Electrical resistivity tomography to investigate geological structures of the earth's upper crust

It is important to have detailed knowledge of the electrical properties of the earth's crust in order to recognize geological structures and to unders tand tectonic processes. In the area surrounding the German Continental Dee p Drilling Project (KTB), we have used DC dipole-dipole soundings to invest igate the electrical conductivity distribution down to a depth of several k ilometres. We have adapted the electrical resistivity tomography (ERT) tech nique, a well-established near-surface method, to large-scale experiments. Independent transmitting and receiving units were used to realize the conce pt of simultaneous multichannel registration of the scalar electrical poten tial at 44 dipoles. The measured data yielded apparent resistivities which were inverted to a 2D resistivity model ranging from the surface down to a depth of 4 km. Two highly conductive structures with steep inclination were detected. They are expected to be major fault zones embedded in a metamorp hic body. The rather low resistivity (rho < 10 Omega m) can be explained by the existence of graphitic minerals and/or electrolytic fluids.