Présentations Colloques

    Oral Presentation
    Session 8.07: Hydrogeophysics: innovative non-invasive technologies for groundwater resources exploitation and management
    Galibert Pierre Yves
    New developments in exploring the saturated zone of karst aquifers- Applications to the Quercy region (Southern France)
    Laboratory and field work show that we can estimate water storage and residence time in the epikarst from seasonal seismic velocity variations. It is however necessary to take into account the rock weakening during dissolution to estimate correctly the water saturation from seismic velocities. We postulate that variations of calcite saturation in the water recharge could lead to the same chemo-mechanical effect in the saturated zone of the karst. Several authors reported significant variations of seismic velocities and amplitudes on cores saturated with different waters. We want to verify in this work if this observation is still valid at the field scale, and helpful in locating karst conduits, a task generally impossible to achieve with traditional methods. We use here seismic amplitudes because they provide better resolution than velocities for deep targets. **Le Ressel test site is located in the Quercy region (Southwest of France). The location of the 100-meter deep conduit is known and piezometers reaching the drain are available. Time-lapse seismic surveys found a signal dimming with seasonal variations above the conduit+ the conduit signature is easier to detect during the dry season. Downhole measurements indicate that the variations could be linked to changing water mixing conditions in the volume surrounding the conduit.**Following this result, the Conseil Général du Lot decided to apply the methodology to the Font Vincent case. This karstic spring supplies drinking water to several communities but does not comply with protection regulations. It is necessary to relocate the water intake upstream to a less vulnerable position by putting in a well to intercept the conduit. The seismic amplitudes map of the area shows that the initial model of a large connection between the spring and a nearby known conduit flow is unlikely. New targets are identified and being evaluated to reduce uncertainty. **


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