Page 479 - Acquisition and Processing of Marine Seismic Data

470 10. NORMAL MOVEOUT CORRECTION AND STACKING

FIG. 10.9 A synthetic CDP gather with a single reflection hyperbola (A) before, and (B) after NMO correction. The close-
ups of NMO corrected gather for (C) near and (D) far offset traces, are indicated by I and II, enclosed by blue and red dashed
rectangles, respectively. (E) and (F) are corresponding mean amplitude spectra of the close-ups in (C) and (D).

point to multiple time locations on the corrected NMO stretch is a frequency distortion in
trace because of their interference nature which the dominant frequencies of the reflection
through more than one individual reflection events shift to the lower frequencies, and it is
event. That is, a specific time sample of an uncor- expressed by
rected trace may represent more than one reflec- Δf
tion hyperbola at the same recording time, ¼ Δt NMO (10.16)
f t 0ðÞ
which ultimately results in a complete degrada-
tion of early arrivals at far offset traces in a CDP where f is the dominant frequency and Δf is the
gather. As an example, a synthetic CDP with two amount of shift in the frequency. NMO stretch
intersecting reflection hyperbolas are illustrated occurs in the early arrivals of the far offset traces,
in Fig. 10.10A, with a close-up to emphasize the which severely distorts the shallow parts of the
interference effect of two independent events in CDPs. The stretched parts of the far offset traces
the intersection zone. A strong degradation are muted out and this process is known as
arises after NMO correction due to the interfer- NMO stretch mute. Muting is performed auto-
ence along the early arrivals for offsets larger matically using the expression given by
than approximately 300 m for shallower reflec- Eq. (10.16), and the mute zone is determined
tion, which even influences the deeper reflection by a percentage value termed the NMO stretch
hyperbola for offsets larger than approximately limit. Fig. 10.11 shows a synthetic CDP and its
750 m (Fig. 10.10B). If two hyperbolas did not NMO corrected versions for different stretch
intersect as in Fig. 10.10C, depending on their limit percentages. The limit can exceed 100%,
velocities, the degradation would occur for the and it does not mean that the entire NMO stretch
offsets greater than approximately 600 m for zone is successfully muted out when a 100%
shallower reflection, and there would be no deg- limit is used. The smaller the percentage mute
radation for deeper reflection at all (Fig. 10.10D). limit, the larger the stretch mute zone. For higher