Conceptually, migration velocity is similar to stacking velocity. Whereas, stacking velocity best characterizes the normal moveout of reflection times across a CDP gather of traces, migration velocity best characterizes the moveout of diffraction times both across different CDP gathers and across traces within those gathers. Hence, migration velocity should be the appropriate parameter for migrating seismic data. Unlike stacking velocity, migration velocity is insensitive to the dip of the reflector to which it pertains and hence is single valued in space and is spatially more correct. For horizontally layered media, the two forms of velocity coincide.
As indicated in the last chapter, accuracy requirements when stacking velocities are considered solely for the purpose of CDP stacking are generally less stringent than those when the goal is to compute interval velocities. Judging from the analogies between CDP stacking and time migration, one can conclude that a similar relationship exists between the accuracy requirements when migration velocities are used for time migration and those for the computation of interval velocities. Since time migration is still a fairly new seismic process, until now most emphasis in the estimation of migration velocities has been put on obtaining well-migrated time sections (Sattlegger, 1975).
One can anticipate that progress in the use of migration velocities will parallel that of stacking velocities. Undoubtedly, in the future, migration velocities will be considered more often for the computation of interval velocities; the tremenÂ—dous success of the time-migration method appears to be a good omen for this expectation.Computation of
Figures & Tables
Interval Velocities from Seismic Reflection Time Measurements
“Over the years, ray theory has furnished the exploration geophysicist with most of the working tools for understanding and interpreting events observed on reflection seismic sections. Even today, notwithstanding the pace at which the more powerful acoustic wave theory is introducing its new tools, ray theory, in the hands of the authors, retains its preeminence for providing insights into fundamental problems in reflection seismology. Professor Krey's earlier contributions are part of ray theory's rich heritage. Alongside C. Hewitt Dix and Hans Durbaum, he elucidated relationships between interval velocity and observed reflection moveout.”