Acoustic scattering by axisymmetric finite-length bodies with application to fish : measurement and modeling

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.

Towards Extra-Luminal Blood Detection from Intravascular Ultrasound Radio Frequency Data

“…When the wavelength of the incident wave is smaller in comparison to the size of the object, the wave is reflected in all directions (i.e., scattering). The power scattered by each object in the direction opposite to the direction of the incident wave depends on the differential backscattering cross section (DBC), which can be considered as a measurement of the effective (acoustic) area of the object [14]. The collective interaction of all the scatterers can be modeled using the Born approximation [2] which implies that the scattered echoes are weak in comparison to the incident signal, and therefore it is possible to use the principle of superposition to represent the total scattered wave as a sum of the individual reflections of each point scatterer.…”