Abstract:

Recent time-reversal mirror (TRM) experiments conducted in the Mediterranean Sea [Kuperman et al., J. Acoust. Soc. Am. 101, 3088(A) (1997)] have demonstrated the spatial focusing and temporal stability of the phase conjugation process over ranges of up to 30 km in a shallow-water waveguide. In the May 1997 experiment, the vertical source-receive array (SRA) consisted of 23 sources and receivers spanning 77 m of a 125-m water column. One series of tests involved a probe source (PS) and 94-m aperture vertical receive array (VRA) located 15 km from the SRA. Various subsets of the SRA sources were used for retransmitting the time-reversed signals received by SRA receivers from the PS. With a single source, the results essentially were the same as those reported previously [Parvulescu and Clay, Radio Elec. Eng. 29, 223--228 (1965)]. Temporal compression is achieved but vertical spatial focusing is not. By using all 23 sources, the retransmitted field focuses distinctly at PS with levels above and below the focal region suppressed by more than 15 dB. Significant spatial focusing also was achieved with as few as six sources where these levels were suppressed by more than 10 dB. [Work supported by ONR, Code 321US.]