Sound propagation from the shelfbreak to deep water

Motivated by a phenomenon in an experiment conducted in the Northwestern Pacific indicating that the energy of the received signal around the sound channel axis is much greater than that at shallower depths,we study sound propagation from the transitional area(shelfbreak)to deep water.Numerical simulations with different source depths are first performed,from which we reach the following conclusions.When the source is located near the sea surface,sound will be strongly attenuated by bottom losses in a range-independent oceanic environment,whereas it can propagate to a very long range because of the continental slope.When the source is mounted on the bottom in shallow water,acoustic energy will be trapped near the sound channel axis,and it converges more evidently than the case where the source is located near the sea surface.Then,numerical simulations with different source ranges are performed.By comparing the relative energy level in the vertical direction between the numerical simulations and the experimental data,the range of the air-gun source can be approximated.     

陆架斜坡海域上坡波导环境中声能量急剧下降现象及其定量分析

深度较浅的声源其辐射声波在陆架斜坡海域上坡传播时,在斜坡顶端会出现声能量急剧下降现象.利用射线声学模型分析了造成这一现象的原因,并根据抛物方程声场模型计算的深海和浅海平均传播损失定义了"声能量急剧下降距离",定量分析了声源位置对该现象的影响.结果表明:声源深度对"声能量急剧下降距离"影响较大,而声源与斜坡底端水平距离对其影响较小;当声源深度变大时,部分掠射角较小的声线最终能够达到斜坡顶端,致使"声能量急剧下降距离"增大,继续增加声源深度,将导致上坡声能量急剧下降现象消失.利用抛物方程声场模型对陆架斜坡海域上坡声传播进行数值仿真,结合"声能量急剧下降距离"的定义,计算并比较了声源位置不同时该距离的变化,数值计算结果验证了理论分析.     

深海海底斜坡环境下的声传播

海底地形变化对声传播具有很大影响,在南海深海区域海底斜坡环境下进行了一次声传播实验,实验显示倾斜海底环境下声传播损失出现了一些不同于平坦海底环境下的现象,分析并解释了海底地形变化对产生声传播差异的原因.结果表明,海底斜坡对声波的反射增强作用可使斜坡上方的声传播损失减少约5 d B.当声波第一次入射到达的海底位置有较小幅度的山丘(凸起高度小于1/10海深)时,海底小山丘即可对声波有反射遮挡作用,导致在其反射区特定传播距离和深度上出现倒三角声影区,比平坦海底环境下相同影区位置处的传播损失增大约8 d B,影响深度可达海面以下1500 m.而海底斜坡对声波的反射阻挡作用使得从海面反射及水体向下折射的会聚区结构消失,只剩下从水体向上折射的会聚结构.因此,海底地形对深海声传播影响较大,在水下目标探测和性能评估等应用中应予以重视.     

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.     

Temporal coherence of sound transmissions in deep water revisited

This paper examines the signal coherence loss due to internal waves in deep water in terms of the signal coherence time and compare to data reported in the literature over the past 35 years. The coherence time of the early raylike arrivals was previously modeled by Munk and Zachariasen ["Sound propagation through a fluctuating stratified ocean: Theory and observation," J. Acoust. Soc. Am. 59, 818-838 (1976)] using the supereikonal approximation and by Dashen et al. ["Path-integral treatment of acoustic mutual coherence functions for arrays in a sound channel," J. Acoust. Soc. Am. 77, 1716-1722 (1985)] using the path integral approach; a -1 [corrected] power frequency dependence and a -1/2 [corrected] power range dependence were predicted. Recent data in shallow water in downward refractive environments with internal waves suggested that the signal coherence time of the mode arrivals follows a -3/2 power frequency dependence and a -1/2 power range dependence. Since the temporal coherence of the acoustic signal is related to the temporal coherence of the internal waves, based on the observation that the (linear) internal waves in deep and shallow waters have a similar frequency spectrum, it is argued that the modelike arrivals in deep water should exhibit a similar frequency dependence in deep and shallow waters. This argument is supported by a brute-force application of the path integral to mode arrivals based on the WKB relation between the ray and mode. It is found that the data are consistent with the -3/2 power frequency dependence but more data are needed to further test the hypothesis.     

深远海线性内波条件下声场时间相关性的空间分布

深海声场特定的干涉结构导致其时间相关性的空间起伏,研究这种空间特性可以为水声信号的探测与处理提供重要参考。利用抛物方程声场仿真模型,联合Monte-Carlo数值方法计算分析了深远海线性内波条件下声场时间相关性的空间分布特性。与现有的研究相比,给出了时间相关性的距离和深度起伏特征。结果表明,当接收达到一定距离,声场时间相关性的空间分布具有与声场干涉条纹类似的结构,声场干涉越强,时间相关性越好。此外,声源频率和声速标准差的变化会引起时间相关性空间分布规律的改变,且会聚区传播模式下的改变强于深海声道传播模式。      

Evidence of Doppler-shifted Bragg scattering in the vertical plane by ocean surface waves

A set of narrowband tones (280, 370, 535, and 695 Hz) were transmitted by an acoustic source mounted on the ocean floor in 10 m deep water and received by a 64-element hydrophone line array lying on the ocean bottom 1.25 km away. Beamformer output in the vertical plane for the received acoustic tones shows evidence of Doppler-shifted Bragg scattering of the transmitted acoustic signals by the ocean surface waves. The received, scattered signals show dependence on the ocean surface wave frequencies and wavenumber vectors, as well as on acoustic frequencies and acoustic mode wavenumbers. Sidebands in the beamformer output are offset in frequency by amounts corresponding to ocean surface wave frequencies. Deviations in vertical arrival angle from specular reflection agree with those predicted by the Bragg condition through first-order perturbation theory using measured directional surface wave spectra and acoustic modes measured by the horizontal hydrophone array.     

大陆坡海域二维声传播研究

针对在西北太平洋大陆坡外海进行的一次实验中观测到的接收信号能量在声道轴深度附近较为集中的现象,分析了存在向下斜坡时声源置于海面附近和斜坡表面两种情况下斜坡分别对声传播的影响。通过数值模拟,得到了不同声源深度下的传播损失和脉冲的时域波形,数值结果表明,当声源置于海面附近时,声波在水平不变深海环境中随距离衰减很快,而大陆坡的存在可实现能量的远距离传播;当声源置于斜坡表面时,大陆坡会改变水体中声波能量的分布,使其在声道轴深度附近比较集中,这种传播条件下小掠射角声线产生的时间展宽很小,      

Long-range reverberation in a randomly inhomogeneous shallow water with the use of focused radiation

The low-frequency bottom reverberation in a randomly inhomogeneous shallow water is investigated within the framework of a numerical experiment using vertical transmitting arrays focusing the acoustic field at various distances from the sea bottom. It is assumed that the main source of sound velocity fluctuations in the medium is represented by background internal waves. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that the reverberation level is mainly determined by the presence of internal waves and may vary by 5–20 dB as the distance from the focusing point to the sea bottom increases up to H/2, where H is the channel depth.     

Sound wave propagation in a vertically inhomogeneous ocean

Propagation of sound waves generated by a time dependent acoustic source in a vertically inhomogeneous ocean is considered. The effect of the solid bottom is included so that both the longitudinal and shear waves can be excited inside the bottom. The possibility of exciting a lateral shear wave by the acoustic source is also discussed. Although the results presented here are formal and general, physical interpretations have been offered whenever possible.     

The wave equation with viscoelastic attenuation and its application in problems of shallow-sea acoustics

A suitable tool for the simulation of low frequency acoustic pulse signals propagating in a shallow sea is the numerical integration of the nonstationary wave equation. The main feature of such simulation problems is that in this case the sound waves propagate in the geoacoustic waveguide formed by the upper layers of the bottom and the water column. By this reason, the correct dependence of the attenuation of sound waves in the bottom on their frequency must be taken into account. In this paper we obtain an integro-differential equation for the sound waves in the viscoelastic fluid, which allows to simulate the arbitrary dependence of acoustic wave attenuation on frequency in the time domain computations. The procedure of numerical solution of this equation based on its approximation by a system of differential equations is then considered and the methods of artificial limitation of computational domain are described. We also construct a simple finite-difference scheme for the proposed equation suitable for the numerical solution of nonstationary problems arising in the shallow-sea acoustics.     

Impact of ocean variability on coherent underwater acoustic communications during the Kauai [corrected] experiment (KauaiEx)

During the July 2003 acoustic communications experiment conducted in 100 m deep water off the western side of Kauai, Hawaii, a 10 s binary phase shift keying signal with a symbol rate of 4 kilosymbol/s was transmitted every 30 min for 27 h from a bottom moored source at 12 kHz center frequency to a 16 element vertical array spanning the water column at about 3 km range. The communications signals are demodulated by time reversal multichannel combining followed by a single channel decision feedback equalizer using two subsets of array elements whose channel characteristics appear distinct: (1) top 10 and (2) bottom 4 elements. Due to rapid channel variations, continuous channel updates along with Doppler tracking are required prior to time reversal combining. This is especially true for the top 10 elements where the received acoustic field involves significant interaction with the dynamic ocean surface. The resulting communications performance in terms of output signal-to-noise ratio exhibits significant change over the 27 h transmission duration. This is particularly evident as the water column changes from well-mixed to a downward refracting environment.     

南海北部浅海声场起伏及统计特性

水下声场的时间积分声强和峰值声强是声呐检测中的重要物理量,而海洋中的内波等动力学过程会造成声场强度的起伏。对南海北部浅海内波环境下定点声起伏实验数据分析,结合数值仿真,总结了试验海域近海底发射和近海底接收条件下内波引起接收信号强度起伏的统计特性.分析结果表明:接收信号的时间积分声强起伏小于峰值声强起伏;时间积分声强与峰值声强起伏的概率分布与对数正态分布较为接近.时间积分声强和峰值声强起伏大小与信号的频率有关,同一收发距离,中心频率650 Hz信号的声强起伏较中心频率310 Hz和375 Hz信号的声强起伏更为剧烈。对于同一频率,当海底较平坦时,声强起伏的闪烁指数随传播距离的增加而增大;当水深随传播距离逐渐变深时,声强起伏规律受内波和水深变化共同影响而更为复杂,闪烁指数先随传播距离增加而增大,之后又随传播距离的增加而逐渐变小.      

On the possibility of using acoustic reverberation for remote sensing of ocean dynamics

The two-point correlation function of diffuse noise fields produced by distributed random sound sources carries useful information on the medium of sound propagation. Such information can be used for performing passive acoustic tomography of the ocean. In a number of cases that are important for practice, the noise field in the ocean is predominated by contributions of individual point sources. Here, a theoretical study is presented on the possibility of determining the sound speed and current velocity in the water column by the correlation processing of reverberation signals measured by two vertical receiving arrays. In other words, we study the possibility of replacing the diffuse noise produced by a great number of delta-correlated sources by waves generated by a localized source and scattered at the rough surface and bottom of the ocean for sensing the medium. The correlation function of scattered waves is calculated by using the method of small perturbations. It is shown that the correlation processing of the scattered waves offers an opportunity of measuring the acoustic nonreciprocity and reconstructing the field of sound speed in the fluid, without using any acoustiLc transceivers.     

基于联合波叠加法的浅海信道下圆柱壳声辐射研究

针对浅海信道下弹性结构声辐射预报尚无高效可靠的研究方法,提出了一种浅海信道下弹性结构声辐射快速预报的联合波叠加法.该方法结合了浅海信道传输函数、多物理场耦合数值计算法和波叠加法理论,运用该方法可对浅海信道下弹性结构辐射声场进行快速预报.经数值法和解析解法验证后,从信道下辐射源、环境影响和辐射声场测量的角度研究分析了浅海信道下弹性圆柱壳的声辐射特性,阐释了进行浅海信道下结构声辐射研究的必要性.研究结果表明,仅在低频浅海信道下弹性结构可近似等效为点源,信道上下边界对声场产生显著的耦合影响,高频段的空间声场指向性分布尤为明显,垂直线列阵进行信道下结构辐射声功率测量时,测量结果受到信道环境边界和潜深的影响较大.     

基于信道传播理论的多基地远程海底混响研究

针对浅海多基地远程海底混响, 以声传播的线性信道近似理论为基础, 通过确定海底界面的不均匀起伏特性, 对入射声波的远距离异地混响脉冲响应函数进行求解. 对非水平分层介质的浅海波导, 利用绝热简正波求解信道的传输函数, 得出了小角度斜坡海底的远程海底混响强度, 以下坡海底为例, 分析对比了不同海底倾角斜坡海底对远程异地混响强度及其衰减规律的影响. 理论仿真表明, 波导的传输效应对多基地远程海底混响强度的影响不可忽略.     

Acoustic Parameters of the Bottom In Lake Baikal

A technique for estimating the effective acoustic parameters of the bottom in shallow water areas under ice cover has been developed. The methodology compares the experimental and simulated dependences of the sound field amplitude on depth at a distance from the source about an order of magnitude greater than the depth of the water area. The effective parameters are the values of the sound speed in the bottom, density, and attenuation coefficient of acoustic waves, which provide maximum agreement with experimental data in the calculations. The methodology was tested in a field experiment on Lake Baikal and can be recommended for developing autonomous acoustic monitoring systems.

     

浅海空气声入水传播特性分析

结合射线和波数谱积分方法,对空气声入水传播途径进行了分析,利用海上试验数据进行了比较检验。结果表明,在浅海环境中,对水下声场有主要贡献的空气声入水传播途径,主要是透射穿过海面边界的折射直达声以及后续的海底反射声途径,其中折射直达声途径的贡献主要集中在声源正下方附近区域,当距离较远时,由于声线扩展损失效应以及直达声影区两方面的限制,折射直达声传播损失显著增加,对接收声场起主要贡献的是可以到达更远水平距离上的海底反射声,包括海底海面多次反射声。      

浅海周期起伏海底环境下的声传播

海底粗糙对水下声传播及水声探测等应用具有重要影响.利用黄海夏季典型海洋环境,分析了同时存在海底周期起伏和强温跃层条件下的声传播特性,结果表明:由于海底周期起伏的存在,对于低频(<1 kHz)、近程(10 km)的声信号,传播损失可增大5—30 dB.总结了声传播损失及脉冲到达结构随声源深度、海底起伏周期及起伏高度等因素变化的规律.当海底起伏周期不变时,起伏高度越大引起的异常声传播的影响随之变大;当起伏高度不变时,随着起伏周期变大,其对声传播的影响逐渐变小.用射线理论分析了其影响机理,由于海底周期起伏改变了声波与海底的入射和反射角度,使得原本小掠射角入射到海底的声线变为大掠射角,导致海底的反射损失增大;另一方面,声线反射角度的改变会使得原本可以到达接收点的声能量,由于与海底作用次数增加或变为反向传播而大幅度衰减.在浅海负跃层环境下,声源位于跃层上比位于跃层下对声传播影响更大.周期起伏海底对脉冲声传播的影响表现在引起不同角度的声线(或简正波号数)之间的能量发生转化,一些大角度声线能量衰减加大,多途结构变少.多途结构到达时间及相对幅度的变化进而影响声场的频谱,会使得基于匹配场定位的方法性能受到影响.所以,声呐在实际浅海环境中应用时,应对起伏海底的影响予以重视.此外,研究结果对海底地形测绘空间精度的提高也具有重要参考意义.     

Mapping the sound field of an erupting submarine volcano using an acoustic glider

An underwater glider with an acoustic data logger flew toward a recently discovered erupting submarine volcano in the northern Lau basin. With the volcano providing a wide-band sound source, recordings from the two-day survey produced a two-dimensional sound level map spanning 1 km (depth) × 40 km(distance). The observed sound field shows depth- and range-dependence, with the first-order spatial pattern being consistent with the predictions of a range-dependent propagation model. The results allow constraining the acoustic source level of the volcanic activity and suggest that the glider provides an effective platform for monitoring natural and anthropogenic ocean sounds.