Effect produced on sound propagation by an internal temperature front moving over the shelf

Results of observing the changes that occur in the vertical distribution of water temperature under the effect of an intense atmospheric cyclone and the influence of these changes on sound propagation in the shelf region of the Sea of Japan are presented. The measurement results refer to the autumn conditions. The measuring equipment includes a vertical acoustic-hydrophysical measuring system, a broadband transmitter (both of them being connected with the shore station by cable lines), and a self-contained resonance (320 Hz) transmitter of the electromagnetic type. The sound (tone signals) propagation is studied on a 510-m-long constant-depth (38 m) track (TON-310 Hz) and a 10.6-km-long track (TON-320 Hz), which is set up by placing the self-contained transmitter at the bottom (at a depth of 65 m). Results of field experiments are presented along with those of numerical simulation of the effect produced by an internal temperature front moving toward the coast and formed by the seasonal thermocline on the propagation of 320-Hz sound signals through it. It is shown that refraction and scattering of sound waves propagating through the temperature front moving along the acoustic track may cause intensity variations of acoustic field at the reception point, which occur synchronously at different depths and have amplitudes of up to 14 dB and a period of about 40 min.     

大陆架低频声传播建模研究

在过去的几十年中大陆架斜坡海域的低频声传播得到越来越多的重视。针对爆炸声作为声源的一次海上实验测量数据,对大陆架海域的低频远距离声传播进行了建模研究。在实验过程中单水听器布放在水下大约240m处接收爆炸声信号,对两条不同测线的传播损失进行了处理。本文结合海底地声模型并考虑了声速剖面的水平变化,利用抛物方程方法对传播损失进行建模。模拟计算该海域的传播损失同实验测量数据相比具有较好的一致性。     

浅海矢量声场干涉结构形成机理及试验研究

浅海低频声场的微观结构特征在于具有可用波导不变量表征的 稳定空间-频率干涉结构.声场兼具标量场和矢量场, 波导条件下二者联合决定声场的全部特性. 本文研究浅海声场空频干涉结构的矢量场特征. 理论分析了声压谱、动能密度谱、声强流谱等矢量场干涉结构的形成机理, 探讨了矢量场干涉结构的波导不变量表征, 数值仿真研究了Pekeris波导中能量和能流密度的干涉特性, 进行了宽带声源辐射矢量声场干涉特性及表征的海上试验.实测结果与理论、仿真分析有较好的一致性. 研究结果表明: 中近程和中远程声场均能模态相干, 有稳定的空频干涉结构, 并且矢量声场空频干涉结构存在多种形式, 除各种能量和能流密度谱图外, 相干系数谱也呈现干涉特征, 这些形式的空频干涉结构均可用波导不变量理论有效表征. 关键词： 矢量声场 干涉结构 波导不变量 浅海低频声场     

浅海环境下数据同化声层析方法研究

讨论了适用于浅海的基于声速局部测量模型、声传播模型及声压场局部测量模型的声学数据同化方法,并给出了具体的执行算法。该算法根据最优化准则把局部声速和声压场的先验测量信息,以及声传播模型进行了有效地融合,提高了海洋环境参数估计的精度,为浅海声层析提供了新思路。同时,利用实验浅海声速测量数据,通过经验正交函数对实际海洋声速剖面进行了估计,并分析了各类噪声对水体声速场及海底声学参数估计精度的影响,验证了该执行算法的有效性。      

声波在含气泡液体中的线性传播

为了探讨含气泡液体对声波传播的影响, 研究了声波在含气泡液体中的线性传播. 在建立含气泡液体的声学模型时引入气泡含量的影响,建立气泡模型时引用 Keller的气泡振动模型并同时考虑气泡间的声相互作用,得到了经过修正的气泡振动方程. 通过对含气泡液体的声传播方程和气泡振动方程联立并线性化求解,在满足 (ω R₀)/c << 1 的前提下,得到了描述含气泡液体对声波传播的衰减系数和传播速度. 通过数值分析发现,在驱动声场频率一定的情况下,气泡含量的增加及气泡的变小均会导致衰减系数增加和声速减小;气泡的体积分数和大小一定时, 驱动声场频率在远小于气泡谐振频率的情况下,声速会随驱动频率的增加而减小; 气泡间的声相互作用对声波传播速度及含气泡液体衰减系数的影响不明显.最终认为气泡的大小、 数量和驱动声场频率是影响声波在含气泡液体中线性传播的主要因素. 关键词： 含气泡液体 线性声波 声衰减系数 声速     

Observationally constrained modeling of sound in curved ocean internal waves: examination of deep ducting and surface ducting at short range

A study of 400 Hz sound focusing and ducting effects in a packet of curved nonlinear internal waves in shallow water is presented. Sound propagation roughly along the crests of the waves is simulated with a three-dimensional parabolic equation computational code, and the results are compared to measured propagation along fixed 3 and 6 km source/receiver paths. The measurements were made on the shelf of the South China Sea northeast of Tung-Sha Island. Construction of the time-varying three-dimensional sound-speed fields used in the modeling simulations was guided by environmental data collected concurrently with the acoustic data. Computed three-dimensional propagation results compare well with field observations. The simulations allow identification of time-dependent sound forward scattering and ducting processes within the curved internal gravity waves. Strong acoustic intensity enhancement was observed during passage of high-amplitude nonlinear waves over the source/receiver paths, and is replicated in the model. The waves were typical of the region (35 m vertical displacement). Two types of ducting are found in the model, which occur asynchronously. One type is three-dimensional modal trapping in deep ducts within the wave crests (shallow thermocline zones). The second type is surface ducting within the wave troughs (deep thermocline zones).     

炉内管阵列中声源辐射特性数值研究

炉内换热器结构类似于声子晶体,基于声子晶体理论,采用数值计算与软件模拟得到管阵列中声源不同方向的声传播特性,并将两种方法所得结果进行对比分析,研究声源在管阵列中不同位置处的辐射特性.结果表明：声指向性特性与声源通过管排数量,声源频率是否处在声波禁带有关.     

Frequency dependence and intensity fluctuations due to shallow water internal waves

A theory and experimental results for sound propagation through an anisotropic shallow water environment are presented to examine the frequency dependence of the scintillation index in the presence of internal waves. The theory of horizontal rays and vertical modes is used to establish the azimutal and frequency behavior of the sound intensity fluctuations, specifically for shallow water broadband acoustic signals propagating through internal waves. This theory is then used to examine the frequency dependent, anisotropic acoustic field measured during the SWARM'95 experiment. The frequency dependent modal scintillation index is described for the frequency range of 30-200 Hz on the New Jersey continental shelf.     

Sound Propagation in Shallow Water with an Inhomogeneous GAS-Saturated Bottom

We present the methods and results of numerical experiments studying the low-frequency sound propagation in one of the areas of the Arctic shelf with a randomly inhomogeneous gas-saturated bottom. The characteristics of the upper layer of bottom sedimentary rocks (sediments) used in calculations were obtained during a 3D seismic survey and trial drilling of the seafloor. We demonstrate the possibilities of substituting in numerical simulation a real bottom with a fluid homogeneous half-space where the effective value of the sound speed is equal to the average sound speed in the bottom, with averaging along the sound propagation path to a sediment depth of 0.6 wavelength in the bottom. An original technique is proposed for estimating the sound speed propagation in an upper inhomogeneous sediment layer. The technique is based on measurements of acoustic wave attenuation in water during waveguide propagation.     

The effect of internal waves on the sound propagation in the shelf zone of the sea of Japan in different seasons

Results of the observation of seasonal variations in the vertical distribution of water temperature in the shelf zone of the Sea of Japan are presented, and the effect of this variability on the parameters of internal waves and on sound propagation is studied. The measurements were carried out in different seasons using a vertical acoustical-hydrophysical measuring system. The propagation of sound (tone and noise signals) was studied on a 510-m-long track at a constant depth of 38 m. Using a self-contained resonance (320 Hz) transmitter of the electromagnetic type, which was bottom-moored at a depth of 65 m, a 10.6-km-long stationary acoustic track crossing the shelf was set up. During the in-sea experiments, the spatial characteristics of internal waves were measured along with the distributions of temperature, salinity, sound velocity, and sea level variations.     

海洋波导中目标声辐射场的计算方法

针对海洋波导中目标声辐射场的计算问题,提出了一种基于波叠加法并在近、远场采用不同水声传播模型的建模方法,可以将近、远场作为一个统一的系统进行高效地分析。该方法通过给定的已知表面振动速度的结构计算出目标内部虚拟点声源的源强,再配合在相应的水声环境中点声源传播模型的Green函数计算出结构外的声辐射场。以Green函数为纽带,在求源强和计算近场声辐射场时采用镜像虚源法,而在计算远场声辐射场时采用简正波法。通过该方法得到的有限水深波导中声速剖面为正梯度、负梯度、负跃层的3种情况下的脉动球、刚体摆动球的声辐射场计算结果与COMSOL的有限元计算结果进行对比,结果表明了该方法在提高计算效率的同时保证了计算精度。      

Acoustic effects caused by high-intensity internal waves in a shelf zone

The sound field fluctuations caused by high-intensity, solitonlike, quasi-plane internal waves crossing a fixed acoustic path at different angles are numerically modeled for natural conditions of the shelf zone of the Sea of Japan. The horizontal refraction of sound is considered for the case of an acoustic path parallel to the internal wave front.     

Matched-field processing gain degradation caused by tidal flow over continental shelf bathymetry

Temporally variable, range dependent sound-speed profiles measured during ebb flow and estimated for slack flow are used to quantify the variability of matched-field signal-processing gain degradation in shallow water propagation channels controlled by tidally driven stratified flow over variable bathymetry. Calculations along a 9.3 km range establish phase changes in the acoustic signal as the primary cause of a 3-9 dB degradation in the coherent matched-field processing output of a full water column vertical array. The work indicates that over a tidal cycle acoustic signal properties and matched-field processing gain can be expected to change continuously in a shallow water stratified channel that has bathymetry variability. Acoustic signals propagating in such tidal flow-controlled environments may be expected to display repeatable (over successive tidal cycles) and predictable changes in their phase coherent properties. These results suggest that matched-field processor replica fields used in the shelf/slope propagation environment will have to be updated regularly during a tidal cycle to maintain maximum processor gain.     

Simulation of low-frequency sound propagation in an irregular shallow-water waveguide with a fluid bottom

A further development of a previously proposed approach to calculating the sound field in an arbitrarily irregular ocean is presented. The approach is based on solving the first-order causal mode equations, which are equivalent to the boundary-value problem for acoustic wave equations in terms of the cross-section method. For the mode functions depending on the horizontal coordinate, additional terms are introduced in the cross-section equations to allow for the multilayer structure of the medium. A numerical solution to the causal equations is sought using the fundamental matrix equation. For the modes of the discrete spectrum and two fixed low frequencies, calculations are performed for an irregular two-layer waveguide model with fluid sediments, which is close to the actual conditions of low-frequency sound propagation in the coastal zone of the oceanic shelf. The calculated propagation loss curves are used as an example for comparison with results that can be obtained for the given waveguide model with the use of adiabatic and one-way propagation approximations.     

用基于声类比的边界元计算管道风扇的管道声传播和辐射

本文介绍了用基于声类比的边界元计算管道风扇声场的新方法,将已被成功地应用于物体对外声场的散射计算方法推广应用到管道风扇的管内声传播和管口声辐射问题。数值结果表明了模型和方法的正确性及其可以作为管道声处理降噪敏果预测的工具。     

The contribution of a lateral wave in simulating low-frequency sound fields in an irregular waveguide with a liquid bottom

A further development of a previously proposed approach to calculating the sound field in an arbitrarily irregular ocean is presented. The approach is based on solving the first-order causal mode equations, which are equivalent to the boundary-value problem for acoustic wave equations in terms of the cross-section method. For the mode functions depending on the horizontal coordinate, additional terms are introduced in the cross-section equations to allow for the multilayer structure of the medium. A numerical solution to the causal equations is sought using the fundamental matrix equation. For the modes of the discrete spectrum and two fixed low frequencies, calculations are performed for an irregular two-layer waveguide model with fluid sediments, which is close to the actual conditions of low-frequency sound propagation in the coastal zone of the oceanic shelf. The calculated propagation loss curves are used as an example for comparison with results that can be obtained for the given waveguide model with the use of adiabatic and one-way propagation approximations.     

On generation of sound by an internal bore

Internal bores are nonlinear internal waves that move toward the coast and have a characteristic steplike shape to their fronts. The bore is a frequent phenomenon for shelf zones of seas and oceans. The passage of the bore leads to a substantial change in the conditions of sound propagation in the sea medium and, generally, to the formation of a near-bottom sound channel. In this work, we theoretically consider another effect of the aforementioned phenomenon on underwater acoustics, namely, the generation of sound by the internal bore propagating over the bottom.     

Anomalous sound propagation due to the horizontal variation of seabed acoustic properties

Sound speed profile and the bottom acoustic property are two of the most important factors, which influence the sound propagation in shallow water. The activities of the internal waves make the temperature of the sea-water varying with the time and space, and then affect the sound propagation in the sea. In 1991 Zhou and his colleagues postu-lated that unusually high transmission losses (TL) over a limited frequency range found in annual measurements taken in the Yellow Sea was due to the in…     

Sound attenuation on an ocean shelf at short ranges from a source in the presence of surface waves

The effect of surface roughness on the attenuation of low-frequency acoustic waves on a shallow ocean shelf is analyzed using numerical simulation. We focus here on transmission loss during propagation at short (less than 50 water layer depths) ranges from the sound source. The effect is considered both for a soft and hard bottom, when the sound velocity in the bottom is, respectively, lower or higher than the sound velocity in seawater. It is shown that to correctly predict losses at a short range in the presence of a rough upper boundary, it is necessary to take into account the interaction of both propagation and leaky modes. In the case of a hard bottom compared to a low-velocity one, the effect of surface roughness on propagation turned out to be the most pronounced.     

Study of how hydrological conditions affect the propagation of pseudorandom signals from the shelf in deep water

The paper examines how hydrological conditions affect manifestation of the acoustic “landslide” effect, which consists in focusing of acoustic energy in the near-bottom layer on the shelf and its transition to the axis of an underwater sound channel in deep water. We compare the results of experiments performed in the Sea of Japan in April 2014 and August 2006 on the same acoustic track, where the distance between corresponding points was more than 100 km. In April, the hydrological conditions in the shelf region of the track and in the upper layer of the deep-water part of the sea were characterized by the presence of a relatively weak (~0.35 s^–1) negative vertical sound velocity gradient, whereas in August 2006, it was ~1.5 s^–1. Experimental and numerical studies showed that the acoustic landslide effect also manifests itself under conditions of a weak negative sound velocity gradient, but the structure of the acoustic field trapped by the underwater sound channel has a more complex character with a time-expanded pulse characteristic. Nevertheless, its ordered, stable, and well-identified structure at all track points chosen for measurements make it possible to reliably create an efficient (with accuracies to hundredths of a percent) underwater navigation systems like GLONASS and GPS for the spring hydrology season.