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Ref.: SiPLAB Report 06/01, FCT, University of Algarve,2001.
Abstract
The main objective of the TOMPACO (TOMografia PAssiva COstiera -
Passive Coastal
Tomography) project is to relax the need for the active sound source to
illuminate
the environment. Active sources are in that case replaced by sources of
opportunity
as passing by ships or natural sound sources. Passing from active to
passive
sources adds several difficulties to the problem: the source emitted
waveform
becomes unknown and uncontrolable, the source position is unknwon and
as
a consequence bathymetry and bottom properties become also unknown. In
other
words, for practical purposes, passing from active to passive shallow
water
tomography is equivalent to passing from a ``simple'' inversion of the
water
column properties to a full inversion of source position, bottom
properties,
bathymetry and water column properties. That represents a dramatical
increase
of the parameter search space with its associated possible
non-uniqueness
solutions and numerical computation problems.
There are several aspects that should be taken into account when
attempting
to demonstrate shallow water passive tomography: one is that it is, at
least
at this stage, irrealistic to attempt to perform a full scale inversion
test
with unknown environmental and source conditions, other is that
environmental
and geometrical effects should be as much as possible tackled
separatelly.
Another important aspect is that care should be taken to isolate
cross-effects
between parameters in order to obtain absolute water column estimates
and
another last important aspect is that care should also be taken when
combining
information across-frequency taking into account that the ultimate goal
is
to operate with unmanned sound sources that are, by nature, highly
non-stationary
and with possibly unpredictible phase evolutions along time.
The INTIFANTE'00 sea trial was partially designed to cope with the
different
situations encountered in passive tomography. The present report
attempts
to demonstrate the concept of TOMPACO using an active source in a
passive
shallow water tomography scenario. By this it is meant that an active
source
is used in a shallow water scenario where hypotheses are progressively
relaxed,
one at a time, leading to more complex problems that will eventually
lead
to a full passive tomography approach.