Page 84 - Acquisition and Processing of Marine Seismic Data

2.2 AIR GUN ARRAYS 75

FIG. 2.32 Side view of a typical gun string. Inset shows a photo of a real air gun hung from its harness by chains. SSMS is
seismic source management system, which consists of a pressure sensor, a digitizer for near-field hydrophone data, a depth
sensor, and a distributed firing system.

where arrays are fired alternatively at each suc- important in terms of frequency content of the
cessive shot points (Section 2.3.5). Dimension of generated signal, and therefore strings are also
each gun array is typically 15 m 20 m. equipped with depth transducers, or sometimes
A common layout of a gun string with a total a pinger, to ensure the operating depths of the
of 10 guns is shown in Fig. 2.32. This example guns. Each gun or cluster in the string has its
string has four clusters with a lateral separation own near-field hydrophone located close to
of 0.75 m and two single guns, and one of the the discharge ports, to obtain the near-field sig-
cluster guns is used as a spare. The inactive or nature of each gun for real-time and offline qual-
spare guns are used to immediately replace ity control implications.
the malfunctioning guns without interrupting Fig. 2.33 shows a plan view of an example lay-
the acquisition in case of gun failures. Strings out of a gun array with three gun strings. Each
have rGPS pods on the flotation tubes to accu- string has 12 guns with two spares and three
rately locate each string during the acquisition. clusters distributed at the frontal and rear posi-
3
The depth of the guns within the strings is tions, forming a total gun volume of 3130 in .