Page 85 - Acquisition and Processing of Marine Seismic Data

76 2. MARINE SEISMIC DATA ACQUISITION

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FIG. 2.33 Plan view of an example air gun array layout consisting of three gun strings with a total volume of 3130 in .
Numbers indicate the gun volume in cubic inches.

The lateral distance between the strings is 10 m that the guns with different volumes produce
while the separations between the guns in each bubbles with different oscillation periods: Bub-
string are variable, slightly larger for outer guns ble periods of larger gun volumes are higher
of larger volume, which makes the overall array because expansion of the bubble takes much
dimension 15 m 20 m. Using specific software more time until it collapses, since the volume
simulations, it is possible to calculate the P-P of the air discharged into the water column is
and PBR amplitudes of such a gun array to test larger when compared to that of small guns.
the performance of the array during the array Therefore, the bubble amplitudes of the guns
design. It should be noted that this simulation with different volumes are summed destruc-
produces a P-P pressure termed back calculated tively, whereas we obtain large primary ampli-
pressure, which is the pressure level obtained if tude because of the constructive summation of
all the guns in the array are consolidated into the the primary pulses from each individual gun.
center of the array with no physical dimensions. Fig. 2.34 shows the superposition of the far-
field signatures of individual guns to strengthen
the primary signal amplitude, since they are all
2.2.6 Tuning and Array Performance
in-phase. If a proper design can be achieved
Array tuning is the process to optimally sup- with suitable volumes of guns, then the bubbles
press the bubble amplitude, or to maximize the cancel out each other in the far-field signature
PBR, while increasing the P-P primary signal since they are out-of-phase. When primary sig-
amplitude by a proper design of the gun array. nal amplitudes are synchronized, the bubble
The bubble effect can be significantly attenuated amplitudes can be suppressed by taking advan-
by suitably tuned arrays composed of air guns tage of the variation of bubble period with
with different volumes. The theoretical basis is respect to the gun size (Larner et al., 1982).