On some computational problems related to the thin-slab approximation and the screen approximation
The thin-slab approximation and the screen approximation are two dual-domain methods for imple-menting the De Wolf approximation,and are widely used in seismic wave numerical simulation and migration imaging.There are several computational problems in implementing the two algorithms,such as the space aliasing in the F-K domain,background velocity selection,thin slab thickness division,the calculation of back-ground Green's function in laterally heterogeneous media,and how to deal with lateral heterogeneity.How-ever,there have been no reports on systematic discussions on the aforementioned computational problems.In response to this situation,this paper first reviews the specific realization of the thin slab approximation and the screen approximation method.Secondly,a systematic study is conducted on the aforementioned computational problems in five specific aspects.①The causes of spatial aliasing in the F-K domain are clarified,and the spa-tial aliasing problem is completely solved by use of vertical and horizontal amplitude attenuation factors.②The selection of background velocity is studied,and the key to selecting background velocity is to ensure that the overall velocity disturbance inside the thin slab is relatively small as much as possible.③ Forward record and computational efficiency of the thin slab approximation method and the screen approximation method are com-pared under different thickness conditions of the thin slab,and bases for the division of the thin slab are dis-cussed.④The reasons for the instability of the background Green's function are studied,as well as the computa-tion error in the case of laterally heterogeneous media.⑤The influence of the lateral heterogeneity of the media on the accuracy of the thin slab approximation and the screen approximation is discussed,and several methods are proposed to improve the algorithm operator's adaptability to the lateral changes of the media.Finally,the discussion results are tested by use of a three-dimensional simple layered model and a two-dimensional actual ve-locity model.The research results,to some extent,can provide theoretical basis for the application of the thin-slab approximation method and the screen approximation method in complex media.
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维普
