Ocean Acoustic Inversion

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Oceans are opaque to electromagnetic radiation but transparent to sound; hence, scientists have devised many methods to use acoustics underwater instead of light, radio or microwaves. The remote acoustic sensing of seabed properties/processes or the locations of sound sources (e.g., submarines, marine mammals) are important problems with geological, geotechnical, environmental and defence applications. However, acoustic remote sensing requires solving a nonlinear inverse problem which is inherently non-unique with uncertainties and resolution that are often not well understood. The proposed research builds on our work in theory and applications of quantitative Bayesian (probabilistic) inversion for ocean-acoustic remote sensing and related seismic problems with several projects. (1) Inversion for seabed acoustic scattering parameters and processes. Our initial work shows that inversion of acoustic backscatter data can resolve in-situ seafloor roughness and sediment heterogeneity spectra (a remote-sensing alternative to laborious direct measurements). We propose to develop quantitative model-selection approaches to determine the relative importance of various physical scattering mechanisms and to estimate relevant scattering parameters and uncertainties for different seabed types, to better understand the physics of seabed scattering and advance geoacoustic databases and sonar models. (2) Estimation of laterally-variant seabed scattering and geoacoustic parameters for naval databases or geotechnical applications such as cable-laying. This approach is based on sequential Bayesian inversion of novel acoustic data sets collected along a track using a mobile system comprising an acoustic source and receiver array towed near the seabed by an autonomous underwater vehicle. (3) Tomographic (2D) inversion for seabed geoacoustic properties using full-field acoustic data recorded at a fixed receiver array. A major component of quantitative nonlinear inversion in 2D involves developing new self-adaptive sparse/irregular numerical grids to parameterize the environment in a manner consistent with the resolving power of the data. (4) Acoustic source localization and uncertainty estimation, including the effects of uncertainty in ocean environmental properties. Our initial work was in naval (passive sonar) applications, but we are moving into marine-mammal studies in collaboration with JASCO Research Ltd, a leading Canadian marine-technology company. New methods include a novel approach to matched-field localization with non-synchronized array systems/sub-systems, and source ranging via modal dispersion (time-frequency analysis) at a single receiver. (5) Tracking an unknown number of vocalizing marine mammals using long-term acoustic monitoring data. This requires developing a Bayesian tracking algorithm to sample over the number of mammals and their tracks, constrained by prior information on swim speed, and provide a probabilistic solution from which information on population and behaviour can be extracted. (6) Passive seismic inversion for crustal/tectonic structure of Cascadia (Canada’s highest earthquake/tsunami hazard), in collaboration with the Geological Survey of Canada. This involves developing a joint tomographic inversion of teleseismic data (receiver functions) and ambient seismic noise (surface-wave dispersion) to resolve 2D structure which can be correlated with historical earthquake foci. The transition from subduction to transform faulting at the Queen Charlotte margin is of particular interest given the 2012 magnitude 7.8 earthquake and local tsunami, and ocean-bottom seismometers from the NEPTUNE system provide new mid-plate data for a section across the Juan de Fuca plate and Cascadia subduction zone.