David Sheley, University of Utah
In this thesis I develop the novel theory of transmitted PS migration and show that PS transmitted arrivals in a Gulf of Mexico VSP data set can be migrated to accurately image a salt sheet even though the receiver array is below the transmitting boundary. I also show that migrating transmitted arrivals is effective in illuminating the base of an ore body invisible to PP reflections. In general, interfaces nearly perpendicular to wavepath propagation, and therefore invisible to PP reflections, can be imaged by migration of PS transmitted waves. These results suggest that migration of PS transmitted waves opens up new opportunities in imaging nearly vertical impedance boundaries that are typically invisible to conventional reflection imaging of crosswell and VSP data.
I also present a new interferometric method, denoted as reduced-time migration, which uses the arrival-time difference between the direct P-wave and subsequent events to increase migration accuracy. Reduced-time migration removes static time shifts in the data, decreases the focusing error due to an incorrect migration velocity model, and relocates reflection or PS transmission events to be closer to their true positions. Synthetic- and field-data examples for crosswell and VSP geometries show that reduced-time migration is noticeably more accurate than conventional migration. This suggests that reduced-time migration may improve the accuracy of migrated images for any data set with static problems or with uncertain knowledge of the migration velocity model.