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Ref.: SiPLAB Report 02/03, University of Algarve,2003.
Abstract: Passive Acoustic Tomography (PAT) is an acoustic tomography variant where the usual cooperative
acoustic source is replaced by a non-cooperative noise source as for example a ship of opportunity. The basic idea
behind PAT is to extend the application of acoustic tomography to areas with heavy or regular ship traffic and where
it would be impossible, or too costly, to deploy a controlled acoustic source in a permanent basis.
PAT differs from classic active acoustic tomography by the fact that in PAT, the source signal is stochastic with unknown characteristics and uncontrolled by the experimenter. There are at least two important implications of the assumptions made under PAT: one is that the emitted signal is possibly fluctuating over time both in strength and bandwidth, the other is that the sound source's position is unknown and possibly changing over time. The fact that the source position is unknown implies that apart from the sound speed profile to be inverted for, the other propagation channel characteristics ( e.g. bottom properties, water depth, etc...) are also unknown. An inverse problem where both the input signal and the channel are unknown is termed a blind deconvolution problem, and is common in the fields of wireless communications, geophysics and in all problems where channel identification is required and where the input signal is not known. The generally adopted methodology is to use higher-order statistics and (in wireless communications) the ciclostationarity properties of the received signal. Such methods have also been used in underwater acoustics for signals with some degree of non-stationarity. Assuming that the noise sources of opportunity are relatively stationary inputs to the propagation channel it is possible to build a model-based cost function where both the source and the channel properties are unknown variables to be estimated.
Project TOMPACO (TOMografia PAssiva COstiera) was proposed in 1999 by DUNE with the goal of testing the feasibility of the PAT concept. To that aim, CINTAL (a TOMPACO subcontractor) has setup an experiment to acquire real acoustic data to support TOMPACO. That experiment took place in October 2000 under the framework of the INTIFANTE'00 sea trial. An overview of the sea trial can be found in elsewhere and intermediate reports specifically dealing with TOMPACO issues were produced regarding the acquired data set and the inversion results with active data. This third TOMPACO report is intended to analyse the results obtained using ship noise data as input signals (passive data) and draw the final conclusion for the practical feasibility of the proposed methodology.
ACKNOWLEDGMENT: this work was supported by TOMPACO project (DUNE, Italy)