SHIPS - Airgun Sound Source
The sound waves generated for SHIPS will be produced by an array of airguns.
The image above is a cross section of one small airgun. At the left end is the connector for an electrical cable. Another connector for a compressed air hose is present, but not shown. Inside the gun, there is a compressed air chamber and a piston. At regular time intervals during the experiment, a computer will send an electrical "trigger" pulse to the guns. This trigger will activate the piston to suddenly release compressed air stored in the chamber, producing a loud bang required for seismic imaging. After the air is released, compressed air from the ship will be supplied through the hose to repressurize the guns for the next "shot".
The figure below shows that the sound output by the airgun array is at frequencies that are well below the frequency band of most sensitive hearing that characterizes many marine mammals, excluding baleen whales. This low frequency should limit the potential impact of the sound produced during the SHIPS project.
* Ocean Studies Board of the National Research Council (1997), based on human hearing in air. TTS stands for temporary threshold shift. This is a protective hearing mechanism in which hearing sensitivity is reduced temporarily to limit the damage caused by loud sounds.
Marine mammal curve derived from:
Kastak, D., and Schusterman, R. J., 1995, Book Aerial and underwater hearing thresholds for 100 Hz pure tones in two pinniped species: Woerden, The Netherlands, De Spil Publishers, p. 71-79.
Richardson, W. J., Greene, C. R., Malme, C. I., and Thomson, D. H., 1995, Book Marine Mammals and Noise: New York, Acadamic Press, 576p.