INTRODUCING PASSIVE MATCHED-FIELD ACOUSTIC TOMOGRAPHY

In acoustic tomography sea-basin environmental parameters such as temp erature profiles and current-velocities are derived, when ray propagat ion models are adopted, by the travel time estimates relative to the i dentifiable ray paths. The transmitted signals are either single frequ ency, or impulsive, or intermittent and deterministic. When the wavele ngth is comparable with the scale lengths present in the propagation s cenario, Matched Field Tomography (MFT) is used, entailing the conside ration of waveguide modes instead of rays. A new concept in tomography is introduced in the paper, that employs passively the noise emitted by ships of opportunity (cargoes, ferries) as source signals. The pass ive technique is acoustic-pollution-free, and if a basin is selected i n which a regular ship traffic occurs data can be received on a regula r schedule, with no transmission cost. A novel al ray pre-processor fo r passive tomography is introduced, such that the signal structure at the pre-processor output is nearly the same as that obtainable in the case of single-frequency source signals. Hence, at the pre-processor o utput all the tomographic inversion methods valid for active tomograph y employing single-frequency sources can be applied. The differences b etween active and passive tomography are pointed out and the potential of passive techniques is illustrated by simple propagation scenarios adopting either rays or waveguide modes.