Robust time-reversal focusing in a random ocean channel

Since time reversal focusing utilizes the reciprocity of a static medium, its performance can be degraded in a nonstatic medium, where the propagation conditions change during the time of forward and backward propagations. In this study, a method for robust time-reversal focusing is investigated based on a method developed for matched-field processing [Krolik, J. Acoust. Soc. Am. 92, 1408–1419 (1992)]. Instead of using a single probe source pulse, the robust method employs multiple probe source signals obtained over a period of time where each ping represents a different propagation condition. A singular value decomposition of the signal matrix of the received probe source pulses shows that over 90% of the acoustic energy is represented by the first few singular values, suggesting that the effective dimension of the pressure field resulting from the sound speed fluctuations is small. The back-propagated field weighted by the linear combination of the dominant singular vectors produces stable focusing for longer time than that with a single probe pulse. The method is useful in nonstatic propagation conditions and in a situation where frequent probe signals are not available. [Work supported by ONR.]