SOUND-SCATTERING BY SEVERAL ZOOPLANKTON GROUPS - II - SCATTERING MODELS

Mathematical scattering models are derived and compared with data from zooplankton from several gross anatomical groups-fluidlike, elastic s helled, and gas bearing. The models are based upon the acoustically in ferred boundary conditions determined from laboratory backscattering d ata presented in part I of this series [Stanton er al., J. Acoust. Sec . Am. 103, 225-235 (1998)]. The models use a combination of ray theory , modal-series solution, and distorted wave Born approximation (DWBA). The formulations. which are inherently approximate, are designed to i nclude only the dominant scattering mechanisms as determined from the experiments. The models for the: fluidlike animals (euphausiids in thi s case)ranged from the simplest case involving two rays, which could q ualitatively describe the structure of target strength versus frequenc y for single pings, to the most complex case involving a rough inhomog eneous asymmetrically tapered bent cylinder using the DWBA-based formu lation which could predict echo levels over all angles of incidence (i ncluding the difficult region of end-on incidence). The model for the elastic shelled body (gastropods in this case) involved development of an analytical model which takes into account irregularities and disco ntinuities of the shell. The model for gas-bearing animals (siphonopho res) is a hybrid model which is composed of the summation of the exact solution to the gas sphere and the approximate DWBA-based formulation for arbitrarily shaped fluidlike bodies. Then is also a simplified ra y-based model for the siphonophore. The models are applied to data inv olving single pings, ping-to-ping variability, and echoes averaged ove r many pings. There is reasonable qualitative agreement between the pr edictions and single ping data, and reasonable quantitative agreement between the predictions and variability and averages of echo data. (C) 1998 Acoustical Society of America.