Reverse time migration (RTM) using a two-way wave equation is able to generate high-quality images of the subsurface. Besides the ability to image complicated geological features, image artifacts can be observed in elastic P and S wave images, where high velocity contrasts/gradients in the velocity model produce back-scattering reflections during wavefield extrapolations. Separating RTM images into components based on wavefield propagation directions is able to effectively remove such image artifacts. We first compare image separation methods, including up/down image separation during, and post, imaging. The post-imaging method is easy to implement using existing RTM software, as it requires only simple operations applied to the stacked RTM images. The separated up/down images are similar to those separated during imaging, but there are also differences. The differences come from the different physical meanings of the up/down separated images. The during-imaging methods separate RTM image into its up/down components using propagation directions of the individual source/receiver wavefields, while the post-imaging method separates images based on the vector sum of the source and receiver wavefield directions. Inspired by these differences, we develop a composite workflow that combines a cost-efficient up/down imaging condition and a post-imaging separation step, which provides an alternative solution for obtaining up/down separated images from full waveform imaging. Numerical example using an elastic version of the Sigsbee 2A model indicates that the up/down separated images from the proposed workflow are effectively equivalent to those obtained using the during-imaging separation methods. More interestingly, we show the pros and cons in the imaging results from different up/down separation methods, especially in converted P–S images.

中文翻译：

---

弹性逆时迁移中的上/下图像分离

使用双向波动方程的逆时偏移 (RTM) 能够生成高质量的地下图像。除了能够对复杂的地质特征进行成像之外，还可以在弹性 P 波和 S 波图像中观察到图像伪影，其中速度模型中的高速对比度/梯度在波场外推过程中会产生反向散射反射。基于波场传播方向将 RTM 图像分成多个分量能够有效地去除此类图像伪影。我们首先比较图像分离方法，包括成像期间和成像后的上/下图像分离。使用现有的 RTM 软件很容易实现后成像方法，因为它只需要对堆叠的 RTM 图像进行简单的操作。分离的上/下图像与成像过程中分离的图像相似，但也存在差异。差异来自于上下分离图像的不同物理意义。成像过程中的方法使用单个源/接收器波场的传播方向将 RTM 图像分离为其上/下分量，而后成像方法基于源和接收器波场方向的矢量和来分离图像。受这些差异的启发，我们开发了一种复合工作流程，该工作流程结合了经济高效的上/下成像条件和后成像分离步骤，为从全波形成像中获得上/下分离的图像提供了替代解决方案。使用弹性版本的 Sigsbee 2A 模型的数值示例表明，来自所提出的工作流程的上/下分离图像实际上等效于使用成像期间分离方法获得的图像。更有趣的是，我们展示了不同上/下分离方法的成像结果的优缺点，特别是在转换后的 P-S 图像中。