Page 214 - Acquisition and Processing of Marine Seismic Data

3.10 OTHER NOISE TYPES 205

FIG. 3.35 (A) High-amplitude spikelike noise on a stack section. (B) If not removed before stacking, they cause smiles after
migration process.

line, which is schematically shown by the inset and mud volcanoes may generate distinctive
in Fig. 3.36, whenever there is sufficient inclina- side-sweep reflections. They are generally
tion on the seabed in the crossline direction. The seen as weak to moderate amplitude dome-
diffractions coming from out-of-plane (or from shaped structures located over the seabed reflec-
the third dimension) are considered as noise tion. Fig. 3.36 shows an example side-sweep
and known as side effects or side-sweeps in from a small-scale mound just above the
2D seismic surveys, whereas they do constitute seafloor reflection. Although in-line (2D) diff-
seismic data itself in 3D surveys. ractions can be eliminated by a 2D migration
Side-sweeps need anomalously high completely, side-sweeps are not easy to
NMO velocities during the NMO correction to remove from 2D seismic data since 2D migration
flatten on the CDP gathers. Particularly is not sufficient to remove the 3D effects from
mounds, canyon walls, slab-type slide material, the data.