A method for robust time-reversal focusing in a fluctuating ocean
In recent years, the authors have demonstrated time-reversal mirrors (TRM) in the ocean. A focus of up to 30 km was achieved with low frequency (445 Hz) transmissions and the focal structure could be maintained over several days at a range of 15 km. However, the stable focus was limited to less than an hour with high frequency (3.5 kHz) transmissions due to the sensitive response of high frequency sound propagation to the medium fluctuations. In this study, an approach for robust time-reversal focusing is investigated based on a method developed for matched-field processing. Instead of using a single probe source pulse, the method makes use of several probe source pulses obtained over a certain period of time where each ping represents a different propagation condition of the medium. The back-propagation from a TRM weighted by a linear combination of the dominant singular vectors obtained from the signal matrix leads to stable focusing for a longer period of time than that with a single probe pulse. The proposed method is useful in non-static propagation conditions and when frequent probe signals are not available.