Air gun: A seismic source that injects a bubble of highly compressed air into the water. Its frequency spectrum depends on the amount of air in the bubble, the air pressure, and the water depth (or water pressure). Arrays of guns of different sizes are often used so that a broader frequency spectrum will be generated.
Air guns are also used in boreholes or pushed down into marsh after being modified to prevent mud, sand etc. from entering and fouling the air gun, and they are sometimes used in bags of water that are set on the ground surface for land work.