Controlled-source electromagnetic sounding in shallow water:: Principles and applications

The marine controlled-source electromagnetic (CSEM) method is being applied to the problem of detecting and characterizing hydrocarbons in a variety of settings. Until recently, its use was confined to deepwater (water depths greater than approximately 300 m) because of the interaction of signals with the atmosphere in shallower water depths. The purpose of this study was to investigate, using a simple 1D analytical analysis, the physics of CSEM in shallow water. This approach demonstrates that it is difficult to simply decouple signals that have interacted with the earth from those that have interacted with the air using either frequency-domain or time-domain methods. Stepping away from wavelike approaches, which if applied without care can be misleading for the diffusive fields of CSEM, we demonstrate an effective way to mitigate the effect of the air in shallow water surveys by decomposing the EM signal into modes and using only the mode least affected by interaction with the atmosphere. Such decomposition is straightforward in a 1D earth, and we demonstrate that the approach remains valid in higher dimensional structures. We also show that the coupling between signals diffusing through the earth and those that have interacted with air can be used to our advantage in the interpretation of marine CSEM data.