Page 87 - Acquisition and Processing of Marine Seismic Data

78 2. MARINE SEISMIC DATA ACQUISITION

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FIG. 2.35 (A) A typical far-field signature of a 3190 in air gun array, and (B) its amplitude spectrum. The performance of an
array can be evaluated by P-P, PBR and effective frequency bandwidth, where the signal amplitude drops 6 dB below its
maximum value.

pressure, size of each gun, and operating depth The amplitude spectra of the far-field signa-
to obtain the far-field signature output of the tures in Fig. 2.36D indicate that the ghost notch
whole array to compute the preceding three moves to higher frequency band as the towing
parameters, and pressure radiation pattern, or depth is decreased. The first notch appears at
source directivity of the array. However, when 125 Hz for 6 m tow depth, which defines the
we record the far-field signature of an array to upper frequency band of the spectrum for most
verify the simulation results, we observe that conventional surveys. Typically, a gun array is
the actual P-P amplitude of a gun array is always towed at shallower depths for higher resolution
approximately 20 dB less than the back calcu- seismic data with a wider spectral bandwidth,
lated value. This occurs due to the partial such as those for site surveys. P-P and PBR values
destructive interference between the signals of aredependentuponthefrequencybandofthefar-
each individual gun in a weakly interacting field signatures of the array: Both values decrease
array, and is termed the array effect. when the array signature is of low-frequency
Towing depth of the gun arrays has a funda- bandwidth, such as for the case of deeper tow.
mental effect on tuning performance since the Far-field signature tests are also important to
ghost time delay depends on the operating observe the source stability from one shot to
depth. Fig. 2.36 shows three example far-field another. It is important for a suitably designed
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signatures of the 6600 in gun array of the R/V array to produce a stable source signature all over
Marcus Langseth computed for three different the survey. Source stability depends on several
tow depths, along with their corresponding factors, including the weather, gun specifications,
amplitude spectra. It is clear that the PBR signif- and stability of operating air pressure, etc.,
icantly decreases as the depth increases, although the most important factor is the design
although P-P amplitude of the primary pulse of the array components. Degradation in the pre-
is not seriously affected by the tow depth. That determined or simulated P-P and/or PBR value
is because the array is tuned for an optimum may result in down times during the acquisition.
deployment at 6 m, which has become a conven- Modern 3D seismic vessels used in the
tional standard for operation depth today. hydrocarbon industry deploy gun arrays with