Abstract:
Benthic shells can contribute greatly to the scattering variability of the ocean bottom,
particularly at low grazing angles. Among the effects of shell aggregates are increased
scattering strength and potential subcritical angle penetration of the seafloor. Sand dollars
(Dendraster excentricus) occur commonly in the ocean and have been shown to be significant
scatters of sound. In order to understand more fully the scattering mechanisms of these
organisms, the scattering from individual sand dollars was studied using several methods.
Using an approximation to the Helmholtz-Kirchhoff integral, the Kirchhoff method
gives an analytic integral expression to the backscattering from an object. This integral was first
solved analytically for a disk and a spherical cap, two high aspect ratio oblate shapes which
simplify the shape of an individual sand dollar. A method for solving the Kirchhoff integral
numerically was then developed. An exact three dimensional model of a sand dollar test was
created from computed tomography scans. The Kirchhoff integral was then solved numerically
for this model of the sand dollar.
The finite element method, a numerical technique for approximating the solutions to
partial differential equations and integral equations, was used to model the scattering from an
individual sand dollar as well. COMSOL Multiphysics was used for the implementation of the
finite element method.
Modeling results were compared with published laboratory experimental data from the
free field scattering of both an aluminum disk and a sand dollar. Insight on the scattering
mechanisms of individual sand dollar, including elastic behavior and diffraction effects, was
gained from these comparisons.
