Ocean acoustic tomography

The technique relies on precisely measuring the time it takes sound signals to travel between two instruments, one an acoustic source and one a receiver, separated by ranges of 100–5,000 kilometres (54–2,700 nmi).

If the locations of the instruments are known precisely, the measurement of time-of-flight can be used to infer the speed of sound, averaged over the acoustic path.

An oceanographic experiment employing tomography typically uses several source-receiver pairs in a moored array that measures an area of ocean.

[3] These properties motivated Walter Munk and Carl Wunsch[4][5] to suggest "acoustic tomography" for ocean measurement in the late 1970s.

Second, the technique naturally averages over the small scale fluctuations of temperature (i.e., noise) that dominate ocean variability.

As the observational technique has matured, so too have the methods of data assimilation and the computing power required to perform those calculations.

One of the intriguing aspects of tomography is that it exploits the fact that acoustic signals travel along a set of generally stable ray paths.

This is an important, unique property, since the ubiquitous small-scale turbulent and internal-wave features of the ocean usually dominate the signals in measurements at single points.

The average of these reciprocal travel times is the measure of temperature, with the small effects from ocean currents entirely removed.

[1] Starting in 1983, John Spiesberger of the Woods Hole Oceanographic Institution, and Ted Birdsall and Kurt Metzger of the University of Michigan developed the use of sound to infer information about the ocean's large-scale temperatures, and in particular to attempt the detection of global warming in the ocean.

This group transmitted sounds from Oahu that were recorded at about ten receivers stationed around the rim of the Pacific Ocean over distances of 4,000 km (2,500 mi).

[21] The ATOC project was embroiled in issues concerning the effects of acoustics on marine mammals (e.g. whales, porpoises, sea lions, etc.).

[22][23][24] Public discussion was complicated by technical issues from a variety of disciplines (physical oceanography, acoustics, marine mammal biology, etc.)

[25] 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.

Various types of man-made sounds have been studied as potential threats to marine mammals, such as airgun shots for geophysical surveys,[29] or transmissions by the U.S. Navy for various purposes.

The western North Atlantic showing the locations of two experiments that employed ocean acoustic tomography. AMODE, the "Acoustic Mid-Ocean Dynamics Experiment" (1990-1), was designed to study ocean dynamics in an area away from the Gulf Stream , and SYNOP (1988-9) was designed to synoptically measure aspects of the Gulf Stream. The colors show a snapshot of sound speed at 300 metres (980 ft) depth derived from a high-resolution numerical ocean model . One of the key motivations for employing tomography is that the measurements give averages over the turbulent ocean.
Propagation of acoustic ray paths through the ocean. From the acoustic source at left, the paths are refracted by faster sound speed above and below the SOFAR channel , hence they oscillate about the channel axis. Tomography exploits these "multipaths" to infer information about temperature variations as a function of depth. Note that the aspect ratio of the figure has been greatly skewed to better illustrate the rays; the maximum depth of the figure is only 4.5 km, while the maximum range is 500 km.
The ATOC prototype array was an acoustic source located just north of Kauai, Hawaii, and transmissions were made to receivers of opportunity in the North Pacific Basin . The source signals were broadband with frequencies centered on 75 Hz and a source level of 195 dB re 1 micropascal at 1 m, or about 250 watts. Six transmissions of 20-minute duration were made on every fourth day.