Page 75 - Acquisition and Processing of Marine Seismic Data

66 2. MARINE SEISMIC DATA ACQUISITION

FIG. 2.23 Schematic illustration of bubble effect formation from an air gun source. (A) An air gun provides a sudden air
discharge into the water as an expanding bubble, which produces the primary seismic signal. (B) A certain time later, the
bubble suddenly shrinks (termed rarefaction) because of the higher ambient pressure than internal pressure of the bubble,
which produces the first bubble signal on the signature. (C) Then bubble expands again (termed compression) because of
its higher internal pressure than ambient pressure, which produces the second bubble signal on the signature. (D) The expan-
sion and shrinkage of the bubble continues until it breaks at the sea surface.

column continues in that way and each individ- amplitude generated by the bubble oscillation
ual expansion and collapse produces a new sig- decreases with increasing time. The bubble
nal (Fig. 2.23D) until frictional losses damp out oscillation period is not constant from one cycle
the bubble oscillations, or the bubble breaks at to another and depends on the temperature and
the surface when it reaches to the sea surface. volume of the air discharged into the water, ini-
Polarities of each bubble signal generated by tial air pressure, and the ambient pressure (i.e.,
the expansion and collapse of the air bubble depth of the air gun). The bubble effect causes
are reversed and the amplitudes are reduced oscillations in the low-frequency components
with respect to the previous bubble signal polar- of the amplitude spectrum of the far-field source
ity and amplitude. signature (Fig. 2.24B) typically between 4 and
The signal shape produced by an air gun can 50 Hz. The effect of bubble oscillations can be
be observed via near-field hydrophone located suppressed by using several air guns with dif-
close to a discharge port of the air gun, that is, ferent volumes instead of deploying single gun
the near-field signature. Fig. 2.24A shows a typ- or instead of using a number of guns with the
ical near-field signature of an air gun, which same volume (Section 2.2.6).
consists of the primary seismic signal generated A specific air gun type, the GI (generator/
during the first expansion of the air bubble and injector) gun, first introduced by Pascouet
the amplitudes from the bubble effects resulting (1991), can eliminate its own bubble effect by tak-
from the successive collapse and expansion of ing advantage of its two independent air cham-
the initial air bubble within the water. The bers (termed generator and injector chambers)