We present a new hybrid approach to simulate acoustic wave propagation in the presence of free surface topography. The method combines spectral-element method (SEM) with an eight-order finite-difference method (FDM) to solve the second-order acoustic wave equation. The SEM is applied to the free surface where the FDM has difficulty accounting for the boundary conditions. The SEM deals with free-surface boundary conditions naturally and leads to the highly accurate modeling of free-surface topography. The FDM is used to propagate waves in the interior regions where the eight-order FDM is computationally more efficient than the SEM. To couple the two methods, an interface is considered between the regions. Modeling is carried out using the two methods in this combinational region. The accuracy of the hybrid method is studied by comparing it with staircase and embedded boundary modeling methods for free-surface topography. The main difficulty of combining the spectral element method with finite difference method is the mesh generation, which we resolve it in this paper by proposing two simple solutions.


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