Page 500 - Acquisition and Processing of Marine Seismic Data

10.5 QC IN NMO CORRECTION AND STACKING 491

limit percentage reduces, even removes, these stretch mute percentage. The gap always occurs
amplitudes (Fig. 10.34B). Consequently, there only at the start-of-line side (Fig. 10.35A). This is
may be a trade-off between the data loss and because the traces in the very first CDP gathers
stack quality by means of the NMO stretch mute located at the start-of-line side are always far off-
percentage. Too-small values result in the loss of set traces, whereas the traces in the very last
far (even near) offsets, whereas too-high limits CDP gathers located at the end-of-line side are
incorporate noise amplitudes in the stack. all near offset traces. Since the early arrivals of
In shallow water data, there is a triangular far offset traces are automatically muted out
data gap located at one border of the seismic line by NMO stretch mute, the early arrivals of the
due to the NMO stretch mute. The size of this traces at the very first CDPs of the lines, which
area is inversely proportional to the NMO consist of only far offsets, are removed. This is

FIG. 10.35 (A) In shallow waters, NMO stretch mute results in a triangular data gap located at the start-of-line side.
Schematic representation of the ray paths of the traces for (B) the very first CDPs at the start-of-line, and (C) the very last CDPs
at the end-of-line. The traces in CDPs 1, 2, and 3 in (B) are composed of only far offset traces, whereas the traces in CDPs 10, 11,
and 12 in (C) are composed of only near offset traces. Because NMO stretch mute removes early arrivals of far offsets, this part in
the very first CDPs is completely muted out. That is why the gap in (A) always occurs only at the start-of-line side. Here, (D) and
(E) show real data examples from the first and last CDPs of a small-spread marine seismic line with a 693-m maximum offset,
where the trace offsets range from 662 to 693 m in the first six CDPs, while they are between 100 and 131 m in the last six CDPs.