Sound scattering by zooplankton with arbitrary shape and orientation

Abstract

One of the challenges in describing the sound scattering by finite elongated objects with applications to zooplankton lies in the complexity of their shape and physical properties. Several approximate models have been developed to date in this field including deformed cylinder model, which allows the cross section of the scatterer to vary along its elongated axis [T. K. Stanton, J. Acoust. Soc. Am. 86, 691–705 (1989)]. While valid for a wide range of material properties, this model is restricted to a normal or near normal incident wave and requires a target to have a high aspect ratio. In this study, taking advantage of the fact that many zooplankton can be approximated as weak scatterers, a distorted wave Born approximation can be applied to compute the backscattering from such scatterers. Different from the conventional Born approximation, a substitution of the internal field by the incident field with a modified phase is used to obtain the backscattered field expressed explicitly as a volume integral over the entire body of the scatterer. This model can be applied to more complicated shapes with arbitrary orientations and its validity has been tested for various cases in which the exact solutions exist. Comparisons with data are made. [Work supported by ONR.]