Air Gun
An air gun is a pneumatic seismic source device that stores a volume of compressed air at high pressure (typically 1,500 to 2,000 psi, or 103 to 138 bar) and releases it instantaneously into the surrounding water column, generating a broadband acoustic pulse that propagates downward through the water and into the subsurface formations, reflects off geological interfaces, and returns to hydrophone receivers towed behind the survey vessel or deployed on the seafloor. The sudden release of compressed air from the gun chamber creates an expanding bubble that generates the primary pressure pulse as it expands rapidly from the ambient water pressure, followed by a series of secondary pulses as the bubble oscillates (compresses and expands) under the inertia of the surrounding water and the restoring force of the compressed gas inside, a phenomenon called the bubble effect. Individual air guns range in chamber volume from 5 cubic inches (80 cm³) for high-frequency Mini-GI or sleeve guns used in shallow-hazard surveys and vertical seismic profiling (VSP), to 2,000 cubic inches (32,800 cm³) for the largest single guns used in deep-crustal reflection surveys, but are almost always deployed in synchronised arrays of 20 to 48 individual guns with different chamber volumes tuned to suppress the oscillating bubble while reinforcing the primary pressure pulse. The result of array tuning is a broadband source spectrum with a dominant frequency of 5 to 150 Hz (depending on gun depth, volume configuration, and array geometry), sufficient to image structure and stratigraphy from the seafloor to crustal depths of 10 to 15 km. In Canada, air gun acquisition is governed by the Canada-Newfoundland Offshore Petroleum Board (CNLOPB), the Canada-Nova Scotia Offshore Petroleum Board (CNSOPB), and the Canada Energy Regulator (CER) for frontier offshore areas, with marine mammal mitigation requirements set by Fisheries and Oceans Canada (DFO) under the Species at Risk Act and the Fisheries Act.
Key Takeaways
- The four principal air gun types differ in firing mechanism, bubble suppression, and frequency content, and are selected based on survey depth target, vessel constraints, and marine mammal mitigation requirements: Sleeve guns (Bolt Technology PAR series) use an annular sleeve that slides axially to expose the chamber port, releasing air uniformly around the circumference and generating a clean spherically symmetric pulse with minimal moving-part turbulence. Bolt guns (Bolt Technology Model 1500) use a T-shaped firing bolt that is hydraulically driven to open the port, a robust and widely used design for standard 3D marine surveys. G-guns (SERCEL) use two opposed firing ports on opposite sides of the gun body that fire simultaneously, with the recoil forces cancelling and reducing horizontal vessel noise. Mini-GI guns (SERCEL) consist of two concentric chambers (a small Generator chamber and a larger Injector chamber) where the Injector fires a short delay after the Generator, injecting air into the first bubble at the moment of maximum expansion to cancel the bubble oscillation, dramatically improving the primary-to-bubble ratio (PBR) and providing a cleaner pulse suitable for high-resolution surveys. The Mini-GI is the preferred source for site survey, shallow geohazard, and VSP work where high frequency (up to 300 Hz) and clean waveform are more important than total acoustic energy.
- The primary-to-bubble ratio (PBR) is the key waveform quality metric for air gun performance, and array tuning using guns of different volumes is the primary engineering method for maximising PBR in standard 3D marine acquisition: The oscillating bubble created by air gun discharge generates secondary and tertiary pressure pulses at frequencies determined by the Rayleigh-Willis formula: Tb = 1.83 × V1/3 / (Pair + Phydrostatic)5/6, where Tb is the bubble period in seconds, V is the chamber volume in m³, and pressures are in bars. Because Tb scales as V1/3, guns of different volumes have different bubble periods. By arraying guns so that each gun's bubble period is a different fraction of the primary pulse period, the bubble pulses from different guns arrive at different times and cancel each other at the composite wavefield level, while the primary pulses from all guns, which are simultaneously fired, add constructively. A well-tuned array of 48 guns in three sub-arrays can achieve a PBR of 25:1 to 50:1 (in field units) compared to a PBR of 3:1 to 8:1 for a single gun, making the array far superior to any single gun for seismic imaging purposes.
- The ghost notch is a frequency-domain notch in the air gun source signature caused by destructive interference between the downward-propagating primary pulse and its sea-surface reflection, and its frequency is determined by gun depth as fnotch = vwater / (2 × d): The sea surface acts as a near-perfect acoustic reflector with a reflection coefficient of -1 (the pressure wave inverts on reflection). When the downward-primary pulse from the gun is followed by the upward-reflected ghost with opposite polarity and a delay of 2d/vwater (where d is gun depth in metres and vwater ≈ 1,500 m/s), the two pulses cancel at frequencies where the two-way travel time equals half a wavelength: fnotch = v/(2d). At a standard gun depth of 6 m, fnotch = 1,500/(2×6) = 125 Hz; at 8 m depth, fnotch = 94 Hz; at 12 m, fnotch = 62.5 Hz. The ghost notch removes a narrow but significant frequency band from the source spectrum that cannot be recovered by processing. Over/under gun arrays mitigate the ghost by deploying pairs of guns at different depths (e.g., 5 m and 10 m) so that the notch frequencies of the two guns interleave, filling each other's notch and producing a broader effective bandwidth. This configuration is now standard practice on Equinor and ExxonMobil deep-water acquisition programmes in offshore Atlantic Canada.
- Marine mammal mitigation protocols govern all air gun operations in Canadian and international offshore waters, requiring exclusion zones, soft-start (ramp-up) procedures, and protected species observers (PSOs) to minimise acoustic injury risk to cetaceans and pinnipeds: Fisheries and Oceans Canada (DFO) specifies that air gun arrays must not fire within 500 m of observed large cetaceans (baleen whales) and must implement a ramp-up procedure (starting with a single lowest-volume gun and adding guns over 20 to 30 minutes) after any shut-down of more than 15 to 20 minutes to allow cetaceans that may have moved into the area without detection to move away. The US Bureau of Ocean Energy Management (BOEM) applies Level A disturbance thresholds of 180 dB re 1 μPa² (cetaceans) and 190 dB re 1 μPa² (pinnipeds) for physical injury, and Level B behavioural disturbance thresholds of 160 dB re 1 μPa², with shutdown required when any marine mammal is detected within the Level A exclusion radius. In UK waters, the Joint Nature Conservation Committee (JNCC) guidelines specify a 500 m exclusion zone around the gun array and require acoustic deterrent devices (pingers) to be operating 30 minutes before gun firing begins at the start of each survey line. Two PSOs must be on watch continuously during gun firing on all Canadian offshore seismic surveys, with a minimum visual observation range of 500 m during daylight and passive acoustic monitoring during low-visibility periods.
- Vertical seismic profiling (VSP) air gun operations use a single small-volume gun or a short sub-array at the sea surface or in a water-filled pit on land, providing a near-wellbore seismic image with higher resolution than surface seismic and enabling direct calibration of surface seismic to well data: In a standard offset VSP, a single 40 to 120 cubic inch air gun or a small two-gun sub-array is positioned at 3 to 6 m water depth adjacent to the wellhead while a string of hydrophone or geophone receivers is suspended in the wellbore at intervals of 5 to 15 m from the surface casing shoe to near total depth. The VSP records the seismic wavefield directly at the reservoir level, eliminating the downward-and-upward two-way path of surface seismic and providing 2 to 3 times the bandwidth of equivalent surface seismic. In offshore Nova Scotia, all major exploration wells drilled by Shell, ExxonMobil, and BP have included walk-away VSP surveys using a 3-gun 210 cubic inch sub-array at 5 m depth to calibrate the Grand Banks 3D seismic to well synthetic seismograms, with the VSP data processed by Schlumberger Wireline at a cost of approximately CAD 280,000 to 420,000 per well-site acquisition and processing programme.
Air Gun Array Design and Signature Modeling
A modern marine seismic source array consists of two to four sub-arrays, each containing 8 to 16 individual guns suspended from a float rig running behind or to the side of the vessel. The guns in each sub-array are arranged in a geometric pattern (typically a line or a cluster) with inter-gun spacings of 1 to 3 m. The total array volume is the sum of all individual gun volumes and is the primary determinant of low-frequency output energy and source depth of investigation: 4,000 to 6,000 cubic inch total volumes are typical for deep-exploration programmes targeting sub-salt or deep-crustal reflectors, while 1,000 to 2,500 cubic inches are standard for shelf 3D programmes targeting features at 2 to 5 km depth. Sub-arrays are typically separated by 50 to 100 m of streamer spread to produce a source that is wide enough to reduce vessel noise coupling to the near-field hydrophones but narrow enough to fit within a practical shooting geometry.
Source signature modeling predicts the far-field pressure wavelet expected from a given gun configuration before acquisition begins, allowing the crew to optimise gun timing (synchronisation offsets of 0 to 200 milliseconds between sub-arrays or between individual guns) to maximise the PBR. Notional source signatures for each individual gun are measured during initial source testing using a near-field hydrophone mounted 1 m from each gun, and the array signature is computed as the vector sum of the individual signatures accounting for travel time from each gun to each receiver location. The computed far-field signature is provided to the processing contractor as the source wavelet for designature (wavelet deconvolution) during seismic data processing.
Air Gun Operations in Vertical Seismic Profiling
VSP operations require a source that is compact, repeatable, and manageable near the drilling vessel without interfering with rig operations. Single air guns or small two-gun sub-arrays of 60 to 240 cubic inches total volume are standard, fired from a gun boat positioned 50 to 300 m from the wellhead or from a purpose-built float rig alongside the vessel. Repeatability is paramount in VSP because multiple shots are stacked at each depth level to improve signal-to-noise ratio: a gun that fires inconsistently (variable muzzle velocity, variable bubble suppression) introduces shot-to-shot wavelet variation that degrades the stack quality and reduces the effective frequency content of the VSP image. Bolt PAR air guns are preferred for VSP applications because of their consistent repeatability (< 0.2% shot-to-shot volume variation) and because the shuttle mechanism provides clean waveform reproducibility across thousands of consecutive shots without mechanical wear degradation.
Fast Facts
The air gun was invented in 1964 by Bolt Technology Corporation's Ray Mifsud and William Dragoset, and rapidly replaced dynamite as the dominant marine seismic source by the early 1970s because of its repeatability, safety, and lack of explosive licensing requirements. The first commercial 3D marine seismic survey using air gun arrays was acquired in the North Sea in 1975 by Western Geophysical, introducing the multi-sub-array towed configuration that remains the industry standard. SERCEL, the seismic equipment subsidiary of CGG, introduced the G-gun in 1979 and the Mini-GI in 1987, both of which became industry standards for bubble suppression and VSP applications respectively. The Canadian offshore seismic acquisition market is served primarily by CGG, TGS, and WesternGeco (SLB), which collectively operate the majority of dedicated 3D marine acquisition vessels operating in the Grand Banks (Newfoundland), Scotian Shelf (Nova Scotia), and Beaufort Sea. Fisheries and Oceans Canada issued updated marine mammal mitigation guidelines for seismic surveys in 2022, extending the mandatory exclusion zone for baleen whales from 500 m to 1,000 m in the Gulf of St. Lawrence during bowhead and North Atlantic right whale migration periods (May through November), affecting the acquisition schedules of all seismic programmes on the Scotian Shelf and Newfoundland continental shelf during these months.