126 A new concept of karst development based on hydrogeology and geophysics • geological description derived from cuttings analysis during drilling (Bernard, 2005), • associated geological layers (general stratigraphy). All electrical resistivity measurements were taken in the open hole section of the boreholes. In borehole M20 (Fig. 6), the open-hole section begins with partially altered oolitic limestone extending down to 56 m. Below this depth, the formation transitions into dolomitized limestone containing several flint layers. The boundary between the oolitic and dolomitic limestones is clearly distinguishable on the OPTV images due to a change in coloration. The OPTV also reveals two dark zones, between 85–89 m and 96–102 m, which correspond to voids that were not identified in the cuttings log. The vertical resistivity profile indicates that the Dogger limestones are generally resistive (ρa >150 ohm·m). However, three lower resistivity anomalies are present. Two of these coincide precisely with the voids observed on the OPTV, while the third is located near the base of the borehole, at a depth of less than 110 m. Figure 6 Comparison of measured resistivities with available geological data for borehole M20. From left to right: vertical electrical resistivity tomography (ERT), optical televiewer (OPTV), geological log (LOG).
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