Technical Contributions in this Dissertation

Chapter 2 presents the noval technique of multisource least-squares migration to efficiently produce high quality images. This algorithm is implemented with Kirchhoff migration method and tested with 320 synthetic shot gathers for the 2D SEG/EAGE salt model. An accurate image is obtained by migrating a supergather composite of all these 320 shot gathers after 60 iterations. Compared to conventional Kirchhoff migration image, the I/O cost of MLSM with static encoding is reduced by 320 times. The MLSM image is much more resolved than conventional Kirchhoff migration image, because the migration artifacts are suppressed, the reflector amplitudes are balanced, the image resolution is enhanced and the crosstalk noise is reduced. Two types of encoding strategies are proposed: static encoding and dynamic encoding. Their performance in crosstalk-noise reduction is studied with the measurements of signal-to-noise ratio of migration images. For the 2D SEG/EAGE salt model example, The MLSM algorithm with static encoding enjoys lower I/O cost compared to the MLSM with dynamic encoding, but the empirical results show that the MLSM with dynamic encoding, on the other hand, is more effective in reducing crosstalk noise introduced by blended sources.

In Chapter 3, the multisource least-squares migration algorithm is implemented with the reverse time migration method and the blended sources processing technique can increase the computational efficiency significantly. To fix the problem that randon encoding strategy is not applicable to marine-streamer data, the plane-wave encoding method is adopted. When an ensemble of prestack images is incorporated into the inversion, the prestack plane-wave least-squares reverse time migration show the following advantages: (1) stable convergence even with velocity errors up to 5% in my example and (2) the common image gathers are available for quality control and migration velocity analysis. I conclude that the least-squares reverse time migration in the plane-wave domain can be an efficient method to improve the quality of RTM images and produce common image gathers for MVA.

In Chapter 4, I proposed a new method for migrating prism waves by RTM. There are two steps to the method: (1) Conventional RTM is applied to the data to estimate the geometry of the horizontal reflectors near the salt flank; (2) Prism wave RTM is applied to the data again, except the prism imaging condition is used rather than the conventional one. A high quality image is obtained by summation of two partial migration images: one from conventional RTM and the other from the migration of the prism waves. The empirical results suggest that the proposed method can migrate the prism waves correctly to delineate salt flanks and improve the image quality.