127 6. Borehole electrical panels:an experiment In borehole M04 (Fig. 7), the open-hole section begins with oolitic limestone down to a depth of 49 m. Below this, the cuttings log describes a succession of clayey and marly layers within the Bajocian formation. Compared to M20, the OPTV data from M04 reveal generally darker intervals, reflecting the clayey-marly nature of the limestone. The first clayey interval, described between 49 and 53 m in the cuttings log, corresponds to a karstic void observed on the OPTV. Two additional karstic zones are identified on the OPTV at depths of 70 m and 91 m. Electrical resistivity tomography (ERT) indicates that the limestone in this section has relatively low resistivity values, often below 100 ohm·m, which aligns with the clayey-marly composition described in both the cuttings log and the OPTV imagery. Two horizontal zones of very low resistivity are detected at the karstic levels observed on the OPTV at 49–53 m and 91 m. In borehole M07, the cuttings log indicates limestone throughout the open-hole section, with a fractured zone around 90 m and clay-rich intervals at depths of 104 m and 124 m. The OPTV imagery reveals well-indurated limestone over most Figure 7 Comparison of measured resistivities with available geological data for borehole M04 and M07. 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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