Geoacoustic reflectivity inversion: A Bayesian approach

Propagation and reverberation of acoustic fields in shallow water depend strongly on the spatial variability of seabed geoacoustic parameters; and lack of knowledge of seabed variability is often a limiting factor in acoustic modelling applications. However, direct sampling (e.g., coring) of vertical and lateral variability is expensive and laborious, and matched-field and other long-range inversion methods fail to provide sufficient resolution.; This thesis develops a new joint time/frequency domain inversion for high-resolution single-bounce reflection data. The inversion approach has the potential to resolve fine-scale sediment profiles over small seafloor footprints (∼100 m). The approach utilises sequential Bayesian inversion of time- and frequency-domain reflectivity data, employing ray-tracing inversion for reflection travel times and a layer-packet stripping method for spherical-wave reflection coefficient inversion. Rigorous uncertainty estimation is of key importance to yield high quality inversion results. Quantitative geoacoustic uncertainties are provided by a nonlinear Gibbs sampling approach together with full data error covariance estimation (including non-stationary effects). The small footprint of the measurement technique combined with the rigorous inversion of both time and frequency domain data provides a powerful new tool to examine seabed structure on finer scales than heretofore possible.; The Bayesian inversion is applied to two data sets collected on the Malta Plateau and the Strait of Sicily during the SCARAB98 experiment. The first application aims to recover multi-layered seabed structure and the second application recovers density and sound velocity gradient structure in the uppermost sediment layer.; An interesting new method of deriving reflectivity data from ambient noise measurements is briefly considered in simulation to examine the resolving power and limits of the approach.