Broadband sparse-array blind deconvolution using unconventional beamforming

Abstract

Sound from a remote underwater source is commonly distorted by multipath propagation. Blind deconvolution is the task of estimating the unknown waveforms for the original source signal, and the source-to-receiver impulse response(s), from signal(s) recorded in an unknown acoustic environment. Synthetic time reversal (STR) is a technique for blind deconvolution of underwater receiving-array recordings that relies on generic features of the propagating modes or ray paths that lead to multipath sound propagation. In prior studies the pivotal ingredient for STR, an estimate of the source-signal's phase (as a function of frequency), was generated from conventional beamforming of the recorded signals. However, through the use of unconventional nonlinear frequency-difference beamforming, STR can be extended to sparse array recordings where the receiving-array elements are many wavelengths apart and conventional beamforming is inadequate. This extension of STR was tested with simple propagation simulations and FAF06 experimental measurements involving broadband signal pulses (11-19 kHz) that propagate 2.2 km in the shallow ocean to a vertical 16-element receiving array having a 3.75-m-spacing between elements (almost 40 signal-center-frequency wavelengths). The cross-correlation coefficient between the source-broadcast and STR-reconstructed-signal waveforms for the simulations and experiments are 98% and 91-92%, respectively. [Sponsored by ONR.]