The fallacy of the “shallow-water problem” in marine CSEM exploration

Abstract

The recent explosion of activity in offshore controlled-source electromagnetic (CSEM) exploration has shown, both theoretically and in practice for the time-harmonic case, that shallow-water environments (depths less than, say, [Formula: see text]) can pose a significant challenge for detection and characterization of thin resistive targets in the subsurface because the weakly attenuated atmospheric response overprints the weaker target response. As an alternative, the transient CSEM experiment is considered to explore the nature of the “airwave” signature when viewed from the perspective of CSEM time series. The signature of a thin, isolated, resistive horizon is computed by using the quasi-analytic solution for a 1D earth in response to excitation by a horizontal electric dipole antenna. That signature, clearly seen in the transient data, generally lies in the time interval between the arrivalof the airwave and the late-time seawater response.The fact that either decreasing the water depth or increasing the source-receiver offset broadens this time interval suggests a target response that is generally easier to identify in shallow water than when viewed from a frequency-domain perspective. The airwave arrival is nearly synchronous across an inline receiver array, whereas the response time of the conducting elements of the model — the sea, the sediments, and the target — increases with increasing source-receiver offset. At short offsets (less than a few kilometers), and in intermediate water depths (approximately [Formula: see text]), these signatures can overlap and be difficult to decouple. However, modeling results also show that moveout rates of the post-airwave arrival may be used to infer the presence and depth of thin resistive targets within the geologic section to depths of [Formula: see text] below [Formula: see text] of water.