The tunnel seismic ahead-prospecting method is used to estimate adverse geology ahead of the tunnel face. Accurately realizing this geology helps to guarantee safety during construction using a tunnel boring machine (TBM). The paper proposes a three-dimensional (3D) seismic ahead-prospecting method by optimizing a filtering method and an imaging algorithm. In this paper, a united filtering method based on $f–k$ and $\tau –p$ filtering is introduced. First, the waves from the interfaces behind the tunnel face are removed and P- and S-waves are separated. Then the interference waves are further eliminated by $\tau –p$ filtering. Mirrored artifacts resulting from the equitravel time plane algorithm are suppressed by the directional weighting integrated into the algorithm. Imaging results indicate that the new method suppresses mirrored artifacts better and produces accurate results. Moreover, the improved algorithm produces reflections that are more consistent with the interfaces than those produced by the original algorithm without directional weighting. Field studies using the 3D seismic ahead-prospecting method have accurately identified and located the main fractured zones, proving the reliability and superiority of the method.

Unexpected geologic heterogeneities in tunnel lines often cause safety issues and economic losses during excavation using tunnel boring machines (TBMs). Therefore, collecting geologic information ahead of the excavation front is important to prevent geologic disasters and ensure excavation safety. To predict the geologic conditions ahead of the TBM-excavated tunnel in the water supply project at the Songhua River in China, the tunnel-seismic-while-drilling (TSWD) method was adjusted and then applied. First, common-source gather data were acquired to record TBM drilling noise (pilot signal and seismic waves); a processing approach (including normalization, crosscorrelation, wavefield separation, and migration) was used to image the major relevant geologic heterogeneities ahead of the excavation front. A synthetic study from a numerical model with a fault verified the feasibility of the modified TSWD method to identify and image geologic interfaces. Then, this method was applied to field data resulting from a TBM tunnel excavation project. The field test of the modified TSWD in the TBM tunnel revealed fractured zones, fault boundaries, and a preexcavated tunnel ahead, providing the required geologic information for TBM tunneling and preventing geologically-caused safety issues.

Severe water inrush currently halts the excavation of the Qinling Water Transportation Tunnel and has even stopped tunneling. Based on the geologic conditions of the construction area, the source of water inrush is abundant groundwater in fractured granite. To accurately predict water inrush, we must know the location, geometry, and boundary information of water-abundant zones before excavation. We have developed a field study of integrated seismic and transient electromagnetic (TEM) applications to map the water-abundant zones ahead of the tunnel face. Before the in-tunnel survey, numerical experiments based on excavation and drilling records verified the feasibility of our scheme. To reveal the fractured zones that may be filled with groundwater, an equitraveltime plane algorithm was used in the seismic data processing. Then, the TEM apparent resistivity was calculated to investigate the water-bearing condition of the fractured zones. To detect the water-abundant zone boundaries, we conducted migration imaging of the TEM pseudowavefield that was extracted from the measured TEM signals. A correlation stacking process to extract the TEM pseudowavefield was used to mitigate the ill-posed effect in the inverse wavefield transformation and obtain boundary information of the water-abundant zones. As expected, the results revealed the depth and geometry of the front and rear boundaries of two water-abundant zones 30 m ahead of the tunnel face, which is consistent with drilling and excavation records after the survey. The results of our case study indicate that the integration of seismic imaging, apparent resistivity imaging, and TEM pseudowavefield migration is an efficient and cost-effective method to provide the location, geometry, and boundary information of water-abundant zones.

Accurate seismic imaging can ensure safe and efficient tunnel construction under complex geologic conditions. As a high-precision migration method, reverse time migration (RTM) has been introduced into tunnel seismic forward prospecting. However, the resolution of traditional RTM imaging results may not meet the requirements in a complex tunnel environment, which affects the interpretation of tunnel seismic forward-prospecting results. We have developed a least-squares RTM method based on the decoupled elastic wave equation in tunnels. The Born forward-modeling operator and its exact adjoint migration imaging operator are derived to ensure a stable convergence of the conjugate-gradient method. Moreover, a pseudo-Hessian-based preconditioning operator is adopted to accelerate the convergence. Numerical examples are provided to verify the efficiency of our scheme. A field test in a traffic tunnel construction site is performed to show the good application effect of the decoupled elastic least-squares RTM in practical situations.