Formation of Brillouin waves in the underwater sound channel

The mechanism of the vertical sound field structure formation in the underwater sound channel is considered. The calculations are performed by the ray method for the rays that have upper turning points at the ocean surface. It is shown that the vertical field structure is formed by the ray pairs producing opposing waves in the vertical. The rays belonging to one pair have the same sign of their departure angles at the source. The pairs are formed because of the presence of a minimum in the ray cycle length as a function of the departure angle. The resulting ray pairs are analogs of Brillouin waves.     

Statistics of caustics in an underwater sound channel

The numerical-analytical phase screen method is used to analyze the statistics of the density of caustics in an underwater sound channel with large-scale random inhomogeneities. Different cases of wave propagation direction with respect to the channel axis are considered, and the influence of the inhomogeneity correlation radius is investigated.     

Ray escape from a range-dependent underwater sound channel

The problem of sound propagation in a spatially inhomogeneous underwater sound channel is considered. The effect of ray escape, i.e., the ray incidence on the absorbing bottom due to the chaotic swing of rays, is studied. With the use of the Poincaré map and maps of escape, the relation of ray escape to the properties of the phase space of the set of ray equations is demonstrated. It is found that the maximum escape occurs under the vertical resonance conditions, i.e., at the resonance of the ray oscillations in the waveguide with the vertical oscillations of the sound velocity perturbation. A qualitative theory of the vertical resonance is developed. It is shown that the ray escape considerably shortens the time spreading of the signal.     

广义相积分简正波理论在双轴海洋声道中的推广与应用

双轴声道常出现于大洋中。双轴声道传播是一检验海洋声学计算方法的\     

Intensity and space-time characteristics of the sound field in the underwater sound channel of the Black Sea

Experimental data are presented on the intensity and space-time characteristics of the sound field generated by explosions in underwater and surface sound channels of the Black Sea. The fine field structure is studied as a function of distance and positions of the source and the receiver. The discreteness of the field structure governed by the deterministic nature of the waveguide is revealed, and the destruction of this structure under the effect of the instability of the waveguide parameters is demonstrated. The effect of the rough sea surface on the sound field in the surface channel is studied, and the diffraction-caused propagation loss is estimated. The mechanism of the forerunner formation is considered. The experimentally observed sound field features are compared with the calculations. A possibility for solving the inverse problem is indicated, and the main parameters that are used in the ray method of determining the source coordinates in the underwater channel (i.e., the method earlier proposed by the author on the basis of the intrinsic structure of the sound field) are pointed out.     

Formation of a narrow sound beam in the underwater sound channel by a vertical array

The possibility of using a vertical array for the generation of a narrow wave beam that propagates in the underwater sound channel along a given reference ray is discussed. The variational problem of choosing the initial field at the array aperture to provide the minimal possible average beam width along the propagation path of a fixed length is solved.     

Space-time and frequency-phase stability of the acoustic field in the underwater sound channel

The space-time and frequency-phase stability of the acoustic field is studied for the case of long-range propagation in the underwater sound channel. The possibility of splitting the field components produced by the Doppler effect in the total interference structure of a monochromatic signal is revealed for different ranges, parameters of the channel inhomogeneities, and frequencies. The experiments are performed in summertime in the northwestern part of the Pacific Ocean, near the Kamchatka Peninsula, on a path of 2100 km. Highly stable sound sources with resonant frequencies of 230 and 380 Hz are used for the measurements. The sources are towed at a depth of 70 m with a speed of 5–6 knots. To receive the signal near the channel axis, a bottom-moored (at a depth of 200 m) stationary system is used. The width of the sound beams is studied, and the broadening limits of the frequency spectra are estimated for the coherent and incoherent field components in the case of super-long-range sound propagation. The phase velocities of the split components are determined.     

Focusing of a wave beam in an underwater sound channel

The use of a vertical radiating antenna array for generation of a wave beam propagating in an underwater sound channel along the reference ray trajectory is discussed. The method for selecting the starting field in the antenna aperture for maximum compression of a beam in the specified vicinity of the reference ray is proposed. The estimates showing up to what distances a beam can propagate while remaining narrow as compared to the range of depths between the rotation horizons have been obtained. The problem concerning the distances from the antenna array at which a beam can still be effectively focused in the vicinity of the selected reference ray point is investigated.     

Interfering modes and an axial wave in an underwater sound channel

Experiments on long-range propagation of low-frequency sound that were conducted starting from the mid-1980s indicate a complex character of propagation in an underwater sound channel, in which a source and a receiver are located close to the channel axis. A burst of energy propagating along the axis follows early arrivals, which are well described by the formulas of geometrical acoustics, in plots of acoustic intensity as a function of propagation time and hydrophone depth. This energy burst cannot be described using geometrical acoustics because of caustics with caustic beaks located near the channel axis. Very complex interference processes occur near these caustics. As the distance from the source grows, the dimensions of the interference vicinity increase and start to overlap producing a peculiar “axial wave.” For an arbitrary two-dimensional underwater sound channel, the axial wave can be represented as a sum of the first normal modes and a residue. This conclusion is based on the use of two representations for an acoustic field. The first of them includes the sum of ray components and an axial wave. The second representation consists of ray addends, the sum of the first normal modes, and a residue. Numerical results are obtained for a canonical profile of sound velocity at the frequency of 200 Hz for the distances of 1600–1650 km.     

单声线水声MIMO信道容量的研究

本文对水声MIMO(Multiple-Input-Multiple-Output)信道容量受收发阵元数目、间距,收发阵位置、方向,平均接收信噪比以及声速剖面的斜率的影响,在收发阵元之间只存一条声线的情况下,通过WKB近似进行了初步的研究。从计算结果可以看出,当收发阵元对之间只存在一条声线时,阵元间距会对MIMO信道容量产生重要影响:当收发阵元间距足够大时, MIMO系统的信道容量将随着接收信噪比和收发阵元数线性增加,一个m×m的MIMO系统的信道容量将为相应的 SISO(Single-Input-Single-Output)系统的m倍,收发阵的方向也会对MIMO系统信道容量产生较大的影响,另外,收发阵的深度、距离也会对水声MIMO信道容量产生影响,声速剖面的斜率在一般水声信道的声速变化范围内,对信道容量的影响不大。     

Space-time structure of the low-frequency acoustic field in an underwater sound channel

Experimental data are presented on the fine structure of the sound field in an underwater sound channel for low and infralow sound frequencies. The experiments are performed in the Black Sea, on a 600-km-long path, with explosive sound sources. The intensity, space-time, and frequency characteristics of the sound field are analyzed. The geometric dispersion of the first normal wave is experimentally studied. The role of the channel inhomogeneities in the violation of the sound field coherence is determined for different frequency bands. On the basis of the experimental data, the vertical distribution of the critical frequencies of the waveguide is obtained, and the validity limits are established for the wave and ray calculation methods. The applicability of the phase methods for calculating the sound fields in waveguides with dispersion is discussed. The frequency-angular dependence of the effective sound attenuation coefficient in an underwater waveguide is revealed and explained.     

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.     

Peculiarities in the formation of the sound field structure of a point source in the Black Sea underwater sound channel

This paper studies unique (characteristic only of the Black Sea) peculiarities of the underwater sound channel (USC). Changes in the sound velocity of at depths of 50–250 m, forming the lower boundary of the Black Sea USC, differ fundamentally from the corresponding areas of the profile c(z) in other regions of the world ocean. With lowering from 40–50 m (by 5–10 m lower than the level of the channel axis) to 200–250 m, the sound velocity gradient decreases monotonically from 0.08–0.22 to 0.02 1/s (and does not increase like in the majority of regions of the world ocean). The end portion of an explosion signal received in the Black Sea USC at a distance of 200 km or more from the source represents a quasi-harmonic signal with a gradually changing frequency. Moreover, the end portion of the signal has an explicitly block structure, which agrees well with the block structure of the spectrum of an explosion signal element. In the truncated τ(R) diagram, there is no sharp bend characteristic of the majority of regions of the world ocean. At comparatively small depths of the Black Sea, a sufficiently rapid increase in the complete duration of a multiray signal with distance is observed. A comparative analysis is conducted of experimental materials obtained with a difference of seven years on virtually the same long-range propagation path of explosion signals. The main reasons for the interannual variability in the conditions of sound channel propagation in the Black Sea are explained.     

Regional distinctions in the time structure of the sound field produced by a point source in the underwater sound channel

Experimental data on long-range propagation of explosion-generated signals in different ocean regions are analyzed. The objective of the analysis is to reveal the regional distinctions in the time structure of the sound fields in the underwater sound channel and to demonstrate the following frequently observed phenomena: splitting of signals in the “classical” quartets, noise background in the quartets, and deviation of the frequency-independent phase shift between signals in the quartets from the expected value (a multiple of 90°) due to the contact with a caustic. Possible mechanisms of these phenomena are discussed.     

水声通信实验信道仿真研究

水声信道的时变多途特性对其中通信信号传输构成严重影响，为了分析评估现实实验信道条件下水声通信系统的性能，给出了一种基于实验测量的水声信道仿真方法。此方法通过发射线性调频脉冲以检测信道多径结构，同时分别采用自回归模型与多普勒时间压扩来仿真信道中各路径的时变幅度与时变延迟。通过一个具体的直接序列扩频水声通信系统，表明在仿真与实验信道条件下系统性能具有较好的一致性。     

Formation of a weakly diverging caustic beam in an underwater sound channel open to the bottom

The paper studies the patterns manifesting themselves in the formation and propagation of caustic and weakly diverging beams using the geometric acoustic approximation and mode theory applied to the simplest model of an underwater sound channel open to the bottom in the form of the bilinear dependence of the square of the refractive index on depth. It is established that when a vertical array emitting a tonal sound signal is located at a certain critical depth, the multimode caustic beam which forms near the reference ray horizontally departing from its center and which predominates in intensity is simultaneously the most weakly diverging beam. It is shown that the reference ray of such a weakly diverging caustic beam corresponds to the smoothest minimum for the dependence of the length of the ray cycle on their angle of departure from the center of the array and to the depth level of revolution at the critical depth characteristic of the given oceanic waveguide.     

利用等离子体声源测量浅海低频段水声信道特性

浅海水声信道的结构特征及其变化特性是浅海环境水下远程探测和通讯应用的基础.为了有效测量低频水声信道特性,我们在实验中采用了拖曳等离子体声源和垂直阵接收系统。通过实验测量与信道模型仿真输出的比较,对等离子体声源特性,特别是脉冲波形、空间指向性和电声转换效率,以及在信道中的传播特性等进行测量分析。结果表明:实验测量与信道模型仿真符合良好,实验中采用的等离子体声源的发射源级和拖曳姿态稳定,波形一致性好,可以满足走航式连续水下信道测量要求。     

深海大深度声传播特性及直达声区水下声源距离估计

深海大深度声传播特性对在深海近海底进行水声目标探测和定位具有重要意义。利用一次南海中南部深海不完全声道中的脉冲声传播实验数据,分析了海底附近大深度声传播损失及脉冲多途传播特性,并根据直达波和海底-海面反射波的时延差与收发距离的关系,提出一种利用深海直达声区脉冲多途到达时间进行水下声源距离估计的方法。结果表明:当接收器深度位于南海深海海底附近而声源深度较浅时,直达声区水平宽度可达30 km,传播损失相对影区来说较小,有利于水下声源探测;直达声区的直达波与海底-海面反射波的到达时延差随着收发距离的增大单调减小,可被用于水下声源距离估计。得到水下声源的距离估计结果与实验GPS测量结果较为一致,距离估计均方误差为0.28 km。     

Surface reverberation spectrum of underwater sound

A general formulation is derived for the power spectrum of fluctuations in the signal received by way of reflection from a rough sea surface, in terms of an arbitrary wavenumber-frequency spectrum of the surface waves and an arbitrary source directivity function. The formulation is then applied to the specific case of backscatter for which the extreme cases of acoustically slightly rough and very rough sea surfaces are examined in detail. The results of the analysis are compared with experiments, and with the somewhat more restricted results of other theoretical work.