Michael J. Buckingham (619) 534-7977 - mjb@mpl.ucsd.edu
Marine Physical Laboratory
Scripps Institution of Oceanography
La Jolla, CA 92093-0213
Popular version of paper 3pID2
Presented Wednesday Afternoon, June 18, 1997
133rd ASA Meeting, State College, PA
Embargoed until June 18, 1997
Since the days of antiquity, sound has been used to provide information about the ocean. The reason is simple: sound waves travel easily through seawater, whereas other forms of radiation do not. Naturally, the early systems for detecting sound in the sea were primitive, often taking the form of a listening rod with one end beneath the surface and the other placed close to the ear of the listener. Todays technology is rather more advanced but the objective remains much the same, to detect sound in the ocean with a view to gaining information about processes occurring at or beneath the sea surface. Such acoustic "inversions" form the essence of acoustical oceanography. A few of the recent developments in acoustical oceanography research are briefly described in this article.
The sounds of water, whether from a stream flowing over rocks or a trickle from a tap, are almost always due to bubbles. Natural sound in the ocean is no different. Breaking waves create large populations of bubbles, which act briefly as efficient acoustic resonators, filling the sea with sound. These same bubbles are also important in transporting air from the atmosphere into the ocean, a process which has a direct bearing on global warming. Is it possible to estimate the gas fluxes across the air-sea interface by simply listening to the sounds of breaking waves? Several techniques are under development which address this question. A preliminary issue is the detailed structure of bubble fields in the turbulent environment immediately beneath the sea surface. The new acoustic approaches are providing information on the bubble size distribution , the proportion of acoustically active to quiescent bubbles, and also the evolution and spatial extent of bubble plumes. Once such factors have been characterized, it may be possible to develop reliable acoustic inversion techniques for quantifying the gas fluxes.
Weather stations are densely distributed on land but are very thinly spread off-shore. Accurate weather forecasting relies heavily on input data such as rainfall rate, which is difficult to acquire over most of the ocean. A simple acoustic technique, using a single hydrophone (underwater microphone) for listening to the sound of rain drops on the sea surface is currently showing promise as an effective means of monitoring rainfall at sea.
The geo-acoustic properties of ocean sediments are important in connection with off-shore engineering structures, marine cable laying, and acoustic propagation modelling in shallow water. Traditionally, the properties of the seabed have been determined using very energetic, dedicated acoustic sources. Recently, however, naturally generated sound in the ocean has been exploited to determine sediment type. In these applications of natural sound, the sources include the low-frequency signals from marine mammals and the higher-frequency, broadband signatures of breaking waves.
Sediment transport is of particular concern in estuarine and coastal environments, where the action of currents and tides may lead to the blockage of seaways. The relative motion of the sediment grains as they roll over one another creates a characteristic acoustic signature. This sound signal provides the basis of an acoustic system for continuously monitoring the condition of the seabed in regions where mobility of the sediment is a concern.
Sound transmissions in the ocean tend to be highly variable because the medium supporting the acoustic field is itself fluctuating. The fluctuations are associated with internal waves, turbulence, temperature variability, fronts and eddies. Recently, the effect of the medium fluctuations on acoustic transmissions has been largely eliminated through the use of a technique known as phase conjugation in which the signal received at an acoustic array is time-reversed and re-transmitted over the original propagation path. As well as stabilizing the final arrival, the time-reversal method offers the possibility of characterizing the fluctuations in the medium, which in turn should provide information on mixing processes and related phenomena in the ocean.
Acoustical oceanography is currently an area of intensive activity. Novel techniques are constantly being developed in which sound is exploited to yield hitherto unattainable information on ocean processes. Many of the new methods are based on naturally generated sound in the ocean, created by both physical and biological sources. These sound signatures are providing information about the sources themselves and the medium through which they travel.