Study On Acoustic Characteristics Of Deep-water In Convergence And Shadow Zone

In the marine environment,sound is the main way for information transmission;study on the propagation rules of sound waves in the deep sea has extremely important meaning in both the military and civilian area.This dissertation takes advantages of the experiments performed in deep water in last several years and analyzes the first-order and second-order propagation rules of sound field characteristics: first-order characteristic is refers to the energy properties,namely the changing rule of the transmission loss;Second-order characteristic refers as the correlation properties of space and time.Most of the theories and research methods validated by computer simulation and sea trial data,and this dissertation has important theoretical and application value.The main research work and conclusions are as follows:1.The deep-water sound propagation modes are analyzed.The definition of trapping coefficient and critical bottom loss is proposed,and the high probability distribution map of the global convergence zone and the sea area of the submarine ejection mode are given.In order to improve the detection performance of the sonar system in the deep sea,several important deep sea propagation modes are analyzed in this study.Firstly,based on the data of hydrological assimilation,the probability distribution map of the convergence zone in the world is given to guide the sonar system to detect;secondly,this study introduces the concept of trapping coefficient,and points out the hydrological conditions which are helpful for the detection with bottom bounce mode;thirdly,this paper analyzes several kinds of typical seabed sediment types,and presents the method of using multilayer equivalent density fluid model to analyze the layered seabed reflection loss;Finally,the concept of critical sea bottom loss is defined: when the bottom reflection loss is less than the critical bottom loss,bottom bounce mode can be used to detect target in acoustic shadow zone.2.The extraction method of seabed reflection loss by deep shadow sound propagation data is studied.On the basis of this,the method of retrieving the equivalent sea bottom parameters using the reflection loss is also studied.Through a deep experimental data obtained in the South China Sea(SCS),bottom reflection loss was extracted with the ray theory.On the base of the bottom reflection loss,approach using the theory of Bayesian inversion method is studied to get seabed sediment parameters.In the approach,the two fluid model,a sediment-basement layer system for the low frequency was assumed,in each layer the sound velocity,density and attenuation coefficient are constan.Then,through the Bayesian theory,seven parameters including sedimentary thickness for the low frequency and three parameters for the mid-frequency were inverted as the equivalent sea parameters.The parameters are used to forecast results,which fit well with the experiment measurements,proving the effectiveness and feasibility.3.The influence of deep-sea linear internal waves on time correlation radius is studied.The relation between time dependent radius and frequency and distance is obtained,and it is found that the acoustic shadow within the time correlation radius is larger than the convergence zones time correlation radius.Based on the Monte Carlo simulation,the relationships between the correlated time radius and frequency and distance are studied within a few tens of kilometers,and the results show that the radius of the relevant time exhibits a-3/2 power frequency dependency,but the relationship between radius and the distance cannot use a simple exponential expression;the distance dependence become more complicated.If considering the linear internal wave effects only,the time radius in acoustic shadow zone is greater than in the convergence zone;the time radius shows a-3/2 power frequency dependency;On the same kind of sound ray path,the time radius is proportional-1/2 power of the distance.4.The mathematical expression of typical deep-sea conditions of low-frequency vertical ship noise directivity function is given.The method is proposed to calculate the spatial and temporal correlation of noise field and shadow of the convergence zone.This dissertation used a numerical calculation method combined with the reciprocity principle to calculate the typical environmental noise field vertical directivity.Numerical results show that under the condition of the deep ocean,the vertical directivity of ambient noise has two symmetrical peak,and there is a groove in the horizontal direction;the position of the peak,and the width of the groove is closely related to the deep sea.Then Von Mises function transformation was used to present a typical vertical directivity function under the condition of deep mathematical form,and with the expression the space and time correlation equation for the type of noise field is obtained at the same time.In the end,the model was verified with the measurement.Through calculation results compared with the experiment,it is found in this paper that the directivity of the established model can approximate the measured results,and the theory of spatial correlation agrees well with experimental data and verify the correctness and effectiveness of the model in this dissertation.