Water Gun
 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.
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| Water gun attached to float (white ellipsoidal-shaped object at top of photograph) and tow harness. The water gun, positioned on deck, is attached to the air-supply hoses (black), exiting from the aft-facing end of the water gun. | 15 cu. in water gun and tow harness |
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System Operation
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| Stages of operation of a water gun. The water gun produces a seismic pulse by the implosion of a cavity created behind a jet of high-pressure water expelled from the gun (diagram modified from Seismic systems, Inc. manual, now Sercel-France). Because no air is released, there is no bubble pulse. Note the changed position of the piston during the firing process; after the gun is fired, air from the upper chamber is released from the valves at the top of the gun. Hydrostatic pressure forces the piston up, the upper chamber is pressurized again, then the gun may be fired again. This cycle requires about 0.5 sec. in the 40 cu. in gun, about 8 sec. or more in larger guns. |
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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 compressor yields an air pressure of 2000-3000 psi. 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).
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| Deployment of the Water Gun | View of the Water Gun in tow behind the ship. | Close-up view of the tow apparatus for the water gun during a R/V Seaward Explorer cruise. Air-supply hose is suspended from the tension-bearing tow wire. The water gun is located under the area of foam. |
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System Usage
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).
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| Example of a water gun profile acquired off southern Long Island, N.Y., collected in 1998 as part of research project to map the Cretaceous strata and Quaternary deposits on the inner-continental shelf of the New York Bight region. In this section, the vertical scale is in seconds, two-way travel time, displayed from 0 to 0.15 seconds. The water bottom is located at approximately 0.025 seconds at the left margin of the image, corresponding to a depth of approximately meters. Sandy layers cut by multiple channels, filled with more mud-like deposits are depicted. The approximate depth of the base of the channel near the right margin of the image is 75 meters. |
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| Another water gun profile from southern Long Island, N.Y. Vertical scale is in seconds, two-way travel time (0 to 0.15 sec. is displayed). The detail of the internal layering within the two channels demonstrates the order of resolution possible with the system. |
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| In 1984, a comparison was shot on a profile crossing a buried valley in Long Island Sound southwest of New Haven, Conn. in about 9-12 meters. of water. The air gun record, both with and without waveshaper, is dominated by bubble-pulse oscillations that heavily obscure the record. A waveshaper chamber is designed to suppress the bubble pulse on an air gun. The general structure of the valley is visible on the air gun with waveshaper record, but details are difficult to resolve. The water gun record yields sufficient detail to allow for interpretation of two acoustically different sediment types. two acoustically different sediment types. |
References
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.
