A new approach to traveltime selection for Kirchoff migration
Stieglitz, Theodore Charles
Doctor of Philosophy
Prestack migration of seismic data is a dominant tool for imaging hydrocarbon bearing structures in the Earth. The standard prestack depth migration is a Kirchhoff approach using single-valued traveltimes computed from eikonal solvers or ray tracing. In simple structures with only one ray path between a surface location and a subsurface point, the single-valued assumption is valid, and the migration yields an acceptable image. However, in complex structures more than one ray path may exist between any image point and any surface location. Although it would be ideal to use all the ray paths, standard practice is to choose a single ray from the multiple arrivals to represent the wave propagation between two points. Standard choices include the ray having the minimum arrival time, the ray with the greatest amplitude or the ray traveling the shortest distance. We are critical of these standard selection rules as these are local comparisons of individual rays of the multi-valued traveltime operator. Single-valued migration operators based upon these selection rules can lead to uncontrolled jumps between the branches of the multi-valued traveltime operator, resulting in poor image quality. In this work we improve the Kirchhoff approach using single-valued traveltimes. We expect that better images can result from selection rules based on analysis of the full multi-valued operator, selecting rays yielding more continuous, yet energetic single-valued operators. We examine a particular selection rule of this type: choosing rays associated with branches of the multi-valued operator having the maximum angular aperture (MAA) about the image point. We provide several examples in 2D and 3D on synthetic and real data comparing the MAA selection rule with standard selection choices. Our results demonstrate that the MAA single-valued operator performs consistently as well as standard single-valued operators.