Ocean effect correction in global inversion of geomagnetic observatory data

Large lateral variations of electrical conductivity are associated with distribution of the oceans at the Earth's surface. Induction signals related to these variations are observed at the geomagnetic observatories, especially in the coastal regions, and must be accounted for before interpreting the magnetic field in terms of conductivity structure of the deep mantle. For this purpose, the forward modelling of the EM induction problem with high spatial resolution is needed. However, such requirement is prohibitive for the inverse problem calculations, where many forward runs are calculated. Correction of observatory-based data, i.e., the frequency-domain C responses, for ocean effect has been used by different authors for some time now. Here we present a similar approach in the time domain. A new inversion scheme is developed that uses intermediate-resolution forward and adjoint modelling, and iteratively subtracts the ocean effect from observatory time series using only limited number of high-resolution forward runs.