Development of Deep Borehole Instruments for Both Multi-Component Observation and <i>in situ</i> Stress Measurement, and Some Interesting Results Obtained

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  • 深部ボアホール観測のための地殻活動総合観測装置および地殻応力測定装置の開発と興味ある成果の例
  • シンブ ボアホール カンソク ノ タメ ノ チカク カツドウ ソウゴウ カンソク ソウチ オヨビ チカク オウリョク ソクテイ ソウチ ノ カイハツ ト キョウミ アル セイカ ノ レイ

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Abstract

We have developed multi-component borehole instruments for borehole stations deeper than 1000m. The instruments are composed of 7 strainmeter components (4 horizontal, 2 inclined, and 1 vertical), 2 tiltmeters, 3 seismometers, 4 magnetometers, and a high-resolution thermometer. The instruments are also equipped with new systems for data transmission and data monitoring, even during installation, and for determining the instrument direction at the bottom of the borehole. We have also developed an intelligent-type strainmeter based on the Ishii-type strainmeter for measuring in situ stresses. It is equipped with strain sensors, an A/D converter, a CPU, a memory, and a battery, and it has no outside cable. It is cemented into a deep borehole with expansion grout and then taken out by overcoring after it has coupled with the basement rock. By this procedure we can estimate in situ rock stresses. Observation in deep boreholes can avoid the problems of both artificial noise and meteorological disturbance. It enables the performance of high S/N ratio observations for detecting very small signals. The results obtained from deep borehole observations on the Izu Peninsula and in the Tono area of Gifu Prefecture have shown some interesting variations of strains and tilts related to the precursory phenomena of earthquakes. An example of in situ stress determination has also been demonstrated. In the case of deep borehole observation, we first measure the in situ stresses and then install a multi-component borehole instrument to monitor crustal activity. By this process we can continuously monitor stress variation. This kind of stress monitoring is very important in earthquake prediction research.

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