Insight into the marine controlled-source electromagnetic signal propagation

In marine controlled-source electromagnetic (CSEM) surveys the subsurface is explored by emitting low-frequency signals from an electric dipole source close to the sea-bed. The main goal is often to detect and describe possible thin resistive layers beneath the sea-bed. To gain insight into how CSEM signals propagate, it is informative to study a stratified model. The electromagnetic field is then given in terms of integrals over TE- and TM-polarized plane-wave constituents. An asymptotic evaluation of the field integrals for large propagation distances results in explicit spatial expressions for the field components and the derived expressions can be used to analyse how the CSEM signals propagate. There are two major signal pathways in a standard CSEM model. One of these pathways is via the thin resistive layer and the resulting response is accounted for by a pole in the reflection response for the TM mode. The signal is propagating nearly vertically down to the resistor from the source, then guided while attenuated along the reservoir, before propagating nearly vertically up to the receiver. The response is slightly altered by the sea-bed interface and further modified in shallow water due to multiple reflections between the sea-surface and sea-bed at both the source and receiver sides. The other major signal pathway is via the resistive air half-space, the so-called airwave. The airwave is generated by the TE mode and interacts with the subsurface via vertically propagating signals reflected between the sea-surface and subsurface at both the source and receiver sides.