Acoustic and elastic reverse-time migration: Novel angle-domain imaging conditions and applications .

摘要

Reverse time migration (RTM) has become a superior choice in seismic imaging due to its capability of handling of rapid spatial velocity variation and imaging without dip angle limitations. By adopting the full-wave propagator, RTM is very accurate to reconstruct the source and receiver wavefields. As the conventional zero-lag cross-correlation imaging condition is applied to the reconstructed wavefields, we can achieve the primary goal of the seismic imaging, which is, to produce a geometrical image of the subsurface structures. However, for further exploration of the subsurface information carried by the reconstructed wavefields, more sophisticated imaging conditions are demanded. For example, to remove RTM low-wavenumber artifacts, we need to incorporate the angle information in the imaging condition. To deal with multicomponent elastic data, we need to separate the wavefields into P- and S- modes before applying the imaging condition. To provide information for migration velocity updating, we need to expand the stacked image into various types of common image gathers, such as in angle, offset, shot, etc. To retrieve the properties of the interfaces, we need to recover amplitude versus angle (AVA) and account for the propagation and acquisition effects in the imaging condition.;Many of those advanced imaging conditions require working in the local angle domain. However, unlike in the ray-based methods, the angle information is not explicitly given in reverse time migration. In this thesis, great efforts are expended to extract the angle information from the migrated wavefields. The local slant stacking is employed to decompose the full acoustic and elastic wavefields into a superposition of localized plane waves and separate the vectorized waves into P- and S-waves. It serves as the fundamental technique in the following part of the thesis to develop angle-domain imaging condition.;In the included chapters, first, we construct local image matrix (LIM) by cross-correlating the decomposed plane-wave components from the source wavefield and those from the receiver wavefield. With the introduction of the angle-domain operators to local image matrix, we remove the strong artifacts existed in PP image and correct the polarization problem in PS image. Second, we generate gathers of various sorts from LIM, such as the common reflection-angle gather (generally called angle-domain common image gather) and common dip-angle gather. The angle-domain common image gather is a powerful tool for migration velocity analysis. Third, we derive the theory of true-amplitude imaging in local angle domain. We calculate the target illumination matrix with the plane wave components of background Green's function under the survey configuration and use it to correct LIM. Similarly, we output dip-angle-dependent illumination (generally called acquisition dip response) and reflection-angle-dependent illumination from target illumination matrix. Fourth, we retrieve the AVA responses by normalizing angle-domain common image gathers with the reflection-angle-dependent illuminations, and cross-plot them to detect the reservoir characterization. Fifth, we compensate the common dip-angle gathers with acquisition dip responses and sum them up to form a true-amplitude image. Due to the tremendous computations involved in the true-amplitude problems, we develop efficient algorithms to make these calculations practical and affordable. Numerical examples demonstrate the applications of angle-domain imaging conditions to angle gather extraction, AVA analysis and true-amplitude imaging.