On the physics of the marine controlled-source electromagnetic method

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 sea...