Page 464 - Acquisition and Processing of Marine Seismic Data

9.4 QC IN VELOCITY ANALYSIS 455

FIG. 9.29 Utilizing NMO correction during velocity picking to discriminate the primary and multiple velocities, especially
in shallow water areas. (A) If the primary reflection velocities are picked (e.g., velocity function of dashed blue line), primaries
(P) are completely flattened after NMO correction while the multiples (M) are still hyperbolic and undercorrected. (B) If veloc-
ities of multiples are picked (e.g., velocity function of dashed red line), then the multiples are flattened whereas the primaries are
overcorrected.

not easy to discriminate, since they may strongly be applied after the dip moveout (DMO) pro-
interfere. In shallow parts of the supergathers, cess, which removes multiple enclosures
even small variations in the picked velocities appearing at the same zero-offset times in the
result in erratic changes on the flattening of semblance plots by solving the conflicting dip
the events, and therefore the processors must issues on the seismic data (Section 11.10).
be extra careful when picking the velocities in The same RMS velocity field, or interval
shallow areas where primaries and multiples velocity field derived from RMS velocities using
interfere. the Dix equation, is also used to migrate the seis-
In a complete seismic processing, generally mic data. Some of the migration algorithms are
more than one velocity analysis is necessary. quite sensitive to lateral velocity variations
After sorting the data into CDP gathers, a first- and may produce artifacts in different shapes
pass velocity analysis is applied. Then, this and sizes if there are issues or erratic lateral var-
velocity field is used in multiple suppression iations in the derived velocity field. Fig. 9.30
since some of the multiple removal methods shows an example velocity issue on a migrated
require 2D or 3D velocity field to be known, such section. There are two apparent normal faults
as a Radon velocity filter. After removing the on the migration section in Fig. 9.30A forming
multiples, it is expected that the semblance plots a small-scale graben, which occurs due to a
will be greatly improved, and therefore a gap in the RMS velocity field (Fig. 9.30B) because
second-pass velocity analysis is applied to of an incorrectly picked velocity function at the
obtain a more appropriate velocity field. Fur- location of the graben. Fig. 9.30C shows a
thermore, a third-pass velocity analysis may migrated section obtained by the correct 2D