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TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 249 use of a transatlantic communication cable is discussed as a possible experimental device to detect such an effect. Earthquakes, Volcanoes, and Geodynamics Since telluric currents are excellently suited for deep-Earth investigations, they are in principle also suitable for monitoring long-scale time variations as well. The most investigated aspect from this viewpoint is concerned with earthquake precursors (e.g., review by Honkura, 1981, and references therein). A clear distinction should be made among three different, possible types of phenomena: (1) geomagnetic effects that can presumably be said to be "very shallow" and are likely related to piezomagnetism, following changes in local stresses in the upper crust, which is an effect strictly local and can completely change in a distance of a few kilometers or less; (2) "shallow" effects that can be detected by ground resistivity changes or by suitably short-period MT or GDS investigations; and (3) "deep" or "very deep" effects that can be most suitably detected by means of long-period GDS investigations. This latter category of effects is more strictly related to telluric currents than are the first two types of effect. Additional possible applications in this area include (1) slowly varying effects correlated with geodynamic and tectonic features, (2) shallow effects related to magma migration in volcanic areas, and (3) the monitoring of temporal variations in underground structures as related to fluid extraction (or reinjection). The shallow effects could, however, possibly be better detected by use of man-made electromagnetic fields than by means of natural fields. Telluric currents are also well suited for investigating ocean-bottom and geothermal areas (see Law, 1983; Berktold, 1983). Communications Within the last 10 to 15 years suggestions have been made that a natural waveguide in the Earth's crust, composed of the insulating layer of dry rocks sandwiched between the upper hydrated conducting and the underlying conducting hot layer, could be used for communication purposes. This suggestion, however, does not seem to have been followed by any known application. Existing literature is referenced in Gregori and Lanzerotti (1982). A practical problem is certainly concerned with the spatial nonuniformity of such a waveguide, the width and depth of which is undoubtedly widely varying (e.g., compare a cratonic area with a mid-oceanic ridge area) and is essentially unknown in many regions. In a similar fashion, magma chambers in mid-oceanic ridges can be considered the natural "equivalent" of man- made submarine communication cables. The conductivities are such that the mid-Atlantic ridge is equivalent to about 1000 such cables in parallel (see Gregori and Lanzerotti, 1982). An interesting communication experiment related to artificial telluric currents was reported by Fraser-Smith et al. (1977). They operated, as a transmitting antenna (using a simple car battery), a circuit loop composed of seawater encircling a small peninsula in a nearly enclosed area. Biological Effects The response of living species to electromagnetic fields (such fields being either responsible for, or a consequence of, telluric currents) is a difficult but important problem. Several examples discussed in the literature include the induced currents in a tree produced by geomagnetic fluctuations (Fraser-Smith, 1978) and the use of magnetic fields for orientation by aquatic bacteria (e.g., Blakemore, 1975) and by migrating birds (e.g., Moore, 1977; Larkin and Sutherland, 1977; Alerstam and HÃ¶gstedt, 1983; Beason and Nichols, 1984). Telluric currents could play a role in some control of fish (e.g., Leggett, 1977; Kalmijn, 1978; Brown et al., 1979; Fainberg, 1980; Fonarev, 1982). Magnetite crystals have been reported as isolated from a sinus in the yellowfin tuna (Walker et al., 1984). Enhanced DNA synthesis has been reported for human fibroblasts exposed to magnetic-field fluctuations with frequencies and amplitudes similar to many geomagnetic occurrences (Li-boff et al., 1984). The entire area is fraught with controversy, particularly that related to magnetic effects, and has been reviewed by Parkinson (1982) and commented on by Thomson (1983). CONCLUSIONS Historically, telluric currents were intensively investigated in the second half of the nineteenth century, particularly because of their influences on long telegraph conductors (see Appendix). The intrinsic difficulties encountered in obtaining fundamental understanding, basically related to the several causes that can be coresponsible for the observed effects, discouraged geophysicists from pursuing such investigations vigorously. However, with respect to a century ago, large improvements have been made in a number of areas, including recording techniques, density of available observations, international data exchanges, computational facilities, mathematical methodologies, and general geophysical