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.     

Phase fluctuations of focused low-frequency sound fields in shallow water

Numerical experiments are carried out to study the phase fluctuations of a focused low-frequency sound field on an oceanic shelf. The focusing of sound at a distance of several kilometers is simulated using the phase conjugation of sound waves. Perturbations of the medium are represented by high-frequency (>1 cph) background internal waves and by the wind waves on the ocean surface. It is shown that, for a focused sound field at frequencies of several hundreds of hertz, the phase fluctuations do not exceed π and can be measured against the background of acoustic noise typical of shallow-water regions of the ocean. The fluctuation magnitude can be reduced approximately by half through the optimal choice of the mode composition. In the presence of such fluctuations, it is possible to measure the relative variations of the length of a stationary acoustic path with an accuracy of 1 m or better at a wind speed no greater than 10 m/s and a typical intensity of background internal waves.     

Experimental studies of pulsed signal propagation from the shelf to deep sea

Results of an experimental study of low-frequency broadband pulsed signal propagation in a waveguide that includes the shelf zone, the continental slope, and the deep sea region are presented. Using phase-manipulated signals with central frequencies of 366 and 600 Hz, pulsed characteristics are measured at six points along the propagation track, the maximal distance from the source being 368 km. It is experimentally demonstrated that, in the presence of a negative sound velocity gradient in the near bottom layer on the shelf with a small bottom slope, the choice of the source position at the shelf bottom near the shoreline provides the formation of a continuous illumination zone in the deep sea near the USC axis and a stable pulsed characteristic with two main sound energy arrivals. The propagation velocity of the pulse that is last to arrive is identical (within the measurement error) to the velocity of sound on the USC axis at the point of reception. Possibilities for practical application of the results obtained from the experiment are discussed.     

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.     

Fluctuating arrivals of short-range acoustic data

Geoacoustic inversion using fluctuating signal observations can be challenging. The origin of these fluctuations needs to be understood so the signals can be used appropriately. A set of experiments [Tang et al., Oceanogr. 20(4), 156-167 (2007)] was carried out in shallow water near the New Jersey shelf break in summer 2006. Significant fluctuations in the direct path and surface-reflected arrivals of short-range chirp transmissions (1.1-2.9 kHz) were observed on a vertical line array. This paper explains the origin of these signal fluctuations through analysis of the arrival amplitudes. It is shown that the strong thermocline combined with an oscillating source motion due to ocean surface waves results in the signal fluctuations.     

Spatial correlation of hydroacoustic signals in a biaxial underwater sound channel

In the hydrological conditions of a biaxial sound channel, the cross-correlation between acoustic signals received at points spatially separated (from 10 to 63 km) along the sound propagation track is investigated. The signals are received by a narrow-beam array scanning in the vertical plane. The beam width is ～2° at the mean frequency (1 kHz) of a pseudo-noise signal. It is noted that, as the distance between the points of reception increases, the correlation decreases. This is mainly caused by the effect of the multipath propagation with an incomplete resolution of signals in arrival angles, rather than by changes in the “water” signal spectrum due to the attenuation.     

Determining the effective parameters of a Shallow-Water bottom from wideband signal spectra under conditions of hydrodynamic variability

We propose a method for determining the effective parameters of the upper marine sediment layer on extended tracks from the spectra of wideband signals in conditions of hydrodynamic variability. As an example, we consider the Shallow Water 2006 experiment on the Atlantic shelf of the United States, which used signals with a band of 300 ± 30 Hz received by a vertical array. The length of the track was ~20 km at a sea depth of ~80 m. Frequency-mode analysis of the received signals showed that spatiotemporal fluctuations of the wave medium lead to random changes in mode amplitudes while retaining the relative stability of the mode phase difference. This is the basis of the proposed method, which makes it possible to determine the track-averaged values of the sound velocity in the bottom and density of the bottom under conditions of hydrodynamic variability.     

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.     

Propagation of pulse pseudorandom signals from a shelf into shallow water in winter hydrological conditions of the Sea of Japan

The paper discusses the results of an experiment conducted in the Sea of Japan in March 2016 on an acoustic track 194 km long under winter hydrological conditions. We have studied the most complex case of propagation of pulse pseudorandom signals from the shelf into shallow water during vortex generation on the acoustic track. Analysis of the experimentally obtained pulse characteristics have shown that the maximum first approach of acoustic energy recorded at all points agrees well with the calculation. This testifies to the fact that at a given reception depth, the first to arrive are pulses that have passed in the near-surface sound channel over the shortest distance and at small angles close to zero. We propose a technique for calculating the mean sound velocity on the track from satellite monitoring data on the surface temperature, which makes it possible to rely on the successful application of the results obtained in acoustic ranging and navigation problems.     

Long-range sound propagation in the shallow-water part of the Sea of Okhotsk

Experimental data on the long-range propagation of explosion-generated sound signals in the shallow-water northern part of the Sea of Okhotsk are analyzed. The propagation conditions in this region are characterized by a fully-developed underwater sound channel that captures the rays crossing the channel axis at angles lower than 3°. The experimental data reveal a small increase in the duration of the sound signal in proportion to the range with the proportionality factor lower than 0.00025 s/km. The frequency dependence of attenuation exhibits a pronounced minimum whose position on the frequency axis is close to the critical frequency of the first “water” mode (about 160 Hz). The increase in the attenuation coefficient at lower frequencies is confirmed by the field calculations performed with the wave-field computer code and is explained by the sound energy loss in the bottom sediments. At frequencies higher than 200 Hz, as in the Baltic Sea, the most probable reason for the attenuation to exceed the absorption in sea water is sound scattering by internal waves.     

深海表面层声速剖面变化对声传播的影响

在我国部分海域，表面声速常常在较短的时间内从弱正梯度变为弱负梯度，这将对表面附近的声传播产生巨大影响。本文通过数值模拟的方法，讨论了表面层声速梯度变化对声传播的影响，同时解释了某次海上实验出现的异常现象：在同一地点同一航向，前后两天海况相同，水文相差不大，第一天的实验可以在50km之外接收到信号，而第二天只能在30km处才能收到信号。     

Range dependence of the vertical structure of the sound field in the ocean

In the ocean without fluctuations, the sound field is calculated by the method of geometrical acoustics with allowance for purely water-path rays in a sound channel of canonical shape with a thickness of 4 km for distances of 500 and 2000 km. The sound field is determined as a sum of individual rays arriving at a given point with their own amplitudes and phases. It is shown that the vertical structure of the sound field consists of a number of caustics separated by regions with a quasi-random distribution of the field whose amplitude is much smaller than that in the caustics. At a fixed distance, the number of caustics is equal to the difference between the numbers of the ray turning points at the boundaries of the departure angle range. As the distance from the source increases, the number of caustics increases proportionally to distance.     

水平阵信号压缩感知用于简正波分离

针对水平阵信号简正波分离过程中常规波束形成分辨率低以及warping模态滤波不适用于复杂声信号的问题,提出水平阵信号压缩感知用于简正波分离的方法。利用压缩感知在方位估计中的高分辨特性,通过估计水平阵接收信号在频率方位角上的二维分布,分离得到各阶简正波的方位谱,并逆Fourier变换得到时域波形。仿真孔径1 km、阵元间隔10 m水平阵接收20~200 Hz伪随机声信号和脉冲声信号,所提方法分离出的各阶简正波与理论波形的相关系数在0.97~1.0。对2011年北黄海声学实验中的海底28元水平阵接收的气枪信号,在合成至1 km孔径后使用压缩感知方法分离简正波,其与warping模态滤波分离得的前5阶简正波相关系数在0.82~0.93。仿真与实验都说明了水平阵信号压缩感知简正波分离方法的有效性。      

Focusing of sound pulses using the time reversal technique on 100-km paths in a deep sea

Numerical and analytical studies are performed on how unstable fluctuations of the parameters of the medium in a deep sea affect the focusing of sound pulses using the time reversal method. The simplest situation, when point sources and receivers are used for emission and reception, is considered. Pulse propagation in the direct and backward directions is numerically simulated by the parabolic equation method. Calculations are performed for sound signals with frequencies of several tens of hertz. It is shown that, in the presence of sound velocity fluctuations caused by random internal waves, noticeable attenuation of the field amplitude at the center of the focal spot can be observed beginning from distances of 200 to 400 km. As the central frequency of the pulsed signal increases, the effect of nonstationarity of the perturbation on the focusing is amplified. This phenomenon is explained qualitatively and quantitatively in the geometrical optics approximation.     

Correlation of underwater acoustic signals simultaneously received by omnidirectional and directional means

A comparison of experimental data on the spatial correlation between acoustic signals simultaneously received by an omnidirectional hydrophone and a directional vertical array is carried out. The spatial correlation was measured between the signals received at different distances in a deep ocean. The points of reception were positioned in two convergence zones along the path of sound propagation with a point-to-point distance of about 64 km. Pseudonoise signals were emitted in the frequency range (0.8–2.0) kHz and received by a vertical array, whose beam had a width of ∼2°. Concurrently, multipath signals received with the central hydrophone of the array were recorded. Signals in the first and second convergence zones were received at different times. Nevertheless, in the case of the directional reception, the coefficients of spatial correlation between such signals appeared to be as high as 0.64–0.74 even under the conditions of incomplete resolution of signals in the angle of arrival in the vertical plane. At the same time, in the case of omnidirectional reception, the coefficients of spatial correlation were below 0.32.     

Temporal coherence of acoustic signals in a fluctuating ocean

Temporal coherence of acoustic signals propagating in a fluctuating ocean is important for many practical applications and has been studied intensively experimentally. However, only a few theoretical formulations of temporal coherence exist. In the present paper, a three-dimensional (3D) modal theory of sound propagation in a fluctuating ocean is used to derive closed-form equations for the spatial-temporal coherence function of a broadband signal. The theory is applied to the analysis of the temporal coherence of a monochromatic signal propagating in an ocean perturbed by linear internal waves obeying the Garrett-Munk (G-M) spectral model. In particular, the temporal coherence function is calculated for propagation ranges up to 10(4) km and for five sound frequencies: 12, 25, 50, 75, and 100 Hz. Then, the dependence of the coherence time (i.e., the value of the time lag at which the temporal coherence decreases by a factor of e) on range and frequency is studied. The results obtained are compared with experimental data and predictions of the path-integral theory.     

Wave method for estimating the sound source depth in an oceanic waveguide

An experimental estimation of the sound source depth, based on measuring the amplitude ratio for neighboring wave field modes, has been performed on the Pacific shelf under the following conditions: path length about 10 km at a sea depth of about 53m; pneumatic source signal in the band of 200±10 Hz. The predictive parameters of the bottom have been reconstructed from the envelope of the signal received by a single detector. The noise immunity of the algorithm and its sensitivity to variations in the bottom parameters have been analyzed. The experimental results are found to be in good agreement with the theoretical predictions.     

海洋信道随机性引起的船舶噪声线谱衰减

阐述准周期性随机声脉冲序列作为船舶噪声模型的普适性,侧重讨论船舶噪声准周期性随机声脉冲序列所产生的线谱经随机海洋信道传输后的变化。指出在大多数工况下,船舶噪声并不是以几种不同频率的加性正弦波直接叠加在宽带连续谱噪声上,而是一类本身并不含有任何有限强度加性正弦波分量的信号。研究结果表明:海洋信道多途效应引起船舶噪声脉冲序列信号时域上时延扩展;海洋信道的随机性使船舶噪声随机声脉冲序列信号准周期性下降,导致船舶噪声线谱相对幅度“额外”快速衰减,对于存在大范围不均匀水团的海洋信道,50 km传播距离上其介质随机起伏引起的船舶噪声线谱“额外”衰减最高可达11 dB。海洋信道随机性引起的船舶噪声线谱衰减不可忽视。      

Comprehensive studies of sound fields in the Kuril-Kamchatka region of the northwestern Pacific

The technique, experimental conditions, and main results of comprehensive studies of sound fields in the northwestern region of the Pacific Ocean are presented. The experiments are carried out on paths up to 2100 km in length. The power-frequency, space-time, and correlation characteristics of the sound fields are studied in sonic and infrasonic frequency bands for long-and extra-long-range propagation with the use of cw and explosion-generated sound signals. Effects of the bottom relief and the spatial distribution of the speed of sound on the frequency characteristics of the sound field are investigated. The role of front zones in the formation of sound fields received at the coastal shelf and in the open ocean is revealed. The loss coefficients are estimated. The space-time stability of the sound field components is studied, and the possibility is shown for the coherent components to be conserved and resolved in frequency at distances up to 2100 km. The phase velocities of these components are determined. The total broadening of the frequency spectra is considered. The correlation characteristics of the total field are obtained for horizontally separated receivers in sonic and infrasonic frequency bands.