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Methane hydrates in controlled‐source electromagnetic surveys – analysis of a recent data example

Data from a recently acquired sea‐bed logging deep‐water survey are analysed for resistive bodies at depths below mudline shallower than about 300 m. A model consistent with known methane hydrate properties is found to explain near‐offset structures over an offset scale of a few hundred metres observed in the data. The lateral near‐seabed resolution of the sea‐bed logging method was determined to less than 100 m for source frequencies of up to 10 Hz. The importance of accurate hydrate maps to improve data processing is demonstrated by placing synthetic reservoirs below hydrates and observing their effects on reference model processing. The phase is shown to be less perturbed by shallow resistors than the amplitude, which is an important quality control of standard anomaly maps. While patchy shallow resistors can generally be mapped with simple normalized magnitude‐versus‐offset and phase‐versus‐offset difference analyses, large area distributions of hydrates over kilometres are hard to distinguish from deeper structures using controlled‐source electromagnetic data only, short of conducting a full 3D inversion of a sufficiently large survey. Beyond, the study confirms the applicability of controlled‐source electromagnetic techniques in general to map shallow resistive structures for drilling hazards and possible future exploration of methane hydrates as an energy source.