The marine controlled-source electromagnetic method in shallow water

We analyzed marine controlled-source electromagnetic (CSEM) acquisition in shallow water by analytic and numerical calculations. The problem at hand was the fact that the amplitude of the airwave due to a horizontal electric dipole becomes larger as the water depth is reduced. The amplitude of the scattered field from a buried thin resistor would be difficult to extract from the increased total field if the scattered-field amplitude were independent of water depth. However, we identified and explained a compensating effect where the amplitude of the scattered field increases with reduced water depth. This amplification effect makes marine CSEM surveys in shallow water feasible. We derived an approximate expression for the total field for a simple but realistic conductivity model. This expression explains why the amplitudes of the term that describes the background field contribution and the term that gives the scattered field from the thin resistor increase when the water depth is reduced. Our results and sensitivity analysis indicated that the sensitivity of marine CSEM data acquired in a water depth of 40 m is comparable to the sensitivity of marine CSEM data acquired in a water depth of 300 m.