Detection of preferential infiltration pathways in sinkholes using joint inversion of self-potential and EM-34 conductivity data
Résumé
The percolation of water in the ground is responsible for measurable electric potentials called self-potentials. These potentials are influenced by the distribution of the electrical conductivity of the ground. Because sinkholes are associated both with self-potential and electrical conductivity anomalies, a joint inversion of EM-34 conductivity and self-potential data is proposed as a way of delineating the location of these features. Self-potential and EM conductivity data were obtained at a test site in Normandy (France) where sinkholes and crypto-sinkholes are present over a karstic area in a chalk substratum overlain by clay-with-flint and loess covers. The presence of sinkholes and crypto-sinkholes is associated with negative self-potential anomalies with respect to a reference electrode located outside the area where the sinkholes are clustered. The sinkholes also have a conductivity signature identified by the EM-34 conductivity data. We used the simulated-annealing method, which is a global optimization technique, to invert jointly EM-34 conductivity and self-potential data. Self-potential and electrical conductivity provide clear complementary information to determine the interface between the loess and clay-with-flint formations. The sinkholes and crypto-sinkholes are marked by depressions in this interface, focusing the groundwater flow towards the aquifer contained in the chalk substratum.