Inversion for seabed geoacoustic properties in shallow water experiments

Low frequency sound propagation features and bottom sediment properties in shallow water were studied in the Shallow Water Acoustic Technology (SWAT) experiments conducted in the East China Sea. In these experiments, a hydraulic-type acoustic source was towed over a range of some 30 km at constant mid-water depth and transmitted low-frequency cw signals, which were received on a bottom-moored vertical line array. After suppressing a time-dependent factor of the received signals, the asymptotic Hankel transform was applied to the acoustic field provided by the resulting synthetic aperture horizontal array that was created at each receiver depth. The horizontal wavenumber spectra thus obtained had peaks corresponding to mode, but these peak positions were observed to be slightly different among the different receiver depths partially due to noise and range dependency. Thus, stochastic mode inversion was exploited by using all of the identified peak positions for estimation of the geoacoustic properties. The sound field simulated using the estimated properties was compared with the measured one for each receiver depth and an excellent agreement was confirmed not only at the frequency used for the inversion but also at the different frequency.