Page 80 - Acquisition and Processing of Marine Seismic Data

2.2 AIR GUN ARRAYS 71

where P and B are the peak primary and bubble negativepeakamplitude.Normally,thepressure
amplitudes, respectively (Fig. 2.28B). value measured at a certain depth below the
A near-field hydrophone located close to the array is multiplied by the source-hydrophone
air gun discharge ports can provide the follow- distance to obtain the source strength. For
ing information which then can be used to eval- instance, a source strength of 50 bar-m implies
uate the performance and reliability of the gun’s that 0.5 bar pressure is measured at a distance
operation by a careful observation and analysis of 100 m from the array center. In practice, higher
during the data acquisition in terms of online or P-Pamplitudeispreferredforabettersignal pen-
real-time quality control applications: etration into the subsurface. The PBR parameter
determines how the bubble is suppressed by the
• The accuracy of air gun aim point
overall design of the whole air gun array. A PBR
• Primary signal shape and amplitude (P)
value as high as possible is desired so that the
• Bubble period
array signature is close to an ideal spike with
• Amplitude and the ratio of P-P to bubble
no side oscillations, with a wide frequency band.
amplitude (PBR)
Both parameters depend on the number of guns
• Possible air leakage or other malfunctions of
and total gun array volume, the air pressure
the gun
applied, and the gun array depth. Near- and
The far-field signature incorporates the far-field signatures of an air gun array can be
source ghost on the near-field signature in addi- computed by dedicated software packages to
tion to the primary and bubble amplitudes evaluate the performance of the designed arrays
(Fig. 2.28C). It is measured with a hydrophone before the surveys.
directly below the gun array, and defines the In towed streamer acquisition, four signals
nominal source strength: that is, the amount of with different ray paths arrive at the receivers
pressure at 1 m away from the source center in for a single reflection event. In Fig. 2.29, these
bar-m. Far-field signatures are characterized individual signals are schematically shown
by their three components: along with their ray paths:
• The initial high pressure pulse produced • A single reflection with no ghost from a tuned
when the air gun is fired air gun array consists of a wavelet, which
• The source ghost produced by the would be recorded if the source and receivers
reflection of the initial pulse from the water were located within an infinite half space
surface (Fig. 2.29A).
• The bubble pulse train produced by the • The recorded reflection signal shape changes
successive collapse and expansion of the when the source ghost from the water surface
initial air bubble is incorporated. The source ghost occurs since
the source is located below the sea surface
A far-field signal can provide the following
(Fig. 2.29B).
information which can be used to evaluate the
• The effect of only receiver ghost on the
overall performance and reliability of the gun
recorded reflection signal shape is similar to
array:
that of the source ghost. The receiver ghost
• Peak-to-peak (P-P) amplitude of the primary occurs since the receiver is located below the
signal sea surface (Fig. 2.29C).
• The ratio of P-P to bubble amplitude (PBR) • The recorded reflection signal shape is
further modified if we incorporate the
P-P amplitude is the distance between the pri-
receiver and source ghost signals together
mary positive peak amplitude and the ghost’s