Page 64 - Acquisition and Processing of Marine Seismic Data

2.1 COMPONENTS OF MARINE SEISMIC ACQUISITION 55

are rather limited, which results in the low pen- market in 1984, the largest available volume
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etration of the seismic signal. Although they are was 40 in , which caused issues in the design of
still in use commonly by academia today, the large arrays since it was required to use several
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production line of water guns stopped in 2014 sleeve guns to form an array of 3000 in volume
and they are no longer manufactured. in total. Today, sleeve guns are manufactured
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TheGI(generator/injector)gunisaspecificair as two series, sleeve gun-IC (10, 20, and 40 in )
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gun type that suppresses its own bubble effect and sleevegun-IIC(70, 100, 150, 210, and 300 in ).
(Fig. 2.13C). Its working principle is given in Sleeve guns are relatively safe to operate since
Section 2.2.2. A GI gun has two independent air they can be deployed into the water and retrie-
chambers termed the generator and injector ved onboard without being pressurized.
chambers, which are fired independently. The One of two industry standard air gun types
generator chamber generates the main seismic used by the hydrocarbon industry is the G gun
pulse, while the injector chamber is fired a few II, which provides recoilless shooting with a reli-
milliseconds later to prevent the collapsing of able acoustic performance (Fig. 2.13E). G guns
the air bubble produced by the generator cham- can also be deployed into the water and retrieved
ber to eliminate the bubble noise. GI guns are onboard without being pressurized. They are
manufactured as three types: 210 in 3 (105 manufactured in a single consistent base design
+ 105), 255 in 3 (150 + 105), and 355 in 3 (250 and various configurations can be achieved by
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+ 105). A specific type of GI gun with 60 in (30 asuitablereplacementofdifferentcasings,which
+ 30) is known as mini GI. GI guns can be allows creating different volume sizes. Today, G
designed either as true GI mode (where the gen- guns are manufactured in four different sizes as
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erator volume is much smaller than the injector 150, 250, 380, and 520 in . It is also possible to
volume), or as harmonic mode (where generator form different chamber sizes with various vol-
and injector volumes are equal). The true GI umes using plastic volume reducers except for
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mode configuration should be used for a com- 520 in . Specific types of G guns with 12, 20, 24,
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plete suppression of the bubble. Even though 40, and 60 in also exist for high-resolution stud-
the maximum output signal amplitude of the ies and are known as mini G guns.
GI guns is lower as compared to the conventional The other industry standard air gun is the
air guns, the primary-to-bubble (PBR) ratio of the Long Life (LL) gun, which produces stable
generated signature is significantly increased source signature with a considerably high peak
since it can suppress its own bubble. A properly signal output (Fig. 2.13F). They are quite reliable
designed GI gun, therefore, can be used as a sin- and require significantly low maintenance dur-
gle gun instead of an air gun array and is gener- ing operations. Today, there are three LL gun
ally preferred for high-resolution seismic models with different volume ranges: 1500LL
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surveys because of its high PBR value and wide (from 70 to 2000 in ), 1900LLX-T (from 10 to
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amplitude spectrum. GI guns have their own 250 in ), and 2800LLX (from 5 to 120 in ).
hydrophones to observe and record the near-
field source signatures and bubble amplitudes. 2.1.4 Gun Control Systems
Sleeve guns are specific type of the conven-
tional air gun, which use single 360-degree dis- Gun controllers are used to control firing of
charge ports to release the compressed air into the air guns within the gun array at each shot
the water column instead of the smaller multiple point. They transmit electrical signals to sole-
ports used by traditional air guns (Fig. 2.13D). noid valves of each individual air gun to fire
This design provides a complete spherical air exactly at the same time, termed the air gun
bubble. When they were first introduced to the aim point. Gun control units of modern gun