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Figure 2. Schematic diagram of a geological structure derived from acoustic survey data. The different colored bands indicate interfaces between rocks of differing density from which the geological structures can be inferred and the geology associated with faulting, volcanism, oil and gas accumulation, or other geological features of interest can be identified. Available at http://www.noia.org/wp-content/ uploads/2014/01/MarineGeophysical.jpg. Accessed August 26, 2016.
Marine seismic surveys may use energy sources on or in the seafloor (e.g., explosives, drilling noise) (Blackburn et al., 2007). The returned acoustic energy from marine inground sources is detected by geophones (“nodes”) as in land sur- veys.
However, in many cases, water depth and the area to be surveyed dictate that towed source seismic surveys are the most practicable and economical approaches. Most marine seismic surveys, the focus of this article, involve an acoustic energy source above the seafloor, which means that sound is also radiated into the surrounding water. Use of the term “seismic testing” is a neologism coined by recent political advocacy campaigns; “seismic survey” has been consistently used historically to describe the process of collecting acous- tic data for geological research.
Although most seismic surveys are associated with the dis- covery, exploration, and development of oil and gas, seismic surveys are also used for other purposes: harbor and ship channel engineering, geological research, earthquake and tsunami preparedness, site selection for offshore renewable energy installations (wind, tidal, and wave energy), siting of buried cables and pipelines, and support of national ex- panded exclusive economic zone (EEZ) claims (Canadian Broadcasting Corporation [CBC], 2016).
Marine Seismic Sound Sources
The first sound source for marine geophysical imaging was a very short acoustic pulse (milliseconds in duration) pro-
duced by an explosive. Explosives as a sound source have obvious safety and environmental concerns that led geo- physicists to explore other sound sources. Consequently, compressed air sources (“airguns”) are now the most widely used source of impulse sound for marine geophysical imag- ing (Parkes and Hatton, 1986). Electrical discharge sound sources (“sparkers” and “boomers”), water guns, various geomagnetic sensing technologies (Houghton, 2011), and multibeam sonars (International Marine Contractors Asso- ciation [IMCA], 2016) are also used for marine geological surveys, but their properties and applications are beyond the scope of the current article.
Compressed air sources do not produce the ultrasonic shock wave that explosives produce and that are the source of baro- trauma or “blast” injuries in animals exposed to explosives (e.g., Ketten et al., 1993). The term blast is sometimes inap- propriately applied to airguns even though the air emerges at only a fraction of the speed of sound (Parkes and Hatton, 1986; R. Laws, personal communication). But then blast is not an American National Standards Institute (ANSI) or International Organization for Standardization (ISO) stan- dardized term and has been used to describe everything from large explosions to whale sounds (e.g., Thompson et al., 1986).
Compressed Air Sources or Airguns
A typical compressed air source (“airgun”) consists of two air chambers surrounding a piston/shuttle (Figure 3). When the pressure is equal in the two chambers, the ports are blocked by the piston. When the air from one chamber is redirected via a solenoid-activated alternative pathway, the piston is pushed out of the way, allowing the air to escape. The escaping air coalesces into a bubble, thereby generating sound by the ensuing expansions and contractions of the released bubble. The term “gun” can be misleading because there is no directed pulse of air or sound as for a piston, ton- pilz, or conical speaker (Massa, 1989). Directivity is only achieved when multiple airguns are configured in an array.
The sound produced by a compressed air source is a func- tion of the volume, size, and shape of the ports by which the air escapes and the air pressure. The amplitude of the sound increases in proportion to the cube root of the volume of the airgun, which means that doubling the amplitude (adding 6 dB of sound pressure) over that obtained from a 1,000- in.3 chamber (16 L) requires an 8,000-in.3 chamber (131 L) (Landrø and Amundsen, 2010). Instead of using larger airguns to achieve greater source levels, multiple smaller
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