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TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 246 Albertson and Van Baelen (1970), Albertson et al. (1970, 1973, 1974), ACRES (1975) (see also references therein), Boerner et al. (1983), and Pirjola (1983). The geomagnetic currents induced in a power system can produce problems of several different types (Albertson et al., 1973, 1974; review by Williams, 1979). First, the arbitrary differential relay operation in power distribution systems during geomagnetic storms can produce a judgmental problem; system operators are unsure of whether the malfunctioning relay indication is an induced-current effect in a transformer or a real transformer malfunction. Second, the currents actually induced in the winding of a power transformer can result in half-cycle saturation of the transformer core. This saturation can produce fluctuations in the transformer operation itself. This local heating can greatly shorten the lifetime of a transformer. Summarizing, the effects of induced telluric currents on power systems produce outages as well as damages to expensive transformers. Gorely and Uvarov (1981) estimated that in the Norilsk region (Siberia) up to tens of amperes can be expected on powerlines of 100 to 150 km length. Since 500-kV transformers capable of withstanding even 3 to 4 A without saturating appear to cause problems for manufacturing (Sebesta, 1979), a way of avoiding such serious damage is to use powerlines of limited total length (e.g., Akasofu and Merritt, 1979, suggest no more than 500 km for Alaska). Pirjola (1983), from measurements made at four locations in Finland, concluded that currents of the order of 100 A lasting about 1 h should damage transformers. Pipelines Varley (1873) reported that large Earth currents on a short length of telegraph cable in London appear to have been related to currents flowing on large, nearby gas pipelines. Studies of induced telluric currents on pipelines took renewed importance when the long, Trans-Alaskan pipeline (1280 km long) was built. The effects of telluric currents appear to be of most importance in affecting electronic equipment related to operational monitoring and corrosion control rather than in producing specific serious corrosion problems. Viewing a pipeline as a man-made part of the natural environment, it is noteworthy to mention the 30-A current reported by Peabody (1979) to cross the Panama Isthmus, from ocean to ocean, a current that also changes direction. Such specific currents can produce corrosion failures at some ocean terminals of the pipelines, even before the pipeline is in operation. Such problems can be avoided most simply by suitable separate ground connections (Peabody, 1979). The Alaskan pipeline has been the subject of careful investigations, principally because of its location across the auroral zone (Hessler, 1974). Campbell (1978, 1979, 1980) and Campbell and Zimmerman (1980) provided a comprehensive account of the problem and concluded that the current I expected to flow within the pipeline is related to the geomagnetic index Ap by the linear relationship I = 5.0 Ap â 0.7. Based on the statistics of occurrence of the Ap index (larger for greater geomagnetic activity), at least once a year about 600 A should be observed, 800 A should be observed at least once every 2 years, and 1200 A should be observed at least once every 5 years. The dimensions of the Alaskan pipeline (diameter of Ë 1.22 m, a mean wall thickness of ~ 1.30 cm, a resistance per unit length of ~ 2.81 Ã 10â 6 â¦/m, and an end-to-end total resistance of 3.6 W; Campbell, 1979) suggest that it is a large man-made conductor that is capable of significantly affecting the local natural regime of telluric currents. Railways Pollution by artificially produced telluric currents associated with railway operations have been investigated from several viewpoints. Burbank (1905) reported the effects in 1890 of the South London Electric Railway on the Earth current records being made at Greenwich. The nuisance for geomagnetic observations of telluric currents associated with return currents from dc electrifield railways has perhaps been the most widely investigated effect (La Cour and Hoge, 1937; RÃ¶ssiger, 1942; Yanagihara and Oshima, 1953; Mikerina, 1962; Yanagihara and Yokouchi, 1965; Yanagihara, 1977). The spatial extent within the ground of telluric currents from railway operations has been investigated by Kovalevskiy et al. (1961) in the southern Urals. They detected telluric current pulses with periods between a few seconds and 20 minutes and amplitudes of about 0.5 to 3 V/ km. They found the effects to drop off rapidly within 10 to 15 km from the railway, although still being dominant over natural telluric currents at 30 km, and still detectable at 60 km (where the measurements stopped). Meunier (1969), following a previous investigation by Dupouy (1950), detected telluric current effects related to a specific operation (lowering and raising the pantograph) of the Paris-Toulouse railway at 115 km distance from the railroad. This effect, in fact, can sometimes be detected on the magnetograms from the Chambon-la-ForÃªt observatory. An example of the effect is shown in Figure 16.12. Jones and Kelly (1966) detected Earth currents in Montreal, clearly correlated with a dc powered railway some 20 km distant. F. Molina (private communication, Osservatorio