Transient electromagnetic in shallow water: insights from 1D modeling

In shallow water environments, the frequency domain, marine controlled source electromagnetic (CSEM) method faces a challenge for detecting thin hydrocarbon reservoirs because the airwave dominates electromagnetic responses and contains little information about resistivity structure of the seabed. In this paper, airwave effects on time domain (or transient) CSEM responses are investigated. The arrival time of the airwave depends on the water depth. The airwave arrives earlier in shallow water and later in deep water environments and it occurs at different time than the signals traveling through the deep resistor. Although in an intermediate water depth the airwave and the signal from the deep resistivity arrive at about the same time, one can still see a clear anomaly compared to the background model without hydrocarbon reservoirs. For shallow water surveys, one has the added advantage that a surface towed system can be used.