A SPECTRAL APPROACH TO 3-D OBJECT SCATTERING IN LAYERED MEDIA APPLIEDTO SCATTERING FROM SUBMERGED SPHERES

A spectral formulation for 3-D object scattering in a layered medium i s described. The formulation is valid when the source and receiver are sufficiently far from the object that multiple scattering between the object and waveguide boundaries can be neglected and the scattered fi eld can be expressed as a linear function of the object's plane wave s cattering function. An analytic expression is then derived for the fie ld scattered from a spherical object in a stratified medium. Since the expression is in terms of 1-D wave number integrals, it is computatio nally efficient to implement, allows detailed investigation of the sca ttered field in the vicinity of the object, and enables scattering of evanescent waves to be incorporated by analytic continuation. Computat ions for a noncompact sphere (1 < ka) illustrate the essential charact eristics of 3-D object scattering in a shallow but multi-modal wavegui de. Relative to free-space scattering, a significant decrease in the l evel of the scattered field in the forward direction as well as a pron ounced directional beaming effect in the vicinity of the sphere are di scovered. Another primary finding is that multistatic observations of the scattered field, distributed over an azimuthal aperture greatly in excess of pi/(ka), will typically be necessary to classify an object submerged in a shallow-water waveguide at ranges exceeding the water c olumn depth. (C) 1998 Acoustical Society of America. [S0001-4966(98)01 010-8]