Forty surveys using Tunnel Reflector Tracing (TRT) developed by C-Thru Ground (CTG) were conducted in Brazil over 3.8 km of the TBM-driven tunnel, excavated mostly in granite-gneissic rock. Each survey extended around 200 m ahead of the face. A hand-held swept frequency source was applied at the rock surface through openings in the TBM shield. Seismic S-waves were measured by an array of accelerometers coupled to the tunnel walls within 20 m behind the sources. Later, the TRT images were compared to the TBM data, the rock types, and the water discharge locations. Approximately 90% of significant features were predicted by the surveys including an anomaly which produced around 3000l/min inflow of water. It was first detected approximately 175 meters ahead of the TBM, and confirmed when at 51 m ahead. The analysis also included: differences in wave polarity, a significance of adjusting the attenuation correction between surveys to accommodate for changing features, particularly for longer image ranges, and for detecting smaller features near larger anomalies. Prospects for assessing wave velocity ahead of the face were also investigated.
Safe, efficient, and economical are the ultimate goals for every tunneling and mining project. And being able to visualize and evaluate ground features in near-real time in front of an advancing tunnel excavation and with negligible intrusion is the Holy Grail for the tunneling and mining community. This is particularly important for modern, fast, but expensive tunnel excavation projects using TBMs. Seismic waves offer one of the most effective means for exploring and mapping the rock mass in 3D. However, specifically when a single tunnel is being excavated, the direct and quick access to the ground for conducting seismic surveys is effectively limited to the perimeter of the tunnel walls.