Page 184 - Acquisition and Processing of Marine Seismic Data

3.2 BUBBLE EFFECT OF THE AIR GUN 175

FIG. 3.3 (A) Linear operational noise at near traces on a shot gather, and (B) mean amplitude spectrum of the dashed rect-
angles in (A).

Because the operational noise is linear on the generated by this sudden air discharge and the
shot records and propagates in the water col- ambient pressure result in periodic expansion
umn with a water velocity of 1500 m/s along and collapse of the air bubble as it ascends
the streamer, it can be removed by a suitable towards the sea surface. Each individual expan-
frequency-wavenumber (f-k) filter. Fig. 3.4 sion and collapse produces a new signal with
shows an application of f-k filter designed to opposite polarity until it reaches to the sea sur-
effectively eliminate the operational noise. face, which is termed bubble noise. Occurrence
of the air gun bubble and its characteristics are
discussed in Section 2.2.2 in detail.
3.2 BUBBLE EFFECT OF THE The bubble effect of the seismic sources
AIR GUN becomes an important issue if a single ordinary
air gun is used as the seismic source. To over-
Air guns are standard seismic sources for come the bubble noise problem, a specific air
conventional 2D and 3D seismic surveys today. gun type known as generator/injector (GI)
When a single air gun is fired at a certain depth gun (Section 2.2.2) can be preferred, especially
in the water column, it discharges high-pressure for high resolution seismic acquisition such as
air from its air chamber into the water to gener- site surveys. In addition, large air gun arrays
ate the primary seismic signal. Interactions consisting of several gun strings with large num-
between the internal pressure of the air bubble ber of air guns, each with different chamber