GEOLOGICAL PROGNOSIS AHEAD OF A TUNNEL FACE

The locations of a specific geological boundary and aquifer were predi cted, prior to reaching them, by a reflection method and the measureme nt of geothermic temperature. The determination was performed during t unnelling work, which passed through the Median Tectonic Line in centr al Japan. The reflection method procedure was as follows. A P-wave was produced on the tunnel face by a small blasting. The location of the velocity layer boundary was predicted by the travel time of the direct wave and the reflected wave. The geothermic temperature was measured by a thermocouple which was set in the borehole at the face. The locat ion of the aquifer was predicted by the thermal deviation between the measured value and the theoretical value that was determined from a me an geothermal gradient. Also, a water quality survey is performed, dis cussing the quality of the fissure water. In addition, we were able to confirm, using the Seismic Tomography method, that the sheared zone f ound, was the layer including lens-shaped low-velocity zone. The metho ds considered proved to be simple and effective for geological prognos is from within the tunnel.