偏移速度建模与成像技术

Migration velocity modeling and imaging techniques

偏移速度建模的准确性直接影响偏移成像的效果。克希霍夫积分法叠前时间偏移,虽能部分适应速度纵横向的变化,但由于没有考虑射线弯曲,所以在速度纵向变化较大时偏移效果并不理想。而弯曲射线各向异性叠前偏移方法则考虑了成像射线的弯曲及速度各向异性,更有利于消除了地层倾角和反射点弥散的影响,建立的速度模型更准确、更符合地质变化规律。采用沿层速度分析技术获得目的层面上准确的速度信息,通过CRP反偏移速度分析技术逐步优化速度模型,借助弯曲射线叠前时间偏移方法取得显著的成像效果。同时指出了在速度纵横向变化剧烈、地下构造复杂时,深度偏移才能恢复地下真实构造形态。采用相干反演法与均方根速度转换法联合建立层速度模型,利用层析成像技术优化地质模型,通过深度偏移方法研究和偏移效果的对比,总结了建立精确偏移速度场的方法和步骤,使速度建模和偏移成像在效果和效率上都得到了很大的改善。

The accuracy of migration velocity modeling directly influences the effects of migration imaging.Although the Kirchoff integral pre-stack time migration can partially adapt to the lateral variation of velocity,the migration results are not ideal when vertical velocity variation is relatively large as it does not take ray-bending into consideration.In contrast,the ray-bending anisotropic pre-stack migration takes both ray-bending and velocity anisotropy into consideration,thus it can effectively eliminate the influences of dip and reflection point diffusion.The velocity model built with this method is more accurate and better accords with real geologic conditions.The along-horizon velocity analysis method is used to obtain accurate velocity on the surface of the targets and the CRP reverse migration velocity analysis method is used to optimize the velocity model progressively.The quality of imaging is greatly improved by using the ray-bending pre-stack time migration method.Moreover,it is pointed out that only depth migration can restore the true structural features when the lateral and vertical velocity variations are strong and the subsurface structures are very complex.The coherent inversion method and root-mean-square velocity conversion method are jointly applied to build the interval velocity model.The tomographic imaging technique is used to optimize the geologic model.Based on the research of depth migration method and comparison of migration effects,the approaches and steps for building accurate migration velocity field are summarized,thus greatly improving the effects and efficiencies of velocity modeling and migration imaging.