Page 73 - Acquisition and Processing of Marine Seismic Data

64 2. MARINE SEISMIC DATA ACQUISITION

FIG. 2.21 Schematic drawing of an air gun (A) in armed position, (B) solenoid valve is open and the air is fed beneath the
upper flange of the shuttle via orifice A, (C) shuttle moves up and the gun is fired, and (D) air gun reverts to its initial armed
position.

the compressed air in chamber II into the water port of an air gun, we get the typical near-field
rapidly (typically in 1–4 ms) through the four source signature. When the ghost interference
discharge ports (Fig. 2.21C). Since the air in is incorporated, the output signal from the air
chamber II is released into the water, the air gun changes into a far-field signature
pressure in chamber I increases again to produce (Section 2.2.4).
a downward force on the shuttle, and it returns The maximum output signal amplitude of an
to its initial or armed position, which allows the air gun is directly proportional to the cubic root
chamber II to fill with the compressed air again of the gun chamber volume. The reason for this
(Fig. 2.21D). is that the expanding bubble is pushing the
The bubble discharged into the surrounding water radially away from its center, and the vol-
water expands rapidly because the pressure ume depends on the radius cubed. In practice,
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within the exhausted air bubble far exceeds an air gun of 250 in (cubic root is 6.3) produces
the ambient hydrostatic pressure at the begin- only 2.3 times higher amplitude than an air gun
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ning. However, it suddenly starts to shrink soon of 20 in (cubic root is 2.7) although it has 12.5
after the initial discharge since its surficial area times larger air chamber volume. This is also
extends during the expansion, which produces true for the gun arrays: the signal amplitude
an additional acoustic signal, termed bubble generated by a larger volume array is not much
effect (Section 2.2.2). The air can only escape higher than a small volume array. For instance,
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from an air gun’s ports at a certain limited rate, an 8000 in array produces only twice the ampli-
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and this throttling effect diminishes the amount tude of a 1000 in array, although the total vol-
of air that can be discharged during the rise time ume ratio between them is 8. This situation
of the first impulse (J. Diebold, pers. comm.). leads us to increase the number of guns within
When we put a hydrophone close to a discharge the array instead of increasing the volumes of