Long-range sound propagation in the northwestern region of the Pacific Ocean

Results of two experiments on long-range propagation of explosion-generated signals in the northwestern Pacific are analyzed. The propagation paths used in the two experiments substantially differ in both their hydrological characteristics and the conditions of signal reception. On the first path, the omnidirectional reception of explosion-generated signals is performed on the coastal shelf. The initial part of the 850-km-long path crosses the cold Kamchatka current and the continental slope. On the second path, the signals are received near the coastline of the Simushir island with a sea depth of more than 2000 m. The path crosses the Kuril-Kamchatka trough and the frontal zone formed at the boundary between the cold Kuril-Kamchatka current and the warm Kuroshio current. The intensity characteristics of the sound fields on the paths, the time structure of the signals, and the phenomena that arise when the path crosses the continental slope and the frontal zone are considered. The results obtained from analyzing the experimental data are compared with computer calculations.     

Long-range sound propagation in the norwegian sea

The data of repeated experiments on the long-range propagation of explosion-generated and cw signals in the Norwegian Sea in summer conditions (with a fully-developed underwater sound channel) are presented. These data are used to analyze the spatial and time structures of the sound field, as well as to estimate the attenuation coefficient at frequencies within 63–630 Hz and to determine its frequency dependence. The spatial variability of the propagation conditions is analyzed on the basis of the experimental data obtained for the propagation of explosion-generated signals along a 815-km-long path crossing the Norwegian and Lofoten Hollows.     

Long-range sound propagation in the central part of the Barents Sea

The data of the experiments on long-range propagation of explosion-generated and tonal sound signals, which had been performed in different years in the central part of the Barents Sea in summertime conditions, are used to analyze the space structure of the sound field intensity, to estimate the attenuation coefficient due to the sound energy loss in the bottom sediments, and to determine the frequency dependence of this coefficient. A comparison of the data on the long-range propagation of explosion-generated signals is performed between two experiments carried out on the same 230-km-long path crossing the Central Basin of the Barents Sea, several years in succession. The two experiments differ in the propagation conditions: in the first experiment, a near-bottom sound channel extends along the entire path, and in the second experiment, the path crosses a frontal zone characterized by fairly complex variations of the sound speed field. Calculations are carried out to show that the specific behavior of the frequency dependence of attenuation can be explained by the powerlaw frequency dependence (with an exponent of 1.4) of the sound absorption in the water-saturated upper layer of the bottom sediments. It is also shown that the difference in the decay laws obtained for the sound field levels in the two experiments is caused by the difference in the corresponding hydrological conditions.     

Long-range sound propagation in the northeastern Atlantic

Experimental data on the long-range propagation of explosion-generated signals are analyzed. The experiments were performed in the northeastern Atlantic under the conditions of a two-axis underwater sound channel. The sound field in the upper channel was governed by the vertical redistribution of the ray structure and sound energy under the influence of a smooth increase in the depth of the channel’s axis along the propagation path. The explosions were produced in the upper sound channel at a depth of 200 m, which was constant along the path. The time structure of the sound field is analyzed for the upper channel (a reception depth of 200 m) and for deeper layers lying somewhat below the boundary between the upper and lower sound channels (a reception depth of 1200 m). The deviation of the decay law obtained for the sound field level in the upper channel from the cylindrical law is used to estimate the attenuation coefficient. The low-frequency (several hundreds of hertz) attenuation coefficients experimentally determined with allowance for the sound field redistribution agree well with the calculated sound absorption in seawater. The attenuation coefficients determined by the differential method also agree well with the absorption calculated by the formulas proposed earlier. The analysis of the time structure of the sound field near the boundary between the upper and lower channels reveals a permanent insonification of this horizon by weak water-path signals propagating with the velocity typical of the signals traveling in the upper channel.     

Year-to-year variability of the long-range propagation conditions in the Norwegian Sea

On the basis of the experimental data on the sound field formation in the Norwegian Sea, the year-to-year variability of the propagation conditions is estimated. A comparative analysis is performed for the data obtained from two long-range-propagation experiments with explosion-generated signals. The experiments were carried out in summertime (August) on a path crossing the central deep-water part of the Norwegian Sea and were separated in time by a period of four years. Noticeable changes are found to occur in the sound speed fields between the two experiments. These changes are related to a change in the distance between the cores of cold waters (observed in the region of the Norwegian Basin) and warm waters (observed in the region of the Lofoten Basin). According to calculations, the observed changes in the sound speed structure can lead to considerable changes in the propagation anomaly and in the range dependence of the sound field decay. In spite of the noticeable difference in the propagation conditions, the experimental coefficients of low-frequency attenuation differ little for the two experiments.     

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.     

The effect of geoacoustic characteristics of the bottom on the long-range sound propagation in an inhomogeneous ocean

The long-range sound propagation from a deep ocean to a receiving system located on the shelf is modeled. The waveguide model is constructed on the basis of the data of an acoustic-oceanographic experiment carried out in the northwestern Pacific. The sensitivity and the frequency dependence of the difference in the sound field levels at the crossing of the frontal zone on the geoacoustic characteristics of the bottom of the shelf and the continental slope are investigated. It is shown that the level difference decreases by 8.2 dB as the velocity of longitudinal waves increases by 100 m/s in the range within 1490–1820 m/s.     

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 Mediterranean Sea

The data of several experiments on the long-range propagation of explosion-generated and tonal sound signals are analyzed. The experiments are performed by the Acoustics Institute in the Mediterranean Sea with a fully developed sound channel. A substantial difference is observed for the propagation conditions in the western and eastern parts of the sea. This difference concerns the vertical sound speed profiles, the time structures of the sound field in the underwater sound channel, the duration of the explosion-generated signal, and the positions of the convergence zones. The experiment is compared with calculations. The observed difference in the experimental and calculated positions of the first convergence zone is explained by the imperfection of the relation used to recalculate the salinity, water temperature, and hydrostatic pressure to the sound speed. In spite of substantial difference in the propagation conditions on two 600-km paths, the experimental low-frequency attenuation coefficients on these paths (and on some shorter ones) agree well with each other for the frequency band of several kilohertz. The data are also close to those published for another 600-km path. All the paths mentioned run in different parts of the Mediterranean Sea. The frequency dependence of sound attenuation (absorption) can be well described by the relation that accounts for the absorption caused by the boron present in the sea water.     

Penetration of sound into the cold eddy of the Sargasso Sea

The results of an acoustic experiment performed in the Sargasso Sea are analyzed. The position of the receiving vessel was in a cold eddy. A 120-km-long acoustic path began in the warm waters of the Sargasso Sea and ended at the receiving vessel. Broadband sound signals were produced by 2.8-kg charges, which were fired at a depth of 290 m by another vessel moving at full speed. The interval between the explosions was 2.5 km. The signals were received by three independent hydrophones positioned at depths of 220, 600, and 1200 m. In processing the experimental data, the main attention was paid to the variability of the parameters of signals belonging to signal quartets with different numbers of bottom reflections. For these signals, on some segments of the path, a considerable intensity decrease (reaching 12 dB at frequencies above 200 Hz) was observed. For signals with different numbers of bottom reflections, the interval of distances corresponding to the intensity decrease was found to be determined by the same range of signal launch angles. It is concluded that the phenomenon under consideration is caused by the presence of a region with high horizontal sound velocity gradients, which correspond to the transition from the cold waters of the eddy to the warm waters of the Sargasso Sea. An attempt is made to reconstruct the shape of the frontal boundary.     

深海海底反射会聚区声传播特性

在深海声道条件下,海水折射效应会使得声场出现会聚效应;在不完全声道条件下,深海海底对声场具有重要影响.利用在中国南海海域收集到的一次深海声传播实验数据,研究了深海不完全声道环境下的海底反射对声传播的影响.实验观测到不同于深海会聚区的海底反射会聚现象,在直达声区范围内的海底地形隆起可导致海底反射会聚区提前形成,并使得部分影区的声强明显提高.由于不平坦海底和海面的反射破坏了完全声道环境下的会聚区结构,在60 km范围内存在两个海底反射会聚区,会聚区增益可达10 dB以上,同时在11 km附近的影区和51 km附近形成高声强区域.当接收深度与声源深度相同时,第二会聚区的增益高于第一会聚区.在第一会聚区内,随着接收深度的增加,声线到达结构趋于复杂,多途效应更加明显.使用抛物方程数值分析结合射线理论对深海海底反射会聚区现象产生的物理原因进行了分析解释.研究结果对于声纳在深海复杂环境下的性能分析具有重要的指导意义.     

The discovery of the underwater sound channel,experimental studies,and regional differences

More than half a century has passed since the discovery of the underwater sound channel. In this period of time, the Acoustics Institute has performed a number of experiments on the long-range propagation of explosion-generated sound signals in different ocean regions. These experiments included the studies of such phenomena as the frequency-dependent sound attenuation in the sea medium and the sound field formation in the underwater sound channel. A combined analysis of the data obtained revealed considerable regional differences in the time structure of the sound field. In the experiments, a number of phenomena were observed that required special explanation and additional theoretical treatment. These phenomena include: the unexpectedly high attenuation of low-frequency sound in the sea medium, the “spectrum-analyzing” properties of the underwater sound channel in the Black Sea, the existence of the reverberation forerunner (the so-called prereverberation), the frequency-independent deviation of the phase shift from the usual value of 90° between the signals in classical quartets differing in the number of contacts with the caustic, the splitting of individual signals into quartets, and the transformation of these quartets into groups of nearly irresolvable signals at long distances. The most interesting data of the aforementioned studies are described in the present paper.     

Effect produced on sound propagation by an internal temperature front moving over the shelf

Results of observing the changes that occur in the vertical distribution of water temperature under the effect of an intense atmospheric cyclone and the influence of these changes on sound propagation in the shelf region of the Sea of Japan are presented. The measurement results refer to the autumn conditions. The measuring equipment includes a vertical acoustic-hydrophysical measuring system, a broadband transmitter (both of them being connected with the shore station by cable lines), and a self-contained resonance (320 Hz) transmitter of the electromagnetic type. The sound (tone signals) propagation is studied on a 510-m-long constant-depth (38 m) track (TON-310 Hz) and a 10.6-km-long track (TON-320 Hz), which is set up by placing the self-contained transmitter at the bottom (at a depth of 65 m). Results of field experiments are presented along with those of numerical simulation of the effect produced by an internal temperature front moving toward the coast and formed by the seasonal thermocline on the propagation of 320-Hz sound signals through it. It is shown that refraction and scattering of sound waves propagating through the temperature front moving along the acoustic track may cause intensity variations of acoustic field at the reception point, which occur synchronously at different depths and have amplitudes of up to 14 dB and a period of about 40 min.     

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.     

Comprehensive studies of sound fields in the Kuril-Kamchatka region of the northwestern Pacific

The technique, experimental conditions, and main results of comprehensive studies of sound fields in the northwestern region of the Pacific Ocean are presented. The experiments are carried out on paths up to 2100 km in length. The power-frequency, space-time, and correlation characteristics of the sound fields are studied in sonic and infrasonic frequency bands for long-and extra-long-range propagation with the use of cw and explosion-generated sound signals. Effects of the bottom relief and the spatial distribution of the speed of sound on the frequency characteristics of the sound field are investigated. The role of front zones in the formation of sound fields received at the coastal shelf and in the open ocean is revealed. The loss coefficients are estimated. The space-time stability of the sound field components is studied, and the possibility is shown for the coherent components to be conserved and resolved in frequency at distances up to 2100 km. The phase velocities of these components are determined. The total broadening of the frequency spectra is considered. The correlation characteristics of the total field are obtained for horizontally separated receivers in sonic and infrasonic frequency bands.     

Sound field structure produced by tone sources in the Kurile-Kamchatka region of the Pacific Ocean with a surface channel and wind waves

Results of the experimental studies of sound signal propagation in the continental wedge of the northwestern Pacific, near the eastern coast of the Kamchatka Peninsula, are presented. The signals are produced by highly stable tone sources. The experiments are carried out in winter, in the presence of a strong surface sound channel and intense wind waves, at frequencies of 100, 230, and 400 Hz, on a 1000-km-long path. The signal transmission is performed by continuously towing the sound sources at a depth of 50±5 m with a speed of 4.5–5 knots, for 115 hours. The decay of the sound field level with distance is studied as a function of the sea state and the frequency. The results of the experiments, including the sound field decay along the path, are compared with the calculations for different sea states.     

Estimation of distortions in the sound field propagating through mesoscale inhomogeneities

Hamiltonian formalism is used to analyze the effect of the mesoscale inhomogeneities of the ocean medium on the ray structure of the sound field. It is shown that the distortions of the structure of the sound field can be successfully estimated by a function that links the values of the canonical variables of the ray action before and after crossing the inhomogeneity. That function is calculated with the use of the standard ray code. Sound propagation through the synoptic eddy and frontal zone is considered.     

Atmospheric effects on voice command intelligibility from acoustic hail and warning devices

Voice command sound pressure levels (SPLs) were recorded at distances up to 1500 m. Received SPLs were related to the meteorological condition during sound propagation and compared with the outdoor sound propagation standard ISO 9613-2. Intelligibility of received signals was calculated using ANSI S3.5. Intelligibility results for the present voice command indicate that meteorological condition imposes little to no effect on intelligibility when the signal-to-noise ratio (SNR) is low (<-9 dB) or high (>0 dB). In these two cases the signal is firmly unintelligible or intelligible, respectively. However, at moderate SNRs, variations in received SPL can cause a fully intelligible voice command to become unintelligible, depending on the meteorological condition along the sound propagation path. These changes in voice command intelligibility often occur on time scales as short as minutes during upward refracting conditions, typically found above ground during the day or upwind of a sound source. Reliably predicting the intelligibility of a voice command in a moderate SNR environment can be challenging due to the inherent variability imposed by sound propagation through the atmosphere.     

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.     

Time structure of bistatic reverberation in the long-range propagation of explosion-generated signals

In experiments on the long-range propagation of explosion-generated signals, a noise background accompanying the classical signal quartets are repeatedly observed. The background smoothly increases before the arrival of the first signal of the quartet, then decreases, and completely vanishes after the arrival of the last signal of the same quartet. The data of an experiment performed in winter in a deep-water region of the northwestern Pacific are considered. These data are used to demonstrate the phenomenon at hand and to show that the noise background of the quartets is the manifestation of the bistatic reverberation caused by sound scattering by the rough ocean surface towards the receiver located a long distance from the source. An easy-to-use technique is proposed for calculating the time relations between the direct signal and the surface-reverberation one. Calculations are performed for the time structure of the bistatic surface reverberation to explain the observed features of the phenomenon.