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 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 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.     

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.     

Acoustic propagation in the subsurface sound channel

Experimental data obtained in the kilohertz frequency band for the sound propagation in the subsurface channel formed by the wind-caused mixing of subsurface waters are discussed. The data were obtained in different years in the northeastern region of the Atlantic Ocean, where the subsurface waters down to the depths of 40–70 m are mixed by both wind waves and the swell that arrives from distant ocean areas. The hydrological conditions in the subsurface waters of this region are characterized by a good reproducibility. The spatial structure of the sound field and the attenuation of sound propagating in the subsurface channel are analyzed. The origin of the additional attenuation (in comparison with the absorption in sea water) is discussed. The data of our experiments are compared with those obtained by other experimenters and with the calculations performed using the computer code by Avilov. The necessity of improving the computer codes to allow for the scattering of sound beyond the channel under the influence of the swell, whose parameters are unrelated to the wind regime at the experimental site, is emphasized.     

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.     

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.     

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.     

西北太平洋副热带模态水区域声传播特性*

西北太平洋副热带模态水(STMW)是在西北太平洋夏季出现的温跃层中温度、盐度和密度具有垂向均一性的水团。由于西北太平洋副热带模态水(STMW)的存在,深海声速剖面呈现出双跃层结构,对深海远程声传播产生较大的影响。本文对比分析了西北太平洋副热带模态水(STMW)区域夏冬两季典型声速剖面环境下的声传播规律。分析结果表明,夏季声速剖面环境下,声源位于浅层声道宽度内时,声传播为浅层声道的类深海声道传播与会聚区传播的复合形式。掠射角较小的声线被限制于浅层声道中,增加了会聚区内的到达结构,并且增强了在影区的声能量,在第二影区内的传播损失比冬季声速剖面环境下最多低近60dB。本文推导了浅层声道的截止频率的近似表达式,分析结果表明,截止频率主要由浅层声道的宽度和正梯度段的声速差值决定,夏季STMW区域浅层声道截止频率主要集中在100Hz左右。     

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.     

Properties of sound attenuation around a two-dimensional underwater vehicle with a large cavitation number

Due to the high speed of underwater vehicles,cavitation is generated inevitably along with the sound attenuation when the sound signal traverses through the cavity region around the underwater vehicle.The linear wave propagation is studied to obtain the influence of bubbly liquid on the acoustic wave propagation in the cavity region.The sound attenuation coefficient and the sound speed formula of the bubbly liquid are presented.Based on the sound attenuation coefficients with various vapor volume fractions,the attenuation of sound intensity is calculated under large cavitation number conditions.The result shows that the sound intensity attenuation is fairly small in a certain condition.Consequently,the intensity attenuation can be neglected in engineering.     

一种低声速沉积层海底参数声学反演方法

软泥底环境下沉积层参数的声学反演是国际水声领域的一个研究热点.浅海中,当高声速基底和海水之间存在一层低声速(小于海水声速)的沉积层时,小掠射角情况下不同频率声传播损失会出现周期性增大现象.基于此现象,提出一种适用于低声速沉积层的海底参数声学反演方法.首先,推导给出小掠射角情况下传播损失周期增大的频率间隔与沉积层声速、厚度及近海底海水声速之间的解析表达式;其次,利用一次黄海实验中软泥底环境下的宽带声传播信号,提取了小掠射角下传播损失增大的频率周期;再次,把该解析表达式作为约束条件,结合Hamilton密度与声速的经验公式,采用匹配场处理反演给出沉积层的声速、密度、厚度及基底的声速、密度;然后,利用声传播损失数据反演得到泥底环境下不同频率的声衰减系数,通过拟合发现泥底声衰减系数随频率近似呈线性关系;最后,给出了双层海底模型和半无限大海底模型等效性的讨论.反演结果为低声速沉积层海底声传播规律研究与应用提供了海底声学参数.     

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.     

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.     

Determination of acoustic attenuation in the Hudson River Estuary by means of ship noise observations

Analysis of sound propagation in a complex urban estuary has application to underwater threat detection systems, underwater communication, and acoustic tomography. One of the most important acoustic parameters, sound attenuation, was analyzed in the Hudson River near Manhattan using measurements of acoustic noise generated by passing ships and recorded by a fixed hydrophone. Analysis of the ship noise level for varying distances allowed estimation of the sound attenuation in the frequency band of 10-80 kHz. The effective attenuation coefficient representing the attenuation loss above cylindrical spreading loss had only slight frequency dependence and can be estimated by the frequency independent value of 0.058 dBm.     

Overview of channel models for underwater wireless communication networks

Acoustic communication in Underwater Wireless Communication Networks (UWCNs) has several challenges due to the presence of fading, multipath and refractive properties of the sound channel which necessitate the development of precise underwater channel models. Some existing channel models are simplified and do not consider multipath or multipath fading. In this paper, a detailed survey on ray-theory-based multipath Rayleigh underwater channel models for underwater wireless communication is presented and the research challenges for an efficient communication in this environment are outlined. These channel models are valid for shallow or deep water. They are based on acoustic propagation physics which captures different propagation paths of sound in the underwater and consider all the effects of shadow zones, multipath fading, operating frequency, depth and water temperature. The propagation characteristics are shown through mathematical analysis. Transmission losses between transceivers are investigated through simulations. Further simulations are carried out to study the bit error rate effects and the maximum internode distances for different networks and depths considering a 16-QAM modulation scheme with OFDM as the multicarrier transmission technique. The effect of weather season and the variability of ocean environmental factors such as water temperature on the communication performance are also shown. The mathematical analysis and simulations highlight important considerations for the deployment and operation of UWCNs.     

Application of the WKB method to calculating the group velocities and attenuation coefficients of normal waves in the arctic underwater waveguide

Algorithms based on the WKB approximation are proposed for the fast and accurate calculation of the group time delays and effective attenuation coefficients of normal waves in the deep-water sound channel of the Arctic Ocean. These characteristics of the modes are determined in the adiabatic approximation by integrating the local group velocity and attenuation coefficient over the horizontal distance between the ends of the propagation path. According to the WKB method, the local group velocity is the ratio of two quantities. The first one is the sum of the length of the ray corresponding to the mode and the side displacement of the ray at the reflection by the ice cover. The second one is the sum of the travel time of the sound signal along the ray cycle and the time delay caused by the side displacement. The grazing angle of the ray is determined from the condition of quantization for the phase integral. According to the WKB method, the local attenuation coefficient of the mode is specified as the ratio of the squared modulus of the coherent reflection coefficient at the lower boundary of the ice cover and the sum of the cycle length and the side displacement of the ray. Simple recurrent relations are proposed to estimate, with fair accuracy and short calculating time, the phase integral, the integral that describes the cycle length, and the related local group velocities and attenuation coefficients. The capacity and efficiency of the algorithms are confirmed by the comparison of the aforementioned mode characteristics calculated by using the proposed relations and the precise computer code. The calculations are performed with the sound speed profiles obtained from the temperature and salinity measurements during the SEVER and SCICEX-1995 expeditions.     

印度洋中北部声速剖面结构的时空变化及其物理机理研究

海洋的声速结构对水下声传播有重要影响,在印度洋中北部复杂多变的海洋物理和水文环境中, 获取声速剖面的时空统计分布规律对水下目标探测和水下声通信有重要意义. 由于垂直梯度法在声速结构分析中的局限性及其在印度洋中北部海域的适用性问题, 采用多元统计分析中的最优分割法对声速跃层进行分析,并应用最近10年的地转海洋学实时观测阵 数据对印度洋中北部海域声速剖面的特征量进行了计算,获得了声速跃层的垂直结构特征和时空变化规律; 还利用经验正交函数(EOF)表示方法,分析了印度洋中北部声速剖面拟合精度随EOF阶次的分布特点. 根据印度洋的海洋物理特征,揭示了声速剖面特征量时空演变的内在物理机理.研究结果表明: 最优分割法是适合印度洋声速结构的跃层判断方法,并提出了相应的判断准则参数; 声速剖面拟合精度随EOF阶次变化的区域性分布特征较明显,其季节性变化较小; 印度洋中北部的深海声道轴只在5°S以南明显存在,在15°S---25°S 附近海域存在三个跃层;印度洋中北部声速剖面结构可分为单跃层、双跃层Ⅰ型、 双跃层Ⅱ型和三跃层四种类型以及春夏秋冬四个季节模态. 声速剖面的分析结果对于水声传播和声纳系统的使用具有一定参考意义.     

用信号处理技术及传播理论还原雷声频谱

以往关于雷声的研究都限于观测点雷声频域范围的探讨, 由于噪音和衰减因素的影响, 观测点的雷声频谱有很大失真. 针对在兰州地区观测的一次雷暴过程中的雷声信号, 利用数字信号处理技术,由观测点处的雷声音频信号得到频率谱,并对雷声频谱进行噪音分析; 然后结合声音在大气中传播的衰减理论, 分析了不同大气环境对声吸收的影响, 并推算出了声源附近的雷声频率谱. 关键词： 雷声频谱 信号处理 传播理论 峰值频率     

海洋信道随机性引起的船舶噪声线谱衰减

阐述准周期性随机声脉冲序列作为船舶噪声模型的普适性,侧重讨论船舶噪声准周期性随机声脉冲序列所产生的线谱经随机海洋信道传输后的变化。指出在大多数工况下,船舶噪声并不是以几种不同频率的加性正弦波直接叠加在宽带连续谱噪声上,而是一类本身并不含有任何有限强度加性正弦波分量的信号。研究结果表明:海洋信道多途效应引起船舶噪声脉冲序列信号时域上时延扩展;海洋信道的随机性使船舶噪声随机声脉冲序列信号准周期性下降,导致船舶噪声线谱相对幅度“额外”快速衰减,对于存在大范围不均匀水团的海洋信道,50 km传播距离上其介质随机起伏引起的船舶噪声线谱“额外”衰减最高可达11 dB。海洋信道随机性引起的船舶噪声线谱衰减不可忽视。