A tuned array of 4 SERCEL's (formerly SODERA-SSI) GI-GUN was used for producing the seismic shots. Two bars hanging two guns each were towed at 6m water depth on the port and left sides 20 m off the stern (Fig.9). Each gun was set in the 'harmonic' 105+105 CI, medium port configuration. The array was energized by a Mod. I21, 37 KW, electrically driven BAUER Compressor, that delivered air (1500 L/min at the pressure of 250 Bars) on a 6x50L bottle reservoir. The guns received the final pressure at the outlet ports trough a pressure regulator (on average 130 to 180 bars). Another K1000 (1000 L/min) by BAUER compressor pumping on the same reservoir was available as a spare.
The array was fired and synchronized by using the ISMAR's synchronizer [Masini and Ligi (1995)] (Fig.10). The instrument is able to control 4 GI-GUNS (or 8 Air-guns) in manual, semi-automatic and automatic modes, up to the array synchronization (generator and injector delays, data visualization, etc). The synchronization is performed by cross-correlating the near-field seismic signal from the hydrophones in the guns, calculating the reciprocal delays and checking and applying trough the line the appropriate timing to each gun (ahead of time or delay) against the first one. A programmable, fixed delay at the time-break can be set for each gun, in this case 10 ms. The time-break can be generated by the internal Main and Delayed Time Base Generators, or can be input by an external system, typically the navigation system, on either TTL or CC mode.
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