Numerical modeling of the reflection coefficients for the plane sound wave reflection from a layered elastic bottom

The matrix method and its numerical realization are considered in calculating the complex reflection coefficients and refraction indices of plane sound waves for geoacoustic models of the ocean bottom in the form of homogeneous elastic (liquid) absorbing layers overlying an elastic halfspace. In calculating the reflection coefficients at high frequencies or in the presence of a large numbers of sedimentary layers, a passage from the Thomson-Haskell matrix approach to the Dunkin-Thrower computational scheme is performed. The results of test calculations are presented. With the aim of developing resonance methods for the reconstruction of the parameters of layered elastic media, the behavior of the frequency-angular dependences of the reflection coefficient are studied for various geoacoustic bottom models. The structure of the angular and frequency resonances of the reflection coefficients is revealed. The dependence of the structure (the position, width, and amplitude) of two types of resonances on the parameters of the layered bottom is considered.     

利用海底反射信号进行地声参数反演的方法

针对现有反演方法的缺点,提出了一种基于海底反射信号的地声参数高分辨反演方法.它利用短距离声源在不同深度上发射宽带线性调频信号,采用垂直阵进行接收,首先通过匹配滤波方法提取多径到达信息,然后利用海底反射损失曲线,反演海底表层的声速和密度,最后利用浅底层反射信号估计沉积层参数.由于海水中直达波受到内波的强烈影响,选择海底表面反射作为参考,用以可靠地计算浅底层反射的相对到达时间和幅度,从而估计出沉积层的厚度、速度和衰减系数.通过海上实验,验证了利用浅底层反射信号反演参数的有效性. 关键词： 海底参数 反演 浅底层反射信号     

固-固界面退化特性的超声反射评价方法

提出了一种基于有限宽超声束反射的固-固界面退化特性评价方法,从理论和数值仿真角度进行了分析和计算。将两固体界面间的薄层简化为界面弹簧模型,以界面法向和切向劲度系数表征界面的退化程度。通过数值计算求得有限宽超声纵波束在不同入射角和界面不同退化程度下的反射横波、反射纵波的镜面反射系数。进一步地,通过建立二维有限元模型,仿真研究了有限宽超声纵波束在给定入射角及界面不同退化程度下镜面反射系数的变化规律。结果表明,反射纵波和反射横波的镜面反射系数随有限宽超声纵波束的入射角及界面劲度系数的改变而变化,且存在镜面反射系数随界面劲度系数单调且敏感变化的入射角,据此可准确评价界面的退化程度。     

兰姆波在搭接焊缝上的反射和透射

采用数值模拟方法和实验研究了兰姆波在搭接焊缝上的反射和透射。数值模拟了频率500 kHz的兰姆波S0与.A0模态,分别在2 mm厚不锈钢板上的不同宽度搭接焊缝模型上的反射和透射,并计算了相应的反射系数和透射系数。发现:当搭接焊缝宽度小于4 mm时,S0模态的反射系数和焊缝宽度具有近似的线性关系,而焊缝余高和两板之间的间隙对S0模态反射系数的影响很小。基于兰姆波S0模态反射系数与焊缝宽度的线性关系。提出了通过测量S0模态反射系数快速测量搭接焊缝宽度的方法。实验测得的2 mm厚不锈钢板上不同宽度搭接焊缝对500 kHz的入射兰姆波S0模态的反射系数,验证了上述线性关系,说明了可用兰姆波反射回波测量搭接焊缝宽度。      

Shallow water sound propagation with surface waves

The theory of wavefront modeling in underwater acoustics is extended to allow rapid range dependence of the boundaries such as occurs in shallow water with surface waves. The theory allows for multiple reflections at surface and bottom as well as focusing and defocusing due to reflection from surface waves. The phase and amplitude of the field are calculated directly and used to model pulse propagation in the time domain. Pulse waveforms are obtained directly for all wavefront arrivals including both insonified and shadow regions near caustics. Calculated waveforms agree well with a reference solution and data obtained in a near-shore shallow water experiment with surface waves over a sloping bottom.     

Resonant reflection of a series of submerged breakwaters

Summary The phenomenon of the resonant reflection of a train of monochromatic waves by an undulated bottom has been largely studied, but in most cases within the hypothesis of smooth variations of the bottom behaviour. Here we consider the case of a series of equally spaced submerged breakwaters of rectangular section, in order to investigate the role of the evanescent modes, neglected in the previous studies, and to evaluate the response of a structure that could be of interest from an engineering point of view. It has been found that the resonant reflection does not occur only in correspondence with the waves of length twice the wavelength of the Fourier components of the bottom behaviour, but also in correspondence with waves of the same length as the distance between the breakwaters. Furthermore, the strength of the resonant reflections,i.e. the height and width of the peaks in the response curve, seems to depend merely on the total amount of material employed in the construction of the breakwaters.     

The reflection and transmission of Lamb waves at overlap joints

The reflection and transmission of Lamb waves at overlap joints are researched by the numerical method and the experiments. The numerical method is used to simulate the reflection and transmission of Lamb waves at welds with different widths on 2 mm-thick stainless steel plates. The reflection and transmission coefficients are calculated as well. When the welds width is less than 4 mm, a quasi-linear correlation is observed between the reflection coefficient of SO mode and the weld width. In contrast to the weld width, both the gap between two plates and the reinforcement height of weld do not have obvious effect on the reflection coefficient. Based on the results, we propose that the weld width could be rapidly measured through examining the echo amplitude of SO mode. Experiments further confirm the quasilinear correlation and the feasibility of our proposed method.     

浅海海底反射系数幅值参数的反演

理论分析了一种通过混响强度衰减特性获取海底反射系数的幅值参数的方法.将海底反射系数的幅值参数和相位参数引入到全波动混响模型中,为海底反射系数的反演提供理论基础。理论分析和数值仿真表明,在小掠射角条件下,利用混响强度衰减特性反演海底反射系数幅值参数的可行性和准确性。该反演方法只需要输入4个变量:本地混响强度的衰减特性,反射系数的相位参数,海深以及海深处的声速,同时要求混响数据具有一定的混响噪声比(大于6 dB)才能够使反演结果准确可信。根据本地静态混响实验数据成功反演得到海底反射系数的幅值参数.      

Method of reflectorβ reflection

A method of reflector reflection is proposed which permits repeated reflection on radiation when only a single surface of the test object is accessible for measurements; it involves the introduction into the gap between reflections from the test object of an auxiliary reflection from a lead or bismuth plate. Experiments are reported which show that the energy and intensity of the reflector reflection are sufficient for detection and that the intensity has a nearly quadratic dependence on the atomic number Z. Instruments are described for analysis by this method.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 6, pp. 111–115, June, 1969.The author thanks Professor A. A. Zhukhovitskii, M. M. Senyavin, and V. I. Bakulin for organizational assistance and valuable advice; and N. S. Podsekal'nikov for preparing the experimental apparatus.     

噪声修正的混响反演海底反射系数幅值参数

利用一种噪声修正方法解决低混噪比条件下混响数据反演海底反射系数幅值参数偏差的问题。将全波动混响理论中的混响非相干平均强度,叠加海洋环境噪声平均强度,获得包含噪声的混响平均强度理论,分析背景噪声平均强度对反演海底反射系数幅值参数的影响。数值仿真表明,背景噪声平均强度的叠加使得混响平均强度衰减趋势减缓,导致海底反射系数幅值参数反演结果偏小,混响噪声比越低,影响越明显。提出一种噪声平均强度相减的修正方法,对混响噪声比为[0,6] dB的混响平均强度数据进行修正,利用修正后的混响数据反演得到偏差较小的海底反射系数幅值参数。对比浅海混响实验数据衰减至不同混噪比时反演所得海底反射系数幅值参数,以及经噪声修正后的反演结果,验证了本文提出方法的可行性。      

A Simple Method for Source Depth Estimation with Multi-path Time Delay in Deep Ocean

A method of source depth estimation based on the multi-path time delay difference is proposed.When the minimum time arrivals in all receiver depths are snapped to a certain time on time delay-depth plane,time delay arrivals of surface-bottom reflection and bottom-surface reflection intersect at the source depth.Two hydrophones deployed vertically with a certain interval are required at least.If the receiver depths are known,the pair of time delays can be used to estimate the source depth.With the proposed method the source depth can be estimated successfully in a moderate range in the deep ocean without complicated matched-field calculations in the simulations and experiments.     

Quantum reflection as the reflection of subwaves

This paper studies quantum reflection with recent research on reflection coefficient. Based on the analytical transfer matrix method, a novel explanation for this phenomenon is proposed that quantum reflection is the reflection of subwaves, which originate inherently from the inhomogeneity of the fields and is always neglected in the semiclassical regime. Comparison with exact formula and the numerical calculations for different potentials has confirmed the reliability and the validity of the proposed theory.     

The reflection of guided waves from simple supports in pipes

Successful ultrasonic guided wave detection of flaws at support locations relies on the ability to distinguish between the reflection produced by a simple support on an undamaged pipe and the reflection produced by pipe flaws. Consequently, it is essential to know how the reflections produced by simple supports behave; very little work has so far been reported on this subject. Through finite element simulations and experiments, this study develops a systematic understanding of how ultrasonic guided waves propagating along a pipe, in particular the T(0, 1) mode, interact with simple supports. It is shown that, unlike the T(0, 1) mode in a free pipe, the torsional mode in a supported region has a cut-off frequency, below which it will not propagate; below this frequency the T(0, 1) reflection coefficient is large, and it quickly reduces beyond the cut-off.     

Diffraction efficiency of sequentially stored gratings in reflection volume holograms

On the basis of the theory of coupled waves the amplitudes of the first-order diffracted waves and the diffraction efficiency is calculated for sequentially superposed phase gratings stored in dielectric reflection holograms. The investigations are restricted to the case of a fixed reference wave during recording and Bragg-angle incidence.By analogy to the transmission volume grating the coupling effects which appear between incident beams and diffracted waves give the possibility of building up beam-couplers and -splitters with certain coupling parameters determined by the recording process. Moreover, reflection gratings allow a favourable combination between coupling and reflecting properties and avoid the diffraction efficiency oscillations that are characteristic of transmission gratings. The performance of the reconstruction process with several waves simultaneously allows the tuning of the intensities of diffracted waves with dependence on amplitude or phase differences between incident beams.     

Ambartsumian's principle of invariance and the reflection of radio waves from plane inhomogeneous slabs

Using Ambartsumian's principle of invariance we investigate for radio waves the reflection coefficient of a plane inhomogeneous slab. We find that the reflection coefficient, as a function of slab thickness, satisfies the Riccati equation. From this equation we deduce a geometric theorem on the upper and lower bounds of the reflection coefficient. We illustrate the theorem by applying it to several special cases.     

The effect of temperature on sound wave absorption in a sediment layer

The effect of temperature on sound velocity, absorption, and reflection coefficient in the seabed sediment layer is investigated. Experimental measurements of sound speed, absorption, and the reflection coefficient in a sandy sediment layer have been carried out at several temperatures. An absorption reduction of 75 dB/m and a velocity increase of 65 m/s have been measured at a frequency of 1 MHz when the temperature increases from 5 to 25 degrees C. Because of the absorption temperature dependence the amplitude of the reflected wave from the back surface of the sub-bottom layer after going back and forth across the layer increases with the temperature.     

一种计算HEMP环境下电缆感应电流的混合算法北大核心CSCD

对高空核爆电磁脉冲(HEMP)环境下、地面上方电缆的感应电流进行了研究。考虑有耗地面的影响,提出了一种将时域积分方程(TDIE)和时域传输模式法结合求解有耗半空间上方电缆瞬态电流的时域方法。通过电磁波本征模的关联性得到斜入射有耗半空间平面波的时域反射系数,不需计算贝塞尔函数的无穷项和便可以得到时域反射系数的精确解。最后给出了核爆脉冲照射下地面上方电缆感应的电流的仿真结果。结果表明:对于水平放置的电缆,当反射脉冲到达电缆后,反射场总是试图抵消入射场,使得电缆上感应电流小于只考虑入射脉冲时的感应电流;电缆的耦合电流会随架设的高度发生显著变化,相对于铺地的电缆,架空的电缆有更大的感应电流。     

Reflection of spin waves from a ferromagnetic multilayer with interfacial coupling of finite strength (reflection of spin waves from multilayer)

The reflection coefficient of bulk spin waves from a ferromagnetic multilayer with periodically modulated parameters of the exchange interaction, the uniaxial magnetic anisotropy and the saturation magnetization (a magnonic crystal) is calculated. The dependence of the reflection coefficient upon the spin wave frequency and the values of the bias magnetic field, the parameter of interfacial coupling, and the internal structure of the unit cell are investigated.      

利用海洋环境噪声直接提取海底反射系数

提出了一种利用海洋环境噪声直接(非迭代)提取海底反射系数的方法,基于海洋环境噪声垂直相关函数与方向密度函数的相似性,结合海洋环境噪声射线理论,由两个垂直排列的水听器实现海底反射系数的被动获取.方法适应性的仿真分析说明较高海况下且近场无航船干扰的海洋环境噪声数据有利于方法的实现;对于两层海底,海洋环境噪声垂直相干函数经高...     

Interference effects in the conical emission of a femtosecond filament in fused silica

It is shown both experimentally and theoretically that interference effects play the key role in the formation of frequency-angular spectrum of the filament conical emission. For the first time, we investigated experimentally the transformation of the conical emission frequency-angular spectrum with an increase in the filament length inside fused silica. We discovered the appearance of fine structure of the conical emission rings produced by lengthy filament. It is shown that the conical emission frequency-angular spectrum is produced by interference of coherent radiation from one or several moving point sources in the filament. The shape of the conical emission spectrum depends on the medium material dispersion, the spectrum structure is determined by length and relative location of filament emitting regions.