Présentations Colloques

Because of its heterogeneous structure, karst formations are one of the most challenging environments in terms of groundwater, engineering and environmental issues. Geophysical methods can provide useful subsurface information in such regions concerning groundwater vulnerability assessment, exploitation or hazard estimation. We present the results of 2D, 3D and time-lapse geophysical studies carried out on dolines and epikarst in South of France.**It is believed that dolines are preferential infiltration pathways towards the phreatic zone. We study some of them for the protection of a karstic aquifer. We show that geophysics provides the lateral and bottom limits of the sinkholes and is able to detect a doline hidden by the soil cover. Information about dolines filling can also be derived from the electrical resistivity and seismic refraction. Time-lapse resistivity measurements show that the studied doline is more vulnerable to infiltration on its sides than at its centre.**Epikarst could be defined as a highly fractured zone above the massive carbonate rocks, which could contain a perched aquifer. 3D seismic refraction was carried out on such epikarst located in dolostones. The results show an important velocity anisotropy linked to the fracturing direction and weathering of the dolostone. The 3D model presents also large heterogeneities- a corridor with highly weathered dolostone and an unweathered pinnacle. The corridor is probably situated on vertical joints, which have drained more aggressive water. The associated weathering with residual weathered-rock keeping its initial volume could create a ghost-rock corridor. Therefore, this epikarst seems to be composed by ghost-rock developed around a specific direction of fractures.**Time-lapse electrical resistivity and seismic refraction velocity were also carried out on this epikarst in order to observe the influence of water saturation on the measurements. The results show important variations for both seismic and electrical methods, particularly in the weathered zone, in the first 6 m. Therefore, time-lapse measurements seem more efficient identifying the storage of the epikarst and its bottom than one-time measurements.**