Marine Controlled-source Electromagnetics and the Assessment of Seafloor Gas Hydrate

Marine controlled-source electromagnetic (CSEM) methods have become an important and valuable tool in the detection of offshore hydrocarbon targets. The formation resistivity of a sediment layer depends on conductive fluids in interconnected pore spaces. Hydrocarbons increase the formation resistivity of a sediment layer if they form in sufficient quantity to block the pores. CSEM has been used for gas-hydrate evaluation for more than a decade. The common published work contains descriptions of theory, apparatus, data analysis, inverse methods, and interpretation. Here, the fundamentals of time-domain electromagnetics are explained using classical dimensional analysis and are illustrated with a simple approach using data from the northern Cascadia margin, to the west of Vancouver Island, British Columbia, where gas hydrates have been extensively studied. Analyses of CSEM data collected from 1996 to 2005 demonstrated the strong correlation between CSEM resistivities, other geophysical imaging data, and subseafloor hydrocarbons. The analysis is consistent with other intensive studies, including a full gamut of seismic and other geophysical experiments, as well as ground truth from direct sampling and the analyses of cores and logs collected by the Ocean Drilling Program (ODP Leg 146) and its successor, the Integrated Ocean Drilling Program (IODP Expedition 311).