We propose a full-wave equation based illuminationanalysis method with target oriented capability. The fullwave finite-difference propagator is used to extrapolate the source and receiver side wavefields to the subsurface target area. A time-domain local-slowness analysis method is used to decompose the wavefields into local angle domain. The local illumination matrix can be constructed and different types of illumination measurements can be derived. This illumination analysis method does not have angle limitations. Thus this approach can handle structures with their dipping angles beyond 90 degrees and is particularly useful to provide illumination analysis for reverse-time migration.
The seismic illumination analysis quantifies the capability that an acquisition system can image the subsurface structures. The illumination measurements can be used to optimize the acquisition survey design, evaluate the image quality, and make corrections to seismic image, resulting in more accurate subsurface physical parameter retrieval. Although the early attempts in illumination estimates adopted oversimplified model e.g., homogeneous velocity model, horizontal target and symmetric ray paths, recently it has developed quickly. The illumination calculation can be considered a special type of modeling. First, seismic waves from sources and receivers need to be propagated through overburden structures to reach to the targets. Second, the propagation directions for both waves need be determined at the target location since targets with different dipping angles can only be illuminated by waves at certain directions. Finally, the illumination measurement can be calculated from incident-scattering waves from different directions.
The ray-based methods can handle smoothly varying heterogeneous velocity models and has no angle limitations. Particularly, because the ray method naturally carries the wave propagation directions, it has been widely used for illumination calculations (Schneider and Winbow, 1999; Muerdter and Ratcliff, 2001; Gelius, et al., 2002). However, the high-frequency asymptotic approximation causes intrinsic difficulties for this method being used in complex overburden structures.
Recently, the one-way propagator is widely used in seismic imaging. At the same time, methods specifically for extracting angle information from the wavefield are developed (Xie et al., 2002; Wu and Chen, 2002, 2003; Sava and Fomel, 2003). Combining the two techniques together, a one-way wave equation based method is developed for illumination analysis (Xie et al. 2003, 2005, 2006; Wu and Chen 2002, 2006; Wu et al. 2003; Jin et al. 2003, 2006). This is an efficient approach and is consistent with most one-way wave equation based migration methods. However this approach cannot properly handle the wide-angle signals such as turning waves.
In the last few years, reverse-time migration regains the attention in seismic community partially because it does not have the angle limitation and can properly image the steeply dipping structures with turning waves. It is also a challenge to conduct the illumination analysis that is consistent with the reverse-time migration. To overcome the angle limitation of the one-way propagator, Xie and Yang (2008) proposed a method which uses the full-wave finite-difference method as the propagator and uses a time domain local slowness analysis method to determine the angle information and calculate the illumination.
