Effect produced on sound propagation by an internal temperature front moving over the shelf

Results of observing the changes that occur in the vertical distribution of water temperature under the effect of an intense atmospheric cyclone and the influence of these changes on sound propagation in the shelf region of the Sea of Japan are presented. The measurement results refer to the autumn conditions. The measuring equipment includes a vertical acoustic-hydrophysical measuring system, a broadband transmitter (both of them being connected with the shore station by cable lines), and a self-contained resonance (320 Hz) transmitter of the electromagnetic type. The sound (tone signals) propagation is studied on a 510-m-long constant-depth (38 m) track (TON-310 Hz) and a 10.6-km-long track (TON-320 Hz), which is set up by placing the self-contained transmitter at the bottom (at a depth of 65 m). Results of field experiments are presented along with those of numerical simulation of the effect produced by an internal temperature front moving toward the coast and formed by the seasonal thermocline on the propagation of 320-Hz sound signals through it. It is shown that refraction and scattering of sound waves propagating through the temperature front moving along the acoustic track may cause intensity variations of acoustic field at the reception point, which occur synchronously at different depths and have amplitudes of up to 14 dB and a period of about 40 min.     

Effect of tide on sound propagation in the shelf zone of the Sea of Japan

Experimental and numerical studies of the effect of surface and internal tides on 315-Hz sound waves propagating along fixed paths, 260 m to 23 km in lengths, oriented across the shelf of the Sea of Japan, are discussed. The measurements are performed using self-contained radio-hydroacoustic receiving stations, which are equipped with hydrophones and scalar-vector receivers, and two vertical acoustic-hydrophysical measuring systems. For the sound signals propagating along the longer paths, the intensity fluctuations are shown to loose their linear relation to the tide-caused changes in the waveguide parameters because of the refraction by the sound speed inhomogeneities induced by different hydrodynamic processes. However, it is established that the phase variations can serve as quantitative indicators of the integral changes in the waveguide parameters.     

The effect of internal waves on the sound propagation in the shelf zone of the sea of Japan in different seasons

Results of the observation of seasonal variations in the vertical distribution of water temperature in the shelf zone of the Sea of Japan are presented, and the effect of this variability on the parameters of internal waves and on sound propagation is studied. The measurements were carried out in different seasons using a vertical acoustical-hydrophysical measuring system. The propagation of sound (tone and noise signals) was studied on a 510-m-long track at a constant depth of 38 m. Using a self-contained resonance (320 Hz) transmitter of the electromagnetic type, which was bottom-moored at a depth of 65 m, a 10.6-km-long stationary acoustic track crossing the shelf was set up. During the in-sea experiments, the spatial characteristics of internal waves were measured along with the distributions of temperature, salinity, sound velocity, and sea level variations.     

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.     

The sound speed in southern deepwater zone of the Caspian Sea,off Anzali Port

Sound velocity determination in seawater is a key component of modern hydrographic surveying; however, little data exists on sound velocity characteristics of the southern Caspian Sea. Hence, a study was undertaken in 2008 to examine the seasonal variability of sound speed in deep-waters of the South Caspian Sea near the Iranian coast. The seasonal cycle of seawater temperature and thermal stratification in the Caspian Sea water created a wide range of spatial and temporal changes of sound speed with relevant differences between shallow water (over the continental shelf) and deep-water area. The collected data showed that seasonal variations of the sound speed were most important in the upper 100 m water depth, while below this level that is in deepwater the changes were small. The maximum values of sound speed were observed at the surface in midsummer around 1517–1519 m s⁻¹ over the continental shelf while the speed of sound was about 1453 m s⁻¹ between 450–470 m depths with no major seasonal variations. Variations in vertical structure of the sound speed were in agreement with temperature changes, while effects of the salinity on the sound speed were little.     

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。      

大陆架低频声传播建模研究

在过去的几十年中大陆架斜坡海域的低频声传播得到越来越多的重视。针对爆炸声作为声源的一次海上实验测量数据,对大陆架海域的低频远距离声传播进行了建模研究。在实验过程中单水听器布放在水下大约240m处接收爆炸声信号,对两条不同测线的传播损失进行了处理。本文结合海底地声模型并考虑了声速剖面的水平变化,利用抛物方程方法对传播损失进行建模。模拟计算该海域的传播损失同实验测量数据相比具有较好的一致性。     

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.     

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.     

Vertical variability of sound fields in a coastal wedge

The results of measuring the vertical variability of sound fields are presented for a coastal wedge off the Pacific shelf of the Kamchatka peninsula. It is shown that the vertical radius of sound field correlation can reach ～30 m for frequencies within 600–800 Hz. The scatter of the specific parameters of the variability is in rather wide limits depending on the location of a vertical chain of receiving hydrophones, the hydrological conditions, the azimuth angle of the sound source, and the relief and bottom structure of the coastal wedge.     

Frequency dependence and intensity fluctuations due to shallow water internal waves

A theory and experimental results for sound propagation through an anisotropic shallow water environment are presented to examine the frequency dependence of the scintillation index in the presence of internal waves. The theory of horizontal rays and vertical modes is used to establish the azimutal and frequency behavior of the sound intensity fluctuations, specifically for shallow water broadband acoustic signals propagating through internal waves. This theory is then used to examine the frequency dependent, anisotropic acoustic field measured during the SWARM'95 experiment. The frequency dependent modal scintillation index is described for the frequency range of 30-200 Hz on the New Jersey continental shelf.     

Annealing of heavy ion tracks in cellulose nitrate plastic track detectors

The effect of heat treatment on the latent tracks in cellulose nitrate plastic track detectors has been studied. The bulk etch rate increases with annealing temperature while the track diameters of different ions in cellulose nitrate decrease with increase in annealing time and temperature. Experimental results show that for heavier ions higher temperatures are needed for their complete erasure. The track length and track etch rate are decreased by the application of heat. Experiments reveal that annealing reduces track density. The vertical tracks are more stable than the oblique tracks and require higher temperature for their complete erasure.     

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.     

The influence of optical properties of shallow shelf seas on temperature structure and acoustic propagation

The penetration of solar radiation below the sea surface is not always taken into account in numerical ocean models, even though its influence on the temperature (and therefore sound speed), structure of the water column can be significant, especially in shallow shelf seas where water turbidity tends to be high. Variations in sound speed structure have a consequent effect on the propagation of sound underwater which, in turn, influences the performance of sonar systems. A double exponential parameterization for the penetration of solar radiation with depth was implemented in an N × 1D turbulence closure model of the UK shelf seas. The model was run along a section through the Celtic Sea Front, with sets of optical extinction coefficients representing different water clarities, for one month to simulate the generation of the front, and the results were compared with a control model run in which all solar radiation was absorbed in the surface layer. Temperature structure and sound propagation were only affected by changes to the optical parameterization on the stratified side of the front, where consequent variations in acoustic propagation loss of up to 10 dB relative to the control were simulated using an acoustic model. Changes in propagation loss were greatest for acoustic sources placed in the stratified water above the thermocline. Similar changes to the optical parameterization made on the well-mixed side of the front had no discernible effect.     

Study of how hydrological conditions affect the propagation of pseudorandom signals from the shelf in deep water

The paper examines how hydrological conditions affect manifestation of the acoustic “landslide” effect, which consists in focusing of acoustic energy in the near-bottom layer on the shelf and its transition to the axis of an underwater sound channel in deep water. We compare the results of experiments performed in the Sea of Japan in April 2014 and August 2006 on the same acoustic track, where the distance between corresponding points was more than 100 km. In April, the hydrological conditions in the shelf region of the track and in the upper layer of the deep-water part of the sea were characterized by the presence of a relatively weak (~0.35 s^–1) negative vertical sound velocity gradient, whereas in August 2006, it was ~1.5 s^–1. Experimental and numerical studies showed that the acoustic landslide effect also manifests itself under conditions of a weak negative sound velocity gradient, but the structure of the acoustic field trapped by the underwater sound channel has a more complex character with a time-expanded pulse characteristic. Nevertheless, its ordered, stable, and well-identified structure at all track points chosen for measurements make it possible to reliably create an efficient (with accuracies to hundredths of a percent) underwater navigation systems like GLONASS and GPS for the spring hydrology season.     

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.     

Effect of the Diffraction of Sound by the Carrier of Hydroacoustic Equipment on the Results of Measurements

The sound field distortions due to the diffraction by the surface of the carrier of hydroacoustic equipment are discussed. A self-contained hydroacoustic buoy in the form of a hollow spherical shell is considered as an example. The sound field formed near such a buoy is calculated with the use of approximate and exact equations of the shell motion. Results of full-scale experiments with a self-contained hydroacoustic buoy are presented. It is concluded that the effect of diffraction by the carrier of hydroacoustic equipment should be taken into account in full-scale hydroacoustic measurements.     

Long-range sound propagation in the Mediterranean Sea

The data of several experiments on the long-range propagation of explosion-generated and tonal sound signals are analyzed. The experiments are performed by the Acoustics Institute in the Mediterranean Sea with a fully developed sound channel. A substantial difference is observed for the propagation conditions in the western and eastern parts of the sea. This difference concerns the vertical sound speed profiles, the time structures of the sound field in the underwater sound channel, the duration of the explosion-generated signal, and the positions of the convergence zones. The experiment is compared with calculations. The observed difference in the experimental and calculated positions of the first convergence zone is explained by the imperfection of the relation used to recalculate the salinity, water temperature, and hydrostatic pressure to the sound speed. In spite of substantial difference in the propagation conditions on two 600-km paths, the experimental low-frequency attenuation coefficients on these paths (and on some shorter ones) agree well with each other for the frequency band of several kilohertz. The data are also close to those published for another 600-km path. All the paths mentioned run in different parts of the Mediterranean Sea. The frequency dependence of sound attenuation (absorption) can be well described by the relation that accounts for the absorption caused by the boron present in the sea water.     

Sound attenuation in a random waveguide when using a radiating system with different patterns of directivity

Using numerical simulations of sound propagation on Russia’s shallow Arctic shelf, low frequency sound attenuation is analyzed for acoustic sources with different patterns of directivity, i.e., vertical discrete radiating arrays of different length. It is shown that sound attenuation depends largely on the parameters of patterns of directivity and the intensity of surface waves even at short ranges r from a source (r < 250H, where H is the waveguide depth).