Source signature data for the water-gun marine seismic source were collected 45 miles south of Grand Isle, La., in water depths of 1,800 ft. Signatures were digitally recorded using calibrated near-field and far-field hydrophones for both a single water gun and a pair of interacting water guns. Single-gun tests were recorded for all combinations of the following two parameters: firing pressures of 2,400, 2,000, 1,800, and 1,000 psi; firing depths of 10, 20, 27, 40, and 80 ft. Gun interaction tests were conducted at 10-ft and 20-ft depths with a firing pressure of 2, 000 psi and gun separation of 4, 8, 12, and 20 ft. In addition, repeatability tests were conducted. The experimental procedures and test results are presented.
The water-gun marine seismic source generates an acoustic signal by means of an implosive mechanism rather than an explosive mechanism. The following characteristics are claimed by the inventor of the water gun (SODERA-Societe pour le development de la Recherche Applique, Toulon, France). (1) The source is a purely implosive device and the bubble pulse effect characteristic of explosive sources is eliminated; (2) the output spectrum is expanded toward higher frequency relative to an air gun while maintaining an output power comparable to an air gun; and (3) the water gun can be interfaced with the same onboard equipment as used for air guns. The above claims quite naturally catch the attention of those of us interested in petroleum exploration by the seismic reflection method. As a result, Amoco, Geophysical Service, Inc. (GSI), and SODERA conducted a joint test to measure the signature characteristics of both a single water gun and pair of interacting water guns. Mica T water guns with 80-cu in. air consumption were used. The tests were conducted in July 1976 aboard a conventionally rigged seismic reflection vessel in about 1,800 ft of water in the Gulf of Mexico.
Fig. 1 shows the operation of the water gun. The water gun is constructed much like an air gun except that one of the chambers is filled with water prior to firing. Upon firing, compressed air accelerates a shuttle through the water-filled chamber, ejecting the water at high velocity through exit ports. The shuttle stops instantly upon reaching the exit ports, while the ejected water continues to move away creating a cavity. The cavity increases in size until the outward velocity of the ejected water is reduced to zero by hydrostatic pressure. Sea water then collapses implosively into the cavity creating the acoustic signal. With the water gun, no air is released into the water to produce subsequent bubble oscillations.
Pressure signatures radiated by the water guns were measured for different firing pressures and firing depths. The single-gun tests were recorded for all combinations of the following two parameters: firing pressures of 2,400, 2,000, 1,800, and 1,000 psi; firing depths of 10, 20, 27, 40, and 80 ft. Signatures from two interacting water guns also were Measured.