跨大洋水声传播建模与绕射效应

Transoceanic underwater acoustic propagation modeling and diffraction effect

跨大洋水声传播通常指传播距离超过一万公里、低频声波与深海海洋环境大尺度、连续相互作用的物理过程,是超远程水声探测的理论基础.水平绕射是指主要由水体声速变化引起的累积效应,是跨大洋水声传播中非常重要的物理现象.跨大洋水声传播水平绕射会引起声传播路径、到达信号方位角和声传播损失的巨大变化,导致因地形遮挡所产生的“声影区”变为“声照亮区”,有利于水声事件的跨大洋水声传播和信号的超远程检测.为了克服传统柱坐标系下水声传播模型空间分辨率逐渐变差的缺点,本文首先在直角坐标系下构建了一种跨大洋三维水下声传播模型,可实现近、远程声场空间分辨率不变的声场仿真.然后在美国声学学会标准斜坡地形等基准问题下,通过比较镜像解和仿真结果以验证该模型的精度.最后以2022年1月15日的汤加海底火山喷发为实例,开展了火山T波传播建模仿真,实现了从太平洋跨越到大西洋的水声场快速计算.声场计算范围超过16000 km.结果表明,太平洋火山所激发的T波在南美洲大陆的南端产生了绕射,能够经非大圆路径到达大西洋中部并为阿松森岛的水听器三联体H10N所探测到.通过对比不同波导条件下的声场模型仿真结果,发现在南极附近海域声速的变化是产生声水平绕射效应的主要原因.定量比较结果表明,当考虑水体声速变化的累积效应时,在水平距离10000~16500 km范围的垂直剖面内声传播损失降幅可达60 dB以上,在850 m深度的水平面内声传播损失降幅也达到60 dB以上.水体声速变化引起的绕射效应是水声观测系统能够接收到跨大洋传播声信号的根本原因.

Transoceanic underwater acoustic propagation refers to the physical process that low-frequency sound waves interact with the deep ocean environment continuously on a large-scale,over a distance of 10000 km.It is the theoretical basis for acoustic detection at ultra-long range.The cumulative effect of the change in water sound velocity causes the diffraction,which is an important physical phenomenon in transoceanic acoustic propagation and leads to significant changes in acoustic propagation paths,arrival angles and acoustic propagation loss.This phenomenon transforms the"acoustic shadow zone"caused by the seafloor topography into an"acoustic illumination zone"and makes it possible to detect the acoustic signal of the event,which propagates over long distances.A three-dimensional underwater acoustic propagation model is constructed in the Cartesian coordinate system to improve the spatial resolution of the traditional acoustic propagation model in the cylindrical coordinate system.The model simulates a sound field with a constant spatial resolution,either near or far.Then,the accuracy of the model is validated by comparing the simulation results with the analytical solution derived from the method of images via Acoustical Society of America slope topography benchmark problems.The Tonga submarine volcano which erupted on 15 January 2022,was used as an example.A model was created to simulate volcanic T-wave propagation across the Pacific Ocean to the Atlantic Ocean,over 16000 km.The results show that T-waves excited by a volcano in the Pacific bypass the southern end of the South American continent and arrive at the mid-Atlantic along the non-great circular path,eventually being detected by the hydrophone triplet H10N at Asunción Island.Comparing simulation results under various waveguide conditions indicates that the variation of sound velocity in the ocean near Antarctica is the main reason for the horizontal bending of sound rays.The quantitative results show that accounting for the cumulative effect of water sound velocity variation,the reduction of sound propagation loss would be over 60 dB in the horizontal plane at 850 m depth and in the vertical profile at the range of 10000 km-16500 km.The diffraction caused by the changes in water sound velocity is the main reason that underwater acoustic observation systems are able to receive acoustic signals propagating through the oceans.