188 A new concept of karst development based on hydrogeology and geophysics wall imaging using an Optical Televiewer (OPTV). A specific acoustic attribute, K-index, was developed to identify karstic bodies. However, due to the seismic data’s lower vertical resolution (on the order of meters) compared to that of logging tools (centimetric to decimetric), some karstic features were detected only by borehole logs. The authors show that 3D seismic imaging enables the construction of a large-scale structural model that can highlight potential karstic zones. These must then be validated through high-resolution borehole investigations, such as full waveform acoustic and OPTV logging. By correlating karstic features with the stratigraphic framework, the discontinuities that control karst formation can be identified, leading to a new karstification model constrained by stratigraphy. At the HES site, the authors demonstrate that karstic horizons in the Dogger limestone are closely associated with stratigraphic discontinuities and early diagenesis. Sedimentation hiatuses, precisely constrained by the absence of specific ammonite zones, together with the emersion of the Poitou Threshold carbonate platform, are interpreted as the primary controls on these levels, through the promotion of hardground formation and subsequent dolomitization. The aquifer is thus composed of multiple productive layers that may be hydraulically connected through faults in the vadose zone and joint systems in the saturated zone. Typically, boreholes are drilled to intersect all karstic levels, which results in mixing waters of varying pressure and chemical composition. These wells are particularly vulnerable to surface pollution, posing a risk of contamination to deeper karst systems. Therefore, a detailed lithological and paleontological description of each borehole is essential to assess the role of stratigraphy in water inflow distribution. Well logging should be systematically performed prior to completion and equipment installation, using techniques such as flowmeter, gamma-ray, OPTV, and other geophysical tools to accurately locate karstic horizons. The karstification model proposed for the Poitou threshold has practical implications. Once a large-scale structural model and karstic levels have been identified, two key recommendations emerge: 1. acquire optical televiewer data from all available boreholes to refine stratigraphic interpretations and accurately locate karst features; 2. consider individually screening each karstic level during borehole completion. This approach allows for a better assessment of groundwater quality and vulnerability and supports the long-term management of deep karst resources as protected reserves. The field experiments at the HES site support a multi-objective strategy for mapping karst conduit networks, based on the following key methods: • identification of water-producing zones in each borehole. This involves logging techniques such as long normal resistivity, temperature, electrical conductivity, and flowmeter logging — the latter being particularly effective in detecting active flow zones. High-resolution borehole imaging via OPTV enables identification, localization, and sizing of fractures, vugs, and karst conduits.
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