Page 205 - Acquisition and Processing of Marine Seismic Data

196 3. NOISE IN MARINE SEISMICS

FIG. 3.26 Two stack sections with guided wave effects (dashed blue polygons).

using these prestack processing techniques. is close enough to the receiving streamer, the
Fig. 3.26 shows two different stack sections con- interference signal has a hyperbolic moveout. If
taminated with strong guided wave effect in it is located at a long distance, then we observe
their shallow parts, consisting of very steep dis- the interfering signal as linear events. In shot
continuous line segments. In such cases, the only gathers in Fig. 3.27, the interference, possibly
way to remove them is to mute out the guided originating from a boomer source, has a linear
waves from shots using a surgical mute. moveout since source is located at a very long
distance away from the receiving streamer. The
time between the successive interference signals
3.9 SEISMIC INTERFERENCE is 1.6 s. After shot number 109, shooting at the
recording vessel was stopped because of the
Seismic interference from other seismic ves- strong interference, but the recording continued
sels shooting close to the survey area may be an to see the character of the interfering noise.
importantissue duringthe acquisition. Since sea- Between shots 110 and 114, records show only
water is a very good conductor for elastic waves, the interference signal.
the echoes from large air gun arrays used by the In tectonically active regions, such as at sub-
oil and gas industry today can be perceived by duction zones, a special case of seismic interfer-
the hydrophones from several tens of miles away ence may be local earthquakes, appearing as
from the seismic source. This type of seismic strong coherent events on the shot records; they
interference has very good trace-by-trace consis- are usually removed by muting. Fig. 3.28, shows
tency on the shot records, and appears at regular an interesting example of an earthquake inter-
time intervals on the shot records, depending on ference on a 3D shot record group consisting of
the shooting rate at the source of noise. Its shape eight streamers. The interference is so strong that
varies with the distance between receiving the records are considered to be bad shots and
streamer and source of interference. If the source should be edited during the processing stage.