Focused Imaging Method Based on Tunnel Borehole Acoustic Detection for Adverse Geological Bodies Ahead of the Tunnel Face and Field Application

Abstract

Accurate identification of concealed geohazards around tunnel advance boreholes is crucial for refined geological prediction and risk control in tunnel construction. This study proposes an acoustic imaging method based on tunnel borehole acoustic detection to detect small anomalies around the borehole, including fractures, cavities, and fractured zones containing water. The method establishes a delay stacking imaging framework driven by traveltime calculation, in which statistical outlier removal and semblance coherence weighting are integrated to suppress incoherent noise, reduce artifacts, and enhance weak reflected signals. Numerical simulations demonstrate that the proposed method produces clearer focusing and more reliable anomaly localization than conventional diffraction stacking. Despite the inherent azimuthal ambiguity of dipole imaging based on a single borehole, the method can effectively determine the axial depth and radial distance of anomalous zones, thereby providing useful constraints for targeted drilling verification. Field application in a deeply buried tunnel shows good agreement between the imaged anomalous zones and subsequent drilling results, which revealed fractured and water rich rock masses. These results indicate that the proposed tunnel borehole acoustic detection method provides a practical high resolution tool for refined advance prediction and risk control in tunnel construction.