Air Gun: Definition, Marine Seismic Source, and Array Design

What Is an Air Gun?

An air gun is a pneumatic seismic source that releases a precisely controlled volume of compressed air at pressures of 1,500 to 2,000 psi (103 to 138 bar) into the water column to generate a broadband acoustic impulse used in marine seismic acquisition surveys, as well as in water-filled pits on land during vertical seismic profile (VSP) operations, enabling geophysicists to image subsurface structure and stratigraphy beneath the seafloor or land surface.

How an Air Gun Works

An air gun consists of two chambers: a storage chamber that is pre-charged with compressed air from the surface compressor system through a tow cable umbilical, and a firing chamber separated from the storage chamber by a solenoid-actuated piston or sleeve valve. When the firing command is transmitted from the seismic vessel's recording system, the solenoid releases the piston or sleeve, allowing the high-pressure air in the storage chamber (maintained at 1,500 to 2,000 psi / 103 to 138 bar) to discharge instantaneously into the surrounding water through ports in the gun body. The discharge duration is measured in milliseconds; a typical 520 cubic inch (8,520 cm3) gun fires in approximately 10 to 15 milliseconds. The abrupt release of compressed air creates a sharp pressure pulse that travels through the water and couples into the seafloor as a downward-propagating seismic wave.

The physical behaviour of the released air bubble is central to understanding air gun performance. After the initial pressure pulse, the compressed air forms a cavity (bubble) that expands rapidly against the water, overshoots its equilibrium radius, and then contracts under water pressure. This expansion-contraction cycle repeats multiple times, producing a series of secondary pressure pulses called the bubble pulse train. Each successive bubble pulse arrives at the seismic receivers with a time delay determined by the bubble period, which is a function of gun volume, operating pressure, and water depth. For a single 520 in3 gun at 2,000 psi (138 bar) and 7 m (23 ft) depth, the primary bubble period is approximately 150 to 180 milliseconds. The ratio of the primary peak pressure amplitude to the first bubble peak amplitude is called the primary-to-bubble ratio (PBR) and is used as a key quality metric for air gun sources. A PBR of 15 to 25 dB is considered excellent; a poorly tuned source or a malfunctioning gun can reduce PBR to 8 to 12 dB, significantly degrading image quality in the processed seismic data.

In practical marine acquisition, individual air guns are never fired alone. They are arrayed in clusters of 2 to 4 guns physically tied together and fired simultaneously, and multiple clusters are suspended from a floatation system called a sub-array or string. A complete seismic source array typically consists of 2 to 4 sub-arrays deployed 50 to 100 m (164 to 328 ft) apart in the cross-line direction, each containing 8 to 16 individual guns totalling 24 to 48 guns overall. The total active volume of the array ranges from 2,000 to 8,000 in3 (32,800 to 131,100 cm3) for most commercial 3D surveys. The gun volumes within the array are selected and tuned to achieve destructive interference of bubble pulses in the far-field signal while maintaining constructive interference of the primary pulses. This is accomplished by choosing a geometric progression of gun volumes (for example, 40, 80, 160, 320 in3) so that the bubble period of each gun volume is a harmonic multiple of the others. When the array fires, the primary pulses reinforce each other, producing a large, clean initial peak, while the bubble pulses arrive at different times from the different guns and largely cancel each other.

Air Gun Across International Jurisdictions

Canada (Offshore Atlantic and Arctic): Marine seismic surveys on the Canadian continental shelf are regulated by the Canada-Newfoundland and Labrador Offshore Petroleum Board (CNLOPB), the Canada-Nova Scotia Offshore Petroleum Board (CNSOPB), and the National Energy Board (NEB) for the offshore Arctic and Pacific. Under the Canada Petroleum Resources Act and the associated conditions of authorisation, operators must submit a detailed seismic program including air gun array specifications, source level calculations, and a Marine Mammal Mitigation Plan (MMMP) before any survey can commence. The mitigation protocols require a minimum 30-minute soft-start (ramp-up) procedure, with a single smallest-volume gun fired first and additional guns added incrementally over a 20-minute period before reaching full array power. Protected Species Observers (PSOs) are mandatory on all vessels, and operations must cease if a marine mammal is observed within 500 m (1,640 ft) of the array during operations. The Department of Fisheries and Oceans (DFO) issues species-at-risk conditions that may impose seasonal restrictions, particularly in the Gulf of St. Lawrence during North Atlantic right whale calving season.

United States (Gulf of Mexico, Atlantic OCS, Alaska OCS): The Bureau of Ocean Energy Management (BOEM) and the Bureau of Safety and Environmental Enforcement (BSEE) administer marine seismic regulations on the US Outer Continental Shelf under 30 CFR Part 551 (Geological and Geophysical Exploration) and Part 250. Operators must submit a G&G permit application to BOEM that includes source array specifications, modelled safety radii for marine mammal impact levels (Level A harassment at 180 dB re 1 microPa rms for cetaceans, Level B at 160 dB re 1 microPa rms), and mitigation and monitoring plans. BOEM requires PSOs aboard all vessels and mandates soft-start procedures. The National Marine Fisheries Service (NMFS) reviews all Gulf of Mexico and Atlantic seismic programs for compliance with the Marine Mammal Protection Act (MMPA) and Endangered Species Act (ESA). The MMPA Section 101(a)(5)(D) Letter of Authorization (LOA) or Incidental Harassment Authorization (IHA) must be in place before seismic operations begin. Airgun surveys near Atlantic coast OCS blocks are particularly sensitive given the proximity of the North Atlantic right whale habitat in the Gulf of Maine.

Norway and the North Sea: The Norwegian continental shelf is one of the most heavily seismically surveyed offshore regions in the world. The Norwegian Offshore Directorate (Sodir, formerly NPD) grants acquisition permits under the Petroleum Activities Act, which requires operators to follow NORSOK G-001 (Marine Soil Investigations) guidelines. The Norwegian Environment Agency oversees environmental compliance, and all seismic surveys must have approved marine mammal mitigation plans. The Joint Nature Conservation Committee (JNCC) guidelines, originally developed for the UK sector but widely referenced across the North Sea, specify a mandatory 2,000 m (6,562 ft) observation zone around the array, a 1-hour pre-clearance period with no marine mammal sightings, and a ramp-up procedure that begins with a single smallest gun and adds guns in stages over a minimum of 20 minutes. The North Sea has been designated a Special Area under MARPOL Annex II for some pollutant categories, and the OSPAR Commission sets overarching environmental standards for North-East Atlantic seismic operations. Equinor, Aker BP, and Shell Norway routinely engage acoustic modelling consultancies to produce seismic source characterisation reports that document near-field and far-field signatures in accordance with ICES (International Council for the Exploration of the Sea) standards.

Middle East (UAE, Saudi Arabia, Qatar): Marine seismic surveys in the Arabian Gulf are conducted primarily by ADNOC (Abu Dhabi National Oil Company), Qatar Energy, and Saudi Aramco Offshore, and are subject to both national regulations and international maritime law. ADNOC's Group Operating Standard ADNOC-AGES-OPS-SE-05 (Environment Management) requires marine mammal impact assessments for any survey involving air gun sources, and pre-survey biological baseline studies are mandatory in environmentally sensitive areas. The Arabian Gulf has shallow average depths (approximately 36 m / 118 ft) that affect air gun bubble dynamics and require careful near-surface corrections in processing. Saudi Aramco's offshore seismic programs in the Red Sea, where water depths exceed 2,000 m (6,562 ft), employ high-capacity tuned arrays. Qatar Energy's North Field seismic programs, covering the world's largest natural gas reservoir, have used 3D and 4D seismic acquisition with fully tuned arrays since the 1990s. The Qatar North Field 4D time-lapse surveys are among the most technically complex in the region, requiring absolute repeatability of source and receiver positioning to detect production-related changes in reservoir properties.