Electromagnetic seabed logging (EM-SBL) is a new marine electromagnetic survey method to map hydrocarbon reserves. This paper gives the general theory for marine electromagnetic wavefield decomposition into upgoing and downgoing components where the dual-sensor (electric and magnetic) data are measured on a plane where the electric permittivity, magnetic permeability and electrical conductivity are constant with respect to space and time. Below the plane of measurement the medium can be arbitrarily inhomogeneous and anisotropic. By integrating Maxwell's equations by the Green's function technique one obtains the integral representation for electromagnetic waves. From the integral representation spatial-dependent decomposition filters are derived. A Fourier transform of the spatial filters over horizontal coordinates yields wavenumber-or slowness-dependent decomposition scalars. EM wavefield decomposition is important for marine EM-SBL exploration in shallow water (i.e., less than about 500 m water depths).

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