| In this paper,the discontinuous finite element method is used to simulate the wave equation of the geological model with complex surface relief(the height difference reaches thousands of meters).The traditional finite difference method can only adapt to the simple undulation(height difference around hundred meters)geological model because of using the rectangular grid to simulate the wave equation.When facing the undulation geological model,the problem of artificial diffraction will be caused because of the step-like approximation of the undulation curve by the rectangular grid,resulting in interference waves and errors in the simulation results.The traditional finite element method is a method suitable for simulating the undulating surface model.This method uses unstructured triangular meshes to describe the undulating curve well,so that the mesh can be divided better.However,the method brings a huge amount of calculation and needs a large amount of storage,so it is difficult to carry out the calculation of the large model.Compared with the above two disadvantages,the discontinuous finite element method can give full play to its advantages,that is,it can depict the undulating curve well,and because of the discontinuity of the method,it can reduce the amount of calculation storage,and it can be better with parallel computing.The calculation efficiency is improved,so that the method can be used for large-scale wave equation simulation.In this paper,the optimization method of PML absorbing boundary condition is proposed to solve the problem of wavefield absorbing in complex surface relief geological model.The surface undulations geological model will cause the problem of uneven thickness of absorption boundary,especially in the case of severe undulation.Conventional PML absorbing boundary conditions,In the face of the above problems,boundary reflections with reflected energy close to the effective wave field energy will occur due to the poor absorbing effect,which will affect the simulation results.The optimized PML boundary condition has two advantages:one is that the boundary region of the undulating surface can be set as required,and the boundary thickness can be adjusted;the other is that the absorbing boundary with different thickness can still be effectively absorbed,and the the boundary reflection energy is very weak.In this paper,the full waveform inversion method is selected for wave equation inversion.Compared with the inversion method which only uses the single information in seismic data,such as Phase Inversion,Travel-Time Inversion and Amplitude Inversion,this method uses more dynamic information and gets higher resolution of inversion results,but also brings the problem of time-consuming.Based on the study of previous studies,this paper will choose to use the grid conversion method to transform the triangular grid to the rectangular grid,so that the discontinuous finite element method forward program can be applied to the full waveform inversion program.In addition,grid partition will be used to generate data,and the parallel mode of MPI and Open MP will be used in the program to improve the the efficiency of full waveform inversion. |
