| Summary: | The magnetotelluric method is an electromagnetic induction method used to image subsurface resistivity. It utilizes Earths' natural time-varying electromagnetic field as a source. The fields propagating into the subsurface are assumed to consist of homogeneous plane-waves. In a marine setting, the subsurface response is measured by a grid of receivers placed at the seabed. The homogeneous plane-wave assumption is a widespread and central argument for how traditional processing schemes are formulated and solved. In this thesis we provide a closer look on this assumption, by approximating the direct source field using up-down decomposition above the seabed. By organizing the data into receiver-pairs, we evaluate the homogeneous plane-wave assumption as a function of receiver-spacing. The method was applied on two offshore case studies located in the North Sea and Barents Sea. The results showed that the homogeneous plane-wave assumption was dependent on the receiver-spacings. The approximation was better for nearest neighbors, but decreased for receivers that were far apart.
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