167 8. Hydro-stratigraphic study of the Hydrogeological Experimental Site of Poitiers, France Additionally, a marked resistivity contrast observed at ~110 m depth across all boreholes is likely associated with the D6 discontinuity and the laminated marl-limestone succession of the Opalinum Biozone (Aalenian). In addition to conventional electrical logging, borehole electrical tests using electrical panels (Moreau et al., 2026) were conducted on the HES. Preliminary results indicate that the electrical panel sections correlate well with OPTV logs, long normal resistivity logs, and acoustic data (Mari, 2026). Electrical resistivity panels have proven to be effective in detecting individual karst conduits within the Dogger Limestone. Stratigraphy and hydrogeological data To complement geophysical data, flowmeter logging was conducted in selected boreholes (Audouin et al., 2008; Bodin et al., 2022; Boulais et al., 2026). These measurements aim to determine whether the porous intervals are hydraulically transmissive and capable of sustaining significant groundwater flow rates (>10 m³/h). The goal is to assess the relationship between observed vugs and cavities and the development of an active karst system with ongoing groundwater circulation. Audouin et al. (2008) identified an upper karstic level developed within granular limestones interbedded with cherty layers in the upper Bajocian, at a depth of approximately 50 m. This horizon is correlated with a bioturbated level situated beneath a chert bed within the Parkinsoni Biozone. An intermediate transmissive zone corresponds to the upper dolomitized interval of the Oolitic Formation, just below the D7bis discontinuity (at ~85 m). The lowermost productive zone, at a depth of ~110–115 m, is associated with the D6 discontinuity. Recent hydrogeological investigations conducted by the University of Poitiers have identified productive intervals that reveal complex and spatially variable flow dynamics. Productive horizons were detected at depths of 62–65 m (MP06, M21), 72–75 m (M2, M11, M16), 80–85 m (M12, M20, M22), and 90–91 m (M11). These transmissive zones correspond to major stratigraphic discontinuities observed in boreholes, even in the absence of significant karst features (D8bis, D8, D7bis, and Upper Dolomitized Zone). Radial convergent tracer tests further demonstrated vertical fluxes in boreholes originating from horizons at 110 m (Lower dolomitized zone), 85 m (D7bis), and 65 m (D8bis). Flowmeter data indicate that transmissive horizons vary between boreholes, suggesting lateral discontinuity of the porous intervals and pointing to a compartmentalized aquifer structure.
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