R & D Activities: Ocean Acoustic Tomography
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.
||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
- Thetis Group, Open-Ocean Deep Convection explored in the Mediterranean,
EOS Trans. Am. Geophys. U., vol. 75, pp. 217-221, 1994.
- U. Send, F. Schott, F. Gaillard and Y. Desaubies, Observation of
a deep convection regime with acoustic tomography, J. Geophys. Res.
- Oceans, vol. 100, pp. 6927-6941, 1995.
- U. Send, Peak tracking by simultaneous inversion: Toward a one-step
acoustic tomography analysis, J. Atm. Ocean. Techn., vol. 13, pp.
- E. Skarsoulis, G. Athanassoulis and U. Send, Ocean acoustic tomography
based on peak arrivals, J. Acoust. Soc. Am., vol. 100, pp. 797-813,
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
- U. Send, G. Krahmann, D. Mauuary, Y. Desaubies, F. Gaillard, T.
Terre, J. Papadakis, M. Taroudakis, E. Skarsoulis, and C. Millot,
Acoustic observations of heat content across the Mediterranean Sea,
Nature, vol. 385, pp. 615-617, 1997.
- E.K. Skarsoulis, U. Send, One-step analysis of non-linear travel-time
data in ocean acoustic tomography, J. Atmos. Ocean. Techn., vol. 17,
pp. 240-254, 2000.
- E.K. Skarsoulis, A matched-peak inversion approach for ocean acoustic
travel-time tomography, J. Acoust. Soc. Am., vol. 107, pp. 1324-1332,
- E.K. Skarsoulis, Multi-section matched-peak tomographic inversion
with a moving source, J. Acoust. Soc. Am, vol. 110, pp. 786-797, 2001.
- E.K. Skarsoulis, U. Send, G. Piperakis, P. Testor, Acoustic thermometry
of the western Mediterranean basin, J. Acoust. Soc. Am, vol. 116,
pp. 790-798, 2004.
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).
- D. Kindler, U. Send, E.K. Skarsoulis, Relative-time inversions in
the Labrador Sea acoustic tomography experiment, Acta Acoustica, Vol.
87, pp. 738-747, 2001.
- T. Avsic, U. Send, E.K. Skarsoulis, Six years of tomography observation
in the Central Labrador Sea, Proc. Conf. Underwater Acoust. Meas.,