Development of seismic observation system in deep borehole

It was found that thick sedimentary structure enables to cause amplification of ground motion during the Niigataken-Chuetsu oki earthquake. In addition, attenuation evaluation from deep basement is important to predict ground motion at surface precisely. Evaluation of these effects due to amplification and attenuation from deep basement is one of important problems to determine design basis ground motion for nuclear power plants. To solve such problem, Japan nuclear energy safety organization started the project to perform vertical-array seismic observation in deep borehole up to 3000 meters depth in Kashiwazaki city, Japan. In this project, we developed seismic observation tools. The remarkable characteristics of tools are as follows: (1) seismometer recordable under high temperature (150 degree Celsius) and pressure (a pressure of 3.0 × 106 kg/m2), (2) seismometer with large dynamic range and relatively broad band frequency ranges (flat amplitude response from 0.2 to 20.0 Hz), (3) cascade type borehole seismometer which enables multi-depth seismic observation in one borehole. We principally introduce these observation system including developed tools for seismic observation in deep borehole.