Velocity discrimination between primaries and multiples

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Seismic Data Analysis
Series Investigations in Geophysics
Author Öz Yilmaz
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store

The CMP gathers in Figure 6.1-8a clearly illustrate the moveout difference between primaries and multiples. A primary p typically has less moveout than a multiple m. From the velocity spectrum in Figure 6.1-8b, note the difference between the velocity trends associated with primaries V P and multiples V M1 and V M2. The V M1 and V M2 velocity functions represent the water-bottom and peg-leg multiples, respectively. If NMO correction is applied using the primary velocities, as is normally done to generate final stacks, then the primaries are aligned while the multiples are undercorrected (Figure 6.1-8c). This suggests that CMP stacking itself is a viable method of multiple attenuation. The CMP stack derived from the gathers in Figure 6.1-8c is shown in Figure 6.1-8d.

The synthetic CMP gather in Figure 6.1-9c contains five primaries, including the water-bottom reflection W and the multiples associated with it. The velocity spectrum shows a significant separation between the velocity functions for multiples V M and primaries V P. Stacking with the primary velocity function should, to a large extent, discriminate against the multiples and result in a section that contains essentially the primary energy as shown in Figure 6.1-10. The stack trace in Figure 6.1-10c is repeated to better examine the relative amplitudes of the primaries and the multiples.

Stacking far offsets works to suppress multiples. However, stacking near offsets works against multiple attenuation, since the moveout difference between primaries and multiples is negligibly small on those offsets as in Figure 6.1-8c. The simplest way around this problem is to apply an inside mute to the CMP gathers before stacking. Another problem then emerges — the outside mute. The severity of this mute governs the amount of far-offset data left at early times for velocity discrimination (Figure 6.1-8c). If there is a severe multiple problem, an effort must be made to preserve the maximum amount of far-offset data associated with target events. The stacked section of Figure 6.1-8d with inside mute applied is shown in Figure 6.1-11a. When compared with Figure 6.1-8d, note that the deeper peg-leg multiple below 4 s has been further attenuated by inside trace muting. The difference between the conventional CMP stack (Figure 6.1-8d) and the inside mute stack (Figure 6.1-11a) shown in Figure 6.1-11b indicates the amount of energy, mostly multiples, that was removed by the inside mute.

A variation of conventional muting, such as optimum-weighted stacking can produce better results. In such a scheme, weights between 0 and 1 are assigned to each offset during stacking. The smaller weights are normally assigned to the near offsets.

In summary, because there is relatively less move-out differential between the primaries and multiples in the near-offset range, the inside mute (or some kind of weighted stacking) helps suppress multiples. Hence, it may help to cascade any one of the multiple attenuation techniques described in this chapter with inside mute during stacking.

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Velocity discrimination between primaries and multiples
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