Reconciling theoretical versus empirical target strengths of krill: effects of phase variability on the distorted-wave Born approximation

Abstract

Abstract A model was recently proposed to predict the target strengths (TS) of Antarctic krill, Euphausia superba, versus incidence angle (θ) (Deep Sea Res. II 45(7) (1998) 1273). Based on the distorted-wave Born approximation (DWBA), the model depends on the coherent summation of scattering from elements of a discretized-bent cylinder. It was empirically validated at 120 kHz near-broadside incidence (θ≈90°), but large discrepancies were observed at other angles away from the main lobe. As the side-lobe measurements were both higher than the model predictions and above the noise floor, the authors noted that the differences were not entirely due to noise. In this study, the accuracy of the DWBA model is further explored. Results indicate that phase variability in the scatter from elements of a discretized-bent cylinder (krill model) causes a dramatic flattening in the side-lobe regions of TS(θ), while negligibly affecting the main scattering lobe. These results are consistent with the krill TS measurements reported by McGehee et al. Thus, by accounting for phase variability in the solution of the DWBA model, a more accurate and thus practical tool is developed for predicting krill TS.