Jing Chen, University of Utah
Prestack depth migration plays an important role in oil and gas exploration. Currently, Kirchhoff 3-D migration is the most important tool employed by the oil and gas industry. The Kirchhoff migration operator is a weighted integration of seismic trace amplitudes over the seismic data space. In this dissertation, the Kirchhoff migration integral is utilized for the following application: resolutions, stationary-phase migration and velocity analysis.
In the far-field approximation which indicates that the imaging depth is at least greater than the surface recording aperture, the migration integrals are estimated by analytical expressions. The migration response of a point scatterer is obtained, from which the resolution limits and the dynamic ranges of migration images are obtained.
The application of stationary-phase theory to the migration operators reveals that the contribution to an image point is dominated by the seismic energy propagating along the specular rays. A specular ray is a ray satisfying Snell's law at the reflection point. This property of the migration operator provides a tool to identify the specular rays and associated parameters during the migration procedure. The associated parameters such as the traveltimes and trace locations found and are used for both migration and tomographic velocity analysis.
I proposed a migration algorithm which first identifies the specular rays and then migrates seismic energy within the Fresnel zones and rejects the seismic energies outside the Fresnel zones. The Fresnel zones are centered at the specular rays, the migration artifacts are suppressed, and the images have better horizon continuity. Synthetic and field data examples show that the stationary-phase migration can produce migration images with apparently fewer artifacts and better horizon continuity.
Another application of estimating specular ray parameters is found in the tomographic migration velocity analysis scheme. I presented a tomographic migration velocity method in which the raypaths are found through the prestack migration. By the stationary-phase procedure, the depth residual moveout is automatically measured in the migrated common image gathers and is backprojected along the raypaths of the specular rays to update the migration velocity. It uses residual moveout information at every CDP station. Synthetic and field data tests show that this method is capable of improving migration velocity.