Acoustic and radar scattering from directional seas

The small-slope approximation is applied to predict acoustic and electromagnetic scattering from directional seas. Results are presented for the scatter of high-frequency fields from fetch-limited seas for which the wavenumber spectrum is isotropic at high wavenumbers but highly directional near the spectral peak. Monostatic backscatter is found to display an upwind-crosswind dependence for a broad range of scattering angles due solely to the directionality of the large-scale waves.     

Acoustic and radar scattering from directional seas

Abstract

The small-slope approximation is applied to predict acoustic and electromagnetic scattering from directional seas. Results are presented for the scatter of high-frequency fields from fetch-limited seas for which the wavenumber spectrum is isotropic at high wavenumbers but highly directional near the spectral peak. Monostatic backscatter is found to display an upwind-crosswind dependence for a broad range of scattering angles due solely to the directionality of the large-scale waves.     

油砂混合水体后向散射系数光谱贡献分离算法Ⅰ: 实验理论

在石油类污染水体中,油会吸附在悬浮颗粒物表面而形成一个双层结构,影响水体后向散射系数光谱特征, 分离水体石油类物质与悬浮颗粒物对后向散射系数光谱的贡献, 能提高水体石油类污染后向散射理论模型的准确性。将美国Wyatt公司生产DAWN HELEOS Ⅱ18角度散射测量仪、美国SEQUOIA公司生产的LISST-100x B粒径仪和美国Hobilabs公司的后向散射仪HydroScat-6 Sprctral Backscattering Sensor(HS6)联动观测,构成后向散射系数光谱测量系统,分别测量不同水样的散射强度电压值、粒径分布及粒径浓度、后向散射系数等参数,提出了利用Mie散射理论计算未知折射系数物质的体散射函数β(λ,θ)的新思路及分离后向散射系数光谱的算法。选择已知折射系数m的石英砂作为颗粒物与采自不同油田区域的油污水进行配比,获取不同特性水样,测定相关数据。首先, 根据Mie散射理论计算出各样本对应的水体体积散射函数β(λ,θ);其次,建立的DAWN HELEOS Ⅱ 18角度激光散射仪测定散射强度对应的电压值V(θ)转化为体积散射函数β(λ,θ)的关系式;再次,根据最优化方法估算出油砂混合的等效折射系数m_os以及油的折射系数m_o;最后,利用β(λ,θ)和估算的m_os值及m_o计算出各类样本的后向散射系数b_b(λ),分别建立油污水b_b,o(λ)和石英砂b_b,s(λ)与油砂混合总b_b,os(λ)的分离算法。分离算法的建立一方面提高了水体石油类污染后向散射理论模型的准确性,另外一方面拓展了米散射理论在海洋水色遥感中的应用。     

Measurements of bubbles in sea water by nonstationary sound scattering

Methods for the characterization of bubbles in sea water by acoustic scattering are analyzed. Nonstationary linear and nonlinear sound scattering methods are proposed. The transient linear and nonlinear sound scattering allows the scattering by resonant gas bubbles to be distinguished from the scattering by other microinhomogeneities. The application of parametric arrays in oceanic experiments, together with the broadband frequency analysis of the backscattering coefficient, allows information about bubbles in sea water to be obtained. Experimental results on sound scattering and gas bubble distribution functions are presented for different conditions in the ocean.     

Controlling Decoherence Speed Limit of a Single Impurity Atom in a Bose–Einstein‐Condensate Reservoir

The decoherence speed limit (DSL) of a single impurity atom immersed in a Bose‐Einstein‐condensed (BEC) reservoir when the impurity atom is in a double‐well potential is studied. It is demonstrated how the DSL of the impurity atom can be manipulated by engineering the BEC reservoir and the impurity potential within experimentally realistic limits. It is shown that the DSL can be controlled by changing key parameters such as the condensate scattering length, the effective dimension of the BEC reservoir, and the spatial configuration of the double‐well potential imposed on the impurity. The physical mechanisms of controlling the DSL at root of the spectral density of the BEC reservoir are uncovered.     

Low-frequency sound absorption and attenuation in marine medium

Original experimental data are analyzed on the low-frequency sound attenuation in the Mediterranean, Black, and Baltic Seas, Sea of Japan, and the north-western region of the Pacific Ocean. In these regions, waters significantly differ in their temperatures and salinities. The analysis is aimed at obtaining an expression for calculating the low-frequency absorption coefficient in sea water. The analysis uses the previously published data on the measured (by the temperature discontinuity method) low-frequency relaxation times associated with boron present in sea water. The dependence of the absorption on the pH value (which was revealed in the 1970s) and the experimental data on sound absorption at frequencies higher than 5–10 kHz are also taken into account. As a result of the analysis based on the assumption that low-frequency relaxation takes place, an expression is proposed that relates the low-frequency absorption to the temperature, salinity, and pH value and equally well describes the experimental frequency dependences of attenuation for the four regions at hand (except for the Baltic Sea). Increased attenuation coefficients are noticed for shallow seas and deep-water regions where waters are influenced by intense currents, strait zones, and zones of mixing waters of different origin, i.e., for the ocean areas where, in addition to the attenuation, sound scattering by inhomogeneities of the marine medium and sound energy leakage into the sea floor are significant.     

Acoustic manifestation of bubbles frozen in an ice sheet

Hydrocarbon sources on the ocean floor produce buoyant bubble plumes, i.e., gas flares. In winter, bubbles reaching the surface freeze in an ice sheet. Such clouds of frozen bubbles are observable in Arctic seas and are usual elements of ice sheets of lakes, e.g., Lake Baikal. Based on the general solution of the problem of scattering by a sphere in an isotropic elastic medium, the frozen bubble scattering cross section is found. The theory of multiple scattering by frozen bubble plume is derived. The structure of low-frequency resonances corresponding to collective oscillations of a bubble cloud is described.     

海面冰层对声波的反射和散射特性

北极海面冰层复杂多变,其对声波的反射和散射严重影响冰下水声信道的传输特性,建立海面冰层的声波反射和散射模型对冰下水声通信研究具有重要意义.假设海面冰层为多层固体弹性介质且冰-水界面粗糙,满足微扰边界条件,导出声波从海水介质入射到海面冰层时相干反射系数满足的线性方程组.对相干反射系数随声波频率、掠射角、冰层厚度的变化进行数值分析.进一步引入根据散射声场功率谱密度计算散射系数的方法,改变掠射角,对冰层厚度、散射掠角对散射系数的影响进行研究.     

Studies of the ocean-bottom reflection coefficient at angles of total internal reflection

For practical-purpose studies in underwater acoustics, a new method is proposed to determine the bottom reflection coefficient on the basis of multiple bottom-surface reflections. The method allows one to obtain the angular dependence of the bottom reflection coefficient at grazing angles from several degrees to several tens of degrees in the audio and infrasonic frequency bands. The sound field structure is studied in deep-water regions of the Black (2000 m) and Arabian (4000 m) seas at frequencies within 10–400 Hz. For the regions under investigation, the frequency-angular dependence of the reflection coefficient is obtained with the use of the proposed method. The data for the Black Sea are compared with those provided by the conventional method based on the use of single and double bottom and bottom-surface reflections. Experimental data on the values and variations of the bottom reflection coefficient are presented for different deepwater and shallow-water regions of the World Ocean. The presence of shear waves in the bottom sediments is revealed, and the effect of these waves on the frequency-angular dependence of the reflection coefficient is demonstrated.     

修正反射系数的Kirchhoff近似方法计算高斯起伏冰面三维声散射

针对随机起伏冰面的声散射计算问题,利用修正反射系数的Kirchhoff近似方法计算了高斯起伏冰面的三维声散射。在计算模型中引入了冰面局部统计平均反射系数的概念,将二维高斯起伏冰面的散射分为相干散射和非相干散射,分别得到两类散射成分的散射系数公式,计算了高斯起伏冰面三维声散射的散射强度。分析了散射强度与随机起伏冰面的均方根高度、声波入射角度及频率的关系。通过实验室水池中高斯起伏冰面的散射强度测量实验,对理论模型的计算结果进行了验证。将实验结果分别与采用冰面局部统计平均反射系数的模型计算结果和文献中采用平整冰面镜反射系数的模型计算结果进行了对比,采用冰面局部统计平均反射系数的模型计算结果与实验测量值吻合较好。      

水雾包裹沙尘颗粒核壳结构的散射特性研究

基于离散偶极子近似法(DDA),对水雾包裹沙尘颗粒的核壳结构光学特性进行研究,计算了长短轴比例为2∶1的椭球形粒子的核壳结构内、外层厚度及散射角度变化对光散射特性的影响。结果表明,内核大小不变,外层厚度由1.2 μm增大到4.8 μm,核壳双层颗粒散射系数和消光系数由3.4和3.43降低到2.543和2.545,且散射相对强度也明显增大。外层厚度不变,内核厚度由0.6 μm增加到2.4 μm,散射系数和消光系数由3.105和3.111变化为2.76和2.9;可见外层厚度对核壳双层颗粒散射特性的影响更大,这是由于散射光主要与外层物质相互作用引起的。散射相对强度随波长的增加而降低,随核壳结构尺度的增加呈现递增的规律。该结果对大气中气溶胶和水雾共同作用时的散射特性,激光在其中的传输特性等研究有参考价值。     

Translational diffusion of water in compacted clay systems

The translational diffusion of water in compacted clays at a high hydration level has been investigated by quasielastic neutron scattering at a time-of-flight spectrometer FOCUS (SINQ). Four compacted clays with systematic structural differences have been studied: Na-montmorillonite, Na-illite, kaolinite and pyrophyllite. The QENS experiments were performed using two different incident wavelengths in order to access a larger Q range and verify the data analysis. The translational diffusion coefficient for water in Na-montmorillonite and Na-illite are lower than those for bulk water, whereas the preliminary results for kaolinite and pyrophyllite show larger diffusion coefficient.     

氘含量对DKDP晶体横向受激拉曼散射增益系数的影响

本文测量了不同氘含量K(H_1-xD_x)₂PO₄晶体(DKDP晶体)在Z(XX)Y散射配置下的自发拉曼散射光谱, 并详细分析了氘含量对与横向受激拉曼散射(TSRS)增益系数有关的拉曼频移、半峰宽和散射强度的影响. 然后通过与去离子水拉曼散射对比得出了不同氘含量DKDP晶体的TSRS增益系数. 结果表明随着氘含量的增加DKDP晶体的TSRS增益系数先减小至KDP晶体的40.1%, 后增大至68.9%; 本文认为掺氘后拉曼半峰宽的变化是引起TSRS增益系数随氘含量变化的主要原因.     

Identification of radiative properties of reticulated ceramic porous inert media using ray tracing technique

The radiative properties of reticulated porous inert media are computationally identified using the real three-dimensional structural data of porous media. The computational grids data are reconstructed from three-dimensional computer tomography scans and magnetic resonance image scans of different reticulated porous media. A ray tracing algorithm is used to track the rays inside the grid structure. Statistically large numbers of rays are traced for their path length and incident angle, which are used to find the probability based equivalent extinction coefficient and scattering phase function. The equivalent extinction coefficients are found for porous media with different porosities and pore densities. The dependency of specular and diffuse scattering phase functions on the porous structure and surface reflectance are also studied.     

Oscillations of a statistical scattering in the Rayleigh limit and the Rayleigh law violation

Oscillations of the Rayleigh wave statistical scattering in the Rayleigh limit are theoretically found. These oscillations are violation of the Rayleigh law of scattering. They are caused by the diffraction of a new form - the extraordinary scattering in the Rayleigh limit, which leads to the different frequency dependencies of the scattering coefficient and thus violates the Rayleigh law of scattering. The fundamental physical conception that a wave does not sense the structure of an irregularity in a long-wavelength scattering, when the wavelength is much greater than the character size of the irregularity, i.e. in the Rayleigh limit, is violated as well.     

Quasi-elastic laser light scattering study of polyacrylamide hydrogel immersed in water and salt solutions

Polyacrylamide (PAAm) hydrogels immersed in water and aqueous NaCl solutions were investigated for their structure and dynamics using static and quasi-elastic laser light scattering (QELS) techniques. Ensemble-averaged electric field correlation function f(q, t) obtained from the non-ergodic analysis of intensity-autocorrelation function for PAAm gel immersed in water and in 5 M NaCl showed an exponential decay to a plateau with an initial decay followed by saturation at long times. The value of the plateau was found to depend on NaCl concentration and was higher than that of water. Collective diffusion coefficient, D, of the polymer network of the hydrogel immersed in water and in different concentrations of NaCl was determined by analysing f(q, t). The measured diffusion coefficient showed linear decrease with increase in concentration of NaCl. The characteristic network parameters were obtained by analyzing f(q, t) with harmonically bound Brownian particle model and from static light scattering studies.     

Acoustical method for measuring the gas content in bottom sediments

Gas bubbles in the bottom of shallow-water seas and freshwater basins lead to a substantial decrease in the sound speed. The gas content in the bottom changes because of the variability of the biological activity and water contamination, therefore serving as an indicator of the environmental equilibrium in the basin. It is proposed to monitor the gas content in bottom sediments by measuring the frequency spectrum of the bottom reflection coefficient in a broad frequency band. The data of calculations are presented for the sound speed in gas-containing sediments and for the values of the bottom reflection coefficient. A method is proposed for monitoring the gas content by measuring the frequency and angular characteristics of the natural ambient noise.     

水云增长过程中的云滴谱及散射特性分析

基于大气物理学研究了水云云滴增长过程中的粒谱及散射特性.研究结果表明,凝结增长使粒谱半高宽和有效半径不断增加,碰并增长使粒谱出现多峰分布,有效半径增加.在凝结增长和碰并增长共同作用下,有效半径的平均增长速率为8 nm/s.凝结增长和碰并增长单独作用下,消光系数和散射系数随时间呈线性变化.在二者共同作用下,除3.2 mm波长外,消光系数和散射系数随时间呈指数增长;1.064, 2.2, 3.7, 12和22μm波长的不对称因子逐渐趋于稳定,200μm的不对称因子呈指数增长,3.2 mm的不对称因子基本保持不变;1.064和2.2μm波长的雷达比在20 sr附近波动,3.7μm波长的雷达比呈大幅振荡.云滴增长过程中,水云在1.064, 2.2和3.7μm波长的单次散射反照率逐渐降低,在12μm, 22μm, 200μm和3.2 mm波长的单次散射反照率逐渐增加,波长指数的绝对值逐渐减小.研究结果可为天气预报、地气辐射平衡研究和遥感数据校正提供重要的参考.