Airgun
This image shows a cross section of a small airgun. Image courtesy of USGS.
An engineer prepares to deploy a single airgun.
An engineer prepares to deploy a single airgun. The airgun will be suspended about 8 m below the water surface from the white float seen at the bottom of the image. Image courtesy of the National Science Foundation.

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This is the sound of air guns operating off Sable Island, Nova Scotia, but being recorded many thousands of kilometers away on the Mid-Atlantic Ridge. Note: This sound has been sped up by a factor of ten.
Courtesy of NOAA Pacific Marine Environmental Laboratory VENTS Program
Description
Seismic airguns are used to examine the layers of the seafloor to study the Earth's history or explore for oil and gas. An airgun is designed to release compressed air, which forms a bubble. The formation of a bubble produces a loud sound that penetrates the ocean floor. Some of the sound energy is reflected and travels back to the surface to be recorded by hydrophone arrays called streamers. 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. For more information see How is sound used to study Earth's history? and How is sound used to explore for oil and gas?

Airguns primarily produce sound at low frequencies (between 10-500 Hz); however, some higher 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.
 
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