The schematic above (derived from Telford and others, 1990) illustrates the basic components of the water gun system (positioned before firing).
Pneumatic sound sources (high air pressure) generate frequencies on the order of 20 Hz - 1500 Hz depending on the size of the air chamber. The water gun is divided into two chambers (see right), the upper firing chamber, which contains compressed air, and the lower chamber, which is filled with water. When the gun is fired, the compressed air forces the shuttle downward and this expels the water from the lower chamber. The shot of water leaving the gun creates a void behind it and the collapse of water into this void creates an acoustic wave. High air pressure and small chamber size yield a higher frequency signal (high resolution and shallow penetration), whereas, low air pressure and large chamber size yield a low-frequency signal (low resolution and deep penetration). The return signals are received by a towed hydrophone array.
The single chamber water gun (approximately 20Hz-1500Hz operating frequency) is a relatively deep penetration source. It can be used in marine, lacustrine, and estuarine environments. Whereas, for example, operation of the sparker system, another low frequency source, would be compromised in fresh or brackish (less saline) environments due to mode of operation.The water gun requires an air compressor on board the ship, so space and power considerations must be met. The water gun has a stable and repeatable pulse in terms of frequency composition and amplitude. It is easily deployed and is towed aft (behind the ship).
The water gun is similar to the air gun, but is more effective at collapsing the bubble pulse, thus generating a cleaner signal. The 15 cu. in. water gun is an excellent source for high-resolution studies and shallow water work, especially in lacustrine locations. It works well with a variety of bottom types. It has a short pulse length, less than 30 ms, and the source signal is rich in high-frequency content. Depending on the sediment type, resolution on the order of less than 10 meters is possible in shallow-penetration (less than about 300 meters) settings. A larger water gun, 80- cu. in., yields resolution on the order of 10-20 meters, with increased penetration, to about 1000-2000 meters. (Hutchinson and Detrick, 1984).
Foster, D.S., McKinney, B.A., and Schwab, W.C., 1999, Stratigraphic Framework Maps of the Nearshore Area of Southern Long Island from Fire Island to Montauk Point, New York: U.S. Geological Survey Open File Report 99-559.
Hutchinson, D.R. and R.S. Detrick, 1984, Water gun vs air gun: a comparison, Marine Geophysical Researches, v. 6, p. 295-310.
Telford, W. M., Geldart, L. P., and Sheriff, R. E., 1990, Applied Geophysics (2nd ed.): Cambridge, Ma., Cambridge University Press, p. 205.