Time-reversing array retrofocusing in simple dynamic underwater environments

A time-reversing array (TRA) has the ability to retrofocus acoustic energy, in both time and space, to the original sound-source location without any information about the acoustic environment in which it is deployed. This un ique capability may be limited or lost when the acoustic medium or its boun daries are time dependent, or propagation losses are prevalent. In this pap er, predictions are made for the size, field-amplitude decay rate (or time) , and location of the retrofocus for a TRA deployed in the presence of thre e dynamic acoustic propagation complexities commonly present in shallow oce an waters: (i) volume scattering from a random superposition of linear inte rnal waves convecting a gradient in the sound speed profile; (ii) reflectio n and volume scattering from a deterministic soliton internal wave travelin g on the thermocline between two water masses with differing sound speed; a nd (iii) surface scattering from a wind-driven dynamic random rough ocean s urface. Analytical propagation models for narrow-band signals are used to h ighlight separately the influence of each propagation complexity on TRA ret rofocusing. As expected, internal wave time scales are long enough so that TRA retrofocusing should persist for several minutes for source-array range s of several kilometers at frequencies approaching 1 kHz. However, the comp aratively rapid motion of ocean surface waves should prevent TRA exploitati on of acoustic scattering from a wind-driven ocean surface at ranges greate r than a few hundred meters, independent of acoustic frequency. Interesting ly, multiple time-invariant propagation paths are not found to consistently enhance retrofocusing unless the TRA has sufficient angular resolution to distinguish them. (C) 1998 Acoustical Society of America. [S0001-4966(98)04 712-2].