Long-term trends in the use of chemical explosive devices also may be taking place. Any speculation on these trends must exclude times of war (the underwater noise created by explosions during the great naval battles in World War II must have been extremely high) since the occurrence of war and resulting contributions to the ocean noise field are highly unpredictable and extremely episodic. Long-term trends in the use of smaller explosive devices also may be taking place. As mentioned previously, explosive sources used in seismic exploration are being replaced by air-gun arrays. However, explosives are routinely used to sever abandoned well-heads so that they can be removed and to decommission the rigs themselves. As oil production in a given region matures and declines, the use of explosives in this way increases. The use of explosives in ocean acoustic and geophysical research has decreased, but these sources still are deployed in a few experiments. Military use of explosive charges as the source component in active sonar systems (e.g., SUS; Urick, 1975) appears to be decreasing. Hull shock tests are rare events and do not appear to be changing significantly in frequency of occurrence. The use of seal bombs has been discouraged by U.S. and international regulations and is being replaced by other types of acoustic deterrent devices. Fishing by the detonation of underwater explosives (a technique whose success improves with its increasing adverse impact on the marine environment) is banned but still is practiced in some regions. In any case, one quantitative measure of the long-term change in numbers and spatial distribution of underwater explosions is possible to obtain, at least for the North Pacific Ocean. The number and estimated source locations of detonations recorded over a modern-day period of time could be compared to those recorded by 20 Missile Impact Location System hydrophones over a one-year period from August 1965 to July 1966 (following Spiess et al., 1968). Nearly 20,000 explosions were detected within this one-year period, with the winter rate of occurrence of 300 explosions per month increasing to 4,000 explosions per month in summer. The highest activity was detected off the west coast of North America, in the Gulf of Alaska, north of Hawaii, and seaward of the Japanese and Kuril Islands (Spiess et al., 1968). The significance of any of these possible changes in the occurrence of underwater explosions to the marine environment is unknown.

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