An Experimental Demonstration of Blind Ocean Acoustic Tomography

S.M. Jesus and C. Soares
SiPLAB - FCT, Universidade do Algarve,
Campus de Gambelas,
PT-8005-139 Faro, Portugal.

E. Coelho
NATO Undersea Research Centre
Viale San Bartolomeo
I-19138 La Spezia, Italy.

P. Picco
ENEA, Marine Environment Research Centre
P.O. Box 224
I-19100 La Spezia, Italy.

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Ref.: Journal of the Acoustical Society of America, 119(3), pp. 1420-1431, March 2006.

Ocean Acoustic Tomography (OAT) was proposed more than 30 years ago but its routine usage is still not a reality today. Despite the advantages clearly demonstrated by OAT when compared to other ocean monitoring techniques, it suffers from several technical-related drawbacks. One is the requirement for rather expensive equipment to be maintained and operated at several locations in order to obtain sufficient source - receiver propagation paths to cover a given ocean volume. In relation with this issue, classic OAT is known to be very sensitive to the precise knowledge of source - receiver relative locations at all times, which imposes severe constraints on the systems to be deployed. This paper presents the preliminary feasibility tests of a new concept that uses ships of opportunity as sound sources for OAT. This concept, sometimes called passive acoustic tomography (PAT), assumes that the acoustic source illuminating the field is emitting an unknown signal, possibly of random nature, at an undetermined location. The approach adopted in this paper views the tomographic problem as a global inversion that includes determining both the emitted signal and the environmental parameters, which is a similar problem to that seen in blind channel identification and was therefore termed Blind Ocean Acoustic Tomography (BOAT). BOAT was tested on a data set acquired in October 2000 in a shallow water area off the west coast of Portugal, including both active and passive (ship noise) data. Successful results show that BOAT is able to estimate detailed water column temperature profiles coherent with independent measurements in intervals where the uncontrolled (ship noise) source signal presents a sufficient bandwidth and signal to noise ratio, which clearly define the limitations of the presented method.

Acknowledgement: this work was supported under projects ATOMS from FCT (Portugal) and TOMPACO, under contract from DUNE, CNR (Italy).