Abstract
In Japan now, spans between electrodes for geoelectric field observations are usually shorter than a few kilometers on land. Electric field observation with a long span of a few tens of kilometers was conducted by making use of the equipment of the Nippon Telegraph and Telephone Public Corporation (NTT). The cables connecting between the repeater stations and the grounded earths at each station were used for the electric field observation.
The observation was conducted over the distance of 26.8 km from Shimodate to Kasama and the distance of 15.7 km from Shimodate to Oyama in northern Kanto. The variations were recorded with a two-penrecorder through low-pass filter with cut-off frequency 0.003Hz (about 1/6 cycle/minutes).
Electric field variations associated with magnetic field variations were recorded in good condition between Kasama and Shimodate, and are very similar to those of the east-west component of the electric field at the Kakioka Magnetic Observatory. On the other hand, the electric field between Oyama and Shimodate was disturbed by large currents leaked from trains driven by direct electric power. In addition to this noisy environment, the amplitudes of electric field variations induced by magnetic ones seem to be much smaller between Oyama and Shimodate than between Kasama and Shimodate. The small amplitudes of the geoelectric variations at Oyama are considered to be closely related to the fact that the high conductive sediment is thick near Oyama.
This test observation shows that the equipment of NTT is available to detect the geoelectric field variations induced by the geomagnetic ones. It was found that these observations are useful in detecting changes of subterranean electric conductivity structure, such as those prior to earthquakes.
