CDP stacking and NMO velocity
So long as layer interfaces in the earth's subsurface have only moderate curvature, CDP reflections for primaries and symmetric multiples fall upon (symmetric) CDP reflection time curves that, for small offsets, are approximately hyperbolic. In contrast, the CDP reflections for asymmetric multiples need not have their apexes at zero offset (Levin and Shah, 1977).
It is difficult to conclude from one CDP reflection time curve alone whether the geologic interfaces are moderately curved or plane-dipping reflectors. So long as lateral velocity changes within layers can be excluded or presumed known the fact that interfaces are dipping or curved can be deduced from the normal reflection time curves of one seismic profile or from neighboring CDP gathers. The more the subsurface is approximated by a homogeneous plane-layer model, the better the CDP reflection time curves approximate hyperbolas.
For CDP reflections from reflectors that are not too shallow, the dynamic characteristics (i.e., amplitudes and wave shapes) generally change little with shot-geophone offset. At earlier times, where critical angles and large deviations from hyperbolas occur, these characteristics are more variable.
CDP stacking, also known as horizontal stacking, involves summing primaries along their interpreted primary CDP reflection time curves. So long as layers are nearly planar and horizontal, and subsurface velocities tend to increase with depth, CDP reflection time curves for multiples will be more curved than those of primaries having the same two-way zero-offset times.
The CDP stacking process can be studied from both a signal- and ray-theoretical point of view. In computing
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.”