浅海小掠射角的海底界面声反向散射模型的简化

Simplification of roughness bottom backscattering model at small grazing angle in shallow-water

在浅海,尤其是负梯度声速剖面和海面较为平静的浅海波导,海底界面反向散射是浅海混响的主要来源.经验散射模型只适用于分析浅海混响平均强度衰减特性,而基于物理机理建立的反向散射模型克服了这一缺陷,但同时也引入了其受地声模型约束的问题.本文结合了海底反射系数的三参数模型,对浅海远场海底反向散射模型进行了简化,以减少地声模型的输入参数.理论分析了海底反射系数的相移参数可以描述海底对声场的散射作用,无需任何海底地声参数的先验知识.通过对海底反向散射模型近似简化,结果表明在临界角附近和甚小掠射角范围内的海底粗糙界面反向散射模型的角度特性和强度特性受海底沉积层的影响不同:在临界角附近,海底反向散射的角度特性受海底反射系数的相移参数加权,而其散射系数则近似与相移参数无关;对于甚小掠射角,海底反向散射的角度特性近似与海底反射系数的相移参数无关,其散射系数则近似与相移参数的4次方成正比.

Bottom backscattering due to roughness seafloor is the main source of shallow water reverberation,especially in the waveguide with downward reflection profile or a calm sea-surface.Empirical backscattering models with a simple form has an important limitation to analyzing other characteristics of reverberation except for the intensity characteristics,which originates from optics and describes the relationship between the bottom backscattering strength and scattering grazing angle of plane-wave in half-infinite space.In the shallow water,such a plane-wave backscattering model cannot be used due to frequency dispersion.The model of bottom backscattering based on physical scattering principle is made to relieve such a limitation,but thereby bringing about another restraint by a geoacoustics model.The bottom backscattering model,which is formulated during modeling the full-wave reverberation theory at small grazing angle in range-independent shallow water waveguide,is simplified by combining with bottom reflection coefficient model which is independent of the geoacoustics model.The bottom reflection coefficient model as referred to the proposed phase parameter P in this paper is equivalent to velocity and density of sediment to describe sound field interacted with sea-bottom.Therefore simplification of bottom backscattering model can be handled by the phase parameter without any knowledge of bottom geoacoustic parameters.The angular dependency and intensity dependency of bottom backscattering due to roughness seafloor at small grazing angle are studied more in depth through such a simplified model.Marking 2/P as the cut-off point,the grazing angle is divided into two stages.Near the critical angle,as grazing angle is greater than 2/P and less than critical grazing angle,the angular dependency of bottom backscattering due to roughness seafloor is weighted by phase parameter of bottom reflection coefficient,while the intensity dependency is independent of phase parameter.At each small grazing angle,as grazing angle is less than 2/P,the angular dependency of bottom backscattering due to roughness seafloor is proportional to incident and scattering grazing angle squared and irrespective of phase parameter of bottom reflection coefficient which is like the empirical bottom backscattering model,while the intensity dependency is proportional to the fourth power of phase parameter.So the bottom has different influences on the angular dependency and intensity dependency of bottom backscattering in different stages of grazing angle.