Problem to be solved: to estimate the reflection surface in front of the face with a high accuracy.A plurality of lock bolts are screwed into one another from one end to the wall surface of the tunnel, and the vibration receiving sensor is mounted on the other end surface of the tunnel as a single component sensor having directionality in the long axis direction of the lock bolt.A seismic wave propagating through each rock bolt is detected by each vibration receiving sensor, and the measured waveform is taken out from the waveform data of each obtained seismic wave to a specific frequency range including the center frequency obtained by the filter processing, and the measured waveform is extracted. The waveform of the reflected wave having characteristics similar to the initial motion waveform is extracted from the individual measurement waveforms, and the reflection plane position of each reflected wave is measured based on the travel time difference of each reflected wave.Diagram
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