Electromagnetic Imaging of Fissured Crystalline Bedrock in Hydrogeology
Sophie Hautot, Pascal Tarits, Corinne Tarits, 1999. "Electromagnetic Imaging of Fissured Crystalline Bedrock in Hydrogeology", Three-Dimensional Electromagnetics, Michael Oristaglio, Brian Spies
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We interpret regional electromagnetic (EM) data and chemical analyses of water, sampled in springs and seepages, from an area in St. Anne, near Brest (Finistére, France). The area is covered by a thin homogeneous arenaceous soil. We surveyed the region with both a controlled-source EM system (Geonics EM31) and a very-low-frequency (VLF) system. The controlled-source survey was sensitive mainly to the soil layer; the VLF system, to structures at depth. Inversion of the VLF data shows evidence of two different structural units below the soil. The first, which is resistive, is interpreted as a massive crystalline gneissic structure. In this zone, the water drainage is restricted to the topsoil. In the southwest, there is a heterogeneous conductive domain that we interpret as a fracture network able to drain water along the structural strike. Differences in water quality in the St. Anne area are associated with the different electrical structures at depth.