Finite-difference Method For Tunnel Seismic Prediction

With the rapid development and expansion of economy, governments aim to improve the infrastructure such as the traffic as a key to the sustainable development of economy. The construction of high-speed railways and highways is an important contribution towards achieving this goal and have made great progress and achievements recently under the good condition of economic development. Railway tunnels and highway tunnels grow with remarkable speed in recent years, especially railway tunnels. With the implementation of National high-speed railway development strategy, the number and the length of railway tunnel has increased significantly.Tunnel engineering belongs to the underground construction engineering and the geological condition of surrounding rock is complicated. Dangerous accidents and expensive delays can result if major voids, water-filled fractures, or cataclastic or breccia zones are encountered unexpectedly during tunneling. Considering the economic and technical limitations, it’s very difficult to predict all characteristics of the adverse geologic bodies accurately only by geological survey on the surface of the earth. In order to avoid engineering disaster, Swiss Amberg Measuring Technology Co., LTD had develop a set of forecasting instrument – TSP(Tunnel Seismic Prediction). The instrument has the characteristics of long detecting distance and high resolution by using the theory of reflection seismology. The prediction method is robust, that does not disrupt the tunneling process, and that yields results quickly and at moderate costs. The TSP often working in complicated geological conditions, as a result, the transformation of seismic wave is very complicated, which leads to the seismic record complex. For karst, which does not meet the basic condition of reflection wave seismic prospecting, the recognition rate of TSP is often lower than usual and even could do nothing.Seismic modelling proves very helpful in supporting the interpretation of TSP or other tunnel seismic data and in validating reflection imaging. Modelling is particularly important in tunnel seismology because poor spatial resolution my often leave room for ambiguities and tunneling noise often compromises data quality. A technique that is suitable for tunnel seismic modelling must be able to treat arbitrarily complex geologies correctly, in particular, seismic interactions at heterogeneities at free surface of the tunnel, must be modelled correctly for arbitrary measurement configurations. The finite difference method suit the situation since they are easy to use and their computation times are only affected by the model size.Based on the previous research, deriving the center finite difference of displacement equation and the any order space and accurate time formulation of oneorder stress-velocity equation. Using perfectly matched layer to avoid wave field being interference by artificial boundary. Treating the tunnel wall as free surface, which is an important topic in tunnel seismic modeling. In this paper, these methods are discussed, as well as the stability and dispersion, and using C-plus-plus language write a software which can do 3D full wave filed simulation.Through the above theoretical basis, using software to do forward modelling, and compare the result with the inversion result. The result shows that this method has high accurate, and is suit for complex medium model. This method can provide a solid theoretical basis for the interpretation of tunnel seismic data, and has important scientific significance for improve the accuracy of tunnel forecast.