Seismic migration is a powerful imaging method developed in the 1960's (French, 1974) to reconstruct the reflectivity distribution from seismic reflection data. Originally developed for common depth point (CDP) data (Hu et al., 1988) it was extended in the 1980's and 1990's to image the reflectivity distribution from both vertical seismic profile (VSP) (Amundsen et al., 1993; Payne, 1994) and crosswell data (Qin, 1994). Although migration of PP reflections is effective in imaging horizontal or obliquely-dipping boundaries, it is notoriously ineffective in delineating near vertical interfaces such as salt flanks or ore body sides.
The inability to image steep flanks is a serious economic liability because oil or minerals are often adjacent to such boundaries. To solve the steep boundary problem I propose migrating the forward-scattered transmitted PS-waves to locate the transmitting boundary. Previous work on forward-scattering by Balch et al. (1991) showed that reverse-time migration of converted waves could image point diffractors in a physical model. This led to the further development of PS-reflection migration of crosswell data, but the imaging of PS transmitted waves remained unexplored.
The earthquake community also uses PS arrivals to image certain reflector boundaries. Using a layered model assumption, earthquake seismologists use P and S transmitted waves from earthquakes to image the Moho, denoting the method as the receiver-function technique (Langston, 1977). Later, Chávez-Pérez (1997) and Chávez-Pérez and Louie (1996; 1998) used forward-scattered P-waves from recorded earthquakes to infer the presence of deep fault-zone diffractors. Chávez-Pérez (1997) proposed using either forward-scattered S-waves or P-SV events to image diffractors but was unable to successfuly do so.
Although these previous researchers utilized PS transmission data, they did not exploit its most useful feature: namely, imaging vertical interfaces with VSP or crosswell data. I will expand on this previous work and define a Kirchhoff migration operator for transmitted converted waves and use it to successfully image transmitting boundaries in both synthetic and field data. This overcomes, to a large degree, the inability of seismic methods to image vertical boundaries.
Other major problems in migration include an incorrect migration velocity model and static shifts in the data. Static shift problems are particularly acute in the mining industry where errors in source initiation time or indeterminate well locations lead to defocusing of the migration image. To mitigate these problems I introduce a new interferometric technique that decreases the sensitivity of migration to velocity model errors and removes static errors; I denote this method as reduced-time migration. I validate reduced-time migration with both synthetic and field data, and show that it can be applied to both transmission and reflection arrivals to improve image quality. By combining these two tools, transmission migration and reduced-time migration, several of the major problems that have long plagued crosswell and VSP migration are now mitigated.
In chapter 2 of this thesis I define both transmission migration and reduced-time migration and show that the imaging-time error will always be less for reduced-time migration. I then test these techniques on synthetic data (chapter 3) and field data (chapter 4). Lastly in chapter 5, I present a summary and conclusions.