Sound propagation in shallow water is significantly influenced by geoacoustic properties.Estimating these geoacoustic parameters is essential for sound field analysis and sonar performance assessment.As a common practice,the seafloor is often treated as a single-layer or two-layer range-independent geoacoustic model to reduce the number of involved parameters.However,acoustic parameters inverted through these two geoacoustic models are typically limited in their applicability to a specific frequency range,thus posing challenges when applied across a broader frequency range.A range-dependent multi-layer geoacoustic model based on experimental measurements obtained with a sub-bottom profiler is proposed in this study.The inversion scheme combines three inversion methods to estimate geoacoustic parameters,considering the different sensitivities of geoacoustic parameters to different physical parameters within the acoustic field.Firstly,modal dispersion is used to invert the geoacoustic parameters of each layer,with the dispersion curve obtained through warping transform and the Wigner-Ville distribution.After that,both the localization using matched field processing and the dispersion curve fitting demonstrate the effectiveness of the inversion results for each layer,although the peak of the probability distribution of sound speed in the first layer is broader than in others.Secondly,matched field processing is employed to invert the geoacoustic parameters of the first layer.This method is based on the theory that as frequency increases,the depth of sound rays penetrating the seabed decreases,revealing changes in the first layer's sound speed with the seabed depth.Lastly,bottom attenuation coefficients at different frequencies are inverted by the transmission loss(TL),and a fitting relationship between the attenuation coefficient and the frequency is derived.The inversion results obtained by using the range-dependent multi-layer geoacoustic model are compared with results estimated by the single-layer geoacoustic model.The findings indicate that the transmission loss(TL)error from the range-dependent multi-layer geoacoustic model in this study is smaller than that from the single-layer geoacoustic model,especially in the lower frequency band.The range-dependent multi-layer geoacoustic model proves to be suitable for a broader frequency range,providing better precision in explaining various acoustic phenomena.

Seamounts have important effects on sound propagation in deep water. A sound propagation experiment was conducted in the South China Sea in 2016. The three-dimensional (3D) effects of a seamount on sound propagation are observed in different propagation tracks. Ray methods (BELLHOP N×2D and 3D models) are used to analyze and explain the phenomena. The results show that 3D effects have obvious impacts on a sound field within a horizontal refraction zone behind the seamount because some sound beams cannot reach the receiver for the horizontal refraction effects, which impacts the sound field within a certain angle range behind the seamount. The arrival structure results show that the eigenrays after horizontal reflection will arrive at the receiver earlier than those obtained from the two-dimensional (2D) model within the horizontal refraction zone behind the seamount. This means that the horizontal reflection effect of a seamount will cause the shortening of sound propagation paths. Finally, in the reflection zone in front of the seamount, the 2D and 3D TL results show that the shape of the reflection zone is similar to an “arch” type, and the horizontal refraction of sound waves has little effect on the TLs in the reflection zone of a seamount.

The seamounts usually have important effects on sound propagation in deep water. A sound propagation experiment was conducted in the South China Sea in 2016. One of the experimental goals is to investigate the three-dimensional(3D) effects of seamounts on sound propagation. Phenomena about horizontal refraction of acoustic waves are observed in different propagation tracks which go through the seamount along different directions when the source depth is 200 m. Ray methods (BELLHOP N×2D and 3D models) which can calculate sound field efficiently and show clear physical images, are used to analyze and explain the causes of the phenomena. The experimental and numerical results show that the convergent zone structures are destroyed by the direct blockage of seamount due to the multiple reflection of acoustic waves, which leads to the increase of transmission loss (TL), and horizontal-refraction zone with obvious boundaries appears behind the seamount. Some experiment phenomena cannot be explained by BELLHOP N×2D model in which the horizontal refraction effects are not taken into consideration. The experimental sound field structure behind the seamount is obviously different from N×2D model numerical result, i.e.the width of shadow zone based on the experimental data is wider than that calculated by N×2D model, and the width of strong horizontal-refraction zone from the experiment is narrower than the N×2D model result. Moreover, the TLs calculated by N×2D model is about 10 dB less than the experimental result in horizontal refraction zone. After analyzing the difference between experimental data and N×2D model numerical results by BELLHOP 3D model which contains the azimuth-coupling capability, it can be concluded that sound waves reach the receiver through horizontal refraction after the interaction with seamount when the source is located behind the seamount. The eigenrays obtained from 3D model are less than N×2D model numerical result because some of sound beams cannot reach the receiver as a result of the horizontal refraction effects, which leads to the experimental TLs larger than the numerical results calculated by N×2D model. Therefore, 3D effect of seamount has an obvious influence on sound field within a certain angle range behind the seamount, and the investigation of 3D effects of seamounts is meaningful for the sound propagation and target detection in deep water.