R & D Activities: Ocean Acoustic Tomography
Large-scale experiments

IACM has participated in the design and analysis of large-scale tomography experiments conducted in the Mediterranean Sea (Thetis and Thetis-2 experiments) and in the North Atlantic Ocean (Labrador-Sea experiment), in cooperation with IfM Kiel (recently renamed to Leibnitz Institute) and IFREMER. The main contribution by IACM was in the development of modeling and analysis methods for the automated processing of large and increasingly complex tomographic data sets.

THETIS

The Thetis experiment was conducted in winter 1991-92 to acoustically observe the development of deep convection (vertical mass transfer from the surface to the deep) taking place in the Gulf of Lions. The experiment involved six tomographic transceivers located at a nominal depth of 150m, daily emitting and receiving tomographic signals of central frequency 400 Hz and bandwidth 100 Hz. The Thetis project was supported by the European Union Marine Science and Technology (EU/MAST) program.
Tomography results: Evolution of average temperature profile along section T3-T6 obtained from wave-theoretic inversion, indicating gradual weakening of stratification in late January - day zero corresponds to 14 Dec. 1991
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THETIS-2

The Thetis-2 tomography experiment was a continuation of the Thetis experiment on the basin scale and served as a pilot for a permanent monitoring system of the western Mediterranean Basin. Seven acoustic transceivers operating at 250/400 Hz were deployed around the basin at a depth of 150 m. Daily transmissions over a period of 9 months (January-October 1994) provided a large set of travel-time data along 13 tomographic sections. Along a particular section (W3-H) a parallel systematic XBT survey was conducted every two weeks to serve as a basis for validation of the acoustic inversion results. The analysis of the complete Thetis-2 data set was performed automatically by applying a matched-peak inversion approach. The Thetis-2 project was supported by the European Union Marine Science and Technology (EU/MAST) program.

Evolution of the heat content (average water temperature in the upper 2000 m) of the western Mediterranean basin obtained from tomography (combination of 13 sections). Comparison with heat content estimated from ECMWF heat-flux data (with corrections applied for the Gibraltar/Sicily straits) and with hydrographic (CTD) data.
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LABRADOR SEA

The Labrador Sea tomography experiment was conducted by IfM Kiel as part of the German SFB460 project from August 1996 to September 2003 - the longest ocean tomography experiment ever conducted. The experiment aimed at acoustically monitoring water mass formation in the Central Labrador Sea, a key process for the renewal of the Atlantic Ocean deep water. The tomographic array consisted of 3-4 transceivers operating at 250/400 Hz redeployed on an annual basis. The routine analysis of 6 years of travel-time data was based on the matched-peak inversion approach developed at IACM.

Evolution of heat content (average over the upper 1300 m) along the southmost section (Kx1-Kx2) obtained from tomograph. Comparison with (i) the time integral of surface heat fluxes from NCEP (ii) data from temperature sensors mounted on the moorings Kx1 and Kx2, (iii) hydrographic data collected close to Kx1 and Kx2 (small circles) and (iv) hydrographic data averaged along the section Kx1-Kx2 (large circles).
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