125 6. Borehole electrical panels:an experiment The lower resistivity values observed in the electrical panels, compared to those obtained from longnormal Rt logs, arise both from the choice of the geometric factor K and, more importantly, from the effect of the borehole fluid (its conductivity), which conducts current and substantially influences the measurements. However, if the borehole geometry remains constant (i.e., the diameter does not vary) and the borehole fluid properties are unchanged, then for a given electrode configuration, the influence of the borehole fluid remains consistent at all depths. Under these conditions, relative variations in apparent resistivity still offer valuable insights for geological interpretation. A GFTC tool, which simultaneously records Gamma Ray, Flow, Temperature, and Water Conductivity logs, was run in boreholes M08, M11, and M20. The measured water conductivity averaged 500 µS/cm, corresponding to a water resistivity of approximately 20 ohm·m. Figure 5 also illustrates the correlation between electrical resistivity measurements and Optical Televiewer (OPTV) imagery (Mari, 2026). The optical televiewer is a downhole probe that provides a continuous, high-resolution, 360° digital unwrapped image of the borehole wall, oriented to magnetic north, using an integrated light source. It functions effectively in air or clear water and typically achieves an average resolution of approximately 2 mm, depending on borehole conditions and logging speed. In boreholes containing mud or turbid fluids, the optical televiewer must be replaced with alternative imaging tools, such as the acoustic televiewer or microresistivity imaging tools. These alternatives also generate continuous unwrapped images of the borehole wall and are extensively used for in situ mapping of natural fracture orientations, as well as for determining the orientation of the principal horizontal stress field—based on the identification of drilling-induced tensile fractures or borehole breakouts (Genter et al., 2025). In this study, the optical televiewer was deployed in boreholes with minimal karstification and long open-hole sections (M01, M09, M10, M14, M15, M17), to enable a very high-resolution hydro-stratigraphic analysis of the HES (Gaillard, 2026b). In boreholes M07, M08, and M11, low resistivity values identified by resistivity logs (Rt and ERT) are correlated with dark zones on the OPTV images between 85- and 90-meters depth. In borehole M04, a conductive horizon is observed between 50.05 and 53.15 meters, where a sequence of vertically stacked cavities— up to 1 meter in height—is clearly visible on both the OPTV and Rt resistivity logs. Geological Interpretation Each borehole is presented in a summary figure that, from left to right, compiles four pieces of information (Fig. 6, 7, and 8): • vertical electrical resistivity panel (ERT), • optical Televiewer image (OPTV),
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