On3D Elastic Finite-difference Modeling With Complex Structures Using Discontinuous Curvilinear Grid With Non-uniform Time Step

Simulating the seismic waves propagation in a complex Earth model is a fundamental subject in Seismology. finite-difference method is one of popular nu-merical methods, for its robustness and easy implementation, and have be widely used in seismology studies. But the conventional finite-difference method, us-ing uniform grid with constant spatial spacing and marching with an constant time step, usually faces the drawback of over-sampling in spatial and temporal discretizations, which cause vast computing resources wasted, when simulating a model containing complex structures (low velocity zone, topographic free surface). While high velocity contrast or some interesting structures exist in the simulation region, fine spatial spacing and time step are needed to keep the simulation accu-rate and stable or depict the details of the concerned structures. This results in spatial and temporal over-sampling in the other regions of the models. So in order to enhance the FD method capability, more flexible and efficient discretization methods need to be developed.A laterally localized discontinuous grid finite-difference method with nonuni-form time step Runge-Kutta (NUTS-RK) scheme on curvilinear collocated-grid is developed for3D seismic wave simulation. We introduce a transition zone contain-ing two parts, spatial transition zone and temporal transition zone, to handle the problem of wave field exchange across the spatial spacing and time step variation interfaces. A Gaussian filter is implied to suppress the artificial numerical noise caused by down-sampling the wavefield from the fine mesh to the coarse mesh. We extend the nonuniform time step Runge-Kutta scheme to discontinuous grid finite-difference method for further increasing the computational efficiency with-out lossing the accuracy of time marching through the whole simulation region. When the topography presents, we carry out the discontinuous grid method on a curvilinear collocated-grid to obtain an accurate free-surface boundary condition implementation.The results of numerical tests show that, through an flexible discretization, the proposed method can simulate the seismic wave propagation accurately, and reduce the resources consumption significantly.