128 A new concept of karst development based on hydrogeology and geophysics of the open-hole section. A darker interval is observed between 86 and 90 m, corresponding to a karstic zone. From a depth of 110 m onward, the OPTV shows a general darkening, suggesting increased clay content or alteration. The electrical resistivity tomography (ERT) data show high resistivity values (ρa >150 ohm·m) down to approximately 110 m, consistent with compact limestone. Below this depth, resistivity decreases (ρa <150 ohm·m), reflecting the presence of clay or increased porosity. A single horizontal low-resistivity anomaly is detected at 90 m, matching the karstic level observed on the OPTV between 86 and 90 m. In borehole M11, the cuttings log indicates limestone interbedded with clayey layers within the Bathonian and Bajocian formations. This is reflected in the OPTV imagery, which generally appears dark, except for a lighter interval between 100 and 110 m. A very dark zone is visible on OPTV between 86 and 89 m, corresponding to an open and highly fractured level. Interestingly, this interval is described as a clay layer in the cuttings log. The ERT profile shows relatively low resistivity starting from a depth of 110 m. Between 90 and 110 m, the resistivity is slightly higher but remains lower than in Figure 8 Comparison of measured resistivities with available geological data for boreholes M11 and M08. Refer to Figure 6 for the ERT resistivity color scale. From left to right: vertical electrical resistivity tomography (ERT), optical televiewer (OPTV), geological log (LOG).
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