人工地震に伴う電磁気現象 : 富山県大山町での調査例

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Electromagnetic properties were measured at the joint observation of the explosion seismic experiment at Ooyama, Toyama Prefecture on Oct. 17 1991. The explosion was induced by 450 kg gun powder placed at a depth of 75 m. This paper represents the results of the experiments. (1) Electric self-potential : The measurement of the electric self-potential (S.P.) was made at two stations. One station was set up around the explosion point, and the other station about 1.5 km apart from the explosion point. At the station about 1.5 km apart from the explosion area, we observed no distinct change in S.P. before the arrival of seismic wave. At the station around the explosion point, four electrodes were arranged in the line of east-west direction crossing the explosion point. The distance of the each electrode from the explosion point ranges from 10 m to 100 m. The seismograph sensor was arranged at the distance of 130 m for the comparion with the change of the S.P. Up to 150 mV changes in S.P. were observed by the electrode couples before the arrival of seismic wave. The detailed analysis revealed that the induced changes in the S.P. consist of two portions : the fluctuation with shorter period (〜0.001 second) and with the frequency of the longer period (〜0.05 second). The propagation speed of the fluctuation with the shorter period was much higher than that of the fluctuation with the longer period. The fluctuation of the shorter period might have been related to the explosion process of gun powder. About the fluctuation with the frequency of the longer period, the following properties were elucidated. The amplitude of the fluctuation in S.P. is in proportion to the span length of the electrode couples. The fluctuation in S.P. arrived faster at the electrode couples situated nearer to the explosion point. (2) Magnetic field The magnetic field was measured by the fluxgate magnetometer at the site 35m north from the explosion point. The apparentchanges of magnetic vertical component and of horizontal component were observed at the time of 0.25 seconds after the explosion. Increase of total magnetic field of 2〜3nT after the explosion was found. This increment was confirmed by the total magnetic field survey around the fluxgate sensor. (3) VLF electromagnetic radiation No clear radiation of VLF electromagnetic wave was seen because of the possible contamination of the other natural radio noises. However, a possibility of VLF radiation connot be denied. Further study of the VLF data should be carried out.
独立行政法人防災科学技術研究所の論文
1995-03-31

人工地震に伴う電磁気現象 : 富山県大山町での調査例

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