84 A new concept of karst development based on hydrogeology and geophysics The HES boreholes are either left uncased or fitted with casings screened across the full thickness of the Supra-Toarcian aquifer. As a result, the piezometric level recorded in the boreholes reflects an average hydraulic head over the aquifer’s thickness. Under natural conditions, piezometric levels range between 20 and 25 m below the ground surface. Drilling consistently revealed dry clayey limestones down to about 30 m, confirming that the Dogger aquifer is confined beneath this low-permeability formation. Two additional boreholes (C2 and IM1, each ~160 m deep) were drilled into the crystalline basement to monitor hydraulic heads in the Infra Toarcian aquifer during hydraulic testing in the other wells. No pressure disturbances were ever detected in the Infra-Toarcian aquifer, demonstrating that they are effectively isolated from the Supra-Toarcian aquifer by the Toarcian marls (Porel, 2024). Scientific studies Since 2002, investigations carried out on the HES have resulted in the collection of a large amount of data concerning the Supra-Toarcian aquifer (Mari and Porel, 2024). These data, now archived in the H+ database (http://hplus.ore.fr/poitiers/ donnees-poitiers), concern: • the geological structure of the reservoir: stratigraphy/lithology on two core drillings, 3D seismic imaging of the reservoir, Vertical seismic profiles (VSP), Full waveform acoustic logging, Gamma ray logs, acoustic imaging, high-resolution camera, and optical televiewers acquisition (OPTV); • petrophysical properties of carbonate rocks: laboratory porosity and permeability measurements on core samples; • flow structure in boreholes: heat-pulse flowmetry, cross-flow logs; temperatureconductivity logs; • groundwater dynamics under “natural” and/or forced flow conditions: pumping tests and slug tests; • transport dynamics (non-reactive) in forced flow: Radially convergent tracer tests (RCT). HES is currently used for the training in hydrogeology and geophysics of young professionals and students coming from universities and high schools. HES is used to study the vadose zone, and data collected since 2002 could be useful to study the impact of climate change on groundwater. The site gives universities and private companies the opportunity to conduct in situ tests and research projects, for example, the “Karst’ogène” project of CPGF HORIZON.
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