16 A new concept of karst development based on hydrogeology and geophysics Gilles Porel: Le Deffend Hydrogeological Experimental Site (HES). Spanning an area of 12 hectares and comprising approximately 35 systematically distributed boreholes, the site represents a significant research initiative. The platform and its associated research objectives are detailed in Chapter 4, dedicated to the description of the experimental hydrogeological site: Le Deffend. Among the investigative techniques implemented at the HES, geophysics—particularly seismic methods—holds a central role, offering a renewed perspective on groundwater flow geometry. In exploration geophysics and reservoir studies, seismic methods are primarily used for constructing subsurface models. These techniques are increasingly significant in geotechnical, hydrogeological, and site characterization studies, particularly in the context of seismic hazard assessments. Surface seismic reflection provides a three-dimensional (3D) representation of subsurface acoustic impedance contrasts, either in terms of time or depth. The resulting 3D seismic blocks generate interpretable images that can be used to develop porosity and karst models. When combined with borehole measurements such as vertical seismic profiles (VSP) in low-frequency ranges (5–200 Hz) and full-waveform acoustic logging in very high-frequency ranges (1–25 kHz), seismic data enable robust estimation of petrophysical parameters, such as seismic porosity. These methods also facilitate the characterization of specific geological attributes, including karstic features. More recently, electrical resistivity tomography (ERT), conducted both at the surface and in boreholes, has been deployed at the Le Deffend site, with findings that corroborate the seismic results. The results of the geophysical investigations are presented across two chapters. Chapter 5 highlights the contributions of various geophysical techniques, including seismic, acoustic, and electrical methods. Chapter 6 is dedicated to a borehole experiment conducted using Electrical Resistivity Tomography (ERT). Based on the geophysical data, a revised geometry of the karstic horizons has been established. Notably, the karstic horizons within the Middle Jurassic limestones were found to be sub-horizontal rather than vertical, challenging hydrogeological interpretations from the 19th and 20th centuries regarding the karstification process. To assess the implications for groundwater flow, logging and tracer tests were conducted alongside initial pumping tests. Pumping and hydraulic slug tests were used to evaluate the interference geometry between boreholes. These tests were initially aimed at identifying a preferential fracturing pattern shaped by the tectonic constraints of the Poitou threshold. However, the three-dimensional seismic block revealed the sub-horizontal geometry of the karst levels, leading to a re-evaluation of hydrogeological flow patterns and the development of a new conceptual model, referred to as the “Le Deffend model.” Details of the logging and hydrogeological tests performed at the site are presented in Chapter 7 entitled “Hydrogeological flow logging and dye tracer tests. The origin of the karstic horizons was also reexamined. While the distribution of karst horizons in limestone massifs is often considered random, a detailed stratigraphic
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