Extraction of a target scattering response from measurements made over long ranges in shallow water

The scattering response of a structure may be extracted from acoustic measurements made in shallow water at relatively long ranges from the target. The procedure described uses an array of sources to approximately generate a low-order mode and an array of receivers to isolate from the received echo a low-order mode. Since low-order modes behave as plane waves in the vicinity of the target, the free-field target response may be extracted from such measurements. In general, such a procedure for the generation of a low-order mode requires a large number of sources. The number is of the order of the number of propagating, nonevanescent modes present in the shallow water environment at the frequencies used. If this number is prohibitively large, temporal discrimination may be used in addition to beamforming to reduce the number of sources necessary for the isolation of the free-field target response. Similarly, the number of receivers required for the isolation of a mode from the received signal may be reduced...