Time‐reversal mirror with variable depth focusing

Recent time‐reversal mirror (TRM) experiments conducted in the Mediterranean Sea demonstrated variable range focusing using the frequency‐range invariant property in a shallow‐water waveguide [Song et al., J. Acoust. Soc. Am. 102, 3171(A) (1997)]. The technique involves retransmitting the data at a shifted frequency according to the desired change in focal range in almost real‐time fashion. As opposed to the variable range focusing, there does not appear to be an environmentally independent elegant solution for variable depth focusing. However, a method based on knowledge of the environment, which translates to being able to compute the acoustic modal structure, permits a focal depth shift. The method involves projecting out the important modes received at the vertical source–receive array (SRA) and appropriately altering their excitation to focus at another depth. In the May 1997 experiment, some limited results demonstrated that this method is feasible. Comparison of data with simulations and theoretical analysis will be presented and discussed. [Work supported by ONR.]