Focusing of low-frequency sound fields in shallow water

A numerical experiment is carried out to study the focusing of a low-frequency (100–300 Hz) sound field in a shallow-water acoustic waveguide typical of an oceanic shelf. Focusing with the use of time reversal of broadband acoustic signals, which is called time reversal mirror (TRM) of waves, is considered along with focusing by phase conjugation (PC) of a monochromatic sound field. It is demonstrated that, in the case of focusing by the TRM method in the waveguide of interest, it is sufficient to have a single source-receiving element. The use of a vertical array improves the quality of focusing. The quality achieved in the latter case proves to be approximately the same as that achieved in the case of focusing by phase conjugation of a monochromatic field at a frequency identical to the carrier frequency of the broadband signals. It is also shown that, in a range-independent waveguide, intense surface waves considerably reduce the quality of focusing. This effect is most pronounced in the case of using phase conjugation.