Acoustic Scattering From Dynamic Rough Surface In Shallow Water Environments

Shallow water acoustic channels are complex time-frequency-space varying channels.Transmission characteristics of the channels are closely related to the acoustic wave frequency,sound velocity profile,seabed terrain,sediment parameters,and the location of the transmitter and the receiver.Moreover,physical ocean phenomena,such as ocean waves,currents and internal waves,have also great influence on the transmission characteristics.Studying the transmission characteristics of the shallow water acoustic channels,especially the time-frequency characteristics,is very important to improve the underwater acoustic signal processing algorithms,which provide a reference for optimizing the underwater acoustic communication system.The random rough sea surface generated by wind and waves on the surface is an essential factor affecting the transmission characteristics of the shallow water acoustic channels.However,its influence mechanism still needs further study.The sea surface is approximate a plane as the average wave height of the sea surface is much smaller than the wavelength of the acoustic waves.In this case,acoustic scattering from the sea surface is assumed to be total reflection.However,the sea surface is a rough interface when the average wave height of the sea surface is comparable or greater than the wavelength of the acoustic waves.Energy will be lost,and the time-frequency-space varying characteristics of the scattering channel will be changed when acoustic waves are scattered from the rough sea surface.In this thesis,the methods of theoretical modeling,numerical simulation and offshore experiments were used to investigate acoustic scattering from the dynamic and rough sea surfaces in the shallow water environments.The main work of this thesis is as follows:1.The Monte-Carlo method was used to simulate a random rough sea surface.The influences of the wave spectrums,which include the PM spectrum,the JONSWAP spectrum and the TMA spectrum,on the random rough sea surface were analyzed.Furthermore,the feasibility and applicability of using the TMA spectrum to describe the shallow sea surface environment was discussed.The influences of the wind speed,fetch,water depth on the random rough sea surface were analyzed.2.The accuracy diference between the approximate method and the numerical method calculating the acoustic scattering strength from random rough sea surface was analyzed.The reliability of using the integral equation method to calculate the acoustic scattering strength from the rough sea surface was discussed.Furthermore,based on the TMA spectrum and the integral equation method,the influence of the acoustic frequency,scattering angle,wind speed,fetch and water depth on the acoustic scattering strength were investigated.The average energy characteristics of acoustic scattering from sea surface were obtained.3.The time-frequency coherence function of the acoustic scattering from dynamic sea surfaces was derived based on the Fresnel approximation theory.The influences of the wind speed,water depth,range between the receiver and the transmitter,acoustic frequency on the time-frequency coherence were analyzed numerically.The relationship between the coherence time,the coherence bandwidth and the wind speed,the water depth was obtained by keeping both the acoustic frequency and the range unchanged4.Shallow water acoustic channel experiments were conducted at Wuyuan Bay in Xiamen Sea.The influence of wind speeds,water depths,and the range between the receiver and transmitter on the channel impulse response were analyzed.Furthermore,the time coherent characteristics of the sea surface scattering channels were analyzed.