108 A new concept of karst development based on hydrogeology and geophysics several meters. These tools offer vertical resolutions of approximately 10–50 cm and are commonly used for lithological interpretation, fluid identification, and fracture detection (Ellis & Singer, 2007). Electric logging tools — including single-point, short normal, and long normal logs (used to determine the resistivity Rtof the virgin zone) — were run in 12 wells: MP4, MP5, MP6, M11, M13, M14, M18, M21, M04, M07, M08, and M09. The Rt logs were used to estimate the formation porosity using Archie’s law (Fig. 6 and 7). Figure 16 presents a comparison between the Rt logs and optical televiewer (OPTV) images for wells M04, M07, M08, and M11. The OPTV imagery confirms that zones of low resistivity correspond to open, water-filled karst conduits in the 85–90 m depth interval at boreholes M07, M08, and M11. A conductive horizon is identified on borehole M04, between 50.05 and 53.15 m depth, where a sequence of vertically stacked cavities up to 1 m in height is observed both on the OPTV log and on the long normal resistivity log (Fig. 16). This karstified interval corresponds to a bioturbated interval. Figure 17 presents a comparison between the short normal (Rxo), caliper, and optical televiewer (OPTV) logs in borehole M07. The karst conduit in the 87–90 m depth interval is detected with a vertical resolution higher than that of the long normal resistivity (Rt) log. The caliper log provides very high vertical resolution and is particularly effective for detecting cavities and karst conduits. Unfortunately, at the SEH site, the caliper tool was run in only a limited number of boreholes. The Electric Cylinder Method (ECM, Lantier and Frappin, 2000) represents an advanced borehole-based resistivity imaging technique. It involves the deployment of a flexible multi-conductor cable equipped with evenly spaced electrodes (spacing typically ranging from 0.3 m to 2 m) over a length of 9 to 60 meters. The cable is inserted into a borehole, and a programmable acquisition unit controls the selection of electrodes for current injections and potential measurements. A potential difference is established between a designated remote electrode (often outside the borehole) and an active current-injecting electrode along the cable. Multiple ΔV measurements are recorded between electrode pairs during each injection cycle, and the procedure is iterated for multiple injection positions along the cable. This configuration enables a quasi-volumetric acquisition within a cylindrical domain centered on the borehole axis. 3D inversion of the acquired dataset yields a highresolution resistivity model within a radius of 2 to 20 meters around the borehole, depending on formation resistivity, electrode spacing, and inversion parameters (e.g., mesh density, regularization strength). The ECM is particularly suited for the detection and characterization of subsurface heterogeneities (Frappin, 2011a) such as lithological discontinuities, fracture networks, altered or weathered zones, fault planes, karstic cavities, or anthropogenic voids (e.g., tunnels or buried structures). Its ability to provide continuous 3D imaging makes it a powerful complement to conventional borehole logging tools, especially in heterogeneous or anisotropic geological settings. The ECM is commonly used in geotechnical applications to measure the diameter of jet grouting columns (Frappin, 2011b).
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