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TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 248 methods are well suited for shallow prospecting but can hardly be applied to deeper layers, because of the skin-depth phenomenon. Such required long-period electromagnetic waves cannot be generated practically. Long powerlines have been used for generating electromagnetic induction fields for prospecting purposes (e.g., Gill and MacDonald, 1967). A distinction should be made between prospecting techniques that use the static geomagnetic field of the Earth and techniques that use electromagnetic induction effects. In handling the standard aeromagnetic and oceanographic magnetic surveys, whose purposes are to understand the static fields, the time-varying fields recorded at a ground- based site "close" to the area of the survey must be subtracted from the air or the ocean signal. This introduces some errors, whose actual values are often difficult to estimate (e.g., Reford, 1979). The time-varying component from such surveys can be used for geomagnetic depth-sounding studies (Gregori and Lanzerotti, 1979a). Deep-Earth Studies Telluric currents are likely eventually to be important tools for prospecting the deep structure of the Earth, thus providing valuable complementary information to that provided by seismic waves. A great advantage of GDS methods is that, while many studies concentrate on magnetic storm events, the studies can also be carried out using inducing signals during more quiet times, signals that are always in existence. The use of induced currents from natural electromagnetic waves for deep-Earth research is being pursued actively in a number of countries, particularly the Soviet Union. Recent reviews include books by Rokityansky (1982), Patra and Mallick (1980), Parkinson (1982), and Berdichevsky and Zhdanov (1984). Tidal Phenomena and Water Flows There are three types of tides: atmospheric, oceanic, and solid Earth. Atmospheric tides generate a large part of the external-origin inducing field of long period; oceanic tides produce a time-varying geomagnetic field associated with water flows; and solid-Earth tides can similarly produce a geomagnetic field because they can produce an eventual water flow that will produce a magnetic field. Theoretical and observational aspects of the phenomena have been discussed by Meloni et al. (1983). Within the past decade extensive use of shorter undersea cables (such as those across the Dover Strait; e.g., Prandle, 1978) has been made for studies of tidal oscillations and water flow. Cables across the Irish Sea have been used for such studies (for example, Prandle and Harrison, 1975; Prandle, 1979). Geomagnetic disturbances can affect the measurement capabilities and, hence, result of such a cable-monitoring system. The data presented in Figure 16.13 are from chart recordings of the cable voltage on the Donaghadee-Port Patrick cable on a day of geomagnetic disturbances (Prandle and Harrison, 1975). The low-frequency variation in the voltage, spanning the record, is produced by tidal flow. The higher-frequency variations, produced by geomagnetic storm induction of currents in the cable, obscure the variations in such a manner that the data cannot be used reliably for water-flow information on such a day. Figure 16.13 Cable voltage on the Donaghadee-Port Patrick cable on a geomagnetically disturbed day (from Prandle and Harrison, 1975). Earlier, Wertheim (1954), in studying water flow across the Florida straits using the Key West-Havana cable, found occasional rapid variations in the cable voltage. He attributed these to geomagnetic effects and tried to model them using magnetometer data from the San Juan Observatory. Recent work in studies of the Florida current were reported by Larsen and Sanford (1985). Earth's Astronomical Motion The variation in the length of the day and the displacements of the positions of the geographic poles are among the most precise and fascinating topics in geophysics and have now become a vast discipline. The problem, however, of a possible role of telluric currents in producing a braking or an acceleration in the Earth's rotation or in displacing the Earth's poles appears still basically unsolved and/or is not considered important by many. This is discussed in some detail by Meloni et al. (1983) (and references therein), where also the possible