Sources of ambient noise in littoral waters

During the so-called "cold war time", less than 10 years ago, the major interest in underwater acoustics was related to deep waters in particular in the North Atlantic and the Pacific Oceans where the deep-going, silent and very large, nuclear powered attack submarines of the "East" and "West" were operating. The considerably more silent regions of the deep waters were one of the main reasons for the particular emphasis on passive listening, use of towed arrays etc., during these years. The political situation changed drastically with the disappearance of the Warsaw Pact and the break down of the Soviet Union in the late eighties and early nineties. The need for operations in the deep waters was strongly reduced, but the Gulf War, the situation in the former Yugoslavia, the risk of local wars at other places around the World, and in particular the proliferation of small, diesel driven submarines developed for operation in shallow waters near their home bases gave rise to a considerably increased interest in underwater acoustical studies of the shallow water in the littoral areas. Active sonars became of increasing interest and as operations against hostile submarines, mine fields, etc. may take place in unknown waters far from the home bases, the concepts of "Rapid Environmental Assessment" were developed. Moreover, exploitation of resources in the Exclusive Economical Zone (EEZ) defined by several countries, and resources in and on the continental shelf, together with the defence of the boundaries for the territorial seas, the EEZ's and the continental shelf have led to a "deadly triangle" of economical growth, marine resources and naval arms, which have called for increased interests in the acoustics of the shallow waters of these zones. The turn in interests from the deep to the shallow littoral waters, however, showed the need for studies of sound propagation in regions where the influence of boundaries - seabed and sea surface -, of inhomogeneities in the water column, of considerable variations in temperature and salinity with decisive effects on the sound velocity profiles, and in particular of much higher levels and another spectral composition of ambient noise were prevailing. The ambient noise, frequently leading to a strongly reduced signal/noise ratio, is of importance at higher frequencies than in deep waters due to shorter ranges in shallow waters. Moreover, the sources of ambient noise constitute another "mix" in shallow than in deep waters. Therefore, the identification of sources of ambient noise has received renewed attention in shallow water areas.