閉山した鉱山の廃坑を利用した自然電位変化の観測

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Using tunnels and shafts in Ikuno mine, which was closed in 1974, we started in 1987 to observe seismic events and temporal change of (1) electric self-potential (Frequency<0.01 Hz), (2) apparent resistivity, (3) underground water level, (4) water seepage from tunnel wall and (5) γ ray intensity. Major ore veins at Ikuno mine are distributed in an area of several kilometers by several kilometers; major ore veins have a width of a few meters, dips of almost vertical and strikes of about north-to-south or northwest-to southeast. Before Ikuno mine was closed, ore veins were excavated down to a depth of about one kilometer from the ground surface. To excavate ore veins, most tunnels of total lengths of 350 kilometers are dense around the ore veins; to bring up the ore there are several shafts with lengths of several hundred meters. Excavated caves as well as tunnels and shafts are now filled up with water. During our observation, underground water table rose about ten meters several times. Heavy rain was the primary causes for all cases, but troubles of drain pumps were also the causes for some cases. Associated with these rises, significant decrease of apparent resistivity across an ore vein was observed during the rises of the underground water; we also found clear correlation in self-potential in some cases. These suggest much water went down to excavated area around ore veins. We found that the seismic events (S-P time<0.3 s; M<0) occur near ore veins; we also found some correlation between the water rises and seismic activities; these suggest something occur near ore veins when water rises. In addition, associated with the rises of the underground water table, significant crustal movement of an order of 10^<-6> was observed that suggests ore veins dislocate when increasing of pore pressure. During our observation we had no opportunity to happen to see the precursor. Although seismic events occurred in the very vicinity of our observation site, they were too small to catch any correlating signals in crustal movement, self-potential, apparent resistivity and γ ray. No significant seismic event larger than M4 occurred in a distance of several ten's kilometers. We were not able to catch neither precursory nor postseismic phenomena associated with significant earthquakes. However, through our analysis of effect of underground water based on various kinds of data, we learned that it is important to discuss from every point of view and easy to find an answer using various kinds of data.
独立行政法人防災科学技術研究所の論文
1995-03-31

閉山した鉱山の廃坑を利用した自然電位変化の観測

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