Ocean Acoustic Tomography - Acoustic Transmissions and Marine Mammals

Acoustic Transmissions and Marine Mammals

The ATOC project was embroiled in issues concerning the effects of acoustics on marine mammals (e.g. whales, porpoises, sea lions, etc.). Public discussion was complicated by technical issues from a variety of disciplines (physical oceanography, acoustics, marine mammal biology, etc.) that makes understanding the effects of acoustics on marine mammals difficult for the experts, let alone the general public. Many of the issues concerning acoustics in the ocean and their effects on marine mammals were unknown. Finally, there were a variety of public misconceptions initially, such as a confusion of the definition of sound levels in air vs. sound levels in water. If a given number of decibels in water are interpreted as decibels in air, the sound level will seem to be orders of magnitude larger than it really is - at one point the ATOC sound levels were erroneously interpreted as "louder than 10,000 747 airplanes". In fact, the sound levels employed, 250 W, were comparable those made by blue or fin whales, although those whales vocalize at much lower frequencies. The ocean carries sound so efficiently that sounds do not have to be that loud to cross ocean basins. Other factors in the controversy were the extensive history of activism where marine mammals are concerned, stemming from the on-going whaling conflict, and the sympathy that much of the public feels toward marine mammals.

As a result of this controversy, the ATOC program conducted a $6 million study of the effects of the acoustic transmissions on a variety of marine mammals. After six years of study the official, formal conclusion from this study was that the ATOC transmissions have "no significant biological impact".

Other acoustics activities in the ocean are not so benign insofar as marine mammals are concerned: air gun shots for geophysical surveys, and some of the louder transmissions by the U.S. Navy for various purposes are documented to present a threat to marine mammals. The actual threat depends on a variety of factors beyond noise levels: sound frequency, frequency and duration of transmissions, the nature of the acoustic signal (e.g., a sudden pulse, or coded sequence), depth of the sound source, directionality of the sound source, water depth and local topography, reverberation, etc.

In the case of the ATOC, the source was mounted on the bottom about a half mile deep, hence marine mammals, which are bound to the surface, were generally further than a half mile from the source. This fact, combined with the modest source level, the infrequent 2% duty cycle (the sound is on only 2% of the day), and other such factors, made the sound transmissions benign in its effect on marine life.