Page 83 - Acquisition and Processing of Marine Seismic Data

74 2. MARINE SEISMIC DATA ACQUISITION

array consists of a combination of both strongly is that their P-P amplitude and PBR is higher
and weakly interacting guns. than one single gun of equal volume. The hori-
Basically there are two main reasons for using zontal distance between the guns in a cluster is
gun arrays in seismic surveys: typically 1 m. If we set a cluster of two guns with
the same volume, we observe that the primary
• to increase the primary signal strength, since signal amplitude increases and the bubble
the signature of a single air gun is too weak to
amplitude and period decrease as the distance
penetrate deeper layers and signal-to-noise
between the two guns decreases.
(S/N) ratio is poor even for moderate
Gun arrays are composed of gun strings,
subsurface depths. Signal amplitude is
which are subarrays consisting of several air
directly proportional to the number of guns
guns of different volumes, or gun clusters. They
within the gun array.
typically involve six to eight individual guns or
• to suppress the bubble amplitudes, since the
gun pairs grouped and hung by chains from
signature of a single gun is contaminated by
their harnesses suspended from large flotation
undesired amplitudes from the bubble effect,
tubes (Fig. 2.31). Each string is hung from a rail
which can be eliminated by carefully tuned
on the back deck to easily deploy the guns. In
air gun arrays.
specifically designed gun strings, all of the com-
In petroleum industry, guns of different sizes ponents may consist of gun clusters.
are grouped in clusters, strings and arrays An air gun array is generally composed of
(Fig. 2.31). Clusters are gun groups consisting three to six strings in total with a maximum of
of two or three closely spaced guns and they 48 guns. In 2D surveys, typically one array with
are more effective with regard to the peak ampli- three strings is used, whereas two identical
tude they produce and suppression of bubble arrays, each consisting of three strings, are used
amplitudes. The advantage of the gun clusters in 3D surveys to perform a flip-flop shooting,

FIG. 2.31 Clusters, strings and gun arrays. Clusters are gun groups generally consisting of two closely spaced air guns.
Gun strings are composed of several air guns or gun clusters. An air gun array is formed by three to six strings with a max-
imum of 48 air guns. One array of three strings is used in 2D surveys while two identical independent arrays, each consisting of
three strings are used in 3D surveys.