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.     

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.     

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.     

Focusing of a sound beam in an underwater waveguide using a vertical antenna

We propose a method for calculating the amplitude-phase distributions of signals on the elements of an emitting antenna for generating narrow wave beams and field focusing at given points of the waveguide. The efficiency of the method is confirmed by the data of a field experiment conducted in Lake Ladoga.     

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.     

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.     

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.     

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.     

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\     

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.     

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.     

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。     

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.     

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。     

The sound field in a hydroacoustic waveguide with an uneven hard bottom

An analytic representation is constructed for a nonaxially symmetric sound field to simulate a hydroacoustic waveguide the bottom of which is hard and has an axially symmetric relief. A numerical analytic method for finding the velocity potential is proposed, for which undetermined coefficients for normal modes are determined from a corresponding infinite system of algebraic equations. The sound fields are studied with for variations of the problem parameters.     

Random sound waves in a weakly stratified atmosphere

The influence of random mass density and velocity fields on the frequencies and amplitudes of the sound waves that propagate along a constant gravity field is examined in the limit of weak random fields, small amplitude oscillations and a weakly stratified medium. Using a perturbative method, we derive dispersion relations from which we conclude that the effect of a space-dependent random mass density field is to attenuate sound waves. Frequencies of these waves are higher than in the case of a coherent medium. A time-dependent random mass density field increases frequencies and amplifies the sounds waves. On the other hand, a space-dependent random flow reduces the wave frequencies and attenuates the sound waves. The time-dependent random flow raises the frequencies of the sound waves and amplifies their amplitudes. In the limit of the gravity-free medium the above results are in an agreement with the former findings.     

A method to accurately measure and respond to an underwater sound pulse in a wide dynamic range

I.IntroductionTomeasurethesound1evelofanundcrwatersoundpulsewithunknownamplitudeisofimportanceinunderwaterapp1ication.Forinstancc,asimu1ationsystemusedtointerfcresomesonarmusthavesuchfunction,whichhastobeabIctomeasurethesoundlcvelatreceivingpointandretransmitasimulatcdsignalwithrequircdsoundlcvelafterpropcrprocessing.Itisdifficultforanorma1peakorthresho1ddetectortocompletethistask.Thereasonforthisisthattherangeofthesignalamp1itudercaches7O-8odBandthereissomedistortionofsignalwaveform,inaddit…