DMO and stacking velocities

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Seismic Data Analysis
Seismic-data-analysis.jpg
Series Investigations in Geophysics
Author Öz Yilmaz
DOI http://dx.doi.org/10.1190/1.9781560801580
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store


We now closely examine an aspect of dip-moveout correction in relation to stacking velocities. Consider the CMP stack in Figure 5.0-3a and the DMO stack in Figure 5.0-3b, and note that the steeply dipping reflections are better preserved by the latter. We know from equation (3-8) that the steeper the dip the higher the stacking velocities. Figure 5.0-4 shows constant-velocity-stack (CVS) panels in the neighborhood of midpoint A as denoted in Figure 5.0-3a using CMP gathers without and with DMO correction. Note that gently dipping reflections and associated multiples stack at low velocities, whereas steeply dipping reflections stack at high velocities. After DMO correction, the velocities for the steeply dipping events have been corrected for the dip effect; hence both gently dipping and steeply dipping reflections stack with equal strength at low velocities.

Consider velocity analysis at midpoint A in Figure 5.0-3a where the steeply dipping reflections are in conflict with the reflections associated with the gently dipping strata. The dip effect on stacking velocities can be clearly observed on the velocity spectrum shown in Figure 5.0-5a. The group of semblance peaks on the spectrum in Figure 5.0-5a denoted by A represent the steeply dipping events which conflict with the nearly-flat events as seen in the stacked section in Figure 5.0-3b in the neighborhood of midpoint A. We would have a problem when trying to pick a velocity function from this velocity spectrum. We normally would pick along the predominant velocity trend as denoted in Figure 5.0-5a. This leads to rejection of the picks associated with the steeply dipping reflections and a significant reduction in their amplitudes on the stacked section (Figure 5.0-3a). Following DMO correction, the duality in velocity picks are eliminated, and the velocities are corrected for dip as shown in Figure 5.0-5b. Also, note the improved velocity trend after DMO correction.

Figure 5.0-6 shows a close-up view of the CMP gather at midpoint A as in Figure 5.0-3a without DMO correction, the velocity spectrum, and the gather after NMO correction using the velocity function denoted in the velocity spectrum. The overcorrected events on the moveout-corrected gather represent the steeply dipping reflections, and the undercorrected events represent the multiple reflections.

The close-up view of the CMP gather in Figure 5.0-6 after DMO correction is shown in Figure 5.0-7. Note the more rigorous delineation of the velocity trend in the spectrum as compared to that in Figure 5.0-6. The absence of the overcorrected events in the moveout-corrected gather is a convincing evidence that the velocities of the steeply dipping reflections have been corrected for the dip effect as inferred by equation (3-8). Where are the overcorrected events that we see in Figure 5.0-6c? Are they still present in the moveout-corrected gather after DMO correction shown in Figure 5.0-7c? They should not be, because both flat events and dipping events could not be flattened simultaneously in the same gather. Moreover, the semblance peak in the velocity spectrum denoted by A in Figure 5.0-5a is absent in Figure 5.0-5b after dip-moveout correction. Did it merge with the semblance peaks aligned with the velocity trend denoted in Figure 5.0-5b? No, it did not; instead, the energy represented by the semblance peak A in Figure 5.0-5a actually has moved to another CMP location in the updip direction. So, we begin to have a clue to another aspect of dip-moveout correction — that it may not be a moveout correction after all. We shall examine this clue closely later in principles of dip-moveout correction.

The case in the neighborhood of midpoint A in Figure 5.0-3a is that of reflections with conflicting dips. Often, on stacked data, diffractions and reflections also form conflicting dips. Figure 5.0-8 shows constant-velocity-stack (CVS) panels in the neighborhood of midpoint B as denoted in Figure 5.0-3a using CMP gathers without and with DMO correction. Note that gently dipping reflections stack at low velocities, whereas the diffraction off the tip of the salt diapir stack at high velocities. After DMO correction, the velocities for the steeply dipping events have been corrected for the dip effect; hence both gently dipping reflections and the diffraction stack with equal strength at low velocities.

Consider velocity analysis at midpoint B as in Figure 5.0-3a where the diffraction off the tip of the salt diapir are in conflict with the reflections associated with the gently dipping strata. The dip effect on stacking velocities can be clearly observed on the velocity spectrum shown in Figure 5.0-9a. The semblance peak on the spectrum denoted by B represents the diffraction off the tip of the salt dome which conflicts with the nearly flat events as seen in the stacked section in Figure 5.0-3b in the neighborhood of midpoint B. Again, we would normally pick along the predominant velocity trend as denoted in Figure 5.0-9a. This leads to rejection of the pick associated with the diffraction, which would mean failure of preserving it on the stacked section (Figure 5.0-3a). Following DMO correction, the duality in velocity picks is eliminated, and the velocities are corrected for dip as shown in Figure 5.0-9b.

Figure 5.0-10 shows a close-up view of the CMP gather at midpoint B as in Figure 5.0-3a without DMO correction, the velocity spectrum, and the gather after NMO correction using the velocity function denoted in the velocity spectrum. The overcorrected event at 3.75 s on the moveout-corrected gather represents the diffraction, and the undercorrected events represent the multiple reflections.

The close-up view of the CMP gather as in Figure 5.0-10 after DMO correction is shown in Figure 5.0-11. Note the more rigorous delineation of the velocity trend in the spectrum as compared to that in Figure 5.0-10. The overcorrected event associated with the diffraction is absent in the moveout-corrected gather — once again, convincing evidence that the diffraction velocity has been corrected for the dip effect. The diffraction energy, however, that gave rise to the peak denoted by B in Figure 5.0-9a is absent in the moveout-corrected gather after DMO correction shown in Figure 5.0-11c. Dip-moveout correction, unlike conventional normal-moveout correction, has caused energy to move from one CMP location to another.

Figure 5.0-12 shows time migrations of the CMP-stacked and DMO-stacked sections in Figure 5.0-3. Because the steep events associated with the diffraction energy and the dipping reflections have been preserved in the DMO stack (Figure 5.0-3b), time migration of this section produces an image of the salt dipair with its boundaries clearly delineated (Figure 5.0-12b). In contrast, the boundaries of the salt diapir in the CMP stack can be delineated only by the terminations of the gently dipping strata (Figure 5.0-12a).

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DMO and stacking velocities
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