Fractured Granite Investigation by Surface Magnetotelluric and Subsurface DC Resistivity Tomography

The performance of the Hungarian National Radioactive Waste Repository programme for the final disposal of low- and intermediate-level radioactive waste started in 1993. Geophysical investigations on the site ongoing since 1997. Two audio magnetotelluric (AMT) field campaigns were carried out in 2002 and 2005 along a 100 m grid with Stratagem EH-4 instrument. Bimodal 2D inversion was applied along 18 profiles perpendicular to valleys and ridges, constrained by the topography. A 3D resistivity distribution model was derived from the 2D inversion results. Two tunnels has been build until 2009, each tunnel is more then 2 km long. High resolution DC resistivity tomography was used in the end sector of tunnels. Both method (AMT and DC) show unusual low resistivity zones in the granite rock. The conductive bands are connected with combined conduit-barrier systems, include clay-rich fracture core, which are fundamental hydrogeological units.