Practical Considerations for 3D Magnetotelluric Surveys around Basalt

Since the quality of sub-basalt seismic imaging is poor, other geophysical techniques must be used. While seismic data can image sills, dikes are mostly invisible. Horizontal currents however are sensitive to vertical resistors such as dikes. This can lead to strong mode splitting in the Magnetotelluric (MT) response because only the currents perpendicular to the dikes are sensitive to the dike resistivity while the currents parallel to the dikes sense the intruded sediments. Feasibility modelling done before collecting the data does not typically account for this type of structural anisotropy. In the following we present a synthetic study of a basalt model containing dikes, which produces splitting of the primary MT modes. We perform 3D isotropic and anisotropic inversions of the synthetic data and show that neglecting structural anisotropy can lead to an incorrect interpretation. Furthermore, we discuss a new data regularization scheme based on spatial interpolation of the frequency response on a finer grid. We show that this scheme drastically increases convergence while reducing artifacts related to sparse station spacing.