BOUNDARY-ELEMENT-MODEL PREDICTIONS OF ACOUSTIC SCATTERING BY SWIMBLADDER-BEARING FISH

摘要

Swimbladder-bearing fish are good acoustic targets because of the large mismatch in the acoustic impedance across the swimbladder wall. The interpretation of the acoustic echoes from such targets depends on an understanding of the scattering process, and this can be greatly assisted by the use of suitable models. Simple models using, for example, finite cylinders to represent the swimbladder are generally inadequate at the higher frequencies used routinely in fish monitoring. In this paper the models are based on morphometric data from specimens of saithe (Pollachius virens) and pollack (Pollachius pollachius). These data in their original form consist of digitized contours of microtomed cross sections of the swimbladders. Details are given of the process used to generate surface meshes consisting of 6-noded triangles and 8-noded quadrilaterals over which the geometric and acoustic variables are interpolated using quadratic functions. A boundary-element method based on the Helmholtz integral equation is used to compute the backscattered pressure for various orientations of the fish at four single frequencies. Comparison is made with measurements of the dorsal and ventral aspect target strength functions for the given specimens.