Hydrogeophysical characterization of the porous and fractured media (chalk aquifer in the Beauvais, France)

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This study is aimed to investigate the principal aquifer in the northern part of Paris (Beauvais region) which is materialized by the Senonian chalk deposits. This aquifer supplies water to the principal region in the north and in particular the Picardie and Artois region. In order to characterize the chalk aquifer of this area, a wide pluridisciplinary research program based on the well logging and near surface geophysics surveys (seismic refraction and electrical resistivity tomography) is in progress. These surveys were carried out within the experimental site of the LaSalle Beauvais Polytechnic Institute and its hydrogeological boreholes. The heterogeneous distribution of the seismic velocities in the complex aquifer and of the physico-chemical properties (conductivity, temperature and gamma-ray) of the chalk aquifer were revealed by the analysis and the interpretation of the seismic lines and data logging, which are linked to changes in lithology and water properties at the place of the hydrogeological wells. The fracturation investigation identified during this work makes it possible to consider the relationship between the degree of the fracturation and the variation of the hydrogeological parameters (hydraulic conductivity, porosity) and physico-chemical parameters (electrical conductivity and temperature). The interface between vadose and saturated zones was assessed by the measurement of the water conductivity, the variation of the average temperature and by the cuttings analysis in particular at the level of the capillary fringe of the chalk aquifer. The presence of the saturated depth is confirmed by the piezometric investigation. The interpretation of the well logging by using gamma-ray is consistent with the seismic and electrical analysis: the three principal layers are composed of silt and clay with cherts, soft chalk, as well as hard chalk.