Page 304 - Acquisition and Processing of Marine Seismic Data

5.8 MUTING 295

FIG. 5.58 Application of top mute. (A) Filtered shot gathers, and (B) after top mute.

Fig. 5.58 shows a top mute application on a known as final top mute, which produces a
number of successive filtered shot gathers. Espe- clearer final seismic section (Fig. 5.59).
cially in shallow waters, the top muting process Fig. 5.60A shows an example application of
prevents the occurrence of apparent closures in surgical mute on two filtered successive shot
the shallow parts of the semblance contours dur- gathers with 500 ms AGC; both have distinctive
ing the velocity analysis later on. In addition, high-frequency harmonic noise in a triangular
removing the direct wave and refracted arrivals zone along the near offset channels. It is not easy
makes the f-k spectrum less complicated and to remove this type of noise using conventional
provides an easy and more correct selection of processing methodology, and the most effective
the f-k polygon. Top mute is also applied to final way to eliminate this noise is to apply a surgical
seismic sections after migration as a final step of mute (Fig. 5.60B).
the processing sequence, to remove the noisy
area just above the seabed reflection produced 5.8.1 QC in Muting
by some specific processing steps, such as f-k fil-
ter or migration. In particular, migration may Muting is the process used to zero out the
produce semihyperbolas with upward flanks amplitudes of specific areas on the seismic data.
aligned on the seabed reflection. Just before Therefore, QC is particularly important after a
the presentation of the data, or before the inter- muting process. A visual inspection of the out-
pretation step, this noisy water column zone is put gather is necessary after muting. A brute
muted out with a suitable top mute process, stack section must be prepared after mute to