Underwater Acoustic Monitoring Research Articles

Underwater noise as source of information on conditions and dynamics of ocean environments

Leonid Brekhovskikh wrote in his book The Ocean and the Human (1987): ‘‘Ocean noise is as important oceanographic parameter as temperature, current, and wind.’’ Brekhovskikh created and headed the Laboratory of Acoustic Methods of Ocean Research in 1956. One of the scientific directions of the Laboratory was investigation of underwater noise both as interference for sound reception and a source of environmental information. Long-term studies on the unique acoustic research vessels created under the initiative of Brekhovskikh resulted in numerous important findings, including ambient noise spectra and envelopes of acoustic fluctuations, depth dependence of noise directivity, and mechanisms of ambient noise generation. Brekhovskikh was always eager to find practical applications of scientific results. Different methods of ensuring noise immunity of hydroacoustic arrays were developed under his supervision. Passive methods of acoustic navigation based on use of natural noise were suggested. Techniques for underwater acoustic monitoring of the ocean based either on ambient noise analysis or reemission of noise from a point away from the receiving system have been developed. The success of the team of scientists headed by Brekhovskikh was determined by the creative atmosphere around him: there was neither competition nor commercial interests. The common goal was knowledge of the ocean.

Hot topics in acoustical oceanography

Since acoustical oceanography was given full Technical Committee status within the ASA in November 1991, there has been a rapid development of novel techniques for interrogating the ocean using sound. For example, the total global precipitation amount is poorly known; however, by examining the underwater noise produced by rainfall, it may be possible to use underwater acoustic monitoring devices to obtain an estimate of this precipitation and from that better estimates of the global heat flux. Recently, it has been demonstrated that the sound pulses can be propagated over extremely large distances in the ocean; consequently, a precise timing of the arrival of these sound pulses over extended periods would enable changes in the average ocean temperature to be monitored, and thus provide information on global warming. Similarly, ambient noise provides the basis of a new acoustic imaging technique, designated acoustic daylight, that is analogous to conventional photography but based on sound rather than light. These and a variety of other topics will be presented.

The underwater radiated noise characteristics of a fishing trawler

The U.S. Coast Guard is responsible for the protection of fisheries resources within the 200 nautical mile offshore zone. In support of this requirement a study was undertaken to examine the feasibility of applying underwater acoustic monitoring techniques, for detection and classification of fishing trawler activities. The approach was to conduct a set of controlled experiments with a cooperating trawler and to employ aircraft/sonobuoy measurement techniques. The specific objectives of the experiments were to (1) determine if significantly distinctive noise characteristics are generated by the different modes of trawler operation, (2) identify the sources of any distinctive noise characteristics and (3) obtain quantitative source levels generated by the trawler for each mode of operation. The experimental objectives were accomplished and the results show a correlation of fishing operations with the radiated noise of a fishing trawler. Noise signature characteristics indicated dynamic acoustic changes during fishing operations, particularly during net deployment and retrieval. In addition, absolute trawler radiated noise levels (source levels) were determined for various fishing operations. [This work sponsored by the Commandant, United States Coast Guard, Washington, DC.]