Large‐amplitude ULF magnetic fields produced by a rapid transit system: Close‐range measurements

Abstract

Measurements at a location close to the San Francisco Bay Area Rapid Transit (Bart) systemshow that Bart can produce ultra low frequency (ULF; frequencies less than 5 Hz) magnetic field fluctuations with amplitudes that are 2-3 orders of magnitude greater than the normal background level. The maximum increase in magnetic field fluctuations over the ambient background level occurs in the range 0.01-1 Hz, but significant increases occur at 0.001 and 10 Hz. Although it is generally accepted that moving trains cause magnetic field disturbances, it has not, until recently, been recognized that modern dc-powered rapid transit systems are such powerful sources of ULF electromagnetic fields, and the existence of these sources in densely populated urban areas could have environmental implications. INTRODUCTION Research in 1975 and 1976 on the origin of exceptionally large and continuous ultra low frequency (ULF; frequencies less than 5 Hz) magnetic field fluctuations on the Stanford University campus revealed that the San Francisco Bay Area Rapid Transit (Bart) system was a powerful generator of ULF electromagnetic fields detectable throughout the Bay Area (--• 100 km 2) and probably also over a large surrounding area [Fraser-Smith and Coates, 1978]. Although there have been a number of suggestions for the controlled generation of ULF fields for communication [Lipa et al., 1975; Davis and Willis, 1974], this appears to be the first observation of a large-scale source whose characteristics can be accurately determined. (High-altitude nuclear explosions produce ULF geomagnetic field fluctuations over a large area of the earth, but the fluctuations are largely uncontrolled, and they persist only for a short time [e.g., Green et al., 1962; Kovach and Ben-Menahem, 1966] .) The observation could prove useful in studies of the communication potential of man-made ULF signals, and it is likely to have important implications for studies of the generation and propagation of naturally occurring ULF geomagnetic pulsations [Jacobs, 1970] and for measurements of geomagnetic •Present address: Corkstown Laboratory, Bell-Northern Re-activity at observatories near populated areas.Furthermore, Bart and other modern dc electricpowered rapid transit systems (which are also likely to be powerful sources of ULF fields) are necessarily located in urban areas with high population densities. The finding that these systems are sources of large-amplitude ULF fields may also be important for environmental reasons. Recognizing these possibilities, we undertook a new series of measurements of the Bart magnetic fields at a location much closer to the Bart system than the sites used for the earlier measurements. We also expanded

Review of electromagnetic coupling between the Earth's atmosphere and the space environment

“…Urban railway system could induce ELF waves in the range 0.01-5 Hz (Ho et al, 1979). The power line of the world's power grid is the source of ELF/VLF waves in the ionosphere/magnetosphere.…”

A network of magnetometers for multi-scale urban science and informatics

“…The spatiotemporal behavior of ultralow-frequency electromagnetic fields throughout the San Francisco Bay Area has led to the identification of the Bay Area Rapid Transit (BART) as a source of electromagnetic noise (Fraser-Smith and Coates, 1978). Subsequent measurements at a distance of 100 m from BART suggested periodic bursts of magnetic activity with roughly the periodicity of the BART train (Ho et al, 1979). Given its observability, reliable periodicity, and the presence of multi-scale signatures (e.g., individual train periodicity versus daily operation schedule), the BART is a useful tool to understand the operation of a magnetometer array in an urban area.…”

A Network of Magnetometers for Multi-Scale Urban Science and Informatics

“…The spatiotemporal behavior of ultra-low-frequency electromagnetic fields throughout the San Francisco Bay Area has led 25 to the identification of the Bay Area Rapid Transit (BART) as a source of electromagnetic noise (Fraser-Smith and Coates, 1978). Subsequent measurements at a distance of 100 m of BART suggested periodic bursts of magnetic activity roughly the periodicity of the BART train (Ho et al, 1979). Given its observability, reliable periodicity, and presence of multi-scale signatures (e.g.…”

“…The increase in spectral complexity is consistent with increases in train frequency, an additional active BART line, and variability associated with schedule changes for commuter hours. Previously efforts to capture period signatures associated with the BART found bursts of activity corresponding approximately to the train schedule, but with an irregular variation in the waveform (Ho et al, 1979 BART train period. The periodicity of the coherent BART signature enables for identification and extraction of the time-series waveform associated with BART operation.…”