Tunnel Geohazard Evaluation Using Improved Seismic Time-Reversal Mirror

Abstract

Tunnel geohazard evaluation method is an important method for evaluating the geo-hazard during tunnel construction. The seismic method has become the most widely used approach for understanding the geological conditions ahead of the tunnel face. Limited by the small aperture in the tunnel and the little offset of the observation system, migration methods for imaging geological structures are mainly based on Ray theory without using the dynamic information of reflected waves, resulting in less accurate imaging results. To improve the detection accuracy, the time reversal mirror method was studied and improved for better imaging geological conditions ahead of the tunnel face. Specifically, for the positioning error caused by the traditional time reversal mirror method (based on constant velocity), the parameter of variable velocity was added and optimized for the calculation process. By calculating each velocity value in different propagation paths, the reflected waves can focus on the scattering point with a more accurate location. Based on several typical adverse geology numerical simulations proved that the improved time reverse mirror method can get more accurate imaging results than the traditional method, achieving the position and imaging of fault, fractured zone, etc. Then the improved method has been applied in a water supply project and successfully predicted the distribution of adverse geologies ahead of the tunnel face, which further verified the applicability and effectiveness of the improved time reversal mirror method. This paper provides reference and guidance for similar water diversion projects and tunnel engineering geological detection.