Page 65 - Acquisition and Processing of Marine Seismic Data

56 2. MARINE SEISMIC DATA ACQUISITION

controllers are mounted onto the gun strings recorded for each shot and analyzed to ensure
and are known as distributed gun control sys- that the guns are working properly and there
tems. There must be exact time synchronization is no misfire, auto fire or air leaks. The signa-
between the integrated navigation system, gun tures are also used to ensure that the time-break
controller and seismic recorder (Fig. 2.14A) to signals are aligned at the aim point (Fig. 2.14B).
synchronize the actual gun firing time and start Commonly, there is an offset (generally less than
of the data recording. The need for this accurate 2 ms) between the time break and actual gun
timing is due to the fact that it is necessary to fire. Recorded near-field signatures can be used
align the peaks from primary bubble pressures to incorporate further signature processing to
of each specific air gun in an array at each shot increase the temporal resolution of the seismic
location. data. Modern gun controllers also have inputs
Gun controllers also have hydrophone inputs for gun array depth and air pressure for each
to record near-field source signatures of the air string provided by depth and pressure sensors,
guns in the arrays. The gun signatures are respectively (Fig. 2.14C).

FIG. 2.14 (A) A schematic display of timing signals between the integrated navigation system (INS), gun controller and
recorder. I: fire command from INS to gun controller to initiate the firing sequence, II: trigger from INS to recorder to initiate
the recording, III: recording starts at the recorder, IV: guns are fired by the gun controller, V: confirmation from the gun con-
troller to INS when the guns are fired. (B) Schematic display of the near-field signature hydrophone output with the specific
time offsets to locate the zero time alignments for different gun types. (C) Block diagram of an example gun system with two
arrays, each consisting of three strings with 16 air guns.