Three-dimensional magnetotelluric modelling in spherical Earth

Magnetotelluric (MT) data interpretation is routinely done by assuming Earth is flat, which may become unjustified for large-scale surveys. This study presents a methodology for calculating the MT impedance tensor in spherical Earth with a 3-D conductivity distribution. The problem of finding an appropriate source model, which leads to valid MT transfer functions, on a sphere is elaborated. We present an accurate and computationally efficient method that allows us to calculate the spherical MT impedance tensor in a spherical shell or a part thereof. The simulation code based on the high-order finite-element method combined with high-order meshes was adopted and used to calculate high-resolution solutions taking into account realistic oceans and Earth’s conductivity. By having the correct spherical MT impedance tensor, we quantify the effects of the Earth’s curvature and errors originating from applying different geographic projections in the presence of realistic coast lines.