Frequency shifts of the spatial interference structure of the sound field in shallow water

A fluctuation model of the frequency shifts of the spatial interference structure formed by the sound field in a randomly inhomogeneous oceanic waveguide is constructed. The relation between the random fields of the perturbation-caused variation of the waveguide’s dispersion characteristic and the frequency deviation of a local interference maximum is analyzed. The applicability of the results to the cases of the waveguide perturbation by the background internal waves and by the roughness of the bottom relief is considered.     

Reconstruction of the spatial spectrum for an anisotropic field of background internal waves

A model reconstruction of the two-dimensional spatial spectrum is considered for an anisotropic field of background internal waves in shallow water. Solution of the inverse problem is based on the data on frequency shifts of the sound field interference maxima. Results of a numerical experiment are presented.     

Method of acoustic sweep monitoring of oceanic inhomogeneities (Review)

Theoretical grounds of the new method of monitoring of the temporal variability of oceanic inhomogeneities, which uses the data on frequency shifts of interference maxima of the sound field, were described. The method is free of limitations on both the resolution of signals coming in individual modes (rays) and the adiabatic approximation underlying the conventional methods of inhomogeneity reconstruction. The monitoring sensitivity was estimated, which allows us to estimate minimum detectable changes in the speed of sound by measurement data on frequency shifts of local maxima. Experimental data on shifts of the frequency spectrum of a broadband low-frequency signal on a stationary path in a shallow sea were presented. On their basis, the possibility of applying this method to diagnose tidal variations was shown. Within a numerical simulation, model reconstruction of the frequency spectrum of background internal waves was considered on the basis of the data on measurements of the spectrum of frequency shifts of the interference maximum. The results of the spectrum reconstruction with and without focusing of the conjugate wave field are presented. The problems of monitoring stability and efficiency with respect to the interference pattern formed by various groups of modes were discussed.     

The sensitivity of monitoring by measuring the frequency shifts of the sound field interference pattern

The sensitivity of the remote method of reconstructing the frequency spectrum of a water medium perturbation from the measured frequency shifts of the sound field interference peaks is investigated. On the basis of the Rayleigh criterion, an expression is derived for estimating the minimal frequency shift that allows the resolution of two neighboring peaks. The detection sensitivity for the sound velocity fluctuations caused by the time variability of the medium along the track is estimated. Results of computations for a perturbation in the form of background internal waves are presented.     

Space-time sound field interference in the horizontal plane in a coastal slope region

The nonstationary interference structure formed in the horizontal plane in the field of a point source in a coastal slope region is analyzed on the basis of the space-time horizontal ray concept. For a model that includes the typical bathymetry and sound velocity profile, the space, time, and frequency dependences of the sound field are considered for different waveguide modes. The characteristic scales of the field variability in space and time are estimated. The frequency-time diagram is analyzed for the region of the interference of direct and reflected waves. The variability of the field interference structure in the presence of mesoscale perturbations is considered.     

Frequency shifts of the sound field interference pattern on oceanic shelf in summer conditions

Considerable shifts Δf/ϕ ≈ 10⁻¹ of the low-frequency sound field interference pattern in the frequency domain, associated with barotropic tide and internal tidal waves, were observed in the Shallow Water’06 experiment on the New Jersey shelf in the summer of 2006. The acoustic frequency shifts appear to be strongly dependent on the modes of the sound field. By examining different modal structure, it is possible to analyze the overall interference pattern and find which part is more sensitive either to the surface tide or the internal waves. This feature can be exploited to acoustically monitor tidal waves of different kinds.     

Reconstruction of spatial spectra of the isotropic field of background internal waves

Model reconstruction of the two-dimensional spatial spectra of isotropic background internal waves is considered in the framework of computer modeling. The solution of the inverse problem is based on information about the frequency shifts of interference maxima of an acoustic field. The results of reconstruction of spectra with and without focusing of the inverse wave field are presented. The possibilities of monitoring are illustrated in relation to the interference pattern formed by different modal composition.     

Frequency shifts of sound field maxima in oceanic waveguides

The frequency shifts of sound field maxima that are caused by the variability of the medium in an oceanic waveguide are described. The condition under which the time spectrum of the frequency shifts of interference maxima is proportional to the time spectrum of fluctuations in the dispersion characteristic of the waveguide is determined.     

Effect of background internal waves on the interference structure of the sound field in a shallow sea

The results of a theoretical analysis of the effect produced by an anisotropic field of background internal waves on the localization of the interference pattern in a shallow sea are presented. The space-time variability of the interference invariant and the smearing of the observation direction of interference fringes are considered in a wide frequency range. The stability of the interference pattern formed by both the superposition of the fields of different mode groups and separate mode groups is analyzed in comparison with the unperturbed waveguide. Numerical calculations are performed for longitudinal and transverse orientations of the acoustic track relative to the propagation direction of internal waves.     

浅海矢量声场干涉结构形成机理及试验研究

浅海低频声场的微观结构特征在于具有可用波导不变量表征的 稳定空间-频率干涉结构.声场兼具标量场和矢量场, 波导条件下二者联合决定声场的全部特性. 本文研究浅海声场空频干涉结构的矢量场特征. 理论分析了声压谱、动能密度谱、声强流谱等矢量场干涉结构的形成机理, 探讨了矢量场干涉结构的波导不变量表征, 数值仿真研究了Pekeris波导中能量和能流密度的干涉特性, 进行了宽带声源辐射矢量声场干涉特性及表征的海上试验.实测结果与理论、仿真分析有较好的一致性. 研究结果表明: 中近程和中远程声场均能模态相干, 有稳定的空频干涉结构, 并且矢量声场空频干涉结构存在多种形式, 除各种能量和能流密度谱图外, 相干系数谱也呈现干涉特征, 这些形式的空频干涉结构均可用波导不变量理论有效表征. 关键词： 矢量声场 干涉结构 波导不变量 浅海低频声场     

Acoustic sweep monitoring of background internal waves

The results of a theoretical consideration of fluctuations that occur in the frequency shifts of the interference pattern under the effect of background internal waves are presented. Possibilities of reconstructing the spectrum of vertical displacements of liquid layers from the measured spectrum of frequency deviations of a local interference peak are analyzed within the framework of a numerical experiment. Problems of stability and the efficiency of the proposed monitoring are discussed.     

Possibility of reconstructing hydrological variability of shallow sea by measuring frequency shifts of interference structure of sound field

Variations in the vertical space-frequency interference structure of the sound field in the oceanic waveguide are described. Reconstruction algorithms of sound speed field, based on information on frequency shifts of interference maxima of the sound field, are presented.     

Acoustic monitoring of background internal waves with the use of the correlation method for measuring the frequency shifts of interference maxima

A numerical experiment is carried out to demonstrate the reconstruction of the frequency spectrum of background internal waves with the use of the correlation method for measuring the frequency shifts of interference maxima. The method is based on monitoring the behavior of the frequency shift for the maximum of the cross-correlation function of signal spectra received at different instants of time. The noise immunity of the correlation method is analyzed in comparison with the direct method based on monitoring the frequency shift of a chosen maximum of the interference pattern.     

Features of the Invariant of the Spatial–Frequency Interference Structure of Acoustic Fields in Oceanic Waveguides

The paper considers the regular patterns that can be manifested in the behavior of the invariant of the spatial–frequency interference structure of an acoustic field in oceanic waveguides that are homogeneous and inhomogeneous along a track. Using the WKB and adiabatic approximations, an analytic expression is obtained for the invariant that, when certain conditions are fulfilled, reduces to the well-known classical expression independent of the mode numbers, their parity, or the emission frequency. It has been established that the approximate classical expression for the invariant correctly describes the slope of interference lines only in ranges of variation in the grazing angles of modes where the dependence of the cycle length of their corresponding Brillouin waves on the ray parameter is quite smooth and monotonic. The paper studies the formation of the spatial–frequency interference structure of an acoustic field propagating from a shallowwater isovelocity waveguide to a relatively deep-water waveguide with a near-surface sound channel.     

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。      

Interference structure of low-frequency reverberation signals in shallow water

Using numerical simulation, an analysis was conducted of the interference structure of a bottomscattered sound field generated by a wideband point source in shallow water under winter and summer conditions. The scattered signals were received from the place where the source was located and were subjected to Fourier transform with a sliding window. The paper demonstrates the possibility of estimating the waveguide invariant for backscattered signals when processing the sound intensity distributions in wide frequency and distance ranges up to the scattering area. A technique is proposed for reconstructing the twodimensional field of internal waves using variations of the interference pattern of reverberation signals. The influence of wind surface waves on the degree of interference band contrast is illustrated.     

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.     

运动目标辐射声场干涉结构映射域特征研究

浅海低频声场呈稳定而显著的干涉特征,并蕴含声源状态和波导特性等信息.本文研究运动目标辐射声场干涉结构的简化映射以及映射域能量分布对目标运动状态的指示特征.理论分析了声压场和矢量声场空(时)频干涉谱图的二维傅里叶变换映射特征,推导了匀速运动目标时频干涉谱图映射域能量脊斜率与波导不变量以及距变率、航向角的关系式,证明了映射域脊斜率符号、脊斜率绝对值变化等对目标来袭或远离以及目标威胁程度的指示,并进行了数值仿真和海上试验研究.实测结果与理论、仿真分析有较好的一致性.研究结果表明:二维傅里叶变换可将声压场和矢量场时(空)频谱图干涉结构简化,匀速运动目标辐射声场干涉结构映射域的能谱脊斜率、距变率、航向角与波导不变量有解析关系式,声强谱、动能密度谱、声强流谱等声场干涉结构经映射后更为一致,映射域脊能简明的指示目标运动状态和威胁程度.     

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.