To address low signal-to-noise ratio of seismic data and poor target visibility in underground engineering applications such as coal mining, hydraulic engineering, and subway construction, we propose a reverse time migration imaging method for tunnel ahead detection based on time-delay stacking. The method designs the relative time delays among the sources in a source array, so that the seismic wavefields generated by the individual sources are aligned in phase along a prescribed direction or at a target point, thereby enhancing coherent signals in reverse time migration imaging. The delays can be implemented physically by controlling the actual source firing times or mathematically by applying corresponding time shifts to independently acquired shot records; the former corresponds to physical implementation, while the latter corresponds to mathematical implementation. Numerical experiments adopting the latter implementation show that compared with the conventional prestack reverse time migration method, the proposed method further improves the signal-to-noise ratio of migration results and enhances diffraction-wave signals, providing a potential approach to tunnel ahead detection imaging under strong-noise conditions. 

underground engineering, tunnel ahead detection, time-delay stacking, reverse time migration, signal-to-noise ratio