Anisotropic field of background internal waves on a sea shelf and its effect on low-frequency sound propagation

The space-time spectral characteristics of the field of background internal waves (IW) are obtained for two oceanic shelf regions (the Atlantic shelf of the United States and the Kamchatka shelf) and analyzed. Within the framework of a numerical experiment, it is shown that the observed anisotropy of the IW field may considerably affect the low-frequency sound fluctuations in the aforementioned regions and, in particular, may change the interference invariant of the sound field.     

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。     

Effect of internal waves on the sea surface in two-dimensional case

A nonlinear model of the interaction between internal and Stokes surface waves is considered. An internal wave (IW) is set in the form of a large-scale variable surface flow, directed at a certain angle to the surface-wave (SW) field. Wave interaction patterns are studied, and a comparison with the results of linear modulation model is performed. The dependence of the refraction-wave pattern on the main parameters of the problem (SW steepness, strength and direction of IW-induced flow, and the angle between the SW and IW) is investigated.     

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.     

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.     

Observationally constrained modeling of sound in curved ocean internal waves: examination of deep ducting and surface ducting at short range

A study of 400 Hz sound focusing and ducting effects in a packet of curved nonlinear internal waves in shallow water is presented. Sound propagation roughly along the crests of the waves is simulated with a three-dimensional parabolic equation computational code, and the results are compared to measured propagation along fixed 3 and 6 km source/receiver paths. The measurements were made on the shelf of the South China Sea northeast of Tung-Sha Island. Construction of the time-varying three-dimensional sound-speed fields used in the modeling simulations was guided by environmental data collected concurrently with the acoustic data. Computed three-dimensional propagation results compare well with field observations. The simulations allow identification of time-dependent sound forward scattering and ducting processes within the curved internal gravity waves. Strong acoustic intensity enhancement was observed during passage of high-amplitude nonlinear waves over the source/receiver paths, and is replicated in the model. The waves were typical of the region (35 m vertical displacement). Two types of ducting are found in the model, which occur asynchronously. One type is three-dimensional modal trapping in deep ducts within the wave crests (shallow thermocline zones). The second type is surface ducting within the wave troughs (deep thermocline zones).     

Effect of internal waves on the intensity and interference structure of the sound field in a shelf zone

Experimental data obtained by studying the effect of internal waves with known space-time parameters on the propagation of a 240-Hz tone signal and on the space-time interference structure of the sound field generated by a noiselike sound source are presented.     

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.     

Long-range sound propagation in the central region of the Baltic Sea

The data obtained from a set of experiments on the long-range, low-frequency (<5 kHz) sound propagation in the central region of the Baltic Sea are analyzed. The experiments were carried out in the summer season, with a fully developed underwater sound channel. Experimental data on the sound attenuation are presented. A significant excess of the attenuation coefficients over the predicted absorption coefficients is obtained. The quantitative estimates indicate that the sound scattering by internal waves is the most probable mechanism responsible for the observed excessive sound attenuation. The frequency dependence of the attenuation coefficient exhibits a minimum whose position on the frequency axis at the beginning of the summer season noticeably differs from that at the end of summer. The analysis of the propagation conditions allows one to relate the position of this minimum to the critical frequency of the water modes. In addition to the intensity parameters of the sound field, the formation of the time structure of explosion-generated signals propagating in the Baltic underwater sound channel is considered for the case of the sound propagation along the 360-km path crossing the Gotland Hollow. The specific role of the bottom waves in the time structure formation at short distances from the sound source is demonstrated.     

Effect of random hydrodynamic inhomogeneities on low frequency sound propagation loss in shallow water

Low frequency (100–500 Hz) sound propagation loss on the US Atlantic continental shelf and in the Barents Sea in the presence of stochastic surface waves, and for the US Atlantic shelf also in the presence of internal waves, is studied for the range of up to 150 km by means of numerical simulations. Qualitative difference between sound propagation loss behavior on the US Atlantic shelf and in the Barents Sea is demonstrated for summertime conditions even without random inhomogeneities. It is shown that whereas internal waves have a weak effect on propagation loss, surface waves result in its considerable increase in both areas under wintertime conditions with a wind speed of more than 9 m/s.     

深远海线性内波条件下声场时间相关性的空间分布

深海声场特定的干涉结构导致其时间相关性的空间起伏,研究这种空间特性可以为水声信号的探测与处理提供重要参考。利用抛物方程声场仿真模型,联合Monte-Carlo数值方法计算分析了深远海线性内波条件下声场时间相关性的空间分布特性。与现有的研究相比,给出了时间相关性的距离和深度起伏特征。结果表明,当接收达到一定距离,声场时间相关性的空间分布具有与声场干涉条纹类似的结构,声场干涉越强,时间相关性越好。此外,声源频率和声速标准差的变化会引起时间相关性空间分布规律的改变,且会聚区传播模式下的改变强于深海声道传播模式。      

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.     

Waves and instabilities in dark interstellar molecular clouds containing ferromagnetic dust grains

The propagation characteristics of magnetization waves, as well as the instabilities of sound waves in a self-gravitating dark interstellar molecular cloud containing ferromagnetic dust grains and baryonic gas clouds, have been theoretically investigated by including the dynamics of both ferromagnetic dust grains and baryonic gases. It has been shown that there exist two types of subsonic or supersonic (depending on the field strength of the magnetization) transverse magnetization waves, which can be regarded as counterparts of Alfvén waves (for the parallel propagation) and magnetosonic waves (for the perpendicular propagation) in a magnetoactive plasma. It has also been found that, in addition to the usual Jeans instability, the sound waves suffer a new type of instability, which is due to the combined effects of the baryonic gas dynamics and self-gravitational field in both weakly and highly collisional regimes.     

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.     

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.     

Experimental and theoretical study of the vertical coherence of the sound field in a shallow sea

The combined effect of multiple scattering by random inhomogeneities of a waveguide and bottomcaused sound absorption is known to be a fundamental factor that governs the formation of the sound field in a shallow sea [1, 2]. A number of publications [2–6] present statistical analyses of the fluctuation phenomena that accompany the sound propagation in shallow sea regions. In these publications, most attention is paid to studying the evolution of the intensities of normal waves under the effect of scattering by the random field of internal waves. It is shown that in natural conditions, in addition to the inhomogeneities of the water column, one should take into account the irregular boundaries of the sound channel, which can also affect the correlation characteristics of the sound field. In this paper, we present experimental data on the vertical coherence of the sound field on a fixed path in the Barents Sea. We also compare the experimental data with theoretical calculations based on a model of sound scattering by the rough sea surface.     

Effect of the variability of hydrological conditions on the sound field interference structure in a shelf region

Computer simulations are performed to study the effect of internal waves and heating of the surface water layer on the behavior of the local spectral maximums of the interference sound field in coastal slope conditions, as applied to the operation of the self-sustained oscillator technique for monitoring the ocean. Results of testing the self-sustained oscillator technique on a 1-km-long propagation path are presented.     

南海北部海域内波环境下声传播损失起伏统计特性

浅海内波会引起声传播能量随时间的起伏变化,进而影响水声设备的工作性能.本文利用2015年南海北部一次浅海声场起伏实验数据,对比分析了浅海线性内波和孤立子内波条件下的声传播损失统计特性.在孤立子内波条件下,声传播损失起伏明显加剧,可达11 dB,且分布明显展宽,相对于线性内波的环境,声传播损失起伏可增加5 dB.从简正波...     

On the feasibility of normal wave selection in a shallow-water waveguide

Long-range propagation of low-frequency narrowband sound signals in the near-bottom acoustic channel with random inhomogeneities caused by internal waves and a rough bottom is investigated in the framework of numerical and field experiments. The feasibility of selecting the signal components corresponding to different normal waves is analyzed. The problem of selecting such components is considered for signals of long duration exceeding the characteristic time of the stationary state of the channel.     

On the propagation of acceleration waves in incompressible hyperelastic solids

The conditions for the propagation of acceleration waves (sound waves) in incompressible elastic media undergoing finite deformation are investigated. The incompressible hyperelastic solid media is considered in accordance with the general constitutive theory of materials subject to internal mechanical constraints. The equation of motion of acceleration waves is obtained using the theory of singular surfaces. A general comparison is made between the magnitudes of the propagation speeds of waves in incompressible and unconstrained solid media by the use of Mandel's inequalities. The magnitudes of the speeds of propagation of acceleration waves in the incompressible hyperelastic material classes of neo-Hookean, Mooney-Rivlin, and St. Venant-Kirchhoff solids are determined. Comparisons are made of the specific results concerning the magnitudes of wave propagation speeds making use of the corresponding material parameters.