It is difficult to understand the geological condition accurately ahead of the tunnel face during tunnel excavation. Accordingly, for the safe and economical tunnel excavation, particularly in complicated ground conditions, survey techniques should be effectively used because they would enable us to avoid large costs and unexpected collapses.

In recent years, a seismic refraction survey and a seismic reflection survey, which are conducted on the ground surface and inside a tunnel, has been customarily carried out as a preliminary survey method. However it does not sufficiently provide proper information of faults and fracture zones.

In order to solve this problem, the authors developed the "Tunnel to surface seismic tomography", which is the new survey method using seismic tomography technique between the inside of a tunnel and the ground surface above the tunnel. This new method enables us to grasp P-wave velocity and more proper prediction over a wide area ahead of the tunnel face.

In order to conduct seismic tomography analysis, arrival time of seismic wave has to be precisely calculated and, for that reason, the wave -generation and reception systems must be synchronized with each other. It is the simplest to use wired channels, but, in reality, it is difficult to synchronize them with wire connection because the receivers on the ground are usually located far away from the source points inside a tunnel. The authors consequently developed a new time synchronization system using GPS satellite time with wireless data transmission solutions. Furthermore, the authors have improved this system so that vibrations from blasting for excavation could be used as a survey source.

This new system contributed to extending the survey limit for depth and improving the survey precision. Furthermore, it is remarkable that this system can be performed without interrupting tunnel excavation.

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