Translated to English from paper in Japanese, originally published in:
Gijutsu Houkoku, Vol.28, No.3 (No.86), 1-77, 1989 with permission of authors.

Geoelectric Observation used by the Telegraphic
Facilities of NTT Corporation
byToshio MORI
Abstract
Unique geoelectric observations have been conducted in land and on the sea floor in the Kantoand Tokai districts in Japan, and progress has been made on original research. The geoelectricobservations on land were conducted using the telegraphic facilities of Nippon Telegraph andTelephone Public Corporation (now NTT Company). The geoelectric observations on the sea floor
have been observed by making use of the power-feeding arrangement of Permanent Ocean-Bottom
Seismograph Observation Systems off Tokai and off the Boso Peninsula installed by the Japan
Meteorological Agency. These geoelectric data are fairly high quality in comparison with conventionalgeoelectric data.
A comparison examination of the geoelectric data observed by NTT facilities was carried out
with geomagnetic changes, geoelectric changes, seismic and volcanic activities, tide level changes,
the weather elements, etc. Moreover, examination of the use of geoelectric noises was alsoperformed.
The geoelectric observations using the electrodes and the underground metallic cables of NTT
have been performed mainly in the Mito and Numazu telephone offices. The observations have thefollowing two advantages in comparison with conventional methods: (1) The contact electric potential
between the electrode and the soil seems very stable. Therefore, we can obtain stable data over along time which is not affected by precipitation. (2) The base lengths for the observation are verylong. The base lengths are 10 to 100 times longer than those for other ordinary observations in Japan.
So, the observed field is affected by a large-scale structure.
The data observed at the Mito group are fairly noiseless, while the data at the Numazu andFujinomiya groups contain large amplitude noises. In order to improve the detectability ofself-potential variations related to tectonic activities, various methods were tried and developed bymaking use of these data. Especially, emphasis was placed on analysis by the method of real-time
detection of anomalous geoelectric changes by removing components induced by geomagneticvariations with a stochastic difference equation.
As a result of investigating the relations between the observed geoelectric variations andtectonic activities, although no related change was observed in the Mito group, some abnormalities
were found in the Numazu group. Anomalous electric potentials at the Numazu group took place
simultaneously with the start of the earthquake swarm off the east coast of Izu on October, 1985. The
electrical potential changes that took place ranged up to 300 mV in Atami, and 100 mV in Ito andShuzenji. The Izu-Oshima volcano erupted on November 15, 1986. Before the start of the eruption,
the geoelectric potentials at ODW were estimated to have decreased abruptly by 500 mV onNovember 6 and attained a maximum decrease of 800 mV on November 9.
Electrical potential changes relevant to tide level changes, and constituents of geoelectric change
unrelated to geomagnetic change and long cycle change, were also detected.
It was shown that the observed electric noises are applicable to investigate the underground40Toshio MORI

structure and to detect underground electrical characteristic changes. If the earth potential isobserved near the conductivity anomaly boundary, it will be thought possible to detect unusual
sensitivity phenomena relevant to tectonic activities.

Comments on the English translation
The following will be noticed if a paper of about 15 years ago is put into English and read overagain.
Mistakes were made in clarity and in some unsuitable expressions, such as in the names ofplaces which are considered to be common sense by Japanese people, and in expressions where asimplified term was found. Corrections were made for those parts and explanation was added.
Although notations of figures had portions which are not unified, since correction was difficult, thenotation was left as it is, but explanations were added as much as possible.
As it is described in the “Circumstances of observation and acknowledgements,” this paper waswritten when research business was being replaced with administrative business. Since it wasthought that it would be difficult to return to research business as before, the data and ideas werehurriedly adjusted. In addition, since there was no Abstract in the text, it was written at this time.
Finally, the author is thankful to the personnel of the Magnetic Observatory who gave us thisopportunity.