二维变密度声波波动方程的衍射层析成像

本文将二维声波衍射层析成像原理推广到二维变密度声波波动方程中，导出了在线源激发条件下，不同观测系统的滤波反传播重建算法，取两个以上不同频率值可得到密度和体积模量的成像．数值实例表明，该方法能得到密度和体积模量较满意的成像。     

Finite element method for strong reflection of water waves

In this study, the propagation of monochromatic water waves over an arbitrarily varying topography is numerically investigated. A finite element model is developed by formulating the diffraction of waves caused by depth changes. Not only the propagating mode but also the evanescent modes are included in the model. The model developed is applied to the study of strong reflection of monochromatic waves over a sinusoidally varying topography. Predicted reflection coefficients are compared with those of available laboratory experiments and the eigenfunction expansion method. A very good agreement is observed.     

浅析干涉与衍射的差异

本文从光的传播是否遵守几何光学定律,以及解两者问题的数学工具不同、条纹分布不同等特点来分析干涉与衍射的差异。     

Application of the edge wave superposition method

Numerical examples of high-frequency synthetic seismograms of body waves in a 2-D layered medium with complex interfaces (faults, wedges, curvilinear, corrugated) are presented. The wave field modeling algorithm combines the possibilities of the ray method and the edge wave superposition method. This approach preserves all advantages of the ray method and eliminates restrictions related to diffraction by boundary edges and to caustic effects in singular regions. The method does not require two-point ray tracing (source-to-receiver), and the position of the source, as well as the type of source, and the position of receivers can be chosen arbitrarily. The memory and the time required for synthetic seismogram computation are similar to ray synthetic seismograms. The computation of the volume of the medium (the Fresnel volume or Fresnel zones), which gives the essential contribution to the wave field, is included in the modeling program package. In the case of complicated irregular interface (or a layered medium with a regular ray field at the last interface), the method displays a high accuracy of wave field computation. Otherwise, the method can be considered a modification of the ray method with regularization by the superposition of edge waves.     

Phase-decoupled refraction–diffraction for spectral wave models

Conventional spectral wave models, which are used to determine wave conditions in coastal regions, can account for all relevant processes of generation, dissipation and propagation, except diffraction. To accommodate diffraction in such models, a phase-decoupled refraction–diffraction approximation is suggested. It is expressed in terms of the directional turning rate of the individual wave components in the two-dimensional wave spectrum. The approximation is based on the mild-slope equation for refraction–diffraction, omitting phase information. It does therefore not permit coherent wave fields in the computational domain (harbours with standing-wave patterns are excluded). The third-generation wave model SWAN (Simulating WAves Nearshore) was used for the numerical implementation based on a straightforward finite-difference scheme. Computational results in extreme diffraction-prone cases agree reasonably well with observations, analytical solutions and solutions of conventional refraction–diffraction models. It is shown that the agreement would improve further if singularities in the wave field (e.g., at the tips of breakwaters) could be properly accounted for. The implementation of this phase-decoupled refraction–diffraction approximation in SWAN shows that diffraction of random, short-crested waves, based on the mild-slope equation can be combined with the processes of refraction, shoaling, generation, dissipation and wave–wave interactions in spectral wave models.     

Simulation of diffraction of ocean waves by a submerged sphere in finite depth

Based on the linear diffraction theory, an investigation is made on the interaction of water waves with a completely submerged sphere in water of finite depth in this paper. The method of multipole expansions is used to obtain the fluid velocity potential in the form of double series of the associated Legendre functions with the unknown coefficients of the infinite set of infinite matrix equations. The truncation property of the matrices and the convergence of the multipole series coefficients are investigated for various wavelengths and depths. The systematic numerical simulation, based on our analytical solution, is carried out and the fields of the hydrodynamic diffraction pressure and fluid velocity around the sphere, the three-dimensional free surface elevation, and total exciting forces acting on the sphere are graphically presented for a wide range of the body submergences, ocean depths and wavelengths.     

衍射CT技术和多源全息成象技术的比较研究

在本文中,我们从理论上比较研究了衍射CT技术和多源全息成象技术,讨论了在弱不均匀介质中多源全息成象技术失真的根源,推导出二维多源全息图象重建的快速算法,利用这些算法和计算机产生的非Born近似的正演数据,系统地研究了异常体的尺寸、异常程度的大小和数据采集方式对重建图象的影响.并且把它与衍射CT重建图象作了数值上的比较,所得到的许多结果（见正文结论部分）对于衍射地震CT技术和多源全息成象技术在生产中的应用有着指导意义.     

衍射地震CT技术的数值研究

本文首先解决了声波方程的非Born近似的正演计算问题,从而获得理论上不带近似的正演数据;然后,推导了井间（CBP）、垂直地震剖面（VSP）和地面反射（SRP）三种不同的数据采集方式下的衍射CT的重建公式;利用这些重建算法和正演数据,系统地研究了影响到地球物理CT成象质量的三种因素,即:（1）数据采集方式,（2）异常程度和（3）成象区域的尺寸,对重建图象的影响;并比较了衍射地震CT和射线地震CT的成象质量。     

二维层状介质中衍射波的计算

本文讨论了衍射波的计算,它是文献[5]的继续和发展,是为了应用射线法解决二维地震人机联作工作的一部分。根据波的物理性质,引入了等效炮点、等效检波点和等效界面等概念,将Berryhill导出的衍射波计算公式推广到倾斜界面和楔形结构等情形。这一工作与文献[1-3]中的工作相结合,在实用上解决了二维地震人机联作解释中的正演计算方法问题。     

Experimental study of wave loads on a small vehicle in close proximity to a large vessel

Operations involving the launch or recovery of a smaller vessel from a larger one are extremely dangerous in high sea states and, therefore, they are normally carried out in low to moderate sea states. However, this can be severely restrictive and in some situations, carrying out such operations in high sea states is unavoidable. Here we report on a detailed investigation of the interaction between two vessels of different size in order to characterise their hydrodynamic interaction under different conditions and to provide insight for operational purposes. Model experiments were conducted to investigate the hydrodynamic interaction between two vessels in close proximity in waves. Previous studies into this interaction have focused on two vessels with comparable size/displacement. This study focused on the interaction between vessels of very different sizes, a platform supply vessel and a lifeboat, at various separation distances between the two models and wave headings. It is found that the effect of the hydrodynamic interaction on the wave loads on the lifeboat model is substantial. The load responses show a strong non-linearity (high order harmonic components). In head waves, the effect of the hydrodynamic interaction on the wave loads is greater in the transverse modes (sway, roll and yaw) than in the longitudinal modes (surge, heave and pitch). The sheltering effects of the larger model on the lifeboat model were also evident from the experiments. The results of this investigation may be used to inform the planning of marine operations, such as the launch and recovery of a lifeboat or an Autonomous Underwater Vehicle (AUV) from a mothership and the transfer of equipment or personnel between vessels. The data will also provide a useful resource for validation of Computational Fluid Dynamics (CFD) codes and other numerical simulations, and can be used to better understand the limitations and potential widening of the operational weather windows and to ensure that operations are carried out safely.