Examination of time-reversal acoustic application to shallow water active sonar systems

Abstract

The ability to employ effectively an active sonar system in the littoral regions is of great interest to the United States Navy. Time-varying multipath propagation introduces significant problems that must be overcome in the employment of shallow water active sonar. The phenomenon of time-reversal acoustics (TRA) has provided hope for a solution to this problem by undoing much of the multipath spreading without the need to have knowledge of the environment in these littoral regions. When an active sonar return is time-reversed (phase- conjugated in the frequency domain) and retransmitted, this second signal focuses in time and space back at the original source location. This thesis investigates the phenomenon of TRA as it applies to an idealized shallow water environment. Numerical modeling was performed for a variety of source and target apertures and ranges. Results demonstrate a significant enhancement in received active sonar signal strength due to the TRA acoustic field focusing effect. Furthermore, the signal strength enhancement remains significant even when the source to target range changes between active sonar transmissions. The results presented in this thesis demonstrate that the use of TRA may provide substantial signal to noise ratio improvements over current active sonar systems. Further modeling and real world experiments could ultimate lead to the development of a practical active TRA sonar system