Geology Reference
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(a)
(b)
Air
Water
Variable
chamber
size
Fig. 3.17 Schematic cross-sections through (a) a Bolt air gun and (b) a Sodera water gun to illustrate the principles of operation. (Redrawn
with permission of Bolt Associates and Sodera Ltd.)
effect, much energy is wasted and the downgoing
seismic pulse is weakened. More sophisticated methods
can be used to overcome the bubble pulse problem
while preserving seismic efficiency. Arrays of guns of
differing dimensions and, therefore, different bubble
pulse periods can be combined to produce a high-
energy source in which primary pulses interfere
constructively whilst bubble pulses interfere destruc-
tively (Fig. 3.18). For deep penetration surveys the total
energy transmitted may be increased by the use of
multiple arrays of air guns mounted on a frame that is
towed behind the survey vessel. Air guns are mechani-
cally simple and can operate with great reliability and
repeatability. They have become the standard marine
seismic source.
Water guns (Fig. 3.17(b)) are an adaptation of air guns
to avoid the bubble pulse problem. The compressed air,
rather than being released into the water layer, is used to
drive a piston that ejects a water jet into the surrounding
Marine sources
Air guns (Fig. 3.17(a)) are pneumatic sources in which a
chamber is charged with very high-pressure (typically
10 -15 MPa) compressed air fed through a hose from a
shipboard compressor. The air is released, by electrical
triggering, through vents into the water in the form of a
high-pressure bubble.A wide range of chamber volumes
are available, leading to different energy outputs and
frequency characteristics. The primary pulse generated
by an air gun is followed by a train of bubble pulses that
increase the overall length of the pulse. Bubble pulses
are caused by the oscillatory expansion and collapse
of secondary gas bubbles following collapse of the
initial bubble. They have the effect of unduly lengthen-
ing the seismic pulse. Steps can, however, be taken to
suppress the effect of the bubble pulse by detonating near
to the water surface so that the gas bubble escapes
into the air. While this does remove the bubble pulse
 
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