Abstract
Geoelectric variations near Numazu have been observed since August 1985 by making use of the equipment of the Nippon Telegraph and Telephone Corporation (NTT). Electrodes used are earths for the communication at the NTT repeater stations. Observed data are stable in a long range of time, because of the low contact resistance and the deep installation of electrodes. Sources of geoelectric variations are so many and complicated that it is necessary to investigate the character of the geoelectric variations in order to clarify the relation between the geoelectric variations and the tectonic activities.
There are many spike-like variations, the time duration of which is shorter than 10 minutes. The sources seem to be leak currents from the railways electrified with direct current power and the electric facilities of factories. The amplitude of spike-like variations becomes larger when there is precipitation, since it lowers the leak resistance of rails to the ground. Therefore, the daily standard deviation, which is the square root of the mean of the squared deviations, correlates to precipitation. In some observation lines, daily mean values also correlate to precipitation. By a filteration process, we were able to identify the geoelectric variations induced by geomagnetic variations. The direction of the largest induced geoelectric variations is nearly east-west. The relation between geoelectric variations and the tide level is not clear, since both these correlate to atmospheric pressure and/or precipitation.
