T. Udagawa
Dept. of Phys., Univ. of Texas, Austin, TX 78712
D. P. Knobles
Univ. of Texas, Austin, TX 78713-8029
Two complementary methods are proposed for exact numerical calculations of the acoustic field scattered by a rigid sphere in an isovelocity fluid layer overlying a horizontally stratified medium. These two methods are based on the partial wave and Fourier expansions of the medium Green's function. It is shown that the method based on the partial wave expansion is useful for clarifying the physical nature of the approximation involved in the method proposed by Ingenito some time ago [F. Ingenito, J. Acoust. Soc. Am. 82, 2051 (1987)]. It is in fact shown that the Ingenito approximation is equivalent to replace the exact medium Green's function by that of the free-field Green's function in obtaining boundary values of the field on the surface of the target. Numerical studies are performed and it is demonstrated that the exact results obtained by using the presently proposed methods are largely different from those obtained by using Ingenito's approximation. In addition more complex cases are examined where the target is nonspherical and the waveguide is bottom limited as is commonly found in shallow water ocean waveguides. The sensitivity of the scattered field to various types of geoacoustic profiles when the target is near the sea floor is also considered.