ABSTRACT
In the ocean waveguide, the sediment sound speed has a simple relationship with the group speed of the highest order mode that propagates close to the critical angle. The paper shows that robust estimates of the sound speed are obtained from estimates of the "critical" mode group speed determined from analysis of the energy distribution of the time-warped spectrum of a broadband signal. The method is applied to experimental data collected in the Yellow Sea of China. Estimated sound speeds agreed closely with expected values for clayey slit (1531 m/s) and sandy silt (1593 m/s) sediment at the sites.
ABSTRACT
On the basis of the principle of stationary phase, Chuprov [Ocean Acoustics: Current State (Nauka, Moscow, 1982)] proposed a scalar parameter-the waveguide invariant ß, to interpret the dispersive properties of underwater acoustic waveguide. It has been found that ß may be useful in many applications in underwater acoustics. A reasonable prediction of the value of ß is often necessary in such applications. It was known that ß has some "canonical" values for waveguides with simple sound speed profiles (SSP). When a thermocline exists, ß for refractive modes (with a turning point) is no longer constant and can even change its sign. In this paper, by dividing the SSP into the non-refractive part and the refractive part, a clear explanation of how the value of ß is affected by the thermocline is presented. The results show that ß can be positive and increase continually from around 1 to +∞; or negative from -∞ to about -3. An analytic criterion of the sign of ß is developed. This method can also be used to analyze the value of ß for any other kind of SSP.
ABSTRACT
The characteristics of scattering due to interface roughness are usually described by the backscattering matrix for reverberation modeling. The backscattering matrix based on the Bass perturbation theory has significant differences from that based on the empirical scattering rule (Lambert's rule), especially at low grazing angles. In a waveguide environment with a point source, it is very difficult to extract the quantitative characteristics of the backscattering matrix at low grazing angles from the experimental data because of the difficulties in acquiring low-grazing-angle scattering data and separating the scattering data between different modes (grazing angles). In contrast, the use of single-mode excitations as sources in shallow-water waveguides enables acquisition of good quality low-grazing-angle scattering data. In this paper, the characteristics of the backscattering matrix were obtained from different single-mode reverberation experiments in shallow-water. The experiments were carried out at different sites during different seasons off the coasts of China. Model-data comparisons were made and the results showed that at low grazing angles (2°-5°), the backscattering matrices based on the Bass perturbation theory were in good agreement with the experimental data, but the backscattering matrices based on Lambert's rule were not.
ABSTRACT
The waveguide invariant ß is affected by the shallow-water environment. The effect due to bottom sediment on ß is investigated in this paper. It is found that the effect of sediment bottom can be concentrated on one parameter P-the bottom reflection phase-shift parameter. For a Pekeris waveguide, under Wentzel-Kramers-Brillouin (WKB) approximation, a very simple analytic relation is given: ß ≈ 1 + P/(k(0)H(eff)), where H(eff) is the "effective depth," and H(eff) = H + P/2 k(0). The value of ß related to different high-speed sediments (including layered sediment) ranges from 1.0 to 1.5. Some numerical examples including the layered sediment case are conducted to verify this result. Good agreement between the results calculated by KRAKEN and by WKB with parameter P has been found. Hence, the application of parameter P provides a model-free platform to investigate the bottom effect on the waveguide invariant ß in shallow-water.