岩盤内の割れ目の分布と音速の変化に関する研究

抄録

A crack-detector using the ultrasonic pulse method has been developed for the borehole measurements, which can obtain the crack distribution and the sonic velocity change in rocks. The BX boreholes required for the measurements are drilled horizontally to the side wall of the opening. The detector is then inserted in a borehole, and the two-pxobes of the detector are pressed against the inner face of the borehole. The sound velocity of rocks between the two-probes is measured by an apparatus for determining the ultrasonic wave velocities. If the cracks exist in rocks between the two-probes, a pulse produced by the ultrasonic wave disappears from the cathode-ray tube included in the apparatus.
The results obtained from the field measurements made at the underground of Yaguki mine. Fukushima prefecture, are as follows;
The cracks are distributed widely in limestone, and it is difficult to find a certain relation between the sound velocity of limestone and the distance from the side wall of the opening. Further, the crack density in the boundary zone between ore body and limestone is much greater than that in ore body.
These observations obtained from the field measurements will be useful for the design of underground structure in rocks.