Ocean Acoustic Tomography

Because this technique integrates temperature variations over a large region, the smaller scale turbulent and internal-wave features that usually dominate point measurements are averaged out and we can better determine the large-scale dynamics.

Although travel times must be measured to a nominal accuracy of 1 millisecond, tomographic transmissions consist of long coded signals lasting 30 seconds or more. These transmissions are audible near the source, but over most of the ocean they are below ambient noise levels, requiring sophisticated spread-spectrum signal processing techniques to recover them. Near the source a typical tomographic signal has a distinctive gurgling sound like this computer generated signal.

Greenland Sea Tomography Experiment

In 1988-89 a six-element tomographic array was moored in the abyssal plain of the Greenland Sea Gyre to study deep water formation and the gyre circulation.

Tomographic observations of range-averaged temperature profiles along one path show a warm surface mixed layer in the fall, which cools to the freezing point by late November. For the next several months dense ice covers this region, but although the surface mixed-layer remains at the freezing point it deepens very little. In February and March this path becomes largely ice free, and the mixed-layer rapidly deepens to about 1500m while dramatic surface cooling occurs. The sun rises above the horizon again in late March, and the heating of the water column results in another warm surface mixed layer.

Daily satellite observations of the surface ice cover are formed into a movie. Note the dense ice cover over the array in December and January, followed by very rapid changes until April. During the later part of the winter, the region above the region discussed above always remains ice-free, whereas the surrounding regions are repeatedly covered by ice.

The Greenland Sea Tomography Experiment was a collaboration with members of the Scripps Institute of Oceanography (La Jolla, CA). More details of this analysis can be found in Pawlowicz et. al, "Thermal Evolution of the Greenland Sea Gyre in 1988-1989", Journal of Geophysical Research, Vol.100, No.C3, pages 4727-2750, March 15, 1995.

Barents Sea Polar Front Tomography Experiment

In August of 1992 a three-element tomography array was deployed in the shallow shelf region east of Bear Island in the Barents Sea to study the frontal dynamics in this region. This movie (courtesy of NPS) shows tomographic observations of temperature across the front at 10 minute intervals for about 10 hours.

The Barents Sea Polar Front Tomography Experiment was a collaboration with the Naval Postgraduate School (Monterey, CA) and SAIC. More details of this experiment can be found in Parsons et. al, "The Barents Sea Polar Front in Summer", Journal of Geophysical Research, 1994.

THETIS-2

In 1994, the THETIS-2 tomography experiment took place in the Western Mediterranean. In collaboration with members of the Institut fur Meereskunde (Kiel, Germany), IFREMER (France), and JAMSTEC (Japan), we deployed one instrument in an seven-element tomographic array which was designed to measure seasonal heat content changes in this region. The S-Tether design of this mooring allowed us to telemeter data in real time using the ARGOS satellite system. Engineering and acoustic data from this mooring were replotted every day as new information was received. On June 6th, after 5+ months of perfect performance, the ARGOS transmitter went off the air.

Postcruise examination of the mooring cable, along with the mooring navigation data, showed that fishing activity likely caused the interruption in the telemetry. However, the tomography data was also stored internally so that no valuable arrival time data was lost during the experiment.

More information can be found in an article by Dr. James F. Lynch in "Sea Technology", Vol. 36, No. 9, September 1995, pages 25-31.