Historically, marine electromagnetic (EM) methods have been employed to study the crust and mantle of the deep ocean, with the earliest applications dating back to Charles ('Chip') Cox's pioneering work on the marine magnetotelluric (MT) method (Cox et al., 1980) and controlled source EM (or CSEM) (Cox, 1981). The rst efforts to use marine MT for petroleum exploration (Hoehn and Warner, 1983) failed to generate enthusiasm largely because the land instrumentation adapted for the task was bulky even by modern terrestrial standards, and the shallow-water environment is, perhaps ironically, much noisier and dif cult to work in than in deeper water. The recent explosion of interest in marine EM has been driven in large part by the challenges presented by deepwater exploration and an associated willingness to manage the high cost of failure by reducing risk through the acquisition of additional data streams. The catalyst for the development of the current generation of marine EM technology was the problem of subsalt exploration in the Gulf of Mexico. An analysis by Hoversten and Unsworth (1994) suggested that EM methods would be capable of identifying the thickness of salt, and that the MT method, with it's superior depth of penetration, would be the tool of choice. The appropriate methodology (Hoversten et al., 1998) and instrumentation (Constable et al., 1998) were developed at Scripps and Berkeley under industry sponsorship, and a vigorous technical transfer program resulted in a commercial marine MT capability being offered by a small service company, AOA Geophysics. Several marine MT surveys were carried out to tackle the sub-salt problem in the Gulf of Mexico (Zerilli, 1999), the carbonate problem offshore Sicily (Oliver et al., 1999), and the sub-basalt problem offshore Faroes (Peace et al., 2002), all areas where the seismic method runs into dif culty because of large impedance contrasts between lithological units which also present electrical targets. Unfortunately, in spite of successes in all three arenas, access to basic structural information through electrical conductivity, however valuable, again failed to excite much enthusiasm within the industry. Interest waned. Around the turn of the century ExxonMobil and Statoil both started investigating the use of CSEM for direct hydrocarbon detection. Both carried out eld trials offshore Angola using an almost identical combination of the UK research vessel the RRV Charles Darwin, a receiver eet made up mostly of SIO instruments, and the Southampton University EMtransmitter called DASI. Statoil pursued this more aggressively, and was the rst to carry out a survey and publish the results (Ellingsrud et al., 2002). ExxonMobil have only recently gone public, mostly through the popular press (Warren, 2004). Both companies have put proprietary spin on what is essentially the public domain methodology of marine CSEM, Statoil calling it 'seabed logging' and ExxonMobil 'R3M'.
Figure 1. Basic methods used in marine EM. See text for description. (Available in full paper)
Figure 1 illustrates the basic methods currently employed in marine EM. Sea oor recorders make timeseries measurements of the electric and magnetic elds at discrete locations.