Mid-to-high-frequency bottom loss in the east China Sea

Bottom-loss measurements made in the East China Sea in May-June 2001 as part of the Asian Sea International Acoustics Experiment as a function of frequency (2-20 kHz) and seabed grazing angle (15°-24°) are presented. The measurements are interpreted as estimates of the modulus of the plane wave reflection coefficient and data are compared to predicted values using a reflection coefficient model, based on a two-layered sediment for which the sound speed in the surficial sediment layer is allowed to vary as a linear k² profile, where k is acoustic wave number. The region below this layer is modeled as a half-space with constant density and sound speed. The reflection coefficient model is driven by eight geoacoustic parameters; these are estimated from the data by minimizing the weighted squared error between the data and the model predictions for a candidate set of parameters. The parameter estimates for the sediment layer are thickness, 0.9±0.5 m; density, 2.0±0.1 g/cm³; and attenuation, 0.25±0.05 dB/m/kHz, with sediment layer sound speed increasing from 1557±4 m/s at the water-sediment interface to 1625±35 m/s at a depth of 0.9 m. The parameter estimates for the half-space are density, 2.0±0.1 g/cm³; attenuation, 0.25±0.15 dB/m/kHz; and sound speed, 1635±52 m/s. Variances for these estimates are derived using the Bootstrap method. This parameter set produced model curves that agreed reasonably well with the observations of bottom loss over the entire frequency range and is consistent with the range of independently measured geoacoustic variables. Since this mid-to-high-frequency data set does not provide detailed information about the sediment structure for depths beyond about 3 m, the geoacoustic parameter set is more properly viewed as description of the sediment layer and sediments in the underlying 2 m. Similarly, a self-consistent construction of a geoacoustic model for the East China Sea should necessarily amalgamate the mid-to-high-frequency results given here with results obtained at lower frequencies.