Computed narrow-band time-reversing array retrofocusing in a dynamic shallow ocean

A time-reversing array (TRA) can retrofocus acoustic energy, in both time a nd space, to the original sound-source location without any environmental i nformation. This unique capability may be degraded in time-dependent or noi sy acoustic environments, or when propagation losses are prevalent. In this paper, monochromatic propagation simulations (based on the parabolic equat ion code, RAM) are used to predict TRA retrofocusing performance in shallow -water sound channels having characteristics similar to those measured duri ng the recent SWARM (shallow-water acoustics in a random medium) experiment . Results for the influence of source-array range, source depth, acoustic f requency, bottom absorption, internal wave strength, and round-trip time de lay are presented. For a fixed channel geometry, higher frequencies; deeper sources, and lower bottom absorption improve TRA performance and allow ret rofocusing at longer ranges. In a dynamic shallow-water channel containing a random superposition of linear internal waves, the size of the retrofocus is slightly decreased and sidelobes are suppressed compared to the static channel results. These improvements last for approximately 1 to 2 min for s ource-array ranges near 10 km at a frequency of 500 Hz. For longer time del ays, the internal waves cause significant TRA retrofocus amplitude decay, a nd the decay rate increases with increasing internal wave activity and acou stic frequency. (C) 2000 Acoustical Society of America. [S0001-4966(00)0610 6-3].