Application of time reversal to reverberation reduction in the ocean

Abstract

Reverberation from rough ocean boundaries causes serious problems in many active sonar applications. Numerous passive signal processing techniques have been developed to effectively extract target signals from reverberation noise. The spatial and temporal focusing capability of the time-reversal method provides a new approach to this problem by actively minimizing the reverberation. A time-reversal process realizes a focus at the probe source position and shadows the boundaries below and above the focus resulting in reduced reverberation. Recent experiments with 3.5-kHz sound showed the possibility of the time-reversal method to the detection problem by demonstrating the enhanced echo-to-reverberation ratio in shallow water. A time-reversal based process for reverberation cancellation from a specified range cell without the use of a probe source has also been developed. An eigen-decomposition of the reverberation data provides the transfer function between the TRM and scatters where the reverberation is generated [Prada et al., J. Acoust. Soc. Am. 99, 2067–2076 (1996); Lingevitch et al., J. Acoust. Soc. Am. 109, 2495 (2001)]. The backpropagated field from TRM with a weight function obtained by projection operations to the null space places an acoustic null at the corresponding boundary. This process results in reduced reverberation returning from the boundary. [Work supported by ONR.]