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Ref.: SiPLAB Report 06/01, FCT, University of Algarve,2001.
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 uncontrollable, the source position is unknown 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, unrealistic 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 separately. 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 unpredictable 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.