| The safe construction and disaster prevention of tunnel construction is of great significance.Tunnel advance detection technology has been an important method to determine the structure of the complex geological body in front of the tunnel face.Among the tunnel advance detection technologies,the seismic method is one of the most accurate methods that have long detection distances.In seismic tunnel advance detection,the cylindrical configuration will aggravate the complexity of wave field in tunnel space and strongly influence the accuracy of detection results.In this situation,it is crucial to simulate an accurate seismic full-wave field of tunnel space and to understand the propagation and wave field characteristics of individual seismic waves for seismic tunnel advance detection.It also provides a technical and theoretical basis for the optimization of the observation system and data processing interpretation of seismic-type tunnel advance detection methods.Usually,in 3D Cartesian coordinates,the tunnel wall is approximated with a staircase boundary,but it is not accurate enough in shape and generates numerical dispersion in simulation,especially with the existence of surface waves.On the contrary,cylindrical coordinates are more adaptive in the tunnel cavity description and allow to use larger spatial grids.Therefore,a variable staggered-grid finite-difference method in cylindrical coordinates is developed and applied in 3D tunnel space.After obtaining the accurate record of seismic wavefield,it is critical to image geologic bodies using an appropriate method for tunnel advance detection.Reverse-time migration is one of the most accurate imaging methods.However,storing source-side wavefields for complete spatio-temporal sequence in 3D space requires huge storage space,which is often unable to be stored completely in large models.On this basis,the reconstruction and injection method of seismic wavefield based on the representation theorem in cylindrical coordinates is developed and applied to reverse-time migration in 3D tunnel space.Based on the finite-difference method in the traditional cylindrical coordinates,this thesis realized seismic full-wave field simulation in 3D tunnel space by eliminating the discontinuity of the pole,setting the variable grid and free boundary according to the practical requirements of the seismic wave-field simulation in tunnel space.Based on reverse-time migration and seismic wavefield reconstruction method in Cartesian coordinates,a reverse-time migration based on the seismic wavefield reconstruction in cylindrical coordinates system is proposed for the tunnel space,and seismic tunnel advance detection based on reverse-time migration is realized.The main results achieved in this thesis are as follows:(1)The accurate propagation operator of seismic wave field with variable staggeredgrid finite-difference method is constructed.A discretization scheme for the variable staggered-grid elastic wave finite difference operator in cylindrical coordinates is constructed based on the elastic wave equations.The problem of drastic radial grid size variation in cylindrical coordinates is solved by using triple variable grid,which effectively improves the stability of the method and reduces the computational consumption.The interference of the instability and discontinuity of the pole axis on the seismic wave field simulation is eliminated by the Rotate Cartesian coordinates method in our scheme.The effectiveness of split and multi-axis perfectly matched layer absorbing boundaries is demonstrated by theoretical derivation and numerical calculations.For the tunnel cavity,free boundary condition is set using the improved vacuum method to solve the surface wave propagation problem on the tunnel wall and face,and the accuracy of the free boundary setting is verified,and then the seismic full-wave field simulation in tunnel space is realized.(2)Seismic full-wave field simulation in 3D tunnel space was realized.According to practical engineering situations,models containing three typical geological bodies in front of the tunnel face are designed and its seismic wave propagation and wave field characteristics are analyzed,concluded that common-source system with shots near the tunnel face is more conducive to obtaining reflection waves.Compared with the simulation results in Cartesian coordinates,results in cylindrical coordinates show that numerical dispersion is negligible,and conclude that a higher signal-to-noise ratio and more accurate seismic wave field can be simulated with cylindrical coordinates in tunnel space.(3)Reverse-time migration in cylindrical coordinates based on seismic wavefield reconstruction was realized.The seismic wavefield reconstruction method based on the representation theorem was introduced into the cylindrical coordinates system,and the effectiveness of the wavefield reconstruction in the cylindrical coordinates system was verified.The accuracy and artifact level of the traditional receiver-side wavefield injection method and injection methods based on the representation theorem were analyzed and discussed,and the effectiveness of injection method based on the representation theorem was verified.The merits and demerits of cross-correlation multicomponent wavefield and vector imaging conditions in reverse-time migration were compared and analyzed,the causes of artifacts in imaging results were analyzed,and methods for artifact suppression was given.Based on the seismic record and theoretical model of wave field simulation in tunnel space,a reverse-time migration process based on the seismic wave field reconstruction in cylindrical coordinates was constructed and the effectiveness of this method was verified.The tunnel advance detection based on seismic wavefield reconstruction with body wave reverse-time migration in cylindrical coordinates is realized.The main innovations of this thesis are as follows:(1)The radial variable grid and free-surface boundary condition with the improved vacuum formulation are introduced into cylindrical coordinates,and the numerical simulation method of finite-difference seismic full-wave field based on variable staggered-grid is proposed.(2)The seismic wavefield reconstruction and injection based on the representation theorem is introduced into cylindrical coordinates,and body wave reverse-time migration imaging method in cylindrical coordinates based on the seismic wavefield reconstruction is proposed. |
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