【摘要】 纵横波波场分离是弹性波偏移方法的必要条件,通过纵横波成像的差异可以获取更多地下介质的信息.目前所用的纵横波波场分离方法多采用Helmholtz分解,这样得到的波场不仅物理意义发生了变化,振幅和相位也会发生改变.本文采用纵横波解耦的弹性波方程,将其应用于三维介质,对比分析了纵横波解耦方法相对传统Helmholtz分解方法在相位、振幅上的优势.将该解耦的波场分离方法应用于弹性波逆时偏移,能得到相位、振幅和物理意义不受改变的偏移结果.但是该解耦方法分离得到的纵横波波场均为矢量场,将该波场分离方法用于弹性波逆时偏移,还需要解决矢量场如何得到标量成像结果的问题.本文引入了Poynting矢量,通过Poynting矢量对矢量波场进行标量化,这样就能得到保振幅、相位,且无极性反转的标量PP和PS成像结果.同时针对S波Poynting矢量求取不准确的问题,采用拟S波应力场和S波速度场得到了更加准确的S波Poynting矢量.理论计算证明了本文采用的3D波场解耦的矢量波场分离方法的正确性和引入Poynting矢量对矢量波场进行标量成像的有效性.更多还原

【Abstract】 Elastic reverse time migration contains information of not only reflected P-wave but also converted S-waves.Converted S-waves have more information of underground,higher resolution,and wider offset.While to get clean PP and PS images,we need to separate P-and S-waves.The traditional P-and S-wave separation method is based on Helmholtz decomposition.It supposes P-waves are no rotation wavefield and S-waves are no divergence wavefield.So,P-waves are calculated by a gradient operator and S-waves calculated by a curl operator.The phase and amplitude information ofdecomposed waves are changed,because of a space difference operator.We adopt the P-and Swave decoupled method to separate P-and S-waves.We extend this decoupled method to 3 D media and compare with the Helmholtz decomposition method.We find this decoupled method can maintain the phase and amplitude information of separated P-and S-waves.By this decoupled method,the separated P-and S-waves both are vectors.We need to get scalar images from these vector wavefields if we want to use the decoupled method in elastic reverse time migration.We introduce Poynting vector of P-and S-waves to scalarize vector P-and S-waves.Then we use a traditional acoustic reverse time migration imaging condition to get scalar PP and PS images.These images can maintain phase and amplitude information,because we use P-and S-wave decoupled method to separate them.And the imaging results have no polarity inversal problem.But we find the Poynting vector of S-waves are inaccurate because the S-wave stress are interfered by P-waves.So,we use the S-wave quasi-stress tensor to calculate the S-wave Poynting vector.The S-wave quasi-stress are not interfered by P-waves.Finally,the S-wave Poynting vector calculated by S-wave quasi-stress will be more accurate.Numerical examples illustrate the validity of 3 DP-and S-wave decoupled separation method and the accuracy of elastic reverse time migration by the Poynting vector.更多还原