| Popis: |
We report an example of the evaluation of seismic wave amplification at a very hard bedrock site by various geophysical investigation methods using deep borehole. The study site is located at the base of the Sadamisaki Peninsula in western Shikoku of Japan where the bedrock consists of the very hard crystalline schist of the Sanbagawa belt. We drilled boreholes to the maximum depth of 2,000 m and measured underground rock properties continuously from ground surface to 2,000 m depth by various geophysical logging methods. The S wave velocity structure as determined by the downhole method is 2.2-2.6 km/s (50-620 m), 3.0 km/s (620-1,280 m) and 3.3 km/s (1,280-2,000 m). The depth of 2,000 m is equivalent to seismic bedrock. According to the results of the walkaway VSP and seismic reflection survey using the 2,000 m deep borehole, deep reflectors in this study site are continuing well horizontally, and there is no apparent fault and fold structure. As a result of 2D FEM analysis using the model based on underground rock properties acquired in this study, the transfer function from seismic bedrock (depth of 2,000 m) to the rock near ground surface is around 1, meaning that seismic waves are hardly amplified. In the future, we are going to improve the accuracy of the evaluation of the seismic wave amplification by seismic observation with a vertical array. |
