The ocean ambient noise field experiences a stochastic process of many such noise sources and the respective interactions of their wave fields with the waveguide boundaries.At frequencies of about 1 kHz and higher,forward scattering from surface wave can strongly affect shallow water sound propagation.However,most of the available ambient forecasting models do not consider the effects of multiple forward scattering from surface wave.Therefore,there is a need for an accurate method of predicting ambient noises at middle and high-frequency which can account for surface scatterings.Aiming at such a requirement,a propagation model based on transport theory method is described which yields the second-order moment of the acoustic field.Monte Carlo simulations of acoustic propagation loss are employed to validate the transport theory method.The mode number dependence of mode coupling phenomenon is demonstrated at 1000 Hz via the competing effects of mode coupling and attenuation ranges.Low and middle propagating modes are seen to have a smaller coupling range than the attenuation range,allowing mode coupling effects to take precedence over attenuation effects.The mode energies and the coherences are also examined,and it is found that the mode coupling rate for surface wave is significant,but strongly dependent on mode number.Mode phase randomization by surface waves is found to be dominated by coupling effects.On the basis of transport theory propagation model,connecting with the properties of ambient noise sources,a spatial characteristic model for ambient noise under surface wave is presented.Further,the effects of surface wave on ambient noise intensity,vertical correlation and vertical directionality are analyzed.Simulation results show that the surface wave may result in energy transfer from medium modes to low modes and high modes,the rate of energy transfer depends on the mode energy difference.Since the medium mode plays an important role in noise intensity,the noise intensity decreases with the increase of surface wave.In addition to noise intensity,the vertical correlation of ambient noise also decreases due to mode phase randomization by surface wave.Besides,mode coupling can also lead to a change of vertical beam intensity distribution,positive high-angle beams associated with direct,surface,and bottom-surface-bounced rays become weaker,while negative high-angle beams associated with bottom bounced rays become stronger.Since the vertical directionality is sensitive to surface wave,the model can be applied to ocean surface parameter inversion.In summary,the model provided in this paper is closer to actual ocean waveguide and has future prospect in ocean acoustic engineering application.%对于1 kHz以上声波,海面起伏会对浅海声传播产生显著影响,现有的噪声预报模型在建模过程中基本没有考虑海面起伏的影响.针对这一问题,本文基于传输理论建立了随机起伏界面下噪声场垂直相关性和指向性模型,仿真分析了海面起伏对噪声强度、垂直相关性与指向性的影响.结果表明,对于表面噪声,海面随机起伏使声波能量从中间阶简正波向低阶和高阶简正波转移,而对噪声强度起主要贡献的一般是中间阶简正波,所以海面起伏使得噪声强度减弱;简正波之间能量的耦合导致垂直平面上不同掠射角方向上到达的声波响应发生变化,经由海面反射大掠射角到达的声波响应以及中小角度到达的声波响应变弱,而经由海底反射大掠射角到达的声波响应变强;海面随机起伏还会扰动各阶简正波相位,使不同阶简正波互相关性变弱,致使噪声场的空间相关性也变弱.
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