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Ref.: Proc. 2nd International Conference and Exhibition on Underwater Acoustics (UA2014), Rhodes (Greece), June 2014.
Acoustic passive methods to infer properties of the oceanic medium have been subject of progressive emphasis in order to obtain low cost, environmental friendly, long time characterization of the ocean. To this end a first step consists in the estimation of the frequency response of the medium or its time domain counterpart, the impulse response. In this work we consider distant ship noise as an opportunity source characterized by a few low frequency discrete tones. Therefore, the frequency response of a shallow water acoustic channel is estimated at these discrete frequencies, between two vertical line arrays (VLA's). The influence on the estimates of several factors of uncertainty such as range, depth and tilt variations in the VLA's is investigated. For validation purposes, a phase conjugation method is considered. In a preliminary approach towards passive ocean acoustic tomography implementation the estimates obtained by the proposed method are applied in a matched field framework to track sound speed profile variations. Simulations are conducted based on experimental setup and environmental parameters gathered during the MREA07 sea trial that took place in the Tyrrhenian Sea, near Elba Island in May 2007. The results show that, although shadowed, the obtained frequency response estimates allows to obtain a permanent focus and enables the tracking of sound speed profile variations in the water column.
This work was funded by National Funds through FCT – Foundation for Science and Technology under Strategic Project - LA 9 - 2013-2014 (Pest-OE/EEI/LA0009/2013)