This image shows a cross section of a small airgun. Image courtesy of USGS.
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). Lower level, higher frequency sounds (up to 20 kHz) may also be created, and they do not travel as far (see Why does sound get weaker as it travels?). A small airgun that releases 0.16 Liters of air can create peak source levels 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. Please note that underwater dB is different than that in-air (see How does sound in air differ from sound in water?).
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.