Self-potential signals associated with preferential groundwater flow pathways in sinkholes
Résumé
Two self-potential (SP) surveys have been conducted at a test site located in Normandy, in the northwest of France, in a chalk karst in spring and summer 2005. The spring survey showed circular negative SP signals associated with the position of sinkholes and crypto-sinkholes, while the survey conducted during the summer showed fewer anomalies with lesser magnitudes. The negative SP anomalies observed in the spring survey were several tens of millivolts less than a reference located outside the ridge along which the sinkholes are located. In addition to the SP surveys, we also performed a DC electrical resistivity survey. The electrical resistivity tomogram shows the position of the interfaces between the chalk and the overlying clay with flint and loess covers and the position of the sinkholes. A linear relationship is observed between the SP signals and the thickness of the loess layer. In addition, large negative SP anomalies are associated with the sinkholes themselves. These SP signals are explained by solving the boundary value problem for the coupled hydroelectric problem. We fixed the boundary condition at the ground surface in terms of hydraulic flux associated with the infiltration of the meteoric water and a no-flow condition on the vertical boundaries. A finite element numerical simulation of this coupling is performed, and a fairly good agreement is obtained between the simulation and the observed SP signals. We show for the first time how the SP signals reflect mainly the topography of the clay with flint/loess interface and help to identify the location of sinkholes. Three-dimensional tomography algorithms are proposed to locate the source of these self-potential signals in the ground.
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