An airgun is designed to release compressed air, which forms bubbles. The formation of bubbles produces a loud sound that is used to explore the geologic structure of the ocean floor. Airguns primarily produce sound at low frequencies (between 10-500 Hz); however, high frequency noise is also created. A small airgun that releases 0.16 Liters of air can create source amplitudes up to 216 underwater dB at 1 meter. A large airgun that releases 32.8 Liters of air can have a source level of up to 232 underwater dB at 1 meter (Richardson et al., 1995).
This image shows a cross section of a small airgun. Image courtesy of USGS.
Sound is released from the airgun and penetrates the ocean floor. Some of the sound energy is reflected and travels back to the surface, which then can be measured. The surfaces that reflect this sound energy are related to different geological formations, such as the layers of the seafloor. This information provides geologists with details of what is beneath the seafloor.
Sound pulses that enter the seafloor are both reflected and refracted (or bent) as they pass into different layers.
Seismic refraction provides more information about the layers. The refracted sound pulse follows a complex path. With seismic refraction, the density of the layers can be determined.
Reflection and refraction can also be done on the seafloor with instruments called ocean bottom seismometers. A seismic survey may make use of both shipboard measurements and measurements from an array of ocean bottom seismometers.
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Courtesy of NOAA Pacific Marine Environmental Laboratory VENTS Program
References
Richardson, W.J., Greene, C.R., Malme, C.I. and Thomson, D.H. 1995. Marine Mammals and Noise. San Diego, CA: Academic Press.
