Spatial coherence measurements from two L-shape arrays in Shallow Water

In the Shallow Water’ 06 experiment, two L-shape arrays (ARRAY52 and ARRAY32) were deployed. The vertical line array (VLA) components of both ARRAY52 and ARRAY32 were exactly perpendicular to the direction of sound propagation. The horizontal line array (HLA) component of ARRAY52 was exactly perpendicular to the direction of sound propagation. The HLA component of ARRAY32 was exactly parallel to the direction of sound propagation. This configuration offered an opportunity to simultaneously analyze the three dimensional (3-D) spatial coherences: vertical, transverse horizontal and longitudinal horizontal. When the source and the receivers were below the thermocline, both the vertical and longitudinal horizontal coherence lengths in units of wavelength increased with increasing range and frequency. When the source was within the thermocline, the transverse horizontal coherence length in units of wavelength decreased with frequency and exhibited weak range dependence. The text was submitted by the authors in English.     

海面大气边界层中聚焦光束漂移各向异性的实验研究

 利用位置敏感型光电倍增管（PSPMT）,设计了测量聚焦光束漂移的实验装置。该装置在80 mm的有效探测面内实现了2维位置信号的直接探测,空间分辨率可达1 mm,明显优于四象限探测器和阵列探测器,最高采样频率可达80 kHz,且动态范围很大,优于一般的成像器件。在近海岸海面上5 m处的大气边界层中进行了距离为1 000 m的聚焦激光传输实验。测量结果表明：聚焦光斑的质心漂移具有各向异性,水平方向光斑漂移幅度一般介于5.61 mm和14.83 mm之间,垂直方向光斑漂移幅度介于3.54 mm和7.3 mm之间,两者之比的平均值为1.69;水平方向和垂直方向的光斑漂移功率谱密度（PSD）在低频段也存在差异,垂直方向光斑漂移的PSD比水平方向光斑漂移的PSD下降速率更快。     

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.     

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。     

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.     

A high frequency amplitude-steered array for real-time volumetric imaging

Real-time three-dimensional acoustic imaging is difficult in water or tissue because of the slow speed of sound in these media. Conventional pulse-echo data collection, which uses at least one transmit pulse per line in the image, does not allow for the real-time update of a volume of data at practical ranges. Recently, a linear amplitude-steered array was presented that allows the collection of a plane of data with a single transmit pulse by spatially separating frequencies in the lateral direction. Later, by using a linear array with frequency separation in the vertical direction and rotating the array in the horizontal direction, volumetric data were collected with a small number of transmit pulses. By expanding the linear array to a two-dimensional array, data can now be collected for volumetric imaging in real time. In this study, the amplitude-steered array at the heart of a real-time volumetric sonar imaging system is described, giving the design of the array and describing how data are collected and processed to form images. An analysis of lateral resolution in the vertical and horizontal directions shows that resolution is improved in the direction of frequency separation over systems that use a broad transmit beam. Images from simulated data are presented.     

海面大气边界层中聚焦光束漂移各向异性的实验研究

利用位置敏感型光电倍增管（PSPMT）,设计了测量聚焦光束漂移的实验装置。该装置在80 mm的有效探测面内实现了2维位置信号的直接探测,空间分辨率可达1 mm,明显优于四象限探测器和阵列探测器,最高采样频率可达80 kHz,且动态范围很大,优于一般的成像器件。在近海岸海面上5 m处的大气边界层中进行了距离为1 000 m的聚焦激光传输实验。测量结果表明：聚焦光斑的质心漂移具有各向异性,水平方向光斑漂移幅度一般介于5.61 mm和14.83 mm之间,垂直方向光斑漂移幅度介于3.54 mm和7.3 mm之间,两者之比的平均值为1.69;水平方向和垂直方向的光斑漂移功率谱密度（PSD）在低频段也存在差异,垂直方向光斑漂移的PSD比水平方向光斑漂移的PSD下降速率更快。     

Transmission beam pattern and echolocation signals of a harbor porpoise (Phocoena phocoena)

The transmission beam pattern of an echolocating harbor porpoise (Phocoena phocoena) was measured in both the vertical and horizontal planes. An array of seven Brüel and Kjaer 8103 hydrophones connected to an amplifier-line driver module was used to measure the beam patterns. The porpoise was trained to station in a hoop and echolocate a cylindrical target located at a range between 7 and 9 m while the array was located 2 m in front of the hoop. The 3-dB beamwidth in both the vertical and horizontal planes was the same at approximately 16 degrees and the beam was pointed toward the forward direction. The individual hydrophones in both the vertical and horizontal arrays measured signal waveforms that were similar throughout the 40-degree span of the array. The porpoise emitted signals with intervals that were 20 to 35 ms longer than the round trip travel time between the animal and the target. The average source level, peak frequency, and bandwidth were 157 dB, 128 kHz, and 16 kHz, respectively.     

孤子内波导致海洋环境噪声垂直指向性异常分析

依据近场波数积分、远场耦合简正波相结合的二维噪声场模型,侧重理论研究孤子内波所在扇区,环境噪声垂直阵响应的变化,分析了某些孤子内波情形下垂直阵环境噪声水平凹槽变深这一异常现象的原因:孤子内波离垂直阵较近时,远离内波的海面噪声源多,其激发的简正波能量由低号耦合到高号,在垂直阵处高号简正波能量对环境噪声场贡献增大,导致环境噪声水平凹槽加深;对于大尺度、多波包孤子内波,其范围相对较大,内波所在区的局部简正波本征值和本征函数产生的变化影响显著,使低号简正波衰减变快,而高号衰减慢,导致接收阵处高号简正波能量增加,低号简正波变弱,这样,无论孤子内波群靠近或离接收阵远,都将使垂直阵环境噪声水平凹槽加深。     

Spatial correlation of sound fields from underwater explosions in the Barents Sea

Results of an experimental study of the spatial correlation of signals from underwater explosions are presented for vertically and horizontally (across the path) separated reception points. The measurements were carried out on two propagation paths about 200 and 110 km long in the Barents Sea for frequencies below 500 Hz. The vertical correlation radius is found to increase with the distance to the source, which is primarily caused by the decrease in the sector of arrival angles of energy carrying signals. The transverse correlation radius weakly depends on the path length for frequencies below 300 Hz, while for higher frequencies, it decreases beginning from a distance of about 40 km, which can be attributed to the effect of short-period internal waves.     

光折变晶体LiNbO_3:Fe中的特殊散射现象

用柱面透镜把经针孔滤波器扩束的激光束沿某一个方向聚焦成细丝状(或长条状),细丝状光束垂直入射到LiNbO_3:Fe晶体上,在远场位置观察散射光.我们发现光散射的方向与晶体的c轴方向不一致,不仅沿着晶体的c轴方向有散射光出现,沿光束的条形方向也出现很强的散射光.     

南海深海典型不平整地形对声场垂直相关性的影响

海底地形变化对深海环境下的声传播和空间相关性有重要影响。基于2014年南海中南部海域不完全深海声道条件下大跨度垂直阵接收的声信号,选取平坦海底和典型不平整(存在小海底山和海底斜坡)海底两个不同传播方向上传播损失差异较大的距离,对声场垂直相关性进行了对比分析,并应用射线理论对相关性差异予以定性分析和机理解释。在不平整海底方向,在第一影区内小海底山遮挡区附近:在部分一次海底反射声线被小海底山遮挡的距离处,接收声信号主脉冲多途干涉结构相对平坦海底方向更加简单,垂直相关性增强;而在全部一次海底反射声线被小海底山遮挡的距离处,对声场起主要作用的变为二次海底反射声线,接收声信号主脉冲呈现多途干涉结构,垂直相关性降低。在第一会聚区附近:平坦海底环境下,由于两组水体中反转声线在空间垂直方向上的干涉效应,使得声场的垂直相关随接收深度增加呈现出周期性振荡的现象;而不平整海底方向,在50 km处,海底斜坡阻挡了一组水体反转声线,在大深度上只出现单会聚结构,观测不到垂直相关的周期性振荡现象;在57 km附近对声场起主要贡献的成分从水体中反转声线变成了多次海底反射声线,主脉冲多途展宽变宽,垂直相关性显著降低。研究结果对深海复杂海底地形环境下声场垂直相关性的深入分析及声呐阵列增益估计等具有重要意义。     

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.     

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.     

Infrasonic waves generated from the volcano eruption in Tonga

The Hunga-Tonga-Hunga-Ha'apai volcano in Tonga erupted violently on January 15, 2022. After 8 hours and 39 minutes, an infrasound array in Kunming at a distance of 10151 km away from the volcano recorded a sequence of strong infrasonic waves. The average period of the first four infrasonic waves is 443 s with propagation speed of 321 m/s. The azimuth angle of this event has been estimated by applying the progressive multi-channel correlation(PMCC) method to process the received signals. During continuous monitoring, the infrasonic waves propagating along and opposite to the direction towards the Kunming array around the earth are recorded respectively.     

A simultaneous multiwavelength dispersive X-ray reflectometer for time-resolved reflectometry

Aiming for the realization of time-resolved specular X-ray reflectivity measurements on the sub-second to millisecond timescales, a conceptually new method of measuring specular X-ray reflectivity curves is developed. Using this method the entire profile of the reflectivity curve of interest is measured in place. A horizontally convergent X-ray beam which has a one-to-one correlation between its direction and energy is realized using a curved crystal or laterally graded multilayers on an elliptic substrate. The X-ray beam is then incident on the surface of the specimen placed at the focus such that the glancing angle in the vertical direction is the same for all X-ray components, which are reflected in the vertical direction by the surface and diverge in the horizontal plane. The perpendicular momentum transfer continuously changes as a function of the horizontal ray direction even with fixed glancing angle since the wavelength (energy) changes. The X-ray intensity distribution across the beam direction measured downstream of the specimen using a one- or two-dimensional detector represents the X-ray reflectivity curve. Specular X-ray reflectivity curves are measured with exposure times ranging from 2?ms to 1?s for a gold film of thickness 14.3?nm on a silicon substrate. The potential of this method for time-resolved measurements is demonstrated by recording reflectivity curves with a time resolution of 20?ms from a rotating specimen.     

浅海孤子内波对水平纵向相关性的影响*

基于理论和模型仿真对孤子内波存在下的水平纵向相关系数的变化进行了研究。理论上推导出浅海孤子内波存在情况下的声场水平纵向相关系数随时间的变化规律,并且运用二维抛物方程模型（Ram）验证理论推导结果。结果表明,孤子内波会导致水声纵向相关系数的变化,并且水平纵向相关系数的变化是由于简正波的干涉所导致。我们给出特定环境下的水平纵向相关系数随时间变化的结果,当环境中第一、二号简正波占主导地位时,水平纵向相关系数会呈现出周期变化的结果。     

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.     

Sidelobe suppression in volume holographic optical correlators by use of speckle modulation

By use of speckle modulation on the object beam of a volume holographic optical correlation system, we show that the sidelobes of the correlation patterns along the vertical direction, as well as those along the horizontal direction, are well suppressed. A theoretical explanation and experimental results are presented and discussed.     

Influence of Anisotropic Wind Waves on the Efficiency of Spatial Processing of Acoustic Signals in Shallow Water

The authors investigate the effect of anisotropy of the frequency-angular spectrum of wind waves on the efficiency of spatial processing signals received by a horizontal array in a shallow water waveguide with a rough surface. The array gain is analyzed for three array signal processors: conventional beamformer, optimal linear processor, and optimal quadratic processor. The numerical simulation results for the hydrological conditions typical of the Barents Sea in winter are presented. The main focus is on the dependence of the array gain on the distance to the source and wind direction with respect to the acoustic path. Simulation results for the anisotropic wind wave spectrum and a simplified model with an isotropic spectrum are also compared.