Shuanglin Wu

This paper extends the seabed reflection phase-shift parameter P from a liquid Pekeris waveguide to an elastic Pekeris waveguide incorporating shear waves, and derives an approximate formula for the waveguide invariant β _mn in the elastic waveguide. Under the small grazing angle approximation, the elastic phase-shift parameter P _e is expressed as the product of the liquid-waveguide parameter P and a factor depending only on the shear-to-water sound-speed ratio L . This factor decreases and then increases with L , reaching its minimum value of zero at approximately L = 0.7. For identical water and seabed properties, the elastic waveguide, with the additional introduction of a shear-wave speed, yields a smaller P _e than its liquid counterpart. Since the phase-shift parameter characterizes the rate of change of the reflection phase with grazing angle, a smaller P _e indicates a weaker angular dependence of the reflection phase, which in turn affects the waveguide invariant through the dispersion equation. The waveguide invariant is positively correlated with Pe, and hence exhibits the same nonmonotonic behavior with L . The optimal linear approximation of the reflection phase shift is jointly determined by the density ratio and L , with a positive correlation between the optimal values of these two parameters. Numerical simulations using the KRAKENC program validate the approximate formula for the waveguide invariant, showing excellent agreement, improved accuracy at higher frequencies, and robust performance across different seabed environments. This work provides a simple analytical tool for acoustic interference field analysis in an elastic Pekeris waveguide.

To solve the problem of array detection performance in environments with mesoscale eddies, this study utilizes the Gaussian eddy model to describe the sound speed structure disturbed by eddies. Through numerical simulations, the corresponding sound field is obtained, and the transmission loss influenced by the eddy is analyzed. Furthermore, to investigate the relation between the array gain and spatial correlation in the eddy environments, the differences in vertical correlation at different positions and their effects on the vertical array gain of conventional beamforming (CBF) are studied. When the source is around the eddy center, the conclusions drawn are as follows: (1) The presence of an eddy changes the turning-point depth and the sound field distribution, significantly affecting the direct sound region and the first convergence zone, while having a minor impact on the first shadow zone. (2) In different eddy-induced environments, the first convergence zone maintains a high vertical correlation, but the vertical correlation of the direct sound region is greatly influenced by the eddy. (3) The array gain of CBF is consistent with the vertical correlation. When the correlation between each element of the sound field is great, the array gain increases with the number of array elements.

Through the experimental data collected in different seasons with the propagation conditions change in the East Indian Ocean (EIO) and the South China Sea (SCS), we observe that the structure of the sound speed profiles (SSPs) in these two seas area is very different and has a significant impact on the CZs and time arrival structure of acoustic pulses. In the EIO environment, sound energy transmitting along the sound channel axis (SCA) is relatively large, and the corresponding signals arrive first, whereas signals propagating off the SCA arrive late, which is totally different with the characteristics of the waveform in the SCS. At the same time, the range of the first CZ in the EIO is 7–8 km farther than that in the SCS, and the width of the CZs in the EIO is about 2–3 km narrower when the water depths in the two experimental areas are similar. Combined with the theoretical model, the influence mechanism of SSP on the time arrival structure and CZ propagation are theoretically analyzed, which well explains the phenomenon observed in the two experiments. The results can provide a reference for the application of communication sonar in the deep-sea remote environment.

The structure of a sound speed profile (SSP) in deep water causes refraction of sound rays and Convergence Zones (CZs) of high intensity where the rays focus at shallow depth. Study of sound field characteristics in the CZs has always been the focus of deep-water acoustics research. Many studies have been conducted on sound propagation in different parts of the oceans with different environments and, in this paper, the range and width of CZ is analyzed in the East Indian Ocean (EIO) and the South China Sea (SCS). Through the experimental data collected in different seasons with the propagation conditions change in the EIO and the SCS, we observe that the SSPs in different marine environments have a significant impact on the CZs of deep water. The sound channel mixing layer and isothermal layer have great effect on the CZ ranges. The water depths in the two experimental areas are similar, the range of the first CZ in the EIO is 7–8 km farther than that in the SCS, and the width of the CZs in the EIO is about 2–3 km narrower than that in the SCS. The surface mixed layer and the thermocline affect the CZ width but has little effect on the CZ range when the sound speed at the source and the bottom are practically the same. As the propagation conditions change along the seasons in the EIO, the range of the first CZ is almost the same, but the width of the CZs in the summer is about 2 km narrower than that in the spring. The water depth affects the CZ width but has little effect on the CZ range if the CZs can be formed. The different CZ characteristics between EIO and SCS are explained by both theoretical calculation and numerical simulation. The influence of the SSP structure and water depth on the CZ range are analyzed and the corresponding mechanism is explained. The research results are of great significance for underwater acoustic detection in deep sea.

The properties of bottom acoustic parameters are greatly influenced on sound field prediction. Acoustic parameters in deep water are not well understood. We will discuss experience with geoacoustic inversion of transmission-loss (TL) data from 100 to 600 Hz from deep water regions. Bottom acoustic parameters are sensitive to the TL data in the shadow zone of deep water. Multiple steps TL inversion method is proposed to invertsound speed, density and attenuation in deep water. Both a uniform liquid half-space and a two-layer geoacoustic profile are assumed. Sound speeds of bottom are inverted by using TL data in the shadow zones at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea (SCS) in summer 2014. Meanwhile, bottom densities are estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the long range TL data by using the known sound speeds and densities as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. The inverted bottom parameters can be used to forecast the transmission loss in deep water area of SCS very well.