Long range sediment tomography

Abstract

The development of long range sediment tomography using shot data and its application in various field experiments is presented. Whereas traditional ocean acoustic tomography has mainly relied on linear inverse theory using the travel time of rays in deep water, our technique relies on nonlinear optimization techniques using the frequency-dependent travel time of normal modes in shallow water. Our experience with broadband sediment tomography using explosive sources started with the Barents Sea Polar Front Experiment where sediment properties were extracted using ground wave analysis. The techniques were further refined during the Shelf Break Primer Experiment in the New England Bight where a nonlinear inversion scheme using the Genetic Algorithm was successfully applied. The inversion scheme was based on modal travel time dispersion characteristics, which in shallow water conditions, is highly sensitive to the bottom properties. Advanced signal processing tools for the space–time–frequency processing and hybrid optimizaton techniques for global search were developed and implemented in parallel. In addition to the individual modal arrival times, emphasis has now been placed on the arrival corresponding to the Airy phase. These inversion techniques are currently being applied to the data from ASIAEX-East China Sea Experiment. Characteristics of this tomography approach such as spatial coverage, resolution, error bars, robustness, and simple instrumentation requirements will be presented. The usefulness of this inversion approach as a rapid environmental assessment tool will be highlighted. [Work supported by ONR.]