The Numerical Simulation Research On Separation Of Wave Field And Inversion Method In Tunnel Seismic Prediction

The construction of tunnel as the underground projects often encounter a variety of undesirable geological and engineering conditions, such as fault zones, and broken zones, etc.. With the development of modern transportation network of China in recent decades, the number of the construction tunnels increases rapidly, and their scale and buried depth are increase greatly. Thus the engineering geology, hydrological and geological conditions encountered in the process of construction become more and more complex and the tunnel geology prediction has become the fundamental means for the safe construction of the tunnel. Among the tunnel geology prediction methods, the tunnel seismic prediction is the most widely used method to prediction poor engineering geology conditions ahead of the working face based on the differences of elastic properties which can cause the variation of the seismic wave field excited by explosive sources.This paper first presents the construction of wave equation with three basic equations: constitutive equation of elasticity, the Cauchy equation and Navier equation. Then by analyzing the principle of finite element method, the study for numerical simulation of the elastic wave field method with finite element method is carried on. The process from the dynamic problems of elasticity to wave field is analyzed, and a2D numerical simulation method of seismic wave field is built using ANSYS software and founded the numerical simulation research of the tunnel seismic field. Numerical simulation of the wave field deepens the understanding of the field features in the tunnel. Based on the numerical simulation data of layered geological models with various dips and the principle of the tunnel seismic reflection method, the research of the wave field separation is conducted using the method of the differential operators, motion product, Radon transform and F-K filtering; Finally, a inversion algorithm for velocity in the depth domain based on the1D wave equation is put forwarded. The following is the main research contents and conclusions:(1)The simulation techniques of seismic wave field is realized by the finite element soft-ware ANSYS. And the key technology of the element type and mesh size in the numerical simulation is analyzed. The effect of dispersion can be eliminated with the mesh size is less than1meter for the field condition simulation. With viscous elastic boundary loading, the boundary reflections can be effetely canceled.(2)The tunnel seismic wave field is simulated and the wave field propagation characteristics of both the TSP measure system and multi-channel tunnel seismic prediction (TMS) are analyzed. The numerical results show that the tunnel seismic wave field is often a quit complex wave field with multi-wave (P-wave, S-wave and surface wave) in multi-components reflected from multi-direction. Although due to the differences of propagation speed and direction, the time-distance curves of multi-component and multi-direction of the reflected wave seismic wave field may be hyperbolic, linear, and other types with a linear measure array, the curves can be well approximate by a linear type as the hyperbolic radius is very large and the curvature is very small because the array length is relatively shorter than prediction distance for the observation space is limited inside the tunnel. the slope of the linear relationship is related to the propagation velocity and direction of the wave field. (3)The conventional seismic wave field separation methods and algorithms are analyzed and applied to the wave field separation of the tunnel seismic wave field based upon the numerical models. The results show that differential operator (or the wave equation) method can well separated P-wave and S-wave field, and is only suitable for the simulated data. Due to the characteristics of the calculation. Movement product method is also a good method for the separation of P-wave and S-wave, but the require a very high S/N ratio of data(4)The Linear on Radon (τ-p) transform can well separate the curve as a straight line or approximated linear one. The research results show that it is an effective method for the separation of both multi-direction and multi-wave to extract P-wave and S-wave reflection events from steep inclination layers in front of work face of the tunnel with dips varying among the900-450Range. Similarly, F-K filtering is also an ideal method for the separation of straight line or approximated linear one, it’s resolution is however related to the signal frequency because of FK filtering using a fan filter area.(5)Hyperbolic characteristics of the interference waves, and the strong energy and high apparent velocity of S-wave can all interfere the separation of the P-wave. On the other hand, S-wave can have the best separation for its low velocity and high energy. Therefore, S-wave profile is of higher ratio of signal to noise and much reliable for in subsequent processing and interpretation.(6) Staggered grid finite difference method for the numerical solution of wave Equation is discussed and the PML boundary conditions analyzed. The numerical simulation differen-ces of the wave field by the finite element method and the staggered grid finite difference method is compared. The latter is, though being a single-wave field method, is more appropriated for a forward model in a inversion for its high calculation effective.(7) Based on wave equation forward modeling shoveled by the staggered grid finite difference method and genetic algorithm for global optimization, a depth (mileage) domain of one-dimensional velocity inversion algorithm is put forward. By analysis and discussion of the influences of various parameters on the inversion algorithm with numerical models, the optimal inversion parameter combination is obtained. And validity and accuracy of the inversion algorithm is verified by the numerical models.(8)Finally,by a successful field application of the tunnel seismic prediction to the Liu Jia Chong tunnel with a length about7km, at Xiang-Gui railway, the practical feasibility and credibility of the wave separation method and inversion algorithm studied in the paper are verified. And also, a practical and flexibility tunnel seismic prediction system with acquisition hard wear and processing and interpretation soft wear are established.