High-Frequency Acoustic Volume Scattering from Biologically Active Marine Sediments.
WASHINGTON UNIV SEATTLE APPLIED PHYSICS LAB
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A thesis on high-frequency acoustic volume scattering from marine sediments with application to remote sensing of benthic biological activity is presented. Small perturbation theory is used to describe bistatic volume scattering in a sediment half-space. The sediment is modeled as an acoustic fluid with random fluctuations in density and compressibility. Insight into determining whether single or multiple scattering is significant in the medium is gained by using the bilocal approximation to Dysons equation. An alternative analysis of volume scattering is made using exact numerical simulations, and a numerical method for two-dimensional volume scattering using the method of moments is presented. Both periodic and nonperiodic random media are considered. Scattering theory is compared with numerical Monte Carlo simulations, and the validity of the small perturbation method is discussed. The effects of the half-space scattering geometry on the coherent field within the sediment and on the bistatic scattering cross section are investigated. Benthic biological activity creates temporal and spatial variations in the sediment physical properties that result in temporal and spatial variations in sediment volume scattering. This acoustic variability is used as a remote sen sing tool to infer parameters of biological activity, or bioturbation. To develop a forward model that relates bioturbation to density fluctuations in the sediment and, therefore, to acoustic scattering, a new stochastic model of bioturbation is presented that describes biological mixing as an inhomogeneous two scale biodiffusion process. Nonlocal mixing due to macrofauna is described as a filtered Poisson process, and local mixing due to meiofauna is described as diffusive. Modeling issues such as the spatial stationarity of bioturbation are discussed. The bioturbation and acoustic scattering models are then combined to produce a model for the decorrelation in time of acoustic backscatter.
- Biological Oceanography
- Physical and Dynamic Oceanography