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Development of and computer code for acoustical inversion technique based on system identification

Within the framework of determining the characteristics of sediments on an acoustic base, a global system identification approach was used for an experimental validation of the wave propagation through sediments and for the determination of its acoustical parameters. In order to achieve this goal, the Maximum Likelihood Estimator in the frequency domain was put to use in several SISO and MIMO representations. Measurements were carried out on fine and medium sorted sands, on viscoelastic materials simulating the seafloor and on a Silicone containing glass particles. For the measurements on real sediments, two configurations were utilized: a water-Plexiglas-Sand-Plexiglas-water and a water-sediment-Plexiglas configuration. The propagation of the multiples in the sediment, and the calibration method, which is incorporated in the global system identification approach, are important assets that are not exploited when carrying out direct measurements of dispersion or absorption with transducers buried in the sediment. The smallest model errors were found with the viscoelastic rational form model, followed by the Buckinghams and viscoelastic CQ-model, which were giving acceptable errors too. This confirms the presence of an intrinsic attenuation almost linear in the frequency for the fine sediments. On the other hand, bigger model errors were found with Biots model, which can be explained by the many parameters that are still unknown and from which some of them need to be fixed in the inverse procedure. For the medium sorted sand the dynamic multi-scattering model of Waterman-Truell was applied. The high-frequency set-up at sea was simulated. The measurements on fine sands in normal and oblique incidence were combined in MIMO sense to estimate the RMS-roughness of the surface, the longitudinal parameters and the frequency independent shear velocity.

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Vrije Universiteit Brussel
2, Pleinlaan
1050 Brussels
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