From 2D to 3D Models of Electrical Conductivity based upon Magnetotelluric Data : Experiences from two Case Studies

Abstract: Magnetotelluric measurements are among the few geophysical techniques capable of imaging the structure both in the shallow subsurface as well as the entire crust of the Earth. With recent technical and computational advances it has become possible to derive three-dimensional inversion models of the electrical conductivity from magnetotelluric data, thereby overcoming the problems arising from the simplified assumption of two-dimensionality in conventional two-dimensional modeling. The transition from two-dimensional to three-dimensional analysis requires careful reconsideration of the classical challenges of magnetotellurics: galvanic distortion, data errors, model discretization and resolution.This work presents two examples of magnetotelluric investigations, where a new three-dimensional inversion algorithm has been applied. The new models are compared with conventional two-dimensional models and combined with the results of other geophysical techniques like reflection seismics and electrical resistivity tomography. The first case presents magnetotelluric investigations of the Kristineberg mining area in the Skellefte district, northern Sweden. This study is part of a joint geophysical and geological project to investigate the present structure and evolution of the whole district. Together with reflection seismic and surface geological information a three-dimensional conductivity model, derived through the inversion of magnetotelluric data, was interpreted. A comparison with two-dimensional modeling gives insights into the capabilities and challenges of three-dimensional inversion strategies with respect to data sampling and model resolution.The second case presents a study of remediation monitoring  with geophysical methods after an oil blow-out in Trecate, Italy. A three-dimensional conductivity model was derived from radiomagnetotelluric measurements. In addition, two-dimensional joint inversion of radiomagnetotelluric and electrical tomography data was performed. Compared with electrical resistivity tomography, radiomagnetotelluric data is more sensitive to conductors and the derived three-dimensional inversion model resolves the vadose zone and the underlying aquifer.