Migration Velocity Analysis Using Seismic Multiples

C. D. Manuel
Curtin University of Technology, Dept. of Exploration Geophysics, Perth, Australia

Migration velocity analysis (MVA) is an essential component of the velocity model building technique in complex geological areas and depth migration is the process whereby seismic data in the timeoffset domain is mapped to the depth domain. After migration with a certain velocity, the curvature remaining on a primary reflection gives an indication of the variance from the true average velocity of the primary. If migration is performed with the correct velocity then the reflector will appear flat. This is done for each primary reflection to find the velocity structure as a function of depth. The process is repeated for other surface locations until a velocity model is generated for the region.

The present research includes the analysis of multiple reflections in MVA. Any initial error in velocity between the migration and interval velocities will be increased as the distance traveled through the Earth increases. Hence in migration, multiples will be more sensitive than primaries to changes in the velocity model. On a coherency plot, multiples will stack in a more localized region. This should result in increased accuracy in determining interval velocity and interval thickness. It should also reduce the number of migration iterations needed to converge on an accurate solution. This has implications for reducing the time and cost of the MVA process. The ability to create a more accurate velocity model will mean that the depth image obtained from seismic data will be much more accurate. This will in turn enable more accurate positioning of wells.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid