Page 74 - Acquisition and Processing of Marine Seismic Data

2.2 AIR GUN ARRAYS 65

FIG. 2.22 Variation of peak amplitude with the cubic root of total air gun volume obtained from the gun array tests of R/V
Ewing (J. Diebold, pers. comm.).

the individual air guns to increase the signal during this expansion, the internal pressure of
amplitude generated by the array. Fig. 2.22 the bubble becomes lower than the ambient
shows the results of air gun tests performed pressure because of its excessively increased
onboard of R/V Ewing by Lamont Doherty volume. This is the point where the bubble
Earth Observatory in 1990. For this gun array, reaches its maximum size, and it starts to shrink
the cubic root rule fails for the gun volume after this point because of the higher ambient
3
above 350 in . pressure, which ultimately produces a second-
ary seismic signal in the water. This is the first
bubble effect (Fig. 2.23B), which is known as rar-
2.2.2 Air Gun Bubble Effect
efaction, expressed by positive numbers on the
The released compressed air from an air gun seismic records. This second signal, however,
into the water column tends to consecutively has reverse polarity with respect to the primary
expand and collapse, ultimately resulting in an seismic signal generated during the first expan-
oscillating seismic source signature. This is sion because it is produced during the collapse
known as bubble effect and may be an important of the air bubble. The collapsing bubble then
issue if a single ordinary air gun is used as a seis- reaches its minimum size and it starts to expand
mic source. When the air gun is fired, it sud- again since its internal pressure exceeds the
denly discharges high-pressure air from its air ambient pressure again, because of its reduced
chamber into the water, and this generates the volume in the water, which produces another
desired pressure spike: the seismic signal seismic signal with the same polarity as the pri-
(Fig. 2.23A). At the beginning, the pressure mary signal since it is generated during the
inside this initial air discharge, namely the air expansion. This is the second bubble effect
bubble, is much higher than the ambient pres- (Fig. 2.23C), which is known as compression,
sure at the air gun’s depth; hence the bubble expressed by negative numbers on the seismic
immediately starts to expand. At a certain point records. This behavior of the bubble in the water