Mitigating urban tunneling risks: Detecting adverse geology with a tunnel-surface-borehole observation system using TBM drilling noise source

Accurately forecasting adverse geological conditions ahead of the tunnel face is a critical challenge for the safety and efficiency of mechanized urban tunneling. Unforeseen features such as faults and karst cavities often pose significant geohazards, including collapse, ground disturbance, and water inrush. To prevent these geohazards, an innovative geological forward-prospecting method with a tunnel-surface-borehole observation system using TBM drilling noise source was proposed. Unlike conventional surface seismic exploration and in-tunnel seismic advanced prediction methods, the proposed method uses the vibration noise generated during TBM tunneling as the passive seismic source, and synchronously deploys observation arrays on the ground surface and in boreholes to acquire seismic wavefield data. This observation configuration increase the effective seismic array aperture. Moreover, this method obtains the formation P-wave velocity distribution from transmitted P-wave data and the layered S-wave velocity structure of the shallow subsurface using high-frequency multimode surface waves. The constrained inversion integrates the two components and provides complementary constraints for imaging adverse geological anomalies in the near-field region ahead of urban tunnels. Synthetic tests indicate that the representative adverse geology (fault, boulder, etc.) can be imaged, supporting the feasibility of the proposed approach. Furthermore, the method was applied to the Zengchengnan Power Tunnel, China, and identified geological anomalies ahead of the TBM, demonstrating its potential for geological risk mitigation in urban shield tunneling.