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.     

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。     

Correlation processing of a noise signal in the arrival-time acoustic monitoring of the water medium in an arctic waveguide

The propagation of a noise signal is considered for an arctic-type waveguide with a varying sound speed profile. The profiles used in the calculations differ from each other due to different depth dependences of salinity. The shape of the envelope of the time correlation function is studied for the coherent and reference signals. For the latter, either the replica of the transmitted noise signal or one of the modes propagating in the waveguide is used. A characteristic feature of the proposed technique is the use of the time cross-correlation between the signals that traveled through the same path in the presence of different sound speed profiles. In this case, from the shape of the envelope of the signal correlation function, one can estimate the changes that occur in the sound speed profile on the path of signal propagation.     

深海不完整声道下反转点会聚区研究

近期南海远程声传播实验数据的处理分析表明在深海不完整声道中声道轴以下存在一种会聚区,该会聚区相比于海面附近的上反转点会聚区在远距离处具有更高的会聚增益.本文利用射线简正波理论确定了水中反转型焦散线和海面反射型焦散线位置,对比发现实验中观测到的深海大深度会聚区位置与水中反转型焦散线位置一致,证明该会聚区是由大量简正波同相叠加形成的下反转点会聚区,其在深海声道轴以下的一定深度范围内都具有会聚效应,研究了该会聚区的形成条件以及声源深度变化对会聚区焦散结构的影响,对比了远距离处上下反转点会聚区的传播损失以及会聚区宽度,分析表明第七个下反转点会聚区的会聚增益仍不小于10 dB,研究了声速垂直结构变化对下反转点会聚区的影响,理论分析结果与实验数据吻合较好.     

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.     

Illumination of the shadow zone in a two-channel oceanic waveguide with a fine structure of sound speed inhomogeneities

The experimental data on the sound field structure, which were obtained by emitting a continuous pseudonoise signal (a midfrequency of 3.2 kHz) in a two-channel oceanic waveguide, are compared with the calculations performed by the wave program with allowance for the fine structure of the sound speed inhomogeneities. A considerable increase in the intensity of the sound field with a definite angular spectrum is observed in the upper channel in the first shadow zone, and, in the experiment, the increase begins nearer to the sound source than predicted by the wave and ray calculations for a smooth sound speed profile. These features of the field structure are explained by the illumination of the shadow zone by the regular scattering of signals from highly anisotropic fine-structure inhomogeneities of the sound speed profile, which are clearly pronounced in the region of the given oceanic experiment.     

Structure of the sound field in convergence zones of the tropical Pacific

Experimental data are presented on the intensity and angular structures of the sound field in the first and second convergence zones for the tropical region of the Pacific Ocean, southwest of the Hawaian Islands. The experiment is carried out using continuous pseudonoise signals in one-third-octave bands around central frequencies of 1.25 and 3.15 kHz with both source and receiver positioned under the discontinuity layer at a depth of 250 m. The experimental data are compared with calculations based on the ray model. The discrepancies arising between the experimental and calculated data at the entry onto the first convergence zone are compared with the corresponding parameters obtained earlier in the tropical regions of the Atlantic and Indian oceans. The detailed intensity structure of the sound field is correlated with the parameters of internal waves observed during the acoustic measurements.     

Stability and a fast calculation method of travel speed of pulse peak in convergence zone

A long-range sound propagation experiment was conducted in the West Pacific Ocean in summer 2013.The signals received by a towed array indicate that the travel speed of pulse peak(TSPP)in the convergence zones is stable.Therefore,an equivalent sound speed can be used at all ranges in the convergence zones.A fast calculation method based on the beam-displace-ment ray-mode(BDRM)theory and convergence zone theory is proposed to calculate this equivalent sound speed.The computation speed of this proposed method is over 1000 times faster than that of the conventional calculation method based on the normal mode theory,with the computation error less than 0.4%compared with the experimental result.Also,the effect of frequency and sound speed profile on the TSPP is studied with the conventional and fast calculation methods,showing that the TSPP is almost independent of the frequency and sound speed profile in the ocean surface layer.     

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.     

深海海底斜坡环境下的声传播

海底地形变化对声传播具有很大影响,在南海深海区域海底斜坡环境下进行了一次声传播实验,实验显示倾斜海底环境下声传播损失出现了一些不同于平坦海底环境下的现象,分析并解释了海底地形变化对产生声传播差异的原因.结果表明,海底斜坡对声波的反射增强作用可使斜坡上方的声传播损失减少约5 d B.当声波第一次入射到达的海底位置有较小幅度的山丘(凸起高度小于1/10海深)时,海底小山丘即可对声波有反射遮挡作用,导致在其反射区特定传播距离和深度上出现倒三角声影区,比平坦海底环境下相同影区位置处的传播损失增大约8 d B,影响深度可达海面以下1500 m.而海底斜坡对声波的反射阻挡作用使得从海面反射及水体向下折射的会聚区结构消失,只剩下从水体向上折射的会聚结构.因此,海底地形对深海声传播影响较大,在水下目标探测和性能评估等应用中应予以重视.     

Broadband sound propagation in shallow water and geoacoustic inversion

Part of an experiment to test a measurement package in a shallow water region in the Gulf of Mexico was designed to gather broadband acoustic data suitable for inversion to estimate seabed geoacoustic parameters. Continuous wave tow acoustic signals at multiple frequencies and broadband impulsive source signals were recorded on a horizontal line array in a high-noise environment. Simulated annealing with a normal mode forward propagation model is utilized to invert for a geoacoustic representation of the seabed. Several inversions are made from different data samples of two light bulb implosions, the measured sound speed profiles at the HLA and at the positions of the light bulb deployments, and for two different cost functions. The different cost functions, measured sound speed profiles, and measured time series result in different inverted geoacoustic profiles from which transmission loss is generated for comparison with measurements. On the basis of physical consistency and from the comparison of the transmission loss and time series, a best estimate geoacoustic profile is selected and compared to those obtained from previously reported inversions. Uncertainties in the sound speed profile are shown to affect the uncertainties of the estimated seabed parameters.     

Investigation of the sound field structure in the ocean with a deep-water reception

The results of studying the angular and energy structures of a sound field at great depths (880 and 1100 m) with the emission of pseudonoise signals (a frequency range of 1–4 kHz) in the upper layers of the ocean below the velocline (at a depth of 200 m) are presented. The results refer to two ocean regions characterized by the presence of a single-axis deep-water sound channel. The sound field structure at great depths is compared with that recorded at the source depth. The experimental data for the first two convergence zones are compared with the calculations by the ray method. A conclusion is made that, on the whole, the basic regularities of the field structure at great depths are adequately described by the ray theory. However, the spatial positions of the convergence zones observed in the experiment differ from those predicted by the ray calculations. Moreover, a closer examination of the field characteristics shows that the parameters of the angular and energy structures noticeably deviate from the calculated values.     

Determination of equivalent sound speed profiles for ray tracing in near-ground sound propagation

The determination of appropriate sound speed profiles in the modeling of near-ground propagation using a ray tracing method is investigated using a ray tracing model which is capable of performing axisymmetric calculations of the sound field around an isolated source. Eigenrays are traced using an iterative procedure which integrates the trajectory equations for each ray launched from the source at a specific direction. The calculation of sound energy losses is made by introducing appropriate coefficients to the equations representing the effect of ground and atmospheric absorption and the interaction with the atmospheric turbulence. The model is validated against analytical and numerical predictions of other methodologies for simple cases, as well as against measurements for nonrefractive atmospheric environments. A systematic investigation for near-ground propagation in downward and upward refractive atmosphere is made using experimental data. Guidelines for the suitable simulation of the wind velocity profile are derived by correlating predictions with measurements.     

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.     

Features of the Sound Field Structure in a Two-Channel Oceanic Waveguide

The results of experimental studies of the energy and angular structure of a sound field in the region of the Iberian Basin in the northeastern Atlantic are discussed. The experiments are carried out in a two-channel waveguide whose axes are located at depths of approximately 450 and 2000 m. A continuous pseudonoise signal in the frequency range 2.52–4.0 kHz is emitted. The signals are received by the omnidirectional hydrophones and, simultaneously, by a 10-m-long vertical array, which allows one to realize a narrow beam reception (～2.5°) in the vertical plane. The source and the receiver are located in a 500-m-thick layer within the upper sound channel. The field characteristics are measured in the course of a continuous change of distance from 1 to 65 km. The comparison of the experimental data with calculations shows that the sound field structure formed by the lower channel is much closer to the theoretical results than the structure formed by the upper channel. In the upper sound channel, the shadow zone manifests itself only slightly and the first convergence zone begins approximately 72–11 km nearer to the source than predicted by the calculations. The corresponding angular sound field structure is fairly pronounced in the vertical plane and bears no evidence of the random behavior that is peculiar to the fields scattered by the inhomogeneities.     

Spectral-energy characteristics of sound fields scattered by fine-structured inhomogeneities of sound velocity into the geometrical shadow zone

In analyzing experimental results obtained with explosive sources in the tropical zone of the Indian Ocean, a good agreement was obtained for spectral-energy characteristics of signals observed in the first geometrical shadow zone with computer calculations of the sound field scattered by fine structure inhomogeneities of the fractal type. From the comparison of the results of calculating the frequency characteristics of sound fields in the shadow zone by the wave code and by the method combining ray acoustics with the wave theory of sound scattering, it was found that both methods are appropriate for describing the real processes of scattering and propagation of sound in the ocean with fine-structured stratification and that these methods can be used for solving inverse problems.     

Long-term stability of the sound field structure in the central part of the eastern Atlantic

Results of studying the angular and energy structures of the sound field formed in the first convergence zone as a result of the transmission of a continuous pseudonoise signal in the frequency range 0.8–1.5 kHz are presented for the case of the corresponding points lying in the upper ocean layer at depths between 150 and 500 m. The sound field characteristics were studied with the use of omnidirectional and narrow-beam (～2°) reception in the vertical plane. The variability of the basic experimental parameters of the field structure is analyzed using the data of three experiments carried out in the eastern part of the Atlantic Ocean, near the Canary Islands, over a period of 11 years. The experimental results are compared with ray calculations. It is shown that, in the region under investigation, the experimental characteristics of the field structure are relatively stable and the extent to which they agree with the calculated parameters is also stable. In addition, some data are presented to illustrate the effect of a small lens of Mediterranean water, which manifests itself in sound velocity variations at the channel axis, on certain field characteristics.     

Theoretical analysis and experiment study on the acoustic parameters of metal rubber materials

The acoustic parameters of metal rubber materials were theoretically and experimentally investigated. Under the assumption that metal rubber materials were homogenous, isotropic and porous structures, formulas were deduced for the calculations of effective sound velocity, characteristic impedance, propagation constant, structural constant and flow resistivity. The structural constant of metal rubber materials with different structural parameters were obtained and analyzed by using experiments. The experimental and theoretical values of characteristic impedance and propagation constant were compared and analyzed. It is shown that the proposed theoretic method based on the homogenous, isotropic and porous material model is suitable to calculate the acoustic parameters of metal rubber materials.     

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.     

Effect of the supersonic transport configuration on the sonic boom parameters

Results of numerical and experimental investigations of the sonic boom parameters for two configurations of civil supersonic transport are presented. Numerical modelling is performed by a combined method based on calculating the spatial flow in the near zone of the aircraft configuration and subsequent determination of disturbed flow parameters at large distances from the examined model. Numerical results are compared with experimental sonic boom parameters measured in the near zone and with results of their recalculation to large distances within the framework of the quasi-linear theory. This validation allows the degree of adequacy of the inviscid Euler model for solving the posed problem to be determined. Reasons for certain disagreement between the calculated and experimental data are discussed. The analysis confirms the possibility of attenuating the sonic boom generated by supersonic transport with an unconventional configuration based on a tandem arrangement of two wings on the fuselage.