We examine the underlying physics of the marine controlled-source electromagnetic (CSEM) method through the use of cross-sectional plots of the vector-current density. A systematic comparison of the cross-sectional current-density distribution within uniform and reservoir-bearing seafloor models reveals that the method induces detectable reservoir responses at the seafloor for source-receiver offsets that are frequency dependent. Higher frequencies generally result in larger anomalous differences between the two models at shorter offsets up to a frequency where induced currents no longer effectively interact with the reservoir due to electromagnetic (EM) attenuation. At zero and low frequencies, the less-attenuated background EM fields mask the reservoir response, although large induced currents are normally incident upon the reservoir. The reservoir response is also masked at larger offsets and/or in shallow environments by theairwave that can be thought of as energy diffusing up and down through the seawater column and propagating along the air-seawater interface. As the background EM fields and airwave are nearly horizontal because of the air-seawater boundary, vertical electric-field (E) measurements are free of the masking effect and, thus, might serve as additional useful information, especially in shallow environments where horizontal E measurements lose resolution of the reservoir. Horizontal magnetic field (B) measurements supplement the horizontal E measurements as E and B decay at different rates away from a source position in the near-field zone of a horizontal electric-dipole source. Comparison of CSEM responses for 1D and 3D reservoirs indicates that the 3D reservoir response is substantially smaller because of the limited surface area of the localized 3D reservoir, and that the pattern of the 3D reservoir response radically varies with a source position relative to reservoir boundaries.

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