Acoustic multipath arrivals in the horizontal plane due to approaching nonlinear internal waves

Simultaneous measurements of acoustic wave transmissions and a nonlinear internal wave packet approaching an along-shelf acoustic path during the Shallow Water 2006 experiment are reported. The incoming internal wave packet acts as a moving frontal layer reflecting (or refracting) sound in the horizontal plane. Received acoustic signals are filtered into acoustic normal mode arrivals. It is shown that a horizontal multipath interference is produced. This has previously been called a horizontal Lloyd's mirror. The interference between the direct path and the refracted path depends on the mode number and frequency of the acoustic signal. A mechanism for the multipath interference is shown. Preliminary modeling results of this dynamic interaction using vertical modes and horizontal parabolic equation models are in good agreement with the observed data.     

南中国海存在孤立子内波条件下的声场时间相关半径

在动态的海洋环境中,由于数据向量和拷贝场之间的失配,匹配场处理器的性能会发生退化。数据向量的时间相关半径是这种退化的一种量度。通过2001年ASIAEX南海实验中垂直阵上水听器接收到的声场数据求取了400 Hz窄带信号的声场时间相关。从实验数据处理结果观察到,伴随着传播路径上非线性内波的进入,声场的时间相关半径减小。同时利用一个二维的平流冻结海洋模型和传播路径上三个温度链的温度数据对声场进行了数值仿真,分析了不同频率下的声场时间相关半径。结果表明:实验结果与仿真的400 Hz信号的声场时间相关较为一致。可见,在时变的海洋环境下,声信道中存在孤立子内波将会使声场的时间相关半径大大缩短。      

Frequency dependent beating patterns and amplitude increase during the approach of an internal wave packet

A frequency-dependent beating pattern in the spectrogram of broadband signals transmitted during the approach of an internal wave packet to an acoustic propagating path is reported. An analytical expression relating the acoustic signal measurements and environmental parameters under certain conditions is obtained. Three-dimensional parabolic equation modeling results compare well with Shallow Water 2006 experiment data.     

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.     

单声阵列的低空目标轨迹估计方法

针对低空飞行目标轨迹估计问题,提出了一种单传声器阵列的目标运动轨迹估计方法。该方法利用窄带信号的多普勒效应估计出匀速直线运动目标飞行参数,结合波达方向估计计算目标的轨迹方向、水平偏置和高度,依据几何模型确定其运动轨迹,并对信号传播时延造成的估计误差进行修正。仿真和实验结果验证了算法有效性,实现了小孔径单声阵列对目标轨迹的估计。      

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.     

Determining the Spatial Properties of a Turbulent Transverse Flow via Multi-Frequency Acoustic Probing

The results are presented from an experimental study of solving the inverse problem of recovering a turbulent flow’s velocity and its position in space by analyzing the spectrum of fluctuations in an acoustic signal of different frequencies intersecting the flow. The conditions for and accuracy of recovering turbulent flow characteristics are discussed in terms of a multifrequency signal propagating along one acoustic path. The study is performed using a muffled acoustic chamber with a jet of air serving as a turbulent flow. Despite the physical character of the experiments, their results can be applied to problems of atmospheric, aero-, and oceanic acoustics.     

南中国海海域存在孤立子内波条件下的 声场统计特性*

利用南海浅海海域低频声传播起伏实验中获取的水文数据，结合二维平流模型重构出声传播路径上的动态声速场，使用蒙特卡洛方法研究了有、无孤立子内波经过声传播路径条件下的声传播损失统计特性，并与实验结果进行了对比分析。仿真和实验结果表明：当孤立子内波经过声传播路径时，声传播损失起伏剧烈；与“下发上收”相比，“下发下收”情况下传播损失的概率分布更加分散。     

Analysis of acoustic channels with a time-evolving sinusoidal surface

Sea-surface movement in the ocean induces a time-varying acoustic channel which affects underwater system including acoustic communication. For an investigation of surface movement effects on communication channel parameters (delay time, amplitude, etc.), acoustic transmission and reception experiments were conducted in the water tank. The delay time and amplitude of single surface bounce path from the measurement data show periodic time dependence, which is caused by a travelling periodic sinusoidal surface. A ray-based propagation model is applied to the experimental environment to estimate communication channel parameters. A comparison between measurement data and model result permit a physical interpretation of the communication channel parameters. The difference of single surface bounce path delay times from the model and measurement data are within small error bound. The delay times oscillate around the delay time of single surface bounce path when the surface is flat and show the periodic sine function. The amplitudes from the model are in agreement with those from the measurement data except at low amplitude region. Slight angle and frequency dependencies of source and receiver and noise in the water tank account for the disagreement in this region. Since the crest and trough of surface wave respectively make the acoustic energy emitted from the source converge and diverge, the amplitudes have high fluctuation and same phase with the delay time. The ray model is applied to an environment in the ocean. A striation pattern appears in surface reflected signal due to shadow zone on the surface.     

Acoustic normal mode fluctuation statistics in the 1995 SWARM internal wave scattering experiment

In order to understand the fluctuations imposed upon low frequency (50 to 500 Hz) acoustic signals due to coastal internal waves, a large multilaboratory, multidisciplinary experiment was performed in the Mid-Atlantic Bight in the summer of 1995. This experiment featured the most complete set of environmental measurements (especially physical oceanography and geology) made to date in support of a coastal acoustics study. This support enabled the correlation of acoustic fluctuations to clearly observed ocean processes, especially those associated with the internal wave field. More specifically, a 16 element WHOI vertical line array (WVLA) was moored in 70 m of water off the New Jersey coast. Tomography sources of 224 Hz and 400 Hz were moored 32 km directly shoreward of this array, such that an acoustic path was constructed that was anti-parallel to the primary, onshore propagation direction for shelf generated internal wave solitons. These nonlinear internal waves, produced in packets as the tide shifts from ebb to flood, produce strong semidiurnal effects on the acoustic signals at our measurement location. Specifically, the internal waves in the acoustic waveguide cause significant coupling of energy between the propagating acoustic modes, resulting in broadband fluctuations in modal intensity, travel-time, and temporal coherence. The strong correlations between the environmental parameters and the internal wave field include an interesting sensitivity of the spread of an acoustic pulse to solitons near the receiver.     

Comparison between ocean-acoustic fluctuations in parabolic-equation simulations and estimates from integral approximations

Line-integral approximations to the acoustic path integral have been used to estimate the magnitude of the fluctuations in an acoustic signal traveling through an ocean filled with internal waves. These approximations for the root-mean-square (rms) fluctuation and the bias of travel time, rms fluctuation in a vertical arrival angle, and the spreading of the acoustic pulse are compared here to estimates from simulations that use the parabolic equation (PE). PE propagations at 250 Hz with a maximum range of 1000 km were performed. The model environment consisted of one of two sound-speed profiles perturbed by internal waves conforming to the Garrett-Munk (GM) spectral model with strengths of 0.5, 1, and 2 times the GM reference energy level. Integral-approximation (IA) estimates of rms travel-time fluctuations were within statistical uncertainty at 1000 km for the SLICE89 profile, and in disagreement by between 20% and 60% for the Canonical profile. Bias estimates were accurate for the first few hundred kilometers of propagation, but became a strong function of time front ID beyond, with some agreeing with the PE results and others very much larger. The IA structure functions of travel time with depth are predicted to be quadratic with the form theta(2)vc0(-2)deltaz(2), where deltaz is vertical separation, c0 is a reference sound speed, and thetav is the rms fluctuation in an arrival angle. At 1000 km, the PE results were close to quadratic at small deltaz, with values of thetav in disagreement with those of the integral approximation by factors of order 2. Pulse spreads in the PE results were much smaller than predicted by the IA estimates. Results imply that acoustic tomography of internal waves at ranges up to 1000 km can use the IA estimate of travel-time variance with reasonable reliability.     

An Investigation of the Effects of Internal Waves on Sound Propagation in a Stratified Medium with a Sloping Bed

Internal waves usually cause temporal and spatial changes of density and consequently affect the acoustic wave propagation in the ocean. The purpose of this study is a laboratory investigation of the effects of internal waves generated by oscillation of a cylinder in a large stratified glass tank with a sloping bed on the sound waves propagation. Results showed that sound waves are affected by internal waves that depend on the slope angle to the direction of internal wave propagation angle ratio. When the ratio is subcritical or supercritical, the acoustic signal is much reduced as compared to the case with no sloped bottom. This can be explained in terms of the internal waves energy reaching the sloped bed and their reflections.     

Modeling three-dimensional propagation in a continental shelf environment

An acoustic propagation model is applied to predict measurements of three-dimensional (3-D) effects recorded off the southeast coast of Florida. The measured signal is produced by a low frequency source that is towed north parallel to the shelf from a fixed receiving array. The acoustic data show the direct path arrival at the bearing of the tow ship and a second refracted path arrival as much as 30° inshore of the direct arrival. Notably, the refracted arrival has a received level more than 25 dB greater than that of the direct arrival. A geoacoustic model of the environment is created to explain the data. It is shown that the topography of the seafloor plays the largest role in controlling horizontal refraction effects, whereas the range-dependent sediment properties have the most influence on the received level. The modeling approach is based on a 3-D adiabatic mode technique in which the horizontal refraction equation is solved using a parabolic equation in Cartesian coordinates. A modal decomposition of the field provides insight into the variability in the arrival angle and received level of the measured signal.     

Azimuthal dependence of reverberation from a harmonic signal propagating in shallow water

The problem of scattering from a rough sea surface is considered for a harmonic low-frequency (200–400 Hz) acoustic signal propagating in a shallow water area. An experimental study of the azimuthal reverberation dependence is carried out on the basis of the bistatic location scheme with the use of linear phased arrays. By comparison with calculations, the formation of the frequency-angular characteristics of reverberation is investigated for various wind directions with respect to the path of acoustic signal propagation.     

A multifrequency scintillation method for ocean flow measurement.

The transverse flow of inhomogeneous fluid produces fluctuation of the acoustic signal passing through it. The coherence of frequency-spaced signal fluctuation is related to the advection of the inhomogeneous medium through the sound path, thus providing a basis for the current velocity measurement. This method can be considered to be the "frequency-domain" version of the conventional scintillation approach to the current velocity registration based on the measurement of the signal correlation transmitted from the source to the two separated in space receivers (space-domain scintillation) [S. Clifford and D. Farmer, J. Acoust. Soc. Am. 74, 1826-1832 (1983)]. The sensitivity of the method depends on the features of the ocean fine structure, which is determined mainly by the internal waves and turbulence. To estimate the sensitivity of the multifrequency method of transverse current probing, the coherence function of two signals propagating through a frozen and moving internal wave field and through the turbulence is considered. The application of the multifrequency signal allows estimation of the fine-structure parameters as well as the current velocity.     

The influence of internal waves on losses during sound propagation on a shelf

We discuss the results of investigations into the influence of internal waves on sound losses during propagation along stationary lanes on shelves of the Sea of Japan and the Sea of Okhotsk. Measurements were conducted with a Mollusk-07 autonomous vertical acoustic-hydrophysical measuring system and TONE-320 and FM-290–330 autonomous electromagnetic-type emitters generating a tonal acoustic signal and a frequency modulated signal, respectively. It is established that sound losses at 290–330 Hz as a result of internal waves do not exceed 5 dB. At the same time, intensive nonlinear internal wave packets that are relatively rare but characteristic of a shelf can increase losses up to 15 dB.     

Determining tomographic arrival times based on matched filter processing: considering the impact of ocean waves

Reflection of high-frequency acoustic signals from an air-sea interface with waves is considered in terms of determining travel times for acoustic tomography. Wave-induced, multi-path rays are investigated to determine how they influence the assumption that the time of the largest matched filter magnitude between the source and receiver signals is the best estimate of the arrival time of the flat-surface specular ray path. A simple reflection model is developed to consider the impact of in-plane, multi-path arrivals on the signal detected by a receiver. It is found that the number of multi-path rays between a source and receiver increases significantly with the number of times the ray paths strike the ocean surface. In test cases, there was always one of the multi-path rays that closely followed the flat-surface specular ray path. But all the multi-path rays arrive at the receiver almost simultaneously, resulting in interference with the signal from the flat-surface specular ray path. As a result, multi-path arrivals due to open ocean surface waves often distort the received signal such that maxima of matched filtering magnitudes will not always be a reliable indicator of the arrival time of flat-surface specular ray paths.     

利用海底反射信号进行地声参数反演的方法

针对现有反演方法的缺点,提出了一种基于海底反射信号的地声参数高分辨反演方法.它利用短距离声源在不同深度上发射宽带线性调频信号,采用垂直阵进行接收,首先通过匹配滤波方法提取多径到达信息,然后利用海底反射损失曲线,反演海底表层的声速和密度,最后利用浅底层反射信号估计沉积层参数.由于海水中直达波受到内波的强烈影响,选择海底表面反射作为参考,用以可靠地计算浅底层反射的相对到达时间和幅度,从而估计出沉积层的厚度、速度和衰减系数.通过海上实验,验证了利用浅底层反射信号反演参数的有效性. 关键词： 海底参数 反演 浅底层反射信号     

Efficiency of sound field focusing at a long distance in an oceanic waveguide with background internal waves

The efficiency of focusing a reversed wave and the possibility of scanning with the focal spot at long distances in a shallow sea in the presence of an anisotropic field of background internal waves are theoretically investigated. The localized fields are controlled by varying the transmission frequency without a change in the distribution of the reversed field over the array aperture. The effect of periodically repeated focal spots is analyzed. Numerical calculations are performed for the longitudinal and transverse orientations of acoustic path with respect to the propagation direction of internal waves. The effect of perturbation on the stability and efficiency of focusing is discussed. A comparative analysis of the data obtained for long and short distances is performed.     

Temporal coherence of acoustic rays and modes using the path integral approach

Acoustic propagation can be described by rays and normal modes. Applying the path integral to refractive rays in three dimensional space, Dashen et al. [J. Acoust. Soc. Am. 77, 1716-1722 (1985)] derived the mutual coherence function of the acoustic field. For shallow water where sound interacts with boundaries, the acoustic field can be described by vertical modes and horizontal rays. Applying the path integral to the horizontal rays, one obtains the mutual coherence function of the normal modes. This paper applies this formulation to the derivation of the temporal coherence function of individual modes and also that of the acoustic field in the presence of linear internal waves. The effects of mode coupling due to internal waves on temporal coherence loss are illustrated with numerical calculations.