浅海中孤立子内波引起的声能量起伏

当孤立子内波的波阵面与声传播路径所成角度较大时,简正波耦合是导致声信号起伏的主要因素。研究了浅海中孤立子内波引起的声能量起伏规律,给出声场起伏的耦合简正波表达式,并使用抛物方程模型进行仿真。数值分析表明,接收点声强随时间变化呈准周期性。在频谱图中能够得到声强起伏的主导频率,主导频率与孤立子内波沿声传播路径的移动速度成正比,与无扰动波导中简正波在距离上的干涉周期(对应于射线理论中临界声线的跨度)成反比,与孤立子内波的形状无关。此外,对声强频谱的垂直结构进行了分析,该结构与对声场起伏起主要作用简正波的本征函数相关。     

浅海内孤立波动态传播过程中声波模态强度起伏规律

内孤立波是一种常见于浅海海域的非线性内波,具有振幅大、周期短和流速强等特点,它通过扰动水体中的温盐分布使声速剖面产生明显的距离依赖性,进而影响水下声传播特性.内波自生成后通常以1 m/s量级的速度传播,运动的内波使声传播路径上的声波模态能量在空间和时间上剧烈起伏.本文定义模态强度为模态系数模值(模态幅度)的平方,并用其衡量各阶模态所含声能量的大小.文中基于耦合简正波理论,推导了内波运动时声波模态强度起伏的表达式,将模态强度表征为振荡项和趋势项的线性叠加.以往的工作大多局限于单独从时域或频域研究内波运动时声波模态强度的时变规律,本文则结合短时傅里叶变换在时频平面上揭示了模态强度的起伏机理.理论推导和数值仿真均表明内孤立波使各阶声波模态之间发生能量交换,即模态耦合.内波的动态传播进一步引起模态干涉,这种干涉效应表现为模态强度中的振荡项并使模态强度随时间快速起伏.受模态剥离效应(不同阶模态之间衰减系数的差异)的影响,趋势项的幅度随时间不断变化,进而对模态干涉引起的振荡叠加了时变的偏置.模态强度的整体走势和振荡项中各频率分量振幅的时变特征均与模态衰减密切相关.同时,本文使用深度积分声强作为总接...     

Sound fluctuations caused by internal waves in a shallow sea

Estimates are presented for the fluctuations of the parameters of low-frequency sound fields in shallow-water regions of the Barents Sea, in the presence of seasonal internal gravity waves. The objective of the experiments is to reveal the main mechanisms that govern the sound fluctuations and their statistical parameters on paths of moderate lengths (50–60 to 100–120 km). Another objective is to determine the features of the sound interaction with internal waves for the sound speed profile of the summer—autumn type for which the water stratification is most pronounced. As the probing signals, continuous tonal ones produced by bottommoored sources at the frequencies about 100 and 300 Hz are used along with the 1/3-octave noise signals with the central frequency 1000 Hz, which are generated by a source deployed from a vessel. For the signal reception, both fixed bottom-moored hydrophones and a vertical chain of hydrophones are used, the chain also being deployed from the vessel. The water temperature, the salinity, and the thermocline displacements are monitored with standard hydrographic sensors. The following main results are presented: the estimate of the degree of correlation between the sound fluctuations and the parameters of the water layer, the comparison of the fluctuations in the signal amplitude envelope with the data obtained in other regions, and the estimate of the statistical parameters of the signal amplitude fluctuations, including their dependence on the path length. One more result consists in the proof of the wave nature of the interaction of sound and internal waves, which manifests itself in a strong dependence of the sound interaction with internal waves of discrete frequencies on the frequency of the probing signal and on the angle at which these wave beams intersect. An attempt is made to explain the observed phenomena by the synchronism in the interacting sound and gravity waves. The data obtained can be used to analyze and compare the fluctuations of the sound fields in the ocean, especially in shallow-water regions.     

典型浅海温跃层内波对声场起伏的影响

本文描述了在夏季典型海区,温跃层内波起伏和不同距离声振幅起伏的实验测量结果。温度起伏记录时间48.96h,温跃层厚度约2-4m,温差达10-15℃。等温线内波垂直位移有5-6m,且主要受最低阶模式支配。测量获得的内波垂直空间频谱特性中,除潮周期占主要成分外,0.05-0.143cpm(7-20min)之间有明显的谱峰,谱衰减系数在-1.5--1.7之间。数值计算表明内波相、群速度在低频段完全相同,均为0.329m/s;随频率增加,相、群速度从约0.03cpm(33.33min)开始分离,且群速度值下降更快。在不同距离接收点,声振幅信号起伏有8-10dB;随距离增加,起伏有加快的趋势,并伴有信号衰落现象。谱衰减系数在-1,45--2之间。最后,用PE算法,模拟计算在内波起主导作用下的声振幅起伏。     

Energy fluctuations of high-frequency sound signals in a shallow water in the presence of nonlinear internal waves

The paper presents an analysis of energy fluctuations of high-frequency (2–4.5 kHz) sound signals propagating in a shallow water in the presence of nonlinear (soliton-like) internal waves (2006 Shallow Water experiment, US Atlantic shelf). Signals were received by three single hydrophones in different directions at distances of ～4, ～12, and ～5 km from the source. The angle between the first two acoustic tracks was ～15°. The third track was almost an extension of the first and was on the other side of the source. A relatively short (one to two solitons) nonlinear internal wave packet first moved approximately along the first two tracks and then along the third track. It is demonstrated that in the presence of solitons on the track in the frequency spectrum of energy fluctuations, there is an isolated frequency that depends, in particular, on the angle between the soliton front and the acoustic track. The experimental results agree well with the theory previously proposed by the authors, where the occurrence mechanism of fluctuations is explained using the ray approach.     

Measurement and modeling of three-dimensional sound intensity variations due to shallow-water internal waves

Broadband acoustic data (30-160 Hz) from the SWARM'95 experiment are analyzed to investigate acoustic signal variability in the presence of ocean internal waves. Temporal variations in the intensity of the received signals were observed over periods of 10 to 15 min. These fluctuations are synchronous in depth and are dependent upon the water column variability. They can be explained by significant horizontal refraction taking place when the orientation of the acoustic track is nearly parallel to the fronts of the internal waves. Analyses based on the equations of vertical modes and horizontal rays and on a parabolic equation in the horizontal plane are carried out and show interesting frequency-dependent behavior of the intensity. Good agreement is obtained between theoretical calculations and experimental data.     

内波、潮导致的声简正波幅度起伏及其深度分布

分析2005黄海声传播—内波实验传播起伏数据,观察（1）号声简正波幅度起伏及其深度分布特征。非线性内波（IW）通过接收阵可以导致脉冲峰值约5 dB起伏,幅度起伏垂向分布与简正波本征函数的微分分布吻合良好,（M2）潮致信号幅度起伏也满足相同深度分布特性。然而内波和潮致起伏的垂向相位变化明显不同:前者本征函数峰值两侧起伏信号反相,而后者近乎同相。前者由接收器与简正波分布的相对移动决定,而后者取决于简正波和水听器位置变化的复合效应。     

Coherence of acoustic modes propagating through shallow water internal waves

The 1995 Shallow Water Acoustics in a Random Medium (SWARM) experiment [Apel et al., IEEE J. Ocean. Eng. 22, 445-464 (1997)] was conducted off the New Jersey coast. The experiment featured two well-populated vertical receiving arrays, which permitted the measured acoustic field to be decomposed into its normal modes. The decomposition was repeated for successive transmissions allowing the amplitude of each mode to be tracked. The modal amplitudes were observed to decorrelate with time scales on the order of 100 s [Headrick et al., J. Acoust. Soc. Am. 107(1), 201-220 (2000)]. In the present work, a theoretical model is proposed to explain the observed decorrelation. Packets of intense internal waves are modeled as coherent structures moving along the acoustic propagation path without changing shape. The packets cause mode coupling and their motion results in a changing acoustic interference pattern. The model is consistent with the rapid decorrelation observed in SWARM. The model also predicts the observed partial recorrelation of the field at longer time scales. The model is first tested in simple continuous-wave simulations using canonical representations for the internal waves. More detailed time-domain simulations are presented mimicking the situation in SWARM. Modeling results are compared to experimental data.     

Frequency shifts of the sound field interference pattern in shallow water because of second-mode soliton-like internal waves

Numerical simulation is carried out to analyze the effect of an internal soliton of the second gravity mode on low-frequency sound propagation in an oceanic shelf region. The simulation is performed using the data of a full-scale experiment performed on the shelf of the South China Sea near Dongsha atoll, where the aforementioned solitons had been detected by stationary vertical thermistor arrays. The calculations take into account the effect of horizontal refraction of sound waves. It is assumed that a stationary acoustic track is oriented across the predominant propagation direction of internal waves. The results of simulation show that, when the soliton crosses the stationary track, some of the sound field modes are focused, whereas other modes are defocused. It is demonstrated that the soliton parameters can be adequately determined from the frequency shifts of the sound field interference pattern. However, such an estimate of the soliton parameters is only possible for a limited length of the stationary track for which the effect of horizontal refraction is sufficiently weak.     

Effect of shallow water internal waves on ocean acoustic striation patterns

Contour plots of underwater acoustic intensity, mapped in range and frequency, often exhibit striations. It has been claimed that a scalar parameter 'beta', defined in terms of the slope of the striations, is invariant to the details of the acoustic waveguide. In shallow water, the canonical value is β=1. In the present paper, the waveguide invariant is modelled as a distribution rather than a scalar. The effects of shallow water internal waves on the distribution are studied by numerical simulation. Realizations of time-evolving shallow water internal wave fields are synthesized and acoustic propagation simulated using the parabolic equation method. The waveguide invariant distribution is tracked as the internal wave field evolves in time. Both random background internal waves and more event-like solitary internal waves are considered.     

Effect of shallow water internal waves on ocean acoustic striation patterns

Abstract

Contour plots of underwater acoustic intensity, mapped in range and frequency, often exhibit striations. It has been claimed that a scalar parameter ‘beta’, defined in terms of the slope of the striations, is invariant to the details of the acoustic waveguide. In shallow water, the canonical value is β=1. In the present paper, the waveguide invariant is modelled as a distribution rather than a scalar. The effects of shallow water internal waves on the distribution are studied by numerical simulation. Realizations of time-evolving shallow water internal wave fields are synthesized and acoustic propagation simulated using the parabolic equation method. The waveguide invariant distribution is tracked as the internal wave field evolves in time. Both random background internal waves and more event-like solitary internal waves are considered.     

Frequency dependence of fluctuations of the repetition period of ion-cyclotron waves in the magnetosphere

We study the fluctuations of the repetition period of the Pc-1 geomagnetic pulsations propagating in the magnetosphere as packets of ion-cyclotron waves. A dimensionless parameter characterizing the frequency dependence of fluctuations is introduced. The value of this parameter and the repetition period of signals at a given frequency are used to evaluate the plasma density at an equator of the Pc-1 trajectory.Institute of Physics of the Earth, Russian Academy of Sciences, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 9, pp. 901–905, September, 1995.     

Rogue waves in shallow water

Most of the processes resulting in the formation of unexpectedly high surface waves in deep water (such as dispersive and geometrical focusing, interactions with currents and internal waves, reflection from caustic areas, etc.) are active also in shallow areas. Only the mechanism of modulational instability is not active in finite depth conditions. Instead, wave amplification along certain coastal profiles and the drastic dependence of the run-up height on the incident wave shape may substantially contribute to the formation of rogue waves in the nearshore. A unique source of long-living rogue waves (that has no analogues in the deep ocean) is the nonlinear interaction of obliquely propagating solitary shallow-water waves and an equivalent mechanism of Mach reflection of waves from the coast. The characteristic features of these processes are (i) extreme amplification of the steepness of the wave fronts, (ii) change in the orientation of the largest wave crests compared with that of the counterparts and (iii) rapid displacement of the location of the extreme wave humps along the crests of the interacting waves. The presence of coasts raises a number of related questions such as the possibility of conversion of rogue waves into sneaker waves with extremely high run-up. Also, the reaction of bottom sediments and the entire coastal zone to the rogue waves may be drastic.     

反演声场简正波耦合系数矩阵

研究了有内波传播时声场的耦合简正波形式,分析表明各阶简正波系数的时间信号包含多个频率成分,各成分的频率为对应的两地波数差与内波速度的乘积,各频率成分的振幅与对应简正波之间的耦合系数成正比。因此即使内波的波形不随其传播而变化,接收器处的各阶简正波系数仍然具有多频的复杂结构。由此并根据简正波耦合强度与声场简正波系数起伏强度的对应关系提出了一种反演简正波耦合系数矩阵的方法;并用实验中获得的内波数据,反演了声场;计算结果表明:该方法有效地反演了内波传播情况下的简正波耦合系数矩阵。     

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.     

The coherence of low-frequency sound in shallow water in the presence of internal waves

The coherence time and transverse coherence length of a low-frequency (100–300 Hz) sound field that is formed by an omnidirectional point source at a distance of 10–30 km in a shallow-water acoustic waveguide, which is characteristic of an open ocean shelf, were estimated analytically and in a numerical experiment. An anisotropic field of background internal waves is considered as a source of spatiotemporal fluctuations. It is shown that the coherence time decreases as the frequency increases, and strongly depends on the perturbation-movement direction. The transverse coherence length is primarily determined by phase incursions that are related to the cylindrical shape of the acoustic-wave front. In the case of transverse propagation, background internal waves may lead to significant variations in this length. The introduction of compensating phase corrections during processing provides a considerable increase in the average transverse coherence length.     

Exact Pollard-like internal water waves

In this paper we construct a new solution which represents Pollard-like three-dimensional nonlinear geophysical internal water waves. The Pollard-like solution includes the effects of the rotation of Earth and describes the internal water wave which exists at all latitudes across Earth and propagates above the thermocline. The solution is provided in Lagrangian coordinates. In the process we derive the appropriate dispersion relation for the internal water waves in a stable stratification and discuss the particles paths. An analysis of the dispersion relation for the constructed model identifies one mode of the internal water waves.     

The effects of the internal waves on temporal correlation of matched-field processing in shallow water

Effects of linear and solitary internal waves on the temporal correlation of matched-field processing(MFP) in shallow water are numerically investigated for acoustic sources with different frequencies and depths based on oceanographic data from an experiment.It is shown that the temporal correlation of MFP decreases as the amplitude of solitary internal waves or the average energy density of linear internal waves increases.For acoustic source with lower frequency or located below the thermocline,the temp...     

Sound field phase front variations in shallow water in the presence of intense internal waves

The method of vertical modes and horizontal rays and the method of parabolic equation in the horizontal plane are used to study space-time fluctuations of the phase front of the sound field in the presence of a train of internal waves passing through the acoustic path. The possibility of measuring the distortions of the phase front with a simultaneous filtering of modes by a horizontal array is analyzed. A numerical simulation is carried out for the conditions corresponding to the Eastern coast of United States and the Barents Sea.