Correcting underwater images distorted by surface waves

An experiment on the correction of underwater images distorted by waves at the air-water interface was conducted using a laboratory modeling installation intended for experimental examination of light and image transfer across a water surface covered with waves. A digital color camera was used for the simultaneous formation of the image of the underwater test object through the disturbed surface and of the superimposed glitter pattern. Both images are spectrally separated. Processing the glitter pattern makes it possible to obtain the values of the surface slopes at a limited number of points and to use these slopes for retrieval of image fragments. The total corrected image is formed by integration of about 300 partially corrected fragments. This image is close to that obtained through a wave-free water surface.     

Microwave scattering by surface waves on water

The electromagnetic backscattering from small-amplitude water waves has been studied using a microwave model (2.5 GHz). The first- and second-order backscatter Doppler spectra have been measured and compared with the theoretical models, and good agreement has been found. In this paper, the specific case of two water waves propagating at right angles to each other will be presented, and the level of contribution of both the hydrodynamic and the electromagnetic effects on the backscatter radar Doppler spectrum will be considered.     

Modeling the generation and gravity-capillary waves using an underwater sound source

This is an experimental study of the acoustic method of surface-wave excitation using an underwater source of high-frequency (950 kHz) sound. The surface waves are excited at the sound-beam modulation frequency (3–55 Hz). For a normal fall onto the free surface, the modulated sound beam efficiently generates waves in the gravity-capillary range. This provides flexible electronic control of the main wave parameters (frequency and amplitude) in the packet and continuous modes. The amplitude-frequency characteristics of the process of surface-wave generation were obtained by numerical calculations (based on equations for the rate of acoustic flux and propagation of gravity-capillary surface waves) and by experiments (based on surface wave measurements by optical and contact methods). Both values are very consistent: on the background of a similar monotonic attenuation with frequency, they have a local dip near the minimum of the phase velocity and oscillation in the frequency range above 20 Hz. The experiments on the excitation of wave packets by single acoustic messages with varying lengths and powers, as well as by falling water drops, indicated that, in all cases, the phase characteristics are satisfactorily consistent with one another and the time needed for the signal to arrive at the measurement point is determined by the group velocity.     

The scattering of regular surface waves by a fixed,half-immersed,circular cylinder

A train of regular surface waves is incident upon a fixed, half-immersed, circular cylinder; the waves are partially reflected and partially transmitted, and also induce hydrodynamic forces on the cylinder. In order to give a theoretical study of this problem, we make the familiar assumptions of classical hydrodynamics and then solve the linear, two-dimensional, diffraction boundary-value problem, using Ursell's multipole method. Accurate numerical results are presented (in the form of tables) for four important (complex) quantities; these are the reflection and transmission coefficients, and two dimensionless coefficients which describe the horizontal and vertical forces on the cylinder. We have also made an experimental study, in which we measured the forces on the cylinder, and the reflection coefficient. These measurements are compared with the linear theory, and also with other experimental data; discrepancies are noted and an attempt to analyse them is made. We have also measured the mean horizontal forces on the cylinder; these results are compared with the predictions of a simple formula obtained by Longuet-Higgins.     

Laboratory investigations on impulsive waves caused by underwater landslide

Laboratory investigations have been performed on the submarine landslide generated waves by performing 120 laboratory tests. Both rigid and deforming-slide masses are considered. The effects of bed slope angle, initial submergence, slide geometry, shape and deformation on impulse wave characteristics have been inspected. Impulse wave amplitude, period, energy and nonlinearity are studied in this work. The effects of bed slope angle on energy conversion from slide into wave are also investigated. Laboratory-based prediction equations are presented for impulse wave amplitude and period in near and far-field and are successfully verified using the available data in previous laboratory and numerical works.     

海洋内波对水声场的扰动

针对海洋内波对水声传播影响的问题，在简要引出内波方程，水声方程和内波解，射线理论之后，以水深2100m等Vaisala频率的全板造波内波解对双线性水声剖面情况计算了内波和内波对声场及声线传播的影响，结果表明，内波对声线传播的路径和时间的影响是不可忽略的。     

Radiation and scattering of surface waves by a group of submerged,horizontal, circular cylinders

One of the major methods available for investigating the interaction of water waves with arbitrarily shaped structures is based on the classical theory of Green's functions. For multiple bodies, however, this technique can become expensive in terms of both computer storage and execution time and it is desirable for special geometrics where possible, to seek simpler methods. In this paper, the radiation and scattering of surface waves by a group of parallel, horizontal, circular cylinders, submerged in deep water is studied using a method involving multipole potentials. The method is developed for any number of submerged, parallel, horizontal cylinders with arbitrary positions and radii. In particular, hydrodynamic coefficients are determined for various configurations of two and three cylinders and a comparison is made with results obtained for a cylinder in isolation.     

A novel synthetic aperture radar scattering model for sea surface with breaking waves

In this study a novel synthetic aperture radar(SAR) scattering model for sea surface with breaking waves is proposed. Compared with existing models, the proposed model considers an empirical relationship between wind speed and wave breaking scattering to present the contribution of wave breaking. Moreover, the scattering weight factor p, and wave breaking rate q, are performed to present the contribution of the quasi-specular scattering term, Bragg scattering term, and wave breaking scattering t...     

Scattering of surface waves by submerged cylinders

A train of surface waves is normally incident upon a cylinder that is totally submerged in a body of deep water. Details are given for the cases of circular or elliptic cross-sections, with estimates for the transmission and reflection constants when the cylinder is many wavelengths below the surface. Corresponding results are suggested for arbitrary smooth cylinders.     

Reflection of surface waves by submerged cylinders

Reflection from submerged cylinders are studied by means of integral equations. By expressing the solution as a distribution of vortices, the integral equations become non-singular for closed contours. It is shown that the method gives a short and easy proof for the classical result that no reflection occurs for the circular cylinder. The reflection power for the elliptic contour and the flat plate are studied when the bodies are situated deeply below the surface.     

Specific features of reverberation in the Black-Sea underwater sound channel

We consider the results of instrumental investigations of specific features of reverberation in the Black Sea and obtain qualitative dependences of the duration of volume reverberation on the parameters of the Black-Sea underwater sound channel, (width, drop of the sound velocity, and dimensions of the inhomogeneities of stratification). We also analyse the behaviour of the intensity of surface reverberation in the far-field zone of acoustic illumination and the influence of bottom reverberation on the detection of underwater objects. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

On the synthetic aperture radar imaging of ocean surface waves

A process of synthetic aperture radar imaging of ocean surface waves is considered on the basis of the two-scale model of microwave scattering by a disturbed sea surface. Analytical expressions are obtained to relate characteristics of a large-scale wave image, averaged over an ensemble of realizations of the small-scale ripple, with the wave, radar, and viewing scheme as parameters. It is shown that the wave image would be defocused as an image of a target moving in the along-track direction with a speed equal to a half of the wave phase speed projection on the line of flight. The defocusing magnitude was measured experimentally for the ocean swells images, obtained with an airborneS-band radar, and the results are found to be in satisfactory agreement with the model prediction.     

Numerical modeling of the long surface waves scattering for the 2011 Japan tsunami: Case study

The March 11, 2011, megaquake caused a catastrophic tsunami recorded throughout the Pacific. This paper presents an analysis of the sea-level records obtained from deep-water tsunami meters (DART and NEPTUNE). To evaluate the effect of the sea-level oscillations’ decay, a statistical analysis of observations and numerical modeling of tsunami generation and propagation have been conducted. The main goal is to uncover physical mechanisms of the tsunami wave field formation and evolution at scales up to tens of thousands of kilometers in space and a few days in time. It is shown that the tsunami lifetime is related to the wave-energy diffusion and dissipation processes. The decay time of the variance of the tsunami-generated level oscillations is about 1 day. Multiple reflections and scattering by irregularities of the bottom topography make the field of the secondary tsunami waves stochastic and incoherent: the distribution of the wave energy in the ocean reaches a statistical equilibrium in accordance with the Rayleigh-Jeans law of equipartition of the wave energy per degree of freedom. After the tsunami front has passed, the secondary-wave energy density turns out to be inversely proportional to the water depth.     

深海浮球内部气压对水下内爆波的抑制机理

深海潜器常携带中空浮球来为其提供浮力。陶瓷因其高强度、低密度等优点成为浮球的理想材料。然而,中空结构在外部高压环境下易发生内爆,产生的内爆波会对周围结构产生毁灭性损害。为了探索浮球内部初始气压对内爆波的影响,首先,利用气泡动力学对其不考虑球壳影响时进行理论分析,得到内爆波压力脉冲沿径向以指数?1 衰减,并指出其物理意义和隐含假设,进而从能量分析得到增加内压使压缩空气消耗的能量增加,从而减弱释放到水中的压力波能量;其次,采用三相流固耦合有限元模型进行计算,考虑水的可压缩性和球壳因挤压引起的脆性破裂的影响,得到更为接近实际的内爆压力的分布。由于两侧挤压球壳,外部的水在不断扩大的缺口处产生向内的射流,造成内部气体非球形塌缩,后续压力波呈现出与球壳碎裂方式有关的方向性差异。通过有限元模型对内部初始气压的研究表明,增加初始内部气体压力到 1 MPa 时,压力脉冲在球壳表面处下降了 15. 6%~24. 8%。这一结果表明,在几乎不增加浮球质量的条件下,增加内部初始气压具有很好地抑制近端内爆波强度的效果。     

水下声学定位中声速改正方法的比较

介绍了水下声学定位原理以及目前常用的几种声速改正方法,提出了一种泰勒级数展开法和声线修正法组合的声速改正方法。通过模拟算例,将加权平均声速法、泰勒级数展开法、等效声速剖面法、声线修正法和组合方法进行了比较,展示声线修正法和组合方法的效果。     

On the generation of lateral internal waves by a surface seiche in a partially mixed estuary

Dyer (1982) observed internal oscillations in Southampton Water of predominantly 8 and 4 min periods. These were thought to be lateral standing waves generated by the interaction of the first- and second-order surface seiches, respectively, with estuarine topography. The present paper extends the work of Rattray (1960) to model the cross-sectional topography of Southampton Water, and shows that internal waves of the observed periods, wavelengths and amplitudes could indeed have been generated in the manner envisaged, but only near the times of certain internal resonant conditions. In general, about two or three such resonances occur per half tidal cycle, and this is in qualitative agreement with Dyer's observations.     

Acoustic scattering by an air-bubble near the sea surface

If the air bubble is near, and strongly interacting, with the (flat) surface of a liquid half-space, then the scattering cross section (SCS) of the bubble differs substantially from its value far-away from the interface. We present the exact solution for this scattering problem which is valid for any incidence direction of the (plane) sound waves, and for any bubble depth, obtained by the general method of images. The bubble is described as a continuous, spatially extended body, having an infinity of modes and resonances. The interaction with the boundary, or the image, is treated by a general, extended continuum approach. This benchmark solution makes use of the addition theorems for the spherical wavefunctions. The resulting SCS contains contributions from the bubble, its image, the incident wave, and the reflected wave from the boundary. It is expressed in terms of coupling coefficients, b_mn, which contain products of Wigner-3j symbols. The formulation is illustrated with many computed plots for bubbles at various depths, and the results compare favorably with earlier experimental observations. The approach also serves to describe scattering by fish near the sea surface, in an exact fashion     

Estimation of underwater scattering radiances up to the third order

Vertical distribution of underwater scattering radiances of the first, the second and the third order is estimated by the Jerlov's method. In the estimation the volume attenuation coefficient and the volume scattering function are assumed to be constant in near surface layer, and the sky radiation separated from solar radiation to be a parallel beam and to fall into the sea in the vertical direction only.In the zenith direction the second scattering radiance is more intense than the first scattering radiance in near surface layer. As for the total radiance which is a sum of radiances up to the third order, the zenith radiation is largest, the nadir radiation is moderate and the horizontal radiation is least.     

顾及声线高度角的水下定位随机模型优化

先验随机模型本身的误差会给参数估计带来随机性影响,水下定位等权随机模型简单却不符合实际。基于这一事实,在水下声纳定位模型和随机模型基础上,讨论了随机模型不完善对参数估计的影响。考虑了影响定位主要误差源与高度角之间的关系,结合统计经验提出了基于声线高度角相关的水下定位随机模型。试验表明,优化后的高度角相关随机模型在定位精度上较等权随机模型有所提高。因此,在水下定位中,应减少低高度角观测值的权重,利用提出的随机模型可以减小高度角观测误差对定位精度的影响。