Marine electromagnetic induction studies - Marine EM Laboratory

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322 S.C. CONSTABLE immediately over the ridge axis with the source-receiver spacing reduced to 5 km. The two models purport to show a magma chamber at a depth of 2 km containing partially molten (2 f~m, circles) and fully molten (0.3 tim, diamonds) rock. Although the 1D models considered above provide an indication of the sensitivity of the controlled source method to ridge structures, they are clearly inadequate for quantitative modelling of what is obviously not a 1D structure. Ridges are good candidates for 2D modelling, and indeed one is struck by how remarkably 2D they are, with hundreds of kilometres between significant offsets. While 2D MT modelling is becoming commonplace, the difficulty associated with controlled source modelling is that the source-field is 3D. However, progress is being made, and forward models of ridges in which the approximation that the source is 2D have been presented by Everett and Edwards (1989) for time-domain sounding and Flosadottir and Cox (1989) for frequency-domain sounding. The models of Everett and Edwards show that the maximum response to a crustal magma chamber for a time-domain system occurs at delay times of about 1 s. 6.3. FUTURE PROGRESS We are seeing diminishing returns from the deployment of single-station MT sites on the seafloor. There is little prospect of significantly expanding the limited frequency band available between long period contamination by oceanic effects and short period cutoff due to screening of the source field, and so resolution is necessarily limited to a narrow depth range in the upper mantle. On the other hand, the expanding use of long period E-field instruments for the study of oceanography is likely to support the continued deployment of seafloor EM arrays, from which we shall continue to extract MT and GDS data. Magnetic measurements are much easier to collect than electric field data, which to date have required either sophisticated water choppers or long antennae, but GDS offers possibilities in the discrimination of lateral changes in structure across features like ridges; thus one expects to see expansion in this area. Already there is a lot of international cooperation, reflecting the fact that once a ship is available for an experiment, more instruments can be deployed than are usually available to one institution. There will be reduced motivation for one facility to maintain a large suite of instruments if this continues. Controlled source experiments are more difficult (and so more costly and with a higher instrument loss) than GDS and MT studies, but offer the chance to study the first 50 km or so which is invisible to MT sounding. Now that ground has been broken in this area, we will see an expanding use of this method. Although the oil and mining industries are currently depressed world-wide, there is academic activity in developing EM exploration techniques for the seafloor which might well blossom during the next rejuvenation of the industry, whenever that may be. A scientific/political emphasis on exploring mid-ocean ridge environments has improved the prospects for a vigorous programme of EM experiments in the intermediate future, and an EM component is being considered important by many
MARINE E.M. 323 of the groups coordinating ridge studies. However, these are not easy areas to work in and represent challenges to the various disciplines; problems include conductive material (controlled source method), dimensionality problems (MT), and topo- graphical inhospitability (all methods). We must be cautious not to promise more than we can achieve. Acknowledgements This review was presented at the ninth workshop on electromagnetic induction, held at Dagomys, U.S.S.R, in September 1988. The author thanks the program commit- tee for the invitation to present the work, and also C. deGroot-Hedlin, who delivered the paper in his absence. Most of the author's marine experience has been obtained during collaborative work with C. Cox over the last six years, an association which has always been instructive and occasionally exciting. He would like to thank A. Chave for making his controlled source forward modelling program available, amd M. Kappus for the translation of Singer et al. (1985). Finally, he expresses his appreciation to all those researchers who responded to his request for material associated with marine EM. This review was produced while working under NSF grant OCE-8719245. References Bahr, K. and Filloux, J. H.: 1989, 'Local Sq Response Functions from EMSLAB data', J. Geophys. Res. 94, 14195-14200. Baines, P. G. and Bell, R. C.: 1987, 'The Relationship between Ocean Current Transports and Electric Potential Differences across the Tasman Sea, Measured Using an Ocean Cable', Deep-Sea Res. 34, 531-546. Becker, K. and 13 others.: 1982, ' In situ Electrical Resistivity and Bulk Porosity of the Oceanic Crust Costa Rica Rift', Nature 300, 594-598. Berdichevsky, M. N., Zhdanova, O. N., and Yakovlev, A. G.: 1984, 'Anomalous Electromagnetic Fields and Electromagnetic Sounding on the Bottom of the Ocean', Geomag. Aeronomy 24, 542-547. Bindoff, N. L., Filloux, J. H., Mulhearn, P. J., Lilley, F. E. M., and Ferguson, I. J.: 1986, 'Vertical Electric Field Fluctuations at the Floor of the Tasman Abyssal Plain', Deep-Sea Res. 33, 587-600. Caress, D. W.: 1989, Some Aspects of the Structure and Evolution of Oceanic Spreading Centers, Unpublished Thesis, Univ. Calif., San Diego. Chan, E., Dosso, H. W., Law, L. K., Auld, D. R., Nienaber, W.: 1983, 'Electromagnetic Induction in the Queen Charlotte Islands Region: Analogue Model and Field Station Results', J. Geomagn. Geoelectr. 35, 501-516. Chave, A. D.: 1983a, 'Numerical Integration of Related Hankel Transforms by Quadrature and Continued Fraction Expansion', Geophysics 48, 1671-1686. Chave, A. D.: 1983b, 'On the Theory of Electromagnetic Induction in the Earth by Ocean Currents', J. Geophys. Res. 88, 3531-3542. Chave, A. D. and Cox, C. S.: 1982, 'Controlled Electromagnetic Sources for Measuring Electrical Conductivity Beneath the Oceans, 1, Forward Problem and Model Study', J. Geophys. Res. 87, 5327-5338. Chave, A. D. and Cox, C. S.: 1983, 'Electromagnetic Induction by Ocean Currents and the Conductivity of the Oceanic Lithosphere', J. Geomagn. Geoelectr. 35, 491-499. Chave, A. D. and Filloux, J. H.: 1984, 'Electromagnetic Induction Fields in the Deep Ocean off California: Oceanic and Ionospheric Sources', Geophys. J. R. Astr. Soc. 77, 143-171.
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