Abstract:
This thesis investigates the complexities of acoustic scattering by finite bodies in general and
by fish in particular through the development of an advanced acoustic scattering model and
detailed laboratory acoustic measurements. A general acoustic scattering model is developed
that is accurate and numerically effcient for a wide range of frequencies, angles of orientation,
irregular axisymmetric shapes and boundary conditions. The model presented is an extension of
a two-dimensional conformal mapping approach to scattering by irregular, finite-length bodies
of revolution. An extensive series of broadband acoustic backscattering measurements has been
conducted involving alewife fish (Alosa pseudoharengus), which are morphologically similar to
the Atlantic herring (Clupea harengus). A greater-than-octave bandwidth (40-95 kHz), shaped,
linearly swept, frequency modulated signal was used to insonify live, adult alewife that were
tethered while being rotated in 1-degree increments over all angles of orientation in two planes
of rotation (lateral and dorsal/ventral). Spectral analysis correlates frequency dependencies to
morphology and orientation. Pulse compression processing temporally resolves multiple returns
from each individual which show good correlation with size and orientation, and demonstrate
that there exists more than one significant scattering feature in the animaL. Imaging technologies
used to exactly measure the morphology of the scattering features of fish include very highresolution
Phase Contrast X-rays (PCX) and Computerized Tomography (CT) scans, which are
used for morphological evaluation and incorporation into the scattering modeL. Studies such as
this one, which combine scattering models with high-resolution morphological information and
high-quality laboratory data, are crucial to the quantitative use of acoustics in the ocean.
