12. Electrical Exploration Methods for the Seafloor

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Abstract
INTRODUCTION Recent developments in instrumentation and submarine geology have spawned increasing interest in the use of electromagnetic (EM) methods for seafloor exploration. Previously, little attention had been given to their use in the marine environment, due both to the success of the seismic techniques in delineating subsurface structure and to a pervasive belief that the high electrical conductivity of seawater precluded the application of EM principles. Marine EM exploration of the solid earth has progressed substantially in academic circles over the past two decades; the adaptation of this technology for commercial purposes is only beginning. Over three-fifths of the earth's surface is covered by oceans. Even though petroleum is produced from huge deposits on the relatively shallow continental shelf, the immense area of the ocean represents a largely unexplored and unexploited resource base. Until recently, little economic interest was shown in the ocean floor environment; with the possible exception of manganese nodule fields (Sorem and Fewkes, 1979) and the heavy metal-rich brines found in intracratonic basins like the Red Sea (Degens and Ross, 1969), the seafloor was assumed to be essentially barren. However, the recent discovery of intense hydrothermal activity and polymetallic sulfide deposits of unprecedented concentration and scale on the crest of the East Pacific Rise (Hekinian et al., 1983; Spiess et al., 1980; Ballard et al., 1981), the Galapagos Ridge (Corliss et al., 1979), the Juan de Fuca Ridge (Normark et al., 1983; Koski et al., 1985), and the Mid-Atlantic Ridge (Rona et al., 1986) has aroused interest in the possibility of deep-sea mining and spurred research into mid-ocean ridge ore genesis as an analog to terrestrial occurrences; see Teleki et al. (1985), Brimhall (1987), and Rona (1987) for comprehensive reviews of these topics. The sulfide deposits were located visually with submersibles or using near-bottom survey tools such as ANGUS (Phillips et al., 1979). While these methods are capable of examining surficial geology, they are not able to adequately assess the actual extent of the deposits and the nature of the geological structures in which they are found. Seafloor conductivity mapping is one of the few geophysical tools suitable for this purpose, just as the EM methods are one of the major geophysical techniques used in mineral exploration on land. Over the past few decades, the search for petroleum reserves has been extended from the continents offshore into progressively deeper water, making the continental shelves a focus for geophysical exploration. The principal geophysical tool for this is the seismic method, and the success of the seismic approach is attested to by the level of offshore drilling activity and the subsequent production of oil. However, there are marine geological terranes in which the interpretation of seismic data is difficult, such as regions dominated by scattering or the high reflectivity that is characteristic of carbonate reefs, volcanic cover, and submarine permafrost. Alternative, complementary geophysical techniques are required to study these regions.

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