Self-consistent response of a galactic disc to vertical perturbations

We study the self-consistent, linear response of a galactic disc to vertical perturbations, as induced, say, by a tidal interaction. We calculate the self-gravitational potential corresponding to a non-axisymmetric, self-consistent density response of the disc using the Green's function method. The response potential is shown to oppose the perturbation potential because the self-gravity of the disc resists the imposed potential, and this resistance is stronger in the inner parts of a galactic disc. For the   m = 1  azimuthal wavenumber, the disc response opposes the imposed perturbation up to a radius that spans a range of 4–6 disc scalelengths, so that the disc shows a net warp only beyond this region. This physically explains the well known but so far unexplained observation that warps typically set in beyond this range of radii. We show that the inclusion of a dark matter halo in the calculation only marginally changes (by ∼10 per cent) the radius for the onset of warps. For perturbations with higher azimuthal wavenumbers, the net signature of the vertical perturbations can only be seen at larger radii – for example, beyond 7 exponential disc scalelengths for   m = 10  . Also, for the high- m cases, the magnitude of the negative disc response due to the disc self-gravity is much smaller. This is shown to result in corrugations of the mid-plane density, which explains the puzzling scalloping with   m = 10  detected in H  i in the outermost regions ∼30 kpc in the Galaxy.     

P‐wave transmission across fractures with nonlinear deformational behaviour

Stress wave attenuation across fractured rock masses is a great concern of underground structure safety. When the wave amplitude is large, fractures experience nonlinear deformation during the wave propagation. This paper presents a study on normal transmission of P‐wave across parallel fractures with nonlinear deformational behaviour (static Barton–Bandis model). The results show that the magnitude of transmission coefficient is a function of incident wave amplitude, nondimensional fracture spacing and number of fractures. Two important indices of nondimensional fracture spacing are identified, and they divide the area of nondimensional fracture spacing into three parts (individual fracture area, transition area and small spacing area). In the different areas, the magnitude of transmission coefficient has different trends with nondimensional fracture spacing and number of fractures. In addition, the study reveals that under some circumstances, the magnitude of transmission coefficient increases with increasing number of fractures, and is larger than 1. Copyright © 2006 John Wiley & Sons, Ltd.     

试论企业组织结构--电子商务企业与传统企业的比较

网络时代的到来，对传统企业的组织结构、运作模式、管理方法均提出了严峻的挑战。传统的企业组织结构是适应大工业生产而形成的等级组织结构形式，它有很大的局限性；电子商务企业的组织结构是适应网络社会而形成扁平化、双向多对多、以信息为基础的新型组织结构形式。电子商务企业以其组织结构扁平化和中空化，决策分散化，运作虚拟化，灵活高效为特点，它将成为传统企业在电子商务发展阶段变革的方向和目标。     

广西老厂铅锌矿床地质特征及其成因探讨

本文是在专题研究基础上,探讨广西老厂铅锌矿床地质特征和矿床成因。该矿床位于海洋山复式背斜南段的老厂穹窿构造核部,铅锌矿化受构造、地层和岩性控制。矿体呈脉状,产于NE向组张扭性断裂构造中;围岩为寒武系白云岩,板岩、砂岩;矿石类型以锌为主,矿物成分简单,以闪锌矿、方铅矿与石英、白云石为主,具充填、交代矿石结构、构造,矿石平均品位含Zn9.94%、Pb1.50%;围岩蚀变主要为硅化、绢云母化和碳酸盐化。据地层岩石元素丰度值、稀土元素配分和稳定同位素数据、资料,探讨了成矿物质来源于围岩,热液主要来自大气降水。因此,认为本矿床实属产于寒武系轻变质岩中的层控矿型大气降水热液铅锌矿床。     

洛坝铅锌矿床的构造演化分析

洛坝铅锌矿床位于黄诸关韧脆性变形带所波及的范围内,断裂、褶皱、劈理、节理等形态构造类型均很发育,经对小型构造的统计及应力场分析证明,矿区变形主要是在海西—印支期长期存在的南北向古应力场持续作用下形成的。