Numerical Simulation Of Elastic Waves With The Undulating Topography

At the present stage, engineering seismic exploration is mainly carried out in areas with complicated surface condition, in which the rolling surface topography has become a new challenge to the exploration. Thorny problems arising from this surface topography, such as low S/N ratio of seismic data, complex disturbing waves, static correction issue and so forth, must be tackled to acquire high-quality seismic profile. As many conventional seismic exploration theories based on horizontal layered elastic medium are no longer applicable to this complicated mountainous area, the study of propagation regularity and field characteristics of seismic waves in the presence of surface topography is of great instructive value to research applicable to this topography on seismic data collection, processing and interpretation.By adopting staggered grid high-order FD method, this paper makes the numerical simulation of elastic waves under staggered topography condition, during which a combination of high-order FD and FCT is used to the numerical dispersion suppression, and the perfectly matched layer(PML) absorbing boundary is used to eliminate artificial reflections from model boundaries. As numerical simulation in the areas of irregular topography focuses on the free-surface boundary condition, the paper develops an improved vacuum formulation that fully satisfies the free-surface boundary condition by choosing an appropriate combination of the staggered-grid form and a parameter-averaging scheme. It uses a parameter-averaging scheme to deal with elastic parameters on free-surface, and meanwhile adds a fictitious layer above the original topographic model surface, which aims to exclusively make the shear stress simply locate on the free surface after shift and make it zero according to the located parameters in staggered grid. As for velocity, a velocity-averaging method is applied and all the elastic parameters and physical quantities should be set to zero above the free-surface boundary because they are in the vacuum.The simulation is designed against geological model of staggered topography common in engineering exploration. The result indicates that the topography complicates the propagation of seismic waves in near surface, which further arouses the conversion of different waves such as surface wave and body wave as well as generation of scattering along surface. Surface topography also leads to event distort, termination and even submerge of reflected waves, and all together results in the low S/N ratio of seismic data.