ABSTRACT: Migration of Velocity Spectra: An Example from the Timor Sea

BEASLEY, J. CRAIG, and ROLF KLOTZ, Western Geophysical Company, Singapore

Successful seismic imaging of complex geology such as that found in the Timor Sea requires accurate migration of the seismic data to collapse diffractions, image faults, and position reflectors in the correct spatial location. Although today's migration algorithms in theory generally are accurate, in practice migration accuracy for steep dips critically depends on the migration velocity.

An approach used routinely to estimate the migration velocity is to minimize the effects of dip and azimuth by applying dip movement (DMO) to the data. However, velocity derived from DMO-corrected data is located at an unmigrated position and should be repositioned prior to use as a migration velocity. The effect of this phenomenon will be demonstrated through an example from the Timor Sea in which, prior to migration, reflections and diffractions from a complex system of faulted carbonates conflict with low-relief primary reflections resulting in poorly resolved velocity analyses.

This problem can be overcome by employing conventional migration to migrate the DMO velocity to the proper spatial location prior to migrating the seismic data. Velocity spectra are generated first from DMO-corrected data on a regular spatial grid and then common-velocity slices are extracted and migrated. Finally, the migrated velocity is reassembled at common spatial locations into velocity spectra, which provide a better estimate of the true subsurface velocity.

By applying conventional migration principles to migrate velocity derived from DMO-corrected data, velocity is placed at the correct spatial location as required for migration. The method provides an automated, efficient, and accurate procedure for determining migration velocity.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)