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Applying numerical modelling approach the accuracy in determining the distance between underwater sound sources and receivers is assessed at a range of several kilometers from each other in the Kara Sea in autumn. It is suggested that the main source of errors in determining the distance is the lack of accurate data on the vertical sound speed profile along the acoustic signal propagation path. Data from September and November were analyzed, in the interval between which significant changes in the profile take place, when the vertical sound speed gradient changes from negative to positive. Characteristic values of sound speed variations were obtained by statistical processing of hydrological data taken from the World Ocean Database. The results are important for analyzing the capabilities of underwater acoustic navigation. [ABSTRACT FROM AUTHOR]

The article presents the results of a high-frequency experiment to detect and determine the bearing of a small-sized underwater noise sound source, carried out in shallow waters of the Black Sea coast. Noise emission from the source was received by three single vector–scalar receivers located at the bottom. Holographic processing was used to detect and determine the bearing of a moving underwater source against a background of heavy shipping in the water area of the experiment. Estimates of the input signal/noise ratio are given. [ABSTRACT FROM AUTHOR]

An algorithm has been substantiated for passive determining the distance to sound sources in convergence zones (CZ), which are observed in most deep-sea areas of the World Ocean. The algorithm is based on the well-known pattern of formation of the ray structure of the acoustic field of the source signal at the input of the receiving array in CZ. The pattern is as follows: when a source enters a CZ by crossing its near boundary, the maximum energy of its signal arrives at the array along rays with negative grazing angles, and when the source enters a CZ by crossing its far boundary, along rays with positive grazing angles. The accuracy in determining distance using the proposed algorithm for detecting surface and underwater sources is assessed. [ABSTRACT FROM AUTHOR]

On the example of an underwater sound channel typical of the Philippine Sea [1–3], it was established by numerical simulation using mode theory that during experimental studies of the propagation of explosive signals by R.A. Vadov [1–3], manifestation of a weakly divergent beam in the spatiotemporal structure of the acoustic field was observed for the first time: at certain locations of the corresponding points in the oceanic waveguide, in addition to classical quadruples of pulses, additional acoustic signals with small time delays with respect to them. [ABSTRACT FROM AUTHOR]

2-1-3 piezoelectric composite is advantageous for its high piezoelectricity, mechanical stability and ease of preparation. Applying piezoelectric shunt damping technology in underwater semi-active sound absorption increases the adjustability of sound absorption performance. This paper analyzes the underwater semi-active sound absorption performance of 2-1-3 composite. The theoretical model and finite element model of 2-1-3 composite are established and the sound absorption coefficient of the composite coating is deduced based on transfer matrix and Mason equivalent circuit theory. The effects of the volume fraction of PZT-5H phase μ, the aspect ratio α and the shunt circuit on the sound absorption coefficient are studied and the root causes are analyzed. The results show that μ and α can affect the peak frequency, bandwidth and the range between open-circuit and short-circuit peak frequencies by changing the longitudinal wave velocity, mechanical loss factor and electromechanical conversion coefficient of the composite. Shunt circuits can adjust peak frequency and bandwidth by changing the elastic constant component c33 and mechanical loss factor of the composite. The conclusions of this study have value as reference for the design of underwater semi-active sound-absorbing material in various application scenarios. [ABSTRACT FROM AUTHOR]

Abstract—The results of studies of the features of the formation of pulse characteristics on the axis of an underwater sound channel in waveguides with different hydrological and bathymetric conditions of the Sea of Japan and the Sea of Okhotsk are discussed. The results of model calculations and experiments characterizing the regularities of propagation of low-frequency pulse signals in complex waveguides including shelf and deep sea for hundreds of kilometers are presented. It is shown that one of the main effects that determines long-range sound propagation in complex waveguides including the shelf and deep sea is the acoustic "landslide" effect. It is also shown that numerical modeling of the process of signal propagation from the shelf to the deep sea on acoustic traces in the Sea of Japan and the Sea of Okhotsk using the RAY program provides good convergence of the calculated and experimentally obtained impulse characteristics. [ABSTRACT FROM AUTHOR]

Abstract—We discuss the results obtained when performing a test acoustic-hydrological experiment in August 2022 at a marine test site from the coast of Sakhalin Island to Kita-Yamato Bank in the Sea of Japan. The methodology of preliminary studies in the water area intended for studying climatic variability of temperature regimes of the aquatic environment based on numerical modeling using the RAY computational program and the NEMO ocean hydrodynamic circulation model is presented. One of the main results is the average temperature of the marine environment calculated with high accuracy on the axis of the underwater sound channel in the Sea of Japan on the 1000-kilometer acoustic trace at the crossing of the vortex system. The shape of the measuring system and the technical and computational means and methods described in the article can be used as a basis for the organization of high-precision operational monitoring of thermodynamic processes in extended sea areas. [ABSTRACT FROM AUTHOR]

Abstract—Numerical modeling with the use of mode theory was used to investigate the regularities of the spatial (depth and horizontal distance) distribution of the acoustic field intensity formed by multiple interaction of a caustic beam with a layered bottom in a shallow oceanic waveguide with an underwater sound channel open to the bottom. It has been established that at the values of the sound velocity at the upper boundary in the sedimentary layer, less than the sound velocity at the bottom in the water layer, the formation of a multi-beam structure of the acoustic field is possible. It was found that, starting from certain distances, newly formed beams can play the main role in the spatial distribution of the acoustic field intensity. [ABSTRACT FROM AUTHOR]

Abstract—The transition from the traditional representation of the wave field in the vertical section of an underwater sound channel as a function of depth and time to the distribution of this field in the 3D phase space "depth–angle–time" is considered. For this purpose, the method of coherent states developed in quantum theory is used. The meaning of the proposed transition is that the field intensity distribution in the specified phase space is less sensitive to sound velocity fluctuations than in the original 2D depth–time space. This fact can be used in solving inverse problems. As an example, we consider the reconstruction of the coordinates of a source in a waveguide from measurements of the field intensity distribution of this source in phase space. [ABSTRACT FROM AUTHOR]

High-speed video filming of surface currents and synchronized acoustic measurements of the underwater sound signals of falling drops were performed in a laboratory tank. During successive falling, the main structural elements of collision of a single drop with the surface are preserved in distorted form in the flow pattern: cavity, splashes, crown, and splash; shock pulses accompanying each contact are stably repeated in the phonogram. In addition, rare resonance packets are observed. For multiple falling drops, the flow pattern changes dramatically: the main structural elements of the drop impact flow disappear, and the surface is covered with floating bubbles. The phonogram assumes the form of a noise signal, in the spectrum of which separate linear sections stand out. [ABSTRACT FROM AUTHOR]

The possibility of controlling the acoustic transparency of resonator boundaries by using piezoactive materials is considered. For a piezoactive structure with a special type of boundary conditions, a stationary electroacoustic vibration problem is solved in the plane-wave approximation. Transient radiation mode of a pumped resonator is considered. Possible applications of resonators with controlled boundaries in underwater acoustics and ultrasonic echo sounding are indicated. [ABSTRACT FROM AUTHOR]

Numerical simulation is applied to consider the propagation of a low-frequency sound pulse in a shallow-water waveguide, which consists of three fluid layers: a water layer (30 m thick, with a sound speed of 1500 m/s), a gas-saturated sediment layer (0.4 m thick, 300 m/s), and a lower half-space (1560 m/s). A similar situation takes place in Lake Kinneret. It is shown that when the source and receiver are located near the bottom at a distance of about 10 waveguide depths from each other, two types of pulses are well observed in the received signal, significantly separated by the arrival time. First, a group of pulses arrives, corresponding to the sound speed in the water layer, then a single pulse, the postcursor, arrives with a significant delay and corresponds to sound propagation in the low-velocity gas-saturated sediment layer. Mode analysis shows that the postcursor is a very unusual first mode (a so-called inleak normal mode) trapped in the gas-saturated layer like an oceanic underwater sound channel. The propagation velocity of the pulse of the first mode is close to the sound speed in the gas-saturated layer in a quite wide frequency range, which makes it possible to estimate this speed from the arrival time of the postcursor. [ABSTRACT FROM AUTHOR]

The paper discusses representation of the wave field on a vertical array in an underwater sound channel as the superposition of components stable with respect to large-scale perturbations of the sound velocity field. Each such component is formed by a narrow beam of rays incident on the array aperture. This representation makes it possible to modify traditional matched field processing by switching from comparison of the measured and calculated fields at the array to comparison of the stable components of these fields. It is shown that the modified approach in solving the problem of source localization makes it possible to weaken the requirements on the accuracy of the mathematical model of the medium. [ABSTRACT FROM AUTHOR]

Using the geometrical acoustics approximation and parabolic equation, we investigated the regularities in the spatial distribution of the acoustic field intensity generated by interaction between a caustic beam and bottom irregularities in a relatively shallow oceanic waveguide with an underwater sound channel open to the bottom. It is shown that when certain conditions are met, a multibeam structure of the spatial distribution of the acoustic field intensity is formed in the oceanic waveguide. [ABSTRACT FROM AUTHOR]

The results of experimental and theoretical studies on the propagation and reception of broadband pulsed signals based on pseudorandom sequences are discussed. The features of impulse response functions for reception of signals with different frequency bands and durations of symbols are investigated. Separation of acoustic energy arrivals in the cross-correlation function of the received and emitted signals is investigated in the framework of the normal mode theory for a deep-sea waveguides. The study concluded that a combination of navigation signals with different symbols duration must be used for solving practical problems of autonomous underwater vehicles positioning. [ABSTRACT FROM AUTHOR]

The results of the evaluation on the spatial and temporal changes of the underwater sound speed in the southern abyssal zone of the Caspian Sea are investigated. Experimental measurements were carried out along three survey line in the west, middle and east parts of the south deep basin during years of 2014–2016. The findings indicated that the variability range of velocity across the deep water layer was 1450–1454 ms–1. The maximum amount of the speed in abyssal water was about 1454 ms–1. Temporal variations of observed data demonstrated the influence of thermal structure of the water column in warm months on the sound speed is more than salinity fields. According to the high correlation of the sound speed and temperature in the Caspian Sea water, their vertical structures have uniformity and similarity from the sea surface to the bottom. Spatial analysis on the underwater sound speeds showed that the vertical gradient of the values across the surface and intermediate waters are more than monthly and seasonal changes in the abyssal area. Vertical difference of the speed across the thermocline layer in August was about 40 ms–1. The range of the measured values of speed in February (winter) was between 1463–1451 ms–1, while it was recorded between 1501–1451 ms–1 in June. Results indicated the formation of the deep sound channel across the intermediate and abyssal water zones. The analyses demonstrated that the axis of the sound channel occurs at 150 m depth approximately (intermediate layer) in warm months, while it appears between 270–300 m depth in February (midwinter). [ABSTRACT FROM AUTHOR]

Using approximate analytical calculations based on the WKB and adiabatic approximations for the mode representation of the field, as well as numerical simulation based on the theory of adiabatic modes, geometric acoustic approximation, and the parabolic equation method, the patterns of the formation and propagation of caustic and weakly diverging acoustic beams in a horizontally inhomogeneous refractive oceanic waveguide are studied. The horizontal distance regions are determined at which the dependences of the acoustic field intensity on the horizontal distance typical of caustic and weakly diverging beams are retained. The study considers the processes involved in reformation of caustic and weakly diverging beams in an oceanic waveguide with increasing water layer depth and constant sound speed profile, which near the source characterized an underwater sound channel open toward the bottom. The conditions are formulated for an effectively matched transition of shallow-water waveguide modes refracted in the thermocline and interacting with the bottom to purely refracted modes of the underwater sound channel in a relatively deep water part of the oceanic waveguide. [ABSTRACT FROM AUTHOR]

The article discusses the application of methods and means of general and satellite oceanology for the high-precision acoustic positioning of autonomous underwater vehicles (AUVs) at distances of hundreds of kilometers from control stations. The authors present the results of analyzing their long-term acoustic ranging studies, which revealed the specific features of low-frequency pulsed signal propagation in different hydrological–acoustic conditions in which underwater sound channels (USCs) of different origin form. Special attention is devoted to the possibility and efficiency of using long-term oceanological observation databases to form the spatiotemporal characteristics of USCs in areas for long-range navigation support to AUVs. Additionally, the authors consider the possibilities of sea surface observation by means of satellites to provide the unit that computes the AUV coordinates with data on the integral (effective) sound speed in the near-surface sound channel (NSSC), which forms in winter. The experimental testing results on applying oceanographic technical tools in the Sea of Japan have shown that they improve the accuracy and reliability of AUV navigation support at distances of hundreds of kilometers from coastal control stations. It is been shown that in summer–autumn and winter hydrological conditions, it is possible to determine the AUV coordinates with errors not exceeding hundredths of a percent at distances of hundreds of kilometers. [ABSTRACT FROM AUTHOR]

The paper considers the interference structure during underwater pulsed sounding in the ocean associated with acoustic field diffraction by wind waves. When calculating the matrix of waveguide modes scattered by an agitated surface in oceanic waveguides, a two-scale surface wave model was used, in which the acoustic field is scattered by a resonance harmonic of waves, the phase velocity of which is modulated by larger waves. It is shown that when wind waves are undeveloped, i.e., the wave spectrum is nonisotropic and described by the JONSWAP model, the frequency matched filter response during observations of surface reverberations becomes asymmetric along the frequency axis and depends on the wind direction. [ABSTRACT FROM AUTHOR]

The article discusses the results of experiments conducted in September 2017 to prove the applicability of positioning underwater objects during their operation at depths substantially exceeding the depth of the underwater sound channel axis. The authors present results of experimental studies and numerical analysis of the effect of focusing of the acoustic energy in the near-bottom layer on the shelf and its transition into deep-water (up to 500 m) layers of the Sea of Japan for summer–autumn hydrological conditions. Experiments on reception of broadband pulsed signals with a center frequency of 400 Hz were carried out at various distances (20, 68, 86, 90, and 198 km) from a source of navigation signals moored at a depth of 35 m at the shoreline near Cape Schulz. The receiving of acoustic signals was performed by a system of distributed over depth up to 500 meters hydrophones, with the possibility of long-term signal recording at fixed depths and during submergence. The experimental results allowed to study the impulse responses of acoustic waveguides, and estimate the effective propagation velocities of navigation signals received at different depths, and to draw conclusions about the possibility of solving positioning problems for autonomous underwater vehicles at depths up to 500 m and distances up to 200 km from the source of navigation signals. Mathematical modeling of acoustic wave propagation in a waveguide reproducing the experimental conditions by the normal mode technique was also carried out. [ABSTRACT FROM AUTHOR]

Abstract: Model experiments are used to obtain the transfer functions of an Arctic-type shallow-water acoustic waveguide in the spring-summer period, including in the presence of surface waves. The calculations were performed for frequencies from 150 to 1000 Hz using the theory of coupling modes. From the numerical simulation results, the usable frequency range is determined for stable underwater communications suitable for waveguides with both acoustically soft and acoustically hard bottoms. It is demonstrated that for certain distances between the sound source and receiver and their location depths, errorless information transmission can be achieved even without special processing methods. Channel compensation with passive time reversal leads to increased transmission reliability predominantly in a waveguide with an acoustically hard bottom. [ABSTRACT FROM AUTHOR]

Abstract: A new sono-elastic substructure method is proposed in this paper to improve the computational efficiency of the hull-substructure coupled and fluid-structure interacted vibration and acoustic radiation of a submerged cylindrical-shell-type vehicle. The typical part of the vehicle structure is divided into the main hull and the internal substructures. The fluid-structure interaction problem of the main hull is solved by an analytical method based on the simplified model of a single-hull ring-stiffened cylindrical shell simply supported at both ends. Meanwhile, the substructures are numerically modeled through the Finite Element Method, with the condensed dynamic stiffness matrices of them obtained via the Superelement Method of Modal Synthesis. The main hull and the internal substructures are then integrated according to the boundary compatibility conditions at the connecting parts. Thus, a Mixed Analytical-Numerical Substructure (MANS) method is formulated. The applicability of this method is validated by two numerical examples as well as the test results of a large-scale submerged structural model. It is shown that the MANS method is suitable for the prediction of vibration and acoustic radiation of typical cylindrical-shell-type submerged structures in the medium frequency region. [ABSTRACT FROM AUTHOR]

Abstract: We consider the patterns manifesting themselves in the formation of the spatial distribution of the intensity of an acoustic field excited by a point source emitting a tone signal in a deep-water oceanic waveguide with an underwater sound channel open to the surface and an acoustically transparent bottom. It is shown that at a certain emission frequency, the diffraction and ray caustics can merge, which leads to an appreciable increase in the acoustic field intensity along characteristic caustic lines on the horizontal distance-depth plane. [ABSTRACT FROM AUTHOR]

Results of verifying the levels and directional characteristics of sound radiation are presented for a finite-element numerical model of a submerged inhomogeneous shell. The error in numerical modeling of directional characteristics is determined for sound radiation from shell parts with one-layer (shell-liquid) and multilayer (shell-liquid-shell-liquid) structures. [ABSTRACT FROM AUTHOR]

Using a computer-based simulation method, we consider the noise immunity of an individual combined receiver consisting of sound pressure and vibration velocity receivers, as well as the noise immunity of a plane acoustically transparent array consisting of such receivers, are considered. The computer simulation results verify the earlier developed analytic method of calculating the noise immunity coefficient of hydroacoustic arrays for the multiplicative processing algorithm for processes in their channels. It is shown that if the anisotropy of the noise field acting on the combined receiver and array varies in a very wide range, the maximum noise immunity is ensured not by multiplicative but by additive processing. [ABSTRACT FROM AUTHOR]

In this study the method of source images for the problem of sound propagation in a penetrable wedge [G. Deane and M. Buckingham, J. Acoust. Soc. Am. 93 (1993) 1319-1328] is revisited. This solution is very important three-dimensional (3D) benchmark in computational underwater acoustics, since a wedge bounded from above by the sea surface and overlying a sloping penetrable bottom is the simplest model of a shallow-sea waveguide near the coastline. The corrected formulae for the positions of the source images and bottom images are presented together with the explanation of their derivation. The problem of branch choice in the reflection coefficient is thoroughly discussed, and the corresponding explicit formulae are given. In addition, numerical validation of the proposed branch choice schemes and the resulting wedge problem solutions are presented. Finally, source images solution is computed for a series of examples with different ratios of shear and bulk moduli in the bottom. The interplay between the acoustic-elastic waves coupling and the horizontal refraction in the wedge is demonstrated. [ABSTRACT FROM AUTHOR]

The study is devoted to statistical modeling of low-frequency acoustic signal propagation in a twodimensionally inhomogeneous random shallow sea with a thermocline and differing penetrability of the bottom. Calculations are performed using the local-mode representation of the solution in the one-way propagation approximation. Plots are presented for the behavior of the mean acoustic field intensity for different sound velocity and density values in the bottom. It is shown that the earlier described effect of a decrease in propagation losses in a model randomly inhomogeneous shallow sea with an absorbing bottom significantly depends on the parameters of bottom sediments and is more strongly manifested for bottom boundaries with greater penetrability. [ABSTRACT FROM AUTHOR]

A model is been developed for the coherence of high-frequency hydroacoustic fields of noise sources in a randomly inhomogeneous ocean. We analyze the dependences of the coherence of hydroacoustic fields of noise sources on the mutual position of the source and receiver system, the type of waveguide, and the parameters of random inhomogeneities of an oceanic waveguide. We discuss the possibilities of underwater acoustic observation. [ABSTRACT FROM AUTHOR]

The paper presents the results of processing measurement data on the spatiotemporal structures of sound fields in Lake Ladoga. Measurements were taken with an extended vertical receiver array. The aim of processing was to isolate the field components that were stable with respect to small variations in the waveguide parameters. Since a model of the medium is inevitably inaccurate, such components can be predicted with greater accuracy than the total field. In terms of the ray approach, a stable component is generated by a beam of rays propagating over close trajectories. The discussed experiment analyzed sound fields excited by a point source that emitted wideband pulses, as well as the fields of wave beams excited by the emitting vertical array at fixed frequencies. In both cases, the processing results showed that the isolated stable components, as expected, coincide substantially better with the prediction of theoretical calculation (by the wide angle parabolic equation method) than with the total wave field. [ABSTRACT FROM AUTHOR]

We solve the problem of layer-by-layer reconstruction of the parameters of bottom layers using parametric models of the formation of signals reflected from a layered half-space for coherent probing of the sea shelf. We use a decision rule based on the sequential application of the MUSIC projection algorithm. We study the operability and stability of the search and decision-making algorithms with limited a priori data. [ABSTRACT FROM AUTHOR]

We consider a recently developed method to analyze the angular structure of pulsed acoustic fields in an underwater sound channel. The method is based on the Husimi transform that allows us to approximately link a wave field with the corresponding ray arrivals. The advantage of the method lies in the possibility of its practical realization by a vertical hydrophone array crossing only a small part of the oceanic depth. The main aim of the present work is to find the optimal parameter values of the array that ensure good angular accuracy and sufficient reliability of the algorithm to calculate the arrival angles. Broadband pulses with central frequencies of 80 and 240 Hz are considered. It is shown that an array with a length of several hundred meters allows measuring the angular spectrum with an accuracy of up to 1 degree. The angular resolution is lowered with an increase of the sound wavelength due to the fundamental limitations imposed by the uncertainty relation. [ABSTRACT FROM AUTHOR]

The paper considers the problem of using a vertical array to focus a monochromatic field in the vicinity of a given point of an underwater sound channel. The solution to the problem is discussed, which has been obtained with a limitation on the initial grazing angles of the emitted waves. The limitation is introduced to increase the stability of the solution to the influence of random perturbations of the sound velocity field. Then, the radiated field represents a wave beam propagating along a reference ray that connects the center of the array and the focusing point. In the geometric optics approximation, analytic estimates of the smallest possible dimensions of the focal spot are obtained. A simplified version of the formula is obtained for the vertical scale of the spot convenient for rough express estimates and comparison of the focusing properties of waveguides with different sound velocity profiles. [ABSTRACT FROM AUTHOR]

This paper investigates a novel approach to processing records of ambient noise in the ocean that are measured concurrently in spatially separated locations. The approach is a synthesis of two well-known phase-coherent signal processing techniques. At the first stage of processing, an approximation to the transient acoustic Green function is found by the method of noise interferometry. At the second stage, the approximate Green function is time reversed and back propagated from the location of one of the receivers, thereby producing a focus in the vicinity of the other receiver. Unlike the earlier work, measurements at just two points (rather than vertical array measurements) are used when the sound-propagation range is large compared to the ocean depth. The requirement for optimal focusing of the back-propagated field is shown to lead to extraction of estimates of the unknown physical parameters of the waveguide and, hence, to passive acoustic remote sensing of the ocean. [ABSTRACT FROM AUTHOR]

Using theoretical analysis and mathematical modeling, we study the dependence of the number of maxima in the correlation function of a wideband signal, whose parameters are related to the source coordinates, on the hydroacoustic conditions, the parameters of the receiver, and the signal/noise ratio. [ABSTRACT FROM AUTHOR]

The paper considers a method for suppressing the reverberant distortions of an underwater sound receiver signal during receiver calibration in a laboratory water tank. The method is based on using the water tank transfer function, which is a complex frequency-dependent coefficient that establishes, for the point of signal reception, the relation between the sound pressures in the reverberant sound field of the water tank and in the free sound field. The procedure for experimentally obtaining the water tank transfer function is considered. Examples of suppressing reverberant distortions during noise and pulse sound reception are presented. [ABSTRACT FROM AUTHOR]

The relationships for optimizing a selective estimate of the correlation matrix of classical (initial and modified) and fast projection algorithms in a complex noise situation were determined. Model investigations on the comparison of the resolution of classical optimized algorithms with the characteristics of two variants of fast projection algorithms were performed. The algorithms were compared using the criterion of a decrease in the time and reduction of the angular zone of the loss of a contact with a weak target when it intersects the path of an intense interfering signal. The relationships that correspond to the most economical realization of the fast algorithm were determined. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

It is very interesting to measure the bistatic characteristics of sound scattering by the ocean bottom and surface for the development of present-day hydrolocation net-centric schemes. Possible methods for measuring scattering bistatic characteristics are evaluated. The angular characteristics of reverberation related to illumination of a water area by active linearly frequency-modulated signals are studied using linear passive arrays to test a measuring scheme in a shallow water area with a depth of ~20 m in the 1-3 kHz frequency range. The bistatic sound-scattering characteristics in the water area are calculated based on the measurements. The obtained characteristics are compared with the known data on the backscattering of sound. [ABSTRACT FROM AUTHOR]

We investigate the specific features of acoustic signals generated in water under the action of cw laser radiation with a power of 3 W at wavelengths of 0.97, 1.56, and 1.9 μm, emerging from an optical fiber. It is established that when a fiber tip without an absorbing coating is used, quasi-periodic pulse signals are generated according to the thermocavitation mechanism due to the formation and collapse of vapor-gas bubbles of millimeter size. In this case, the maximum energy of a broadband (up to 10 MHz) acoustic signal generated only at wavelengths of 1.56 and 1.9 μm is concentrated in the range of 4-20 kHz. It is shown that when there is no absorbing coating, an increase in the laser-radiation absorption coefficient in water leads to an increase in the frequency of generated acoustic pulses, while the maximum pressure amplitudes in them remain virtually constant. If there is an absorbing coating on the laser-fiber tip, a large number of small vapor-gas bubbles are generated at all laser-radiation wavelengths used. This leads to the appearance of a continuous amplitude-modulated acoustic signal, whose main energy is concentrated in the range of 8-15 kHz. It is shown that in this case, increasing the absorption coefficient of laser radiation in water leads to an increase in the power of an acoustic emission signal. The results can be used to explain the high therapeutic efficiency of moderate-power laser-fiber apparatus. [ABSTRACT FROM AUTHOR]

We analyze the results of an experiment using an explosive sound source in the tropical part of the Indian Ocean. We consider the time structure of sound signals in geometric shadow zones to a distance of 270 km and the scheme of how the sound field in the shadow zone is formed by rays reflected from horizontally extended fine-structured sound velocity layers. From the results of calculation using a wave program that realizes the method of psuedodifferential parabolic equations, we analyze the influence of signal scattering by fine-structure sound velocity inhomogeneities on the sound field distribution in a waveguide. We show that the field formed by spots of light in each of the shadow zones is generated by a regular field and propagates in parallel to it, taking energy from the regular zone in the near field and in each subsequent convergence zone. This mechanism causes an additional decrease in the field in illuminated zones, which can be interpreted as additional attenuation of the regular sound field. [ABSTRACT FROM AUTHOR]

Generation of infrasonic radiation into a water medium by sea surface waves is analyzed. The analysis is carried out for the situation in which the infrasound is generated by surface waves with frequencies close to those of dominant waves. The presence of two wave systems on the sea surface is assumed: swell and wind waves. It is shown that if the frequencies of spectral peaks of wind waves and swell diverge by 20%, the maximum value of the radiation spectrum decreases by approximately 40% (if the general directions of the two wave systems are oriented strictly towards each other). A deviation of the general directions of the two wave systems from the opposite direction by 45° leads to a decrease in the maximum value of the radiation spectrum by more than two times. [ABSTRACT FROM AUTHOR]

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) negative vertical sound velocity gradient, whereas in August 2006, it was ~1.5 s. 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. [ABSTRACT FROM AUTHOR]

It is shown that scalar, horizontal, and vertical vector receivers efficiently split modes of different numbers, which makes it possible to analyze the mode structure and estimate the characteristics of surface layers of a shallow sea bottom. To analyze the mode structure of propagating pulses from a towed pneumatic source, Winger transform was applied, with which seven modes were isolated by vertical vector receivers, whereas the scalar receivers and horizontal vector receivers isolated only three modes. It is established that the use of four-component vector-scalar receivers makes it possible to increase the accuracy in estimating the parameters of a layered bottom model. [ABSTRACT FROM AUTHOR]

A technique for detecting direct signal pulses based on steep rising edges of acoustic pressure is developed. The technique consists in calculating the mirror derivative of the received signal and normalizing it in a specific manner. This makes it possible to amplify weak direct signals and suppress strong reflected ones. A key feature of this technique is that it ensures a high probability of detection of direct signal pulses while keeping the number of false detections at a minimum. [ABSTRACT FROM AUTHOR]

When performing calibrations, it is expedient to measure the set of detailed frequency responses of an underwater sound receiver so that it can receive particular characteristics adequate to the tasks and conditions of using it according to its intended use. We demonstrate the possibility of using the reverberation field of a noise signal for this in conjunction with processing by complex moving weighted averaging. The paper presents the results of calibrating underwater sound receivers in the noise reverberation field of a laboratory tank. [ABSTRACT FROM AUTHOR]

We propose and test in a field experiment a method that uses an emitting vertical array to focus a field to a given depth interval in a vertical section of an underwater sound channel at the observation distance. The amplitude-phase distribution of signals at the array elements is found by solving the variational problem of maximizing the ratio of mean sound field intensities inside and outside the chosen area of the section. The introduction of an additional restriction on the range of allowable grazing angles of excited waves has made it possible to obtain a solution in the form of a wave beam propagating without bottom reflections. Such solutions can be used for field focusing without information on the bottom parameters. [ABSTRACT FROM AUTHOR]

The paper presents a description and the technical characteristics of an autonomous hydroacoustic station developed at the Pacific Oceanological Institute, Far Eastern Branch, Russian Academy of Sciences (TOI DVO RAN) for stationary measurements of variations in acoustic pressure in the frequency range of 2-15 000 Hz on the shelf at the bottom. To organize real-time monitoring of the parameters of seismic survey impulse signals and industrial acoustic noise, the station is additionally equipped with a digital UHF-FM radio telemetry channel. Acoustic data measured in the frequency band of 2-2000 Hz is transmitted through the channel to a shore post, as well as via an Iridium satellite radio telemetry channel. It controls the operation of the UHF-FM radio transmitter and transmits the results of special analysis of acoustic data obtained in subsequent 1 min time intervals. [ABSTRACT FROM AUTHOR]

A three-dimensional analytic solution is constructed for the model of an inhomogeneous hydroacoustic waveguide with a cylindrically shaped body drifting on the surface. A numerically analytic method is proposed to find the velocity potential for which the unknown coefficients for normal modes are determined from the corresponding infinite system of linear algebraic equations. The amplitude-frequency characteristics of the waveguide are studied as a function of the model parameters. [ABSTRACT FROM AUTHOR]

The state of the art of matched field hydroacoustic signal processing is described from the viewpoint of estimating the signal parameters in adaptive antenna arrays. The focus is on methods for solving the problem of source localization in an oceanic waveguide under mismatching effects of different nature, caused by disagreement between the received acoustic field and its model. Different approaches to increase the stability of the algorithms for source position estimation are discussed, which allows an increase in their efficiency in natural conditions. [ABSTRACT FROM AUTHOR]