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TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 243 here, however, that these depths are very approximate; the actual structures are generally much more differentiated and complex; see, e.g., Hermance (1983).] The problem of the deep electrical conductivity structure of the Earth has usually been treated in terms of a concentric spherical shell model of the Earth, where each shell has a uniform electrical conductivity. The interested reader may refer to such classical treatments as Chapter XXII of Chapman and Bartels (1940), Rikitake (1966), Rokityansky (1982), or Parkinson (1982) (a more concise treatment can be found in Price, 1967). A simple but effective procedure was proposed by Schmucker (1970) whereby the Earth is simplified to a two-layer body having an insulating outer layer underlain by a conductor (he considered both flat and spherical Earth models). Using only the ratio of the horizontal to the vertical component of the geomagnetic field (at a prechosen frequency), Schmucker (1970) provided simple formulas by which the thickness of the insulating layer and the electrical conductivity of the underlying conductor can be promptly evaluated. By considering fields of different frequencies it is possible to evaluate different estimates of depth and conductivity. This "Schmucker inversion" technique, which often appears to agree with results obtained by means of other, more involved, methods of handling geomagnetic data, is a simple way of treating the inversion problemâa difficult and much debated problem (e.g., Rokityansky, 1982; Hohmann, 1983; Parker, 1983; Varentsov, 1983; Gough and Ingram, 1983; Berdichevsky and Zhdanov, 1984). INTERACTIONS OF TELLURIC CURRENTS WITH MAN-MADE SYSTEMS The natural telluric current environment can significantly affect man-made systems. Conversely, human technology can "pollute" the natural telluric current environment. The mechanisms by which these interactions occur, as well as their modeling, are far from being understood satisfactorily and comprehensively. Geophysicists have often viewed such interactions as an unwanted, unnatural nuisance. Engineers have almost always been concerned with thresholds of system reliability and with a system's capability to react positively to any sudden change in the natural environment, always on a strict basis of yield/cost ratio. Moreover, technological improvements have been progressively introduced within systems to ensure a higher and higher reliability (e.g., Axe, 1968; Anderson, 1979), so that it becomes difficult to compare effects observed on different systems in different years. Seldomly have man-made systems been viewed as scientific instruments that are useful for studying the natural environment. Often a man-made system can be considered part of the natural environment itself. Geophysicists can then imagine such large man-made tools as similar to specifically designed measuring instruments that, unlike laboratory instruments that are normally presumed to negligibly affect the system, actually interfere with the natural phenomena, often quite seriously. Such huge and expensive man-made systems can allow, in principle, some complex experiments and measurements, which otherwise could not be carried out. For this reason this topic has particular scientific value, much beyond a matter of scientific curiosity or of a more or less minor nuisance affecting the operation of huge engineering systems. The literature on the subject tends to be rather sparse. However, five principal areas of interest can be considered. These are discussed below; some of this material has been previously reviewed elsewhere (Axe, 1968; Lanzerotti, 1979a, 1979b, 1979c, 1983; Paulikas and Lanzerotti, 1982). Communication Cables Historically, this is the best investigated and documented effect of telluric currents on technological systems. In fact, after the lightning rod, the telegraph was essentially the earliest of man-made electromagnetic devices in use. Subsequently, telegraph lines have been progressively supplanted by telephone lines, and submarine cables have supplanted the former radio links between the telephone networks of different continents (e.g., Blackwell, 1928; Bown, 1930, 1937; Schelleng, 1930). Even with the advent of communication satellites, cable systems are still of major economic importance for long-distance communications. The first detection of effects on a telegraph wire dates back to the years 1847-1852. The first observations appear to be from England by Barlow (1849). As stated by Prescott (1866): M. Matteucci had the opportunity of observing this magnetic influence under a new and remarkable form. He saw, during the appearance of the aurora borealis of November 17, 1848, the soft iron armatures employed in the electric telegraph between Florence and Pisa remain attached to their electromagnetics, as if the latter were powerfully magnetized, without, however, the apparatus being in action, and without the currents in the battery being set in action. This singular effect ceases with the aurora, and the telegraphs, as well as the batteries, could operate anew, without having suffered any alteration. Mr. Highton also observed in England a very decided
TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 244 action of the aurora borealis, November 17, 1848. The magnetized needle was always driven toward the same side, even with much force. But it is in our own country that the action of the aurora upon the telegraph-wires has been the most remarkable. . . . In September, 1851, . . . there was remarkable aurora, which took complete possession of all the telegraph lines in New England and prevented any business from being transacted during its continuance. The days between August 28 and September 2, 1859, were also quite remarkable, not only for some wonderful auroral displays (Clement, 1860; Hansteen, 1860; Prescott, 1860, 1866). Clement's (1860) book had a self-explanatory title: The Great Northern Light on the Night before 29 August 1859 and the Confusion of the Telegraph in North America and Europe. According to Chapman and Bartels (1940), this aurora was seen in the Atlantic at a latitude as low as 14Â° N, while in France 800 V were induced on a wire over a distance of 600 km. From Prescott (1866): We have, however, the second yet more wonderful effects of the aurora upon the wires; namely, the use of auroral current for transmitting and receiving telegraphic dispatches. This almost incredible feat was accomplished . . . on the wires of the American Telegraph Company between Boston and Portland, upon the wires of the Old Colony and Fall River Railroad Company between South Braintree and Fall River, and upon other lines in various parts of the country. . . . Such was the state of the line on the September 2nd, 1859, when for more than one hour they held communication over the wires with the aid of celestial batteries alone. Other studies of historical interest on telluric currents in communication cables are mentioned in the Appendix. In 1910 work was begun in Norway by Carl StÃ¶rmer of measuring the height of polar aurorae (StÃ¶rmer, 1955). StÃ¶rmer used photographs taken simultaneously from two sites separated by a few tens of kilometers. He was able to send a message of alert to his co-workers about an imminent night of photographic work whenever he measured disturbances in the local telegraph wires. A geomagnetic storm in Sweden in May 1921 (Germaine, 1942; Sanders, 1961) produced voltages of 6.3 to 20 V/ km (i.e., 1 kV or more over 100 to 200 km, with 2.5 A, while the threshold for serious troubles was 15 mA). A large magnetic storm on April 16, 1938, produced potentials of several hundred volts over local wires in Norway (Chapman and Bartels, 1940). On March 24, 1940 (Germaine, 1942; Harang, 1951; Brooks, 1959; Sanders, 1961), a geomagnetic storm damaged the Norwegian wirelines ~ 600 V, >4 A), while in the United States, more than 500 V were estimated to have occurred along some lines. Reports from two sites near Tromso, Norway, stated â¦ Sparks and permanent arcs were formed in the coupling racks and watch had to be kept during the night to prevent fire breaking out. . . . One line was connected to earth through a 2 mm thick copper wire, which at once got red hot, corresponding to a current more than 10 amps (Harang, 1951). In the second half of the nineteenth century, Earth currents in submarine cables were rather extensively investigated. Saunders (1880, 1881) and Graves (1873) reported some of their work, which included a cable between Suez and Aden and a cable between Valentia and Newfoundland. Wollaston (1881) concluded that his current measurements on a submarine cable across the English Channel resulted from tidal currents and related an 1851 conversation with Faraday on the matter. The latter was quoted as quite enthused about this confirmation of his earlier predictions. Axe (1968) listed several geomagnetic storm-induced effects on submarine cables occurring in 1957-1967 (total voltage drops range from 50 V to 2700 V for the different occurrences). The largest voltage drop (Figure 16.9) occurred across a transatlantic cable (equivalent to 0.75 V/km) at the time of the huge storm on February 11, 1958, which produced a well-known spectacular auroral display down to low latitudes (Brooks, 1959; Winckler et al., 1959; Sanders, 1961; Akasofu et al ., 1966). It is noteworthy that "the cable to Hawaii which originates about 140 miles north of San Francisco exhibited no major voltage swings" (Winckler et al., 1959). A major geomagnetic event on August 4, 1972, caused the outage of a continental cable in the midwestern United States. The outage has been investigated (Anderson et al., 1974; Anderson, 1979) by modeling the tel Figure 16.9 Output voltage of the power-feed equipment at the Oban, Scotland, end of the Oban-Clareville, Newfoundland, cable. The voltage variation in North America was somewhat larger, leading to a total variation of about 2700 V across the cable. From Axe (1968).