Geoacoustic reflectivity inversion : a Bayesian approach




Dettmer, Jan

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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. How-ever, 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 strip-ping 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 to¬gether 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.



underwater acoustics, inversion