基于光弹测试技术的颗粒材料细观变形试验研究

针对颗粒材料剪切变形的细观尺度研究,介绍了基于密度匹配原理的颗粒实现完全移除底部摩擦力的光弹试验技术。通过对密实度为82%的光弹颗粒系统进行直剪变形研究,研究结果表明颗粒系统在直剪试验过程中,力链的主方向即主应力方向沿着剪切室短对角线方向发展,并贯穿于整个颗粒系统后在剪切室的边界处达到应力平衡; 而颗粒系统中各颗粒的位移场在归一化后表现出很好沿剪切室中轴的对称性。     

用于地震波场模拟的PML边界衰减因子研究(英文)

在地震波场数值模拟中,为了消除由人为边界产生的边界反射,需要引进边界吸收条件。本文从声波方程完全匹配层吸收边界的经典方法出发,基于高斯函数任意阶光滑的特点,提出了一种高斯型衰减因子,分析比较该因子与一般衰减因子的性质,并基于均匀与层状速度模型分别进行了数值模拟计算。数值结果显示,当选择相同的PML边界吸收层层数时,高斯型衰减因子的吸收效果明显优于一般的衰减因子,边界反射更少,信噪比更高;对比最近提出的正弦型衰减因子,在信噪比接近的情况下,高斯型衰减因子所需的PML吸收层层数更少。     

高阶旋转交错网格有限差分方法模拟TTI介质中横波分裂

笔者给出了一种能够模拟弹性波在任意各向异性介质中传播的二维三分量高阶有限差分算法.相对于常规交错网格有限差分方法,旋转交错网格有限差分方法在介质具有强差异性时能更精确地模拟地震波的传播,避免常规交错网格中因对弹性系数进行插值而带来的误差.采用高阶旋转交错网格有限差分方法模拟并分析了零偏移距横波分裂现象随裂缝介质方位角和倾角变化的响应特征.结果表明:结合完全匹配层(PML)吸收边界条件的高阶旋转交错网格有限差分方法能获得高精度的地震波场模拟数据,并且在边界具有良好的吸收效果;横波分裂现象主要受裂缝走向与波的极化方向之间的夹角影响,受裂缝倾角影响较小,且快慢横波的能量也跟裂缝走向与波极化方向间的夹角有关.具有倾斜对称轴的横向各向同性(TTI)介质倾角的变化可能会导致记录中波到达时的变化,影响快慢横波的时差.利用横波分裂的能量分布和方位各向异性特征,可以帮助检测裂缝的方位角和倾角.横波在多层TTI介质中传播时会发生多次分裂的现象.     

提取精密控制震源信号的自适应加权匹配滤波方法

为克服短时突发噪声对提取精密控制震源信号的影响, 本文研究并提出了加权匹配滤波方法。 通过评估记录中的噪声时间分布取得相对应的信号段的权重, 对噪声水平高的时段分配较小的权重, 噪声水平较低的时段分配较大的权重, 进行加权匹配滤波, 达到降低短时突发噪声的影响、 有效提高输出信号信噪比的目的。     

Application of a perfectly matched layer in seismic wavefield simulation with an irregular free surface

Recently, an effective and powerful approach for simulating seismic wave propagation in elastic media with an irregular free surface was proposed. However, in previous studies, researchers used the periodic condition and/or sponge boundary condition to attenuate artificial reflections at boundaries of a computational domain. As demonstrated in many literatures, either the periodic condition or sponge boundary condition is simple but much less effective than the well‐known perfectly matched layer boundary condition. In view of this, we intend to introduce a perfectly matched layer to simulate seismic wavefields in unbounded models with an irregular free surface. We first incorporate a perfectly matched layer into wave equations formulated in a frequency domain in Cartesian coordinates. We then transform them back into a time domain through inverse Fourier transformation. Afterwards, we use a boundary‐conforming grid and map a rectangular grid onto a curved one, which allows us to transform the equations and free surface boundary conditions from Cartesian coordinates to curvilinear coordinates. As numerical examples show, if free surface boundary conditions are imposed at the top border of a model, then it should also be incorporated into the perfectly matched layer imposed at the top‐left and top‐ right corners of a 2D model where the free surface boundary conditions and perfectly matched layer encounter; otherwise, reflections will occur at the intersections of the free surface and the perfectly matched layer, which is confirmed in this paper. So, by replacing normal second derivatives in wave equations in curvilinear coordinates with free surface boundary conditions, we successfully implement the free surface boundary conditions into the perfectly matched layer at the top‐left and top‐right corners of a 2D model at the surface. A number of numerical examples show that the perfectly matched layer constructed in this study is effective in simulating wave propagation in unbounded media and the algorithm for implementation of the perfectly matched layer and free surface boundary conditions is stable for long‐time wavefield simulation on models with an irregular free surface.     

OBC与拖缆数据的子波域匹配滤波方法研究

海底电缆（OBC）采集方式的出现,导致OBC采集与拖缆采集数据进行拼接成为三维连片地震资料处理中的一个重要环节.由于采集方式不同,数据在振幅、频率、相位等方面均存在较大差异.为了消除和减小这些差异,为后续的地质解释和油气预测提供便利,本文提出了一种在子波域的1范数匹配滤波方法来计算匹配滤波因子,以用来提高OBC数据和拖缆数据的匹配精度,改善连片处理的数据品质,为后续的地质解释和油气预测提供便利.通过实际资料检验,子波域匹配滤波方法在OBC与拖缆数据拼接应用中取得了较好的效果.     

基于平均导数方法的声波方程频率域高阶正演

本文首先阐明了基于旋转坐标系的频率域正演算法只能适用于相同横纵向空间采样间隔的局限性,并发展了一种新的基于平均导数方法（average-derivative method,简称ADM）的25点有限差分格式来实现声波方程频率域高精度正演.这种基于平均导数方法的算法将声波频率域方程中空间导数项的差分近似表示为正交方向上5个网格点的加权平均形式,能适用于不同的横纵向空间采样间隔,因此能作为四阶声波频率域正演的一种统一格式,具有很好的适用性.通过优化方法求取空间导数项和加速度项的加权优化系数,从而使数值频散达到极小化,每个波长所需要的网格点数在1%的误差范围内仅为2.78个网格点数.本文通过引入完全匹配层（perfectly matched layer,简称PML）吸收边界条件,有效地消除了人工边界反射.数值模拟结果验证了本文25点ADM算法的有效性和准确性.     

Seismic wave modeling in viscoelastic VTI media using spectral element method

Spectral element method (SEM) for elastic media is well known for its great flexibility and high accuracy in solving problems with complex geometries. It is an advanced choice for wave simulations. Due to anelasticity of earth media, SEM for elastic media is no longer appropriate. On fundamental of the second-order elastic SEM, this work takes the viscoelastic wave equations and the vertical transversely isotropic (VTI) media into consideration, and establishes the second-order SEM for wave modeling in viscoelastic VTI media. The second-order perfectly matched layer for viscoelastic VTI media is also introduced. The problem of handling the overlapped absorbed corners is solved. A comparison with the analytical solution in a two-dimensional viscoelastic homogeneous medium shows that the method is accurate in the wave-field modeling. Furtherly, numerical validation also presents its great flexibility in solving wave propagation problems in complex heterogeneous media. This second-order SEM with perfectly matched layer for viscoelastic VTI media can be easily applied in wave modeling in a limited region.     

On the stability of a non-convolutional perfectly matched layer for isotropic elastic media

The multi-axial perfectly matched layer (M-PML) is a material boundary condition for wave propagation problems in unbounded domains. It is obtained by extending the formulation of the split-field perfectly matched layer to a more general absorptive medium, for which damping profiles are specified along all dimensions of the problem. Under the hypothesis of small damping, it has been demonstrated that the stability of the system of partial differential equations of the M-PML can be related to the ratio of the damping profiles, and stable M-PML terminations for isotropic and orthotropic elastic media have been constructed. In the present work, we use the Routh–Horwitz determinants to demonstrate that the conclusions regarding the stability of M-PML for isotropic media for small damping are in fact valid for the more general case of damping coefficients of any (positive) value. The effectiveness of the M-PML is demonstrated by constructing stable terminations for the abovementioned media. The stability analysis is presented for 2-D in-plane (P-SV) wave propagation in elastic isotropic continua.     

基于对流尺度集合预报的飑线结构特征模拟与改善

对流尺度集合预报是研究飑线等强对流天气的新方向。当前对飑线系统结构的研究主要采用卫星和雷达资料结合高分辨率确定性预报的方法,而本文从集合预报技术的角度分析飑线结构特征。针对2014年7月30日中国江淮地区的一次强飑线过程,利用WRF模式开展了对流尺度集合预报试验,采用概率匹配平均法对集合预报结果进行综合处理,重点考察集合预报对飑线结构特征的模拟能力。结果表明:对流尺度集合预报能够模拟出飑线系统的基本结构特征。集合平均和概率匹配平均法相比控制预报而言,对飑线回波、热力场、动力场和微物理量场结构有明显的改善作用。同时模拟出了飑线系统近地面冷池和环境垂直风切变的相互作用,与RKW理论相一致。概率匹配平均法在回波强度上较集合平均更接近实况,应用于对流尺度集合预报研究极端天气事件具有指示意义。