Geology Reference
In-Depth Information
(a)
Single 270 in
3
air gun
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8 s
(b)
Seven - gun array (1222 in
3
total volume)
Fig. 3.18
Comparison of the source
signatures of (a) a single air gun (peak
pressure: 4.6 bar metres) and (b) a seven-
gun array (peak pressure: 19.9 bar metres).
Note the effective suppression of bubble
pulses in the latter case. (Redrawn with
permission of Bolt Associates.)
water.When the piston stops, a vacuum cavity is created
behind the advancing water jet and this implodes under
the influence of the ambient hydrostatic pressure, gener-
ating a strong acoustic pulse free of bubble oscillations.
Since the implosion represents collapse into a vacuum,
no gaseous material is compressed to ' bounce back' as a
bubble pulse. The resulting short pulse length offers a
potentially higher resolution than is achieved with air
guns but at the expense of a more complex initial source
pulse due to the piston motion.
Several marine sources utilize explosive mixtures of
gases, but these have not achieved the same safety and
reliability, and hence industry acceptance, as air guns. In
sleeve exploders
, propane and oxygen are piped into a sub-
merged flexible rubber sleeve where the gaseous mix-
ture is fired by means of a spark plug. The products of
the resultant explosion cause the sleeve to expand rapid-
ly, generating a shock wave in the surrounding water.
The exhaust gases are vented to surface through a valve
that opens after the explosion, thus attenuating the
growth of bubble pulses.
Marine Vibroseis
®
.Whilst vibrators were developed for
land surveys, it is of interest to note that experiments
have been carried out using marine vibrator units, with
special baseplates, deployed in fixtures attached to a
survey vessel (Baeten
et al
. 1988).
Sparkers
are devices for converting electrical energy
into acoustic energy. The sparker pulse is generated by
the discharge of a large capacitor bank directly into the
sea water through an array of electrodes towed in a frame
