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Estimation of sediment properties using frequency domain identification and marine acoustics

In the field of underwater acoustics, the importance of the characterization of the seafloor is well known and of interest for scientific, economical and military applications. An acoustic method would reduce the present needs for sediment sampling, while producing continuous profiles of sediment properties. However, for studying the functional relations between acoustical and geo-physical properties, an accurate determination of the acoustical parameters is necessary first. In this paper, a global system identification approach is used for the experimental validation of wave propagation models through sediments and for the determination of its acoustical parameters. In order to achieve this goal, laboratory experiments on calibrated degassed sediments with broadband Panametrics piston transducers (300 kHz-700 kHz) are carried out. For this, two configurations of acoustic measurements are used: a water-Plexiglas-Sand-Plexiglas-water and a water-sediment-Plexiglas configuration. The Maximum Likelihood Estimator is applied in the frequency domain to retrieve the sediment parameters. The propagation of 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.