DMO and multiples
Multiples can be enhanced or attenuated by DMO correction. Consider the situation depicted in the velocity spectrum shown in Figure 5.2-25. Suppose you have a flat primary and a flat multiple. After DMO correction, the velocity contrast Δv between the primary and the multiple would not change; therefore, DMO correction does not have any impact on the success or failure of a multiple attenuation technique in such a case. A similar situation exists for a dipping primary and a dipping multiple. If you have a dipping primary and a flat multiple — a case where the multiple associated with a flat water-bottom interferes with a deeper dipping primary, then, after DMO correction the flat multiple will not be affected. However, the dipping primary will shift to the left on the velocity spectrum, causing a decrease in the velocity contrast between the two events. This suggests that multiple attenuation based on velocity discrimination between primaries and multiples would be less effective on DMO-corrected gathers. Finally, the situation would favor multiple attenuation when you have a flat primary conflicting with a dipping multiple.
A field data example is shown in Figure 5.2-26. Note the conflicting primary and multiple events below midpoint 1716 at 1 s on the section without DMO correction (Figure 5.2-26a). In this case, following DMO correction, the velocity contrast between the primary and multiple event has increased, and thus has led to attenuation of the latter during stacking of the DMO-corrected gathers (Figure 5.2-26b).
Figure 5.2-25 A velocity spectrum which indicates a primary event (P) and a multiple event (M) arriving at about the same zero-offset time. The multiple event is associated with a shallow primary. Depending on the flat or dipping character of the primary and multiple reflection, the velocity contrast Δv between the two changes after DMO correction (see text for details).
The practical question as to whether a multiple attenuation technique should be applied before or after DMO correction is an important one. This question is relevant only for multiple attenuation techniques based on velocity discrimination (noise and multiple attenuation). Often, for reasons of efficiency, the multiple attenuation step precedes DMO correction. This is especially the case in processing 3-D data — following 3-D DMO correction, data often are stacked concurrently without creating DMO-corrected gathers.
- Salt flanks
- Fault planes
- DMO and coherent linear noise
- Other considerations
- Aspects of DMO correction — a summary