Comments: download file (pdf )
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
uncontroled
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 strenght 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 unknwon 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)