109 5. Geophysical methods In addition to the 3D ERT method, the HES was also investigated using borehole electrical methods of the ERT type. The results are presented and discussed in Chapter 6, entitled Borehole Electrical Panels: an experiment (Moreau et al., 2026). For example, the borehole electrical panels also detect the karst level identified through acoustic methods (Fig. 10 and 11) at a depth of 100 m in borehole M20 (Fig. 18). A comparison between the resistivity panels and optical wall imagery confirms that low-apparent resistivity anomalies correspond to open, water-filled karst conduits. (a) (b) (c) (d) Figure 16 Comparison of resistivity (Rt) logs and optical televiewer (OPTV) images in boreholes M04, M07, M08, and M11. On the OPTV images, dark zones — corresponding to low Rtvalues — indicate open, water-filled karst conduits in the 85–90 m depth interval in boreholes M07, M08, and M11.
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