Particle‐scale effects on global axial and torsional interface shear behavior

An extensive literature on the shear behavior of continuum–particulate interfaces has been developed during the last four decades. However, relatively limited work regarding the behavior of interfaces under different loading conditions has been published. This paper presents a discrete element modeling study, along with comparisons from experimental data, of interface behavior under axial and torsional drained loading conditions. Detailed studies allow for links between micro‐scale particle behavior and observed global response to be developed and for the latter to be evaluated in light of particle–particle and particle–continuum interactions. The results of this study indicate that axial and torsional interface shear induce inherently different loading conditions, as shown by the different failure envelopes, stress paths, and induced soil volume changes and deformations. Furthermore, the results presented in this paper indicate that particle‐level mechanisms, such as particle rotations and contact slippage, play different roles in axial and torsional shear. Coordination number, polar histograms, particle displacements, particle rotations, and local void ratio measurements provide further insights into the fabric evolution, loading conditions, and failure mechanisms induced by these two shear modes. This study expands the current understanding of interface behavior and discusses potential improvements to geotechnical systems that leverage the characteristics of different imposed loading conditions. Copyright © 2016 John Wiley & Sons, Ltd.     

Numerical simulation of the quicksand phenomenon by a 3D coupled Discrete Element ‐ Lattice Boltzmann hydromechanical model

This paper deals with the numerical simulation of the quicksand phenomenon using a coupled Discrete Elements – Lattice Boltzmann hydromechanical model. After the presentation of the developed numerical model, simulations of ascending fluid flow through granular deposits are performed. The simulations show that the quicksand actually triggers for a hydraulic gradient very close to the critical hydraulic gradient calculated from the global analysis of classical soil mechanics, that is, when the resultant of the applied external pressure balances submerged weight of the deposit. Moreover, they point out that the quicksand phenomenon does not occur only for hydraulic gradients above the critical hydraulic gradient, but also in some cases with slightly lower gradients. In such cases, a more permeable zone is first gradually built at the bottom of the deposit through a grain rearrangement, which increases the hydraulic gradient in the upper zones and triggers the phenomenon. Copyright © 2016 John Wiley & Sons, Ltd.     

Fully coupled simulation of a hydraulic fracture interacting with natural fractures with a hybrid discrete‐continuum method

A hybrid discrete‐continuum numerical scheme is developed to study the behavior of a hydraulic fracture crossing natural fractures. The fully coupled hybrid scheme utilizes a discrete element model for an inner domain, within which the hydraulic fracture propagates and interacts with natural fractures. The inner domain is embedded in an outer continuum domain that is implemented to extend the length of the hydraulic fracture and to better approximate the boundary effects. The fracture is identified to propagate initially in the viscosity‐dominated regime, and the numerical scheme is calibrated by using the theoretical plane strain hydraulic fracture solution. The simulation results for orthogonal crossing indicate three fundamental crossing scenarios, which occur for various stress ratios and friction coefficients of the natural fracture: (i) no crossing, that is, the hydraulic fracture is arrested by the natural fracture and makes a T‐shape intersection; (ii) offset crossing, that is, the hydraulic fracture crosses the natural fracture with an offset; and (iii) direct crossing, that is, the hydraulic fracture directly crosses the natural fracture without diversion. Each crossing scenario is associated with a distinct net pressure history. Additionally, the effects of strength contrast and stiffness contrast of rock materials and intersection angle between the hydraulic fracture and the natural fracture are also investigated. The simulations also illustrate that the level of fracturing complexity increases as the number and extent of the natural fractures increase. As a result, we can conclude that complex hydraulic fracture propagation patterns occur because of complicated crossing behavior during the stimulation of naturally fractured reservoirs. Copyright © 2017 John Wiley & Sons, Ltd.     

Characterisation of confinement effect on jointed rock pillars using a Synthetic Rock Mass approach

Confinement effect on jointed rock pillars is numerically characterised in this research using a Synthetic Rock Mass (SRM) approach. The SRM is an integrated model incorporating a discrete fracture network within a Particle Flow Code 3D particle assembly. In this paper, the confinement effect on a 3D jointed pillar SRM model is investigated in a series of simulations, including biaxial compression tests and true and conventional triaxial compression tests. The numerical results suggest that the applied confining stresses generally result in higher pillar strengths and ductile post‐peak responses. More brittle post‐peak behaviour is simulated in the biaxial and true triaxial tests when the pillar is confined by a high stress in one lateral direction and by a zero/low stress in the other lateral direction. This phenomenon is attributed to significant lateral pillar dilation in the less confined direction. Detailed pillar failure modes are monitored in the uniaxial and triaxial tests. Axial splitting fractures and long shear zones cutting through the pillar are simulated when the pillar is able to dilate in the direction of least confinement. Localised shearing along joints and failed rock blocks is the dominant failure mode when the pillar dilation is resisted by the applied confining stresses. The pillar remains relatively intact with limited cracking in the pillar core in the highly confined triaxial tests. Copyright © 2016 John Wiley & Sons, Ltd.     

CCD星点提取方法研究

电荷耦合器件CCD星敏感器的关键问题是星图识别,星图识别就是从星敏感器获得的原始星图中获取星的位置和星等信息。在介绍星敏感器工作原理的基础上,初步探讨了电荷藉合器件（CCD）天文定位的应用和优点,重点分析了Moravec算子、Frstner算子、Harris算子等几种点特征提取算子的原理和其适用性。     

基于多尺度预处理的GPS/SINS组合导航系统

基于多尺度分析的思想,以离散小波变换为工具,利用小波对惯性元件输出的信息进行并行阈值消噪以削弱惯性元件误差对SINS及组合系统性能的影响;然后,对GPS输出的信息进行并行多尺度预处理,并结合传统的Kalman滤波方法,对系统进行综合滤波;将上述方法引入到GPS/SINS组合导航系统中,利用实测数据进行验证,并给出了基于不同方法的大量实验曲线。实验结果表明,该方法可以有效削弱惯性元件以及GPS误差对系统的影响,提高了GPS/SINS组合导航系的精度和可靠性。     

利用DFT相位的矢量地理空间数据水印方法

基于矢量地理空间数据自身的特点,运用离散傅立叶变换(DFT)技术研究了基于DFT相位的矢量地理空间数据数字水印算法。由于DFT相位的较小变化也会导致原始数据的较大改变,因此引入自相关检测来辅助判定数字水印的存在。实验表明,DFT相位水印算法在抗矢量地理空间数据处理中常见的删点、数据格式转换、平移、旋转等攻击方面具有较好的效果,并且针对浮点型数据的效果要明显优于整数型数据的结果。     

震级离散下的震级频度关系

运用最大熵原理,研究震级离散条件下震级频度关系,得到震级离散条件下震级概率分布函数,结果表明:(1)震级大于等于某一震级的地震次数应通过离散求和的方式得到,而不应通过积分的方式得到;(2)震级上限取为∞的情况下,古登堡意义和里克特意义两种震级频度关系式的b值相等;震级上限有限的情况下,里克特意义震级频度关系式可能不是直...     

True amplitude imaging by inverse generalized Radon transform based on Gaussian beam decomposition of the acoustic Green's function

True amplitude migration is one of the most important procedures of seismic data processing. As a rule it is based on the decomposition of the velocity model of the medium into a known macrovelocity component and its sharp local perturbations to be determined. Under this decomposition the wavefield can be considered as the superposition of an incident and reflected/scattered waves. The single scattering approximation introduces the linear integral operator that connects the sharp local perturbations of the macrovelocity model with the multishot/multioffset data formed from reflected/scattered waves. We develop the pseudoinverse of this operator using the Gaussian beam based decomposition of acoustic Green's functions. The computation of this pseudoinverse operator is done pointwise by shooting Gaussian beams from the target area towards the acquisition system. The numerical implementation of the pseudoinverse operator was applied to the synthetic data Sigsbee2A. The results obtained demonstrate the high quality of the true amplitude images computed both in the smooth part of the model and under the salt body.     

A generalized derivative operator for potential field data‡

The enhancement of potential field data using filters based on horizontal and vertical derivatives is common. As well as the direct use of the gradients themselves they are used in filters such as sunshading, total horizontal derivative, analytic signal, horizontal and vertical tilt angles, the Theta map and other filters. These techniques are high‐pass filters of different types and so enhance noise as well as detail in the data. A new derivative operator is introduced in this paper, which generalizes the effects of some of the previously mentioned filters. This filter is a linear combination of the horizontal and vertical field derivatives, normalized by the analytic signal amplitude. The filter is demonstrated on aeromagnetic and gravity data from South Africa.