Sonar Systems and Oceanographic Research
Sonar systems have been extensively used since the Second World War, and aquatic animals have apparently been able to co-exist with their use. Conventional pulsed signals are used in sonar systems for echo sounding, bottom imaging (side scan and multibeam sonars), bottom and surface scattering studies, fish finding, navigation, communication, and Acoustic Harassment Devices (AHDs). Typical operating frequencies range from a few kHz for a large towed sonar to 100 kHz and above for short range imaging systems. Their output levels range from 180 to 230 dB re 1 µPa at 1 m . The high frequencies used in these systems are rapidly attenuated in range by spreading, scattering and absorption, and therefore the sound usually has a limited range of effect.
More recently, use has been made of low-frequency sonar systems in a range of military and research applications. At the time of writing, full scale tests of marine mammal reactions to the US Navy's Low Frequency Active (LFA) sonar, which operates at sub-kilohertz frequencies, are underway.
Probably the man made source of noise that has generated the most controversy is the low frequency signal used in the Acoustic Thermometry of the Ocean Climate (ATOC) experiment. The ATOC programme was commenced in 1991 and was a collaboration between the Scripps Institution of Oceanography and the Applied Physics Laboratory of the University of Washington.
The initial feasibility trial, based on Heard Island and reported by Munk et al. , generated considerable public outcry. The source for the ATOC programme is a 75 Hz phase modulated sinusoidal signal at a Source Level of 195 dB re 1 µPa at 1 m. Mercer et al.  describes how the ATOC source was installed at a depth of 900 metres on the Pioneer Seamount. Au et al.  reports on a theoretical study of the environmental effects of the ATOC signal, and conclude that small odontocetes such as Pseudorca and Grampus swimming directly above the ATOC source would not hear the signal unless they dived to a depth of 400 metres. Au et al concluded that with the ATOC source on the axis of the deep sound channel it would have minimal physical and physiological effects on cetaceans.
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