![]() Here, ‘dB re 1 mPa-m’ means decibel value peak-to-peak relative to the reference pressure one micropascal at a reference distance of one metre. By filtering out high frequencies there is less deviation between amplitude levels vertically and horizontally. ![]() The amplitude levels emitted horizontally tend to be 15-24 dB lower. The broad band of frequencies from the array form a pulse with peak-to-peak amplitude in the range 14-28 bar-m, corresponding to 243-249 dB re 1 mPa-m vertically downward. The energy sent out by air-gun arrays is dominantly directed vertically downwards. The main purpose of clustering is to improve signal characteristics, since the bubble motion (see below) is reduced by this configuration. ![]() It is common to arrange several (2-4) air-guns in a cluster, with the guns so close together that they behave as a larger single gun. The gun pressure mostly used by the seismic industry is 2,000 psi (138 bar) and during a survey, the guns fire every 10-15 seconds. The air-guns hang in the sea beneath floats between 3m and 10m below the sea surface, generally at about 6m, except for refraction studies when a deeper deployment is needed. The air-gun array volume is the sum of the volumes of each gun, and is typically in the range 3,000-8,000 in 3 (49.2-131.6 l). An air-gun array consists of 3 - 6 sub-arrays called strings, each string containing 6 - 8 individual guns, so that the array usually involves between 18 and 48 guns, although in special cases as many as 100 guns an array can be used. Typical volumes of individual air-guns used by the exploration industry vary from 20 in 3 (0.3 l) to 800 in 3 (13.1 l), while academic seismic refraction studies can use volumes up to 1600 in 3 (26.2 l). Top: Single air-gun and three-gun cluster (Bottom: Pictures of air bubbles 1 ms, 1.5 ms, and 7 ms after firing a small air-gun in a tank (Langhammer 1994).Īn air-gun volume is measured in litres (l) or more commonly, by the conservative petroleum geophysicist, in cubic inches (in 3). The air gun is the most commonly used source because the pulses are predictable, repeatable and controllable, it uses compressed air which is cheap and readily available, and it has only a minor impact on marine life. ![]() The air-gun can be described as a chamber of compressed air that is released rapidly into the surrounding water to create an acoustic pulse. The major source type in marine exploration is the air-gun array, which since the 1970’s has been by far the most popular. A seismic source is defined as any device which releases energy into the earth in the form of seismic waves. ![]()
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