Topographic distortions of magnetotelluric transfer functions: a high-resolution 3-D modelling study using real elevation data

This study aims at quantifying the influence of topographic distortions on magnetotelluric transfer functions, including the impedance tensor, tipper and phase tensor. To this end, we performed a series of numerical experiments using real topography. We utilized a high-order 3-D finite element modelling code combined with locally refined unstructured meshes to achieve an accurate representation of topography and low modelling errors. We calculated electromagnetic fields for a region in the Hangai Mountains (Mongolia) for a wide period range and showed that, depending on the period and size of topographic features, distortions due to both galvanic and inductive effects can become significant. In addition, we quantify the effect of the vertical electric field component for tilted electric dipoles located on slopes. For real topography examples considered here, we found that galvanic distortions dominate at periods >10 s, resulting in a static shift effect. At shorter periods (<1 s), inductive effects cause significant variations of the impedance, the phase tensor and the tipper.