162 A new concept of karst development based on hydrogeology and geophysics Stratigraphy versus seismic and acoustic data Following earlier studies (Bourbiaux et al., 2007; Audouin et al., 2008), a 3D seismic survey was carried out (Mari & Porel, 2008; Mari, 2026) to obtain a volumetric image with extensive horizontal coverage. Structural interpretation revealed a nearly horizontal stratigraphy with a gentle westward dip of about one degree, confirming the absence of significant vertical tectonic displacements. The 3D seismic volume was subsequently converted into a pseudo-velocity model, calibrated using acoustic velocity logs, and then into a pseudo-porosity model (Fig. 10; Mari et al., 2009; Delay et al., 2022). This seismic-derived porosity model highlighted three high-porosity—and presumably water-bearing—layers at depths of 35–40 m, 85–87 m, and 110–115 m, which were interpreted as karstic horizons. These porous levels are subhorizontal but laterally discontinuous across the HES study area. An integrated approach has since been developed to identify effective three-dimensional (3D) discrete karst conduit networks, constrained by tracer tests and geophysical data (3D seismic velocity block) (Bodin et al., 2022). The karstic horizons were independently confirmed through acoustic logging and borehole wall imaging with an Optical Televiewer (OPTV). In addition, a specific acoustic attribute, the Karstic Index, was introduced to identify karstic bodies (Fig. 11). However, owing to the relatively low vertical resolution of seismic data (meter scale) compared with borehole logs (centimeter to decimeter scale), some karstic features were detected only through borehole investigations. Figure 10 3D Seismic porosity block in the 30–120 m depth interval. Modified after Mari et al. 2009, Delay et al., 2022.
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