Title

High Frequency Seafloor Acoustic Backscattering With the Presence of Turbulence In Seawater

Author

Li Zhang

Date of Award

1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Marine Science

First Advisor

Ralph R. Goodman

Advisor Department

Marine Science

Abstract

Theoretical models are developed for high frequency acoustic backscattering from a statistically isotropic seafloor, especially on continental shelves. In addition to surface admittance inhomogeneity scattering and surface roughness scattering, discrete scatterers are introduced to simulate the appearance of high contrasts in admittance or roughness such as shells, rocks, or sediment structures created by the process of marine bioturbation in shallow-water environments. Surface admittance scattering is determined by the seafloor admittance anomaly. Its angular dependence can be described by Lambert's law for almost all types of sediments. Roughness backscattering depends on the mean square micro-relief height and its spectral characteristics. For an exponential roughness spectrum, spectral slope controls the frequency dependence, which can be either positive or negative. A maximum in backscattering strength occurs at normal incidence when the roughness spectrum is concentrated in lower spatial frequency components. For discrete scattering, individual scatterer's target strength and the distribution of the scatterers are both important. Temporal and spatial fluctuations of sound speed in seawater due to the presence of dynamic oceanographic processes creating turbulence are shown to be responsible for backscattering variability. A criterion is given to judge whether measured backscattering strength can represent seafloor backscattering when variations in sound speed in the water column are present. For scattering from continuously distributed scatterers, the standard deviation of backscattering coefficient is linearly related to the standard deviation of sound speed. This is also true for scattering from many discrete scatterers. More variability is expected when there are only a few scatterers inside the insonified area. Besides the angular and frequency dependence, variability of backscattering provides additional information for the inverse problem (sediments characterization) using remotely sensed acoustic backscattering data. Comparisons between the model and the measurements suggest that, at the West Florida Sand Sheet experiment site, scattering from discrete shells is an important and is often the dominant component of the total scattering field. Laboratory experiments show that at high frequency a shell's normal incidence target strength can be approximated by solid sphere scattering theory.