Abstract: Three-dimensional (3D) tunnel geological advance prediction methods currently face several challenges: (1) The arrangement of shot and geophone points is complex and nonlinear, preventing the use of directional filtering techniques to extract reflected waves in the advance direction, which results in a low signal-to-noise ratio (SNR); (2) the direct wave arrival is unsuitable for estimating the surrounding rock velocity; and (3) geophones are positioned on the tunnel walls, far from the tunnel face, leading to low illumination energy for the target body in the forward direction, which decreases the accuracy of anomaly imaging. To enable true 3D fine detection and imaging of unfavorable geological bodies in the tunnel advance direction, the author proposes a 3D seismic advance prediction method for tunnels with wide illumination and high SNR. A 3D symmetric observation system (STSP) is designed, and its advantages are evaluated through numerical simulation, illumination analysis, and 3D reverse time migration method. Results from the analysis of irregular karst cave and weak interlayer models highlight the following advantages of STSP: (1) The shot points in 3D STSP are arranged linearly, which facilitates extracting reflected waves in the advance direction via directional filtering, resulting in high data SNR. (2) In terms of illumination energy and range of the target layer, 3D STSP outperforms 3D true reflection tomography (TRT). (3) 3D STSP provides high offset imaging accuracy, whereas TRT shows insufficient accuracy in imaging the inclination and trend of geological bodies.

Key words: tunnel, three-dimensional (3D) seismic advance prediction, 3D symmetrical observation system, seismic illumination analysis, 3D reverse time migration