. DEEP ERT/IP for geothermal exploration and de-risking 5. starting model for 5 mV/V chargeability inversion, 6. estimated data noise of 1.5% for resistivity processing and 5% for IP data processing. The resistivity and chargeability inversions both converged, respectively in 6 and 4 iterations, with an excellent concordance of the modeled measurements with respect to the site acquisitions. This is illustrated on the examples in Figure 5.13, showing the cross-plot of the measured data compared to the modeled data at the end of the inversions. Figure 5.13 Graphs of resistivity inversion progress. Left: trend of the measure-data misfit modeled as the iterations progress. Right: cross-plot of modeled data vs. measured data at the end of the inversion. 5.3.7 Results This 3D Deep Electrical Resistivity Tomography approach allowed to highlight potentially aquifer sandy portions within the first 250–300 meters from the ground level. Figure 5.14 shows the geological map of the investigated area as well as a schematic geological section of the same area. These potential sandy portions are characterized by electrical resistivity values around 20–30 Ω·m and represented by a red-orange color Figure 5.15. The aquifer is well known from existing boreholes in the same region and is related to a fine sands layer (P3 in Figure 5.16). It is confined at the top by a more conductive layer related to sandy clays (Q1 in Figure 5.16). With regard to investigations for the deep geothermal aquifer, the 3D model shows the presence of zones with very low electrical resistivity (<20 Ω·m) starting from 300–350 m below sea level down to the base of the investigated volume. These more conductive zone can be interpreted as related with the blue clays layer (P in Figure 5.16 and 5.14). We notice that within the conductive layer we can isolate a plume characterized by even lower resistivity values, below 12 Ω·m (A, Figure 5.16).
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