抄録
Continued to the previous work at Iwatsuki the similar experiment on the shear wave velocity measurement in deep soil deposits was carried out in the earthquake observation well at Shimofusa, Chiba Prefecture, which was constructed by the National Research Center for Disaster Prevention of Japan.
Shear waves were produced by means of the SH wave generator on the ground surface. In 15 different depths at intervals of 100-250m, sequent measurement down to the borehole bottom with depth of 2.3km were conducted by using a set of three component geophones installed in the specially designed capsule with a clamping device to the borehole wall. The generated SH signals could clearly be detected as deep as the bottom.
The obtained velocity profile is essentially composed of three layers and substratum of which shear wave velocities are, from the top to the bottom, 0.40km/s, 0.82km/s, 1.20km/s and 2.60km/s respectively. The disclosed discontinuities by this experiment agree completely with the known data such as sonic log, density and geologic profile etc. A comparison with those velocities at Iwatsuki was also made as the first step to visualize the underground structure in Tokyo area three-dimensionally.
