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 approximat ely generate a low-order mode and an array of receivers to isolate fro m 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 respo nse may be extracted from such measurements. In general, such a proced ure 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, none vanescent modes present in the shallow water environment at the freque ncies used. If this number is prohibitively large, temporal discrimina tion may be used in addition to beamforming to reduce the number of so urces necessary for the isolation of the free-field target response. S imilarly, the number of receivers required for the isolation of a mode from the received signal may be reduced by the use of temporal discri mination in addition to beamforming techniques. This article presents the detailed procedure used as well as numerical simulations to demons trate the feasibility of such a process.