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TELLURIC CURRENTS: THE NATURAL ENVIRONMENT AND INTERACTIONS WITH MAN-MADE SYSTEMS 247 Geofisico Monte Parzio, Italy), in close examination of standard magnetogram records from the observatory at L'Aquila, has found a difference in the width of the trace depending on whether the Italian railways are on strike or not. In the former case, the Z-component trace is about a factor of 2 less thick than during normal operations. The noise introduced appears to be about 0.5 nT. The closest railway is about 30 km away. Figure 16.12 Artificial Earth currents (north-south direction) measured (Fournier and Rossignol, 1974) at Nozay (France) and concurrent fluctuations in the total magnetic field measured at Chambon-la-ForÃªt, on opposite sides of the Paris-Toulouse electrified railway line. A most impressive telluric current effect (Fraser-Smith and Coates, 1978; Fraser-Smith, 1981) in the San Francisco Bay area has been produced by BART (the San Francisco Bay Area Rapid Transit system). ULF waves (frequency less than 5 Hz) are observed, having energy at a frequency predominantly below about 0.3 Hz. Their amplitudes are at least ten times greater than the natural background environment, i.e., they are comparable with the levels reached during great geomagnetic storms. The effect originated by BART appears to occur over an area of about 100 km2. A similar effect has been detected by Lowes (1982) in Newcastle upon Tyne (U.K.), produced by the dc rapid transit underground railway system. F. J. Lowes (University of Newcastle upon Tyne, private communication, 1985) also noted that when the system starts up in the morning he can follow individual train movements over about 12 km of track before there is too much superimposition of the signals. Corrosion Corrosion in buried metal structures (in addition to pipelines) is significantly enhanced by the occurrence of telluric currents, presumably via electrolytic processes. This is a well-known phenomenon to people routinely working on repairs of telephone cables or of pipes (for water or otherwise). Severe damage comes mainly from man-made telluric currents when the conductors are buried close to dc electrified railways or tramways. A simple insulating coating, provided that it has no holes, appears to be the best protection. The problem is discussed to some extent by Peabody (1979). A much older reference (given by Kovalevskiy et al., 1961) is Tsikerman (1960). The problem can exist also for buried powerlines that have, unlike aerial power lines, some relevant problems of heat flow (Salvage, 1975). APPLICATIONS OF TELLURIC CURRENT MEASUREMENTS Listing all possible applications of telluric current measurements is presumptuous and almost impossible. A tentative scheme is given here, which, perhaps, can provide a first approach to such a complex topic. Detection of Electromagnetic Signals from Space The Earth (including natural conductivity structures and man-made systems, such as communication cables and powerlines) can be treated as a receiving antenna, useful for monitoring the external origin electromagnetic fields. Communication cables of varying length can provide information on the spatial scale as a function of frequency of the external signal. See Meloni et al. (1983) for additional discussions of this point. Prospecting of Underground Structures Prospecting of underground structures is the most developed application of telluric currents. The methodology is quite extensive. There are two principal approaches, viz., Magneto-Tellurics (MT), which uses measurements of the two horizontal components of both the geomagnetic and the geoelectric fields, and Geomagnetic Depth Sounding (GDS), which uses measurements of all three components of the geomagnetic field. "Active" methods (see, e.g., Keller, 1976; Ward, 1983) make use of man-made electromagnetic fields. Such