Research On The Influence Of The Dispersion Of Sound Veloticy Of Marine Sediments On The Characteristics Of Sound Propagation

As the lower boundary of ocean waveguides,the acoustic characteristics of marine sediments have always been a hot research topic in sound propagation of sound and field modeling.For a long time,although various parameters of marine sediment have been widely considered in the study of sound propagation,the phase speed of marine sediment is often treated as a constant,which leads to inevitable errors in theoretical modeling and simulation analysis.Existing research has shown that marine sediment is a kind of dispersion medium,and its phase speed is not only related to the frequency of sound,but also changes with the depth.Therefore,In order to solve the problem that people often ignore the dispersion of sound velocity in sediment and the variation of sound velocity with depth in practical applications,this paper uses four methods based on the theory of normal mode to solve the dispersion equations of normal mode in different environments.The sound field is analyzed based on the eigenvalues and eigenfunctions of normal mode,and the influence of sound velocity dispersion in marine sediments and variation with depth on sound propagation is studied.In this paper,the parabolic Newton’s method is first used to solve the dispersion equation to obtain the eigenvalues of normal mode of each order in the complex field.The phase speed and group speed of normal modes of each order are obtained by using the eigenvalues,and the dispersion characteristic curve is drawn.The influence of environmental parameters on the dispersion characteristics of the group speed and phase speed of normal mode is studied.In response to the significant impact of changes in marine sound velocity on dispersion characteristics,after considering the dispersion of sound velocity in marine sediments,the sound field is simulated and compared with the case where sound velocity is constant in marine sediments to study the impact of sound velocity dispersion in marine sediment on sound propagation.Then,considering the continuous distribution of sediment sound velocity,based on the approximation theory named W.K.B,the modal equation was solved to obtain the eigenvalues of normal mode and the reflection coefficient of sound waves on the sediment under the continuous distribution of sound velocity and density of marine sediment.The results were compared with the case where the sound velocity of marine sediment was constant,and the influence of the continuous distribution of sound velocity with depth on the sound propagation characteristics of marine sediment was studied.The work of this paper indicates that:1.The calculation results of the parabolic Newton’s method are accurate and reliable,and the accuracy is high.All roots of the dispersion equation in the real number and complex number fields can be calculated.In addition,the parabolic Newton method can make up for the deficiency that KRAKENC cannot calculate the normal wave eigenvalue near the cut-off frequency.2.The changes of environmental parameters have an impact on the dispersion characteristics of normal mode.For the same order of normal mode,the greater the depth of seawater,the greater the group speed;The higher the sound velocity,the smaller the group speed,the smaller the minimum group speed,the faster the change in group speed,and the shorter the length of Airy waves;The higher the density of the sediment,the smaller the minimum value of normal mode group speed,but the change is not significant;When considering the sound absorption of the medium,the Airy frequency disappears.After considering the dispersion of sound velocity in marine sediment,there is a significant broadening phenomenon in the time-domain waveform of broadband pulses received in the water,and more energy is returned to the water during the propagation process of sound waves,reducing the propagation loss of sound waves in the water.3.After considering the continuous distribution of sound velocity in marine sediments with depth,the surface sound velocity of the seabed has a strong impact on the sound reflection coefficient.The higher the surface sound velocity,the greater the sound reflection coefficient.However,as the depth increases,the impact of changes in sound velocity on the sound reflection characteristics becomes smaller.When a certain depth is reached,even if the sound velocity changes greatly,it will not have a significant impact on the sound reflection characteristics.In addition,due to the limited range of changes in seabed density itself,the continuous distribution of seabed density has no significant impact on the acoustic reflection characteristics.