Real-time detection of anomalous geoelectric changes

Abstract

Aiming at an accurate and quick detection of anomalous changes of the electric field so that we can detect tectonic signals, we developed observation methods for use both on land and on the ocean floor. With these observation methods, that is, observation on land using electrodes and the underground cables of the Nippon Telegraph and Telephone Company, and on the ocean floor using the power-feeding arrangement of the ocean-bottom seismograph of the Japan Meteorological Agency, we can obtain more reliable data than those from ordinary observation methods. Using these data, we further developed two techniques of analysis in real-time which are based on a statistical model, eliminating components induced by the variations of the geomagnetic field and/or the movement of the seawater. For both cases, the geomagnetic field at the Kakioka Magnetic Observatory and/or the seawater level data observed on the ocean floor were used as associated data. The predominant induced component in the variation of the geoelectric field is almost completely separated in the cases of the geoelectric field in the Mito region and on the ocean floor. Owing to the characteristic periodicity of the monopolar variation of noise in the Numazu region, in spite of the large amplitude of the noise (mainly due to the leak-current of the d.c. electric trains) in the original data from the Numazu region, the noises were considerably separated, with these methods, into the induced component part or tidal component part. With these methods, consequently, the detectability of anomalous changes was improved about ten times and about five times in the cases of the geoelectric field in the Mito region and on the ocean floor, and in the case of the geoelectric field in the Numazu region, respectively.