Study On Seismic Wave Simulation Based On Orthogonal Curvilinear Mesh Under The Complex Geological Conditions

Numerical simulation of seismic waves is an efficient tool to study the propagation law of seismic waves,whose results have great theoretical and practical significances for the processing,interpretation and inversion of seismic data under various complex conditions.Among the various methods of numerical simulation of seismic waves,the finite difference method,whose features such as flexibility,high efficiency,and good applicability are valued by the researchers and engineers,has been one of the most popular methods up to now.However,when simulating the seismic waves under the condition of complex boundary,the subsurface curved interface and rugged topography can only be represented by staircase lines if the regular grid in the Cartesian coordinate system is employed.Thus,the spurious diffractions will be caused at this kind of boundary,which will reduce the precision of numerical simulation.Furthermore,when performing the numerical simulation in the media containing surface low-velocity zone or low-velocity layer,the grid interval needs to be set very small to guarantee the computational precision and stability if the global grid is adopted.Therefore,the memory requirement is increased when the computational efficiency is reduced.For the above limitations of the finite difference method,this paper makes improvements from the following aspects.(1)The improved Ryskin and Leal(RL)orthogonal body-fitted grid is adopted to implement the mesh generation for the rugged topography model.Compared with the conventional staircase grid approximation method,the body-fitted grid can not only suppress the spurious diffractions caused by the staircase approximation efficiently,but also improve the computational precision.(2)The adaptive mesh generation technique is introduced to the grid division of the complex inner boundary.The adaptive mesh generation technique can automatically adjust the density of the grids according to the physical parameters of the model.Consequently,the computational precision can be improved without increasing calculated amount.(3)The curvilinear grid is combined with the local adaptive grid to avoid the problem of spatial oversampling.(4)The tensor wave equation which is more suitable to the curvilinear coordinate system is adopted as the control equation of numerical simulation of seismic waves under the complex geological condition.(5)To avoid the problems of the requirement of interpolation and the low precision for staggered grid in the curvilinear coordinate system,the high order selective filtering non-staggered finite difference method is adopted to solve the tensor wave equation.(6)The traction image method is used to implement the free boundary condition.(7)The auxiliary differential equation PML(ADE-PML)in the curvilinear coordinate system is employed to eliminate the artificial boundary reflections.At the end,this paper adopts the above methods to perform the numerical simulations in several typical complex geological models and discusses the propagation characteristics of the wavefields in them.