Air Shooting: Definition, Poulter Method, and Seismic History

What Is Air Shooting?

Air shooting is a seismic acquisition method in which explosive charges are detonated in free air, either suspended from poles above the ground surface or carried aloft by balloons, to generate elastic waves that travel into the subsurface. Also called the Poulter method after American geophysicist Thomas Poulter, it was widely used from the 1930s through the 1950s and remains a niche technique in environments where drilling shot holes is impractical.

How Air Shooting Works

In a conventional reflection seismic survey using dynamite, the explosive charge is placed at the bottom of a drilled shot hole, typically 6-30 m (20-100 ft) deep and below the base of the low-velocity weathered zone. This positions the charge in competent bedrock or consolidated sediment, maximizing seismic energy transmission directly into the high-velocity subsurface and minimizing the generation of horizontally propagating surface waves (ground roll). Air shooting inverts this geometry: the charge hangs on a wooden or steel pole 1-3 m (3-10 ft) above the ground surface, or is suspended from a helium-filled balloon at heights of 3-15 m (10-50 ft), and is detonated electrically by the recording crew. The explosive's pressure wave radiates spherically outward in air. When the downward-propagating wave reaches the ground surface, a portion of the energy transmits into the solid earth as a P-wave (compressional wave), while the remainder reflects back upward. This transmission is governed by the acoustic impedance contrast between air (density approximately 1.2 kg/m³; velocity 343 m/s / 1,125 ft/s) and the near-surface soil or rock (density typically 1,500-2,200 kg/m³; velocity 300-1,500 m/s / 980-4,920 ft/s), which is a very large impedance contrast and results in only a small percentage of the incident energy transmitting into the ground, typically 0.1-1% of the total explosive energy.

To compensate for this poor coupling efficiency, air-shooting surveys historically used large charge sizes, ranging from 5-50 kg (11-110 lb) of dynamite per shot point, compared to 0.25-2 kg (0.5-4.4 lb) typical for in-hole detonations at comparable depths. Charge size was constrained by blast radius safety requirements: Canadian Standards Association (CSA) blasting regulations and US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) guidelines both specify minimum standoff distances from structures and persons as a function of net explosive weight (NEW). The pole height also influenced the frequency content of the downward-propagating wave. Field experiments in the WCSB during the 1940s by Imperial Oil and Socony-Vacuum Geophysical (later Mobil) showed that poles of 1.5-2.5 m (5-8 ft) produced the most useful reflection energy in the 20-60 Hz band for exploring Devonian carbonate targets at 1,500-3,000 m (4,900-9,800 ft) depths.

Signal processing of air-shooting data requires specific corrections that differ from those applied to in-hole shot data. The uphole time correction, which normally uses the direct arrival at a surface geophone to determine the shot hole depth and the weathering layer velocity, does not exist for air shots because the source is above the surface. Instead, processors use the air-wave arrival (traveling at 343 m/s / 1,125 ft/s) recorded on the nearest geophones to establish the shot time datum and to model the near-surface velocity structure. Air-wave suppression is a mandatory processing step: the air blast generates a high-amplitude, 343 m/s (1,125 ft/s) coherent noise train on all geophones that must be muted or FK-filtered before reflection processing. Modern reprocessing of heritage air-shot data employs surface-consistent deconvolution to remove the source signature, but the inherent bandwidth limitation of the air-coupled source means that reprocessed data typically achieves a dominant frequency of 30-55 Hz rather than the 60-100 Hz achievable with modern vibroseis or in-hole dynamite. See also vertical seismic profile for how modern borehole seismic methods overcome near-surface coupling problems that plagued air-shooting surveys.

Air Shooting Across International Jurisdictions

Canada: Western Canada Sedimentary Basin Heritage Surveys

Air shooting has deeper historical roots in Canada than in any other jurisdiction. The first commercial air-shooting surveys in Canada were acquired in Alberta and Saskatchewan between 1939 and 1952 by seismic contractors including Geophysical Service Incorporated (GSI, later the parent of Texas Instruments' defense division), Seismos GmbH (the German-origin contractor that introduced reflection seismology to North America), and Imperial Oil's exploration division. The technique was chosen because the vast majority of the Western Canada Sedimentary Basin lies on flat agricultural prairie or northern muskeg and boreal forest, where drilling shot holes required expensive water hauling or was logistically impossible in winter freeze-thaw conditions. The Alberta Energy and Utilities Board (now the Alberta Energy Regulator, AER) and the Saskatchewan Ministry of Energy and Resources do not formally catalog air-shooting surveys as a distinct data type, but the Canadian Society of Exploration Geophysicists (CSEG) and Canada-Newfoundland and Labrador Offshore Petroleum Board (CNLOPB) archives contain hundreds of air-shot field records from the WCSB that have been digitized since the 1990s. These heritage surveys were legally required to be submitted to provincial energy regulators as part of exploration well license conditions, and many are now publicly available through the Alberta Geological Survey and the Saskatchewan Geological Survey digital data portals. In the Mackenzie Delta and Yukon regions, air shooting was used through the early 1970s because permafrost made conventional shot-hole drilling impractical without heated drilling fluids, which were both expensive and environmentally concerning long before formal northern development regulations were enacted under Canada's Mackenzie Valley Resource Management Act.

United States: Appalachian Basin and Gulf Coast Origins

Air shooting was introduced to the United States in the mid-1930s and saw its greatest commercial application in the Appalachian Basin of Pennsylvania, West Virginia, and Ohio, and in parts of the Rocky Mountain Overthrust Belt where surface topography made conventional drilling extremely difficult. The Appalachian surveys of the 1940s and early 1950s targeted the Knox Dolomite, Oriskany Sandstone, and Clinton Sandstone plays at depths of 600-2,400 m (2,000-8,000 ft). The technique was later supplanted by weight-drop and vibroseis methods as environmental regulations restricting near-surface blasting became more common during the 1960s and 1970s. The Gulf of Mexico shallow-water transition zone, where water depths of 0.3-3 m (1-10 ft) preclude conventional marine air-gun deployment and make shot-hole drilling impractical, saw brief revival of elevated air-charge techniques in the 1970s using rubber-mounted poles on shallow-draft barges, though these were quickly replaced by marine vibrators and boomers. The US Geological Survey (USGS) maintains an archive of Appalachian air-shot field records at its National Center in Reston, Virginia, and several academic institutions including Penn State University have used these legacy datasets to build structural models of the Valley and Ridge Province that would be impossible to acquire today under Pennsylvania's Act 13 of 2012 oil and gas regulations, which impose strict setback requirements from waterways, buildings, and agricultural land.

Middle East and North Africa: Desert Terrain Applications

While air shooting is primarily associated with high-latitude and vegetated terrains, it was also deployed in desert environments during the earliest phases of Arabian Peninsula exploration. Saudi Aramco's predecessor entities, along with IPC (Iraq Petroleum Company) and AIOC (Anglo-Iranian Oil Company, predecessor to BP) carried out air-shooting surveys in the Rub' al Khali (Empty Quarter) and Iraqi desert during the 1940s and 1950s, where the extremely hard calcrete and gypsite desert pavement at the surface made conventional shot-hole drilling with percussion methods time-consuming and expensive. Saudi Aramco's historical archives, partially declassified and published in its centennial history books, describe survey designs using 15-25 kg (33-55 lb) charges at pole heights of 2-3 m (7-10 ft) and geophone spreads of 600-2,400 m (2,000-8,000 ft) aperture. The resulting seismic sections, reprocessed in the 1980s using digital techniques, contributed to the structural understanding of the Ghawar field anticline flanks. In Libya and Algeria, AGIP (Eni's predecessor) and CFP (Total's predecessor) used air shooting to establish stratigraphic frameworks for the Sirte Basin and Hassi R'Mel areas before upgrading to vibroseis in the late 1960s. Neither Saudi Aramco's regulator (the Saudi Ministry of Energy) nor the National Oil Companies of Iraq or Algeria maintain explicit technical standards for air shooting because the method is no longer practiced; however, historical data custodianship responsibilities are addressed in each country's petroleum data law.

Norway and the North Sea: Limited Historical Application

Air shooting saw limited application in the North Sea region because the exploration frontier rapidly moved offshore after the 1959 Groningen gas discovery and the 1969 Ekofisk oil discovery, where marine air-gun sources were entirely adequate. However, in the onshore areas of southern Norway, Sweden's Gotland Basin, and Denmark's onshore Jutland region, air shooting was used by Geophysical Service Incorporated and Prakla-Seismos (West German contractor) in the early 1960s to build regional structural maps ahead of offshore licensing rounds. The Norwegian Petroleum Directorate (NPD, now Norwegian Offshore Directorate) and the Danish Energy Agency do not formally archive these onshore heritage surveys under their petroleum data regulations, which focus on the continental shelf. The onshore surveys are held by the Geological Survey of Norway (NGU) and the Geological Survey of Denmark and Greenland (GEUS). In northern Norway (Finnmark) and Swedish Lapland, air shooting was considered for Arctic swamp and peat bog terrain similar to the Canadian muskeg context, but most exploration in these areas shifted directly to vibroseis technology in the 1970s, so the Norwegian-specific legacy of air-shooting data is thinner than in Canada or the United States. Modern seismic acquisition on the Norwegian Continental Shelf uses only marine sources and no air-shooting analogs, though the Norwegian Environment Agency (Miljodirektoratet) has published guidelines on marine seismic sound emissions that use air-shooting's historical energy coupling principles to model near-surface geological noise in coastal acquisition.