Numerical thin-sheet modelling of the telluric field distortions by the hybrid technique. I. Theory and an example for the Baltic Shield

The distortions of the electromagnetic-plane wave field, due to near-surface conductivity inhomogeneities, are considered. The real three-dimensional distribution of the shallow conductivity is approximated by Price's thin-sheet model. As low-frequency magnetotelluric variations are considered it is possible to neglect the self-induction in the thin sheet.The calculation of the current function has been carried out by the alternative Schwarz method. The method combines, in the iterative sense, the finite element solution in the inhomogeneous region and the analytical solution outside of it. The primary electric field is assumed to be circularly polarized. An example for the Baltic Shield shows that the telluric field is mostly very distorted.