Migrants, markets and multinationals: competition among world cities for the highly skilled

Urban areas compete with one another for people, goods, capital, ideas and other inputs of economic activity. Under the rubric of globalization, instead of only maintaining or improving the initial stock of assets in a city, the power of a place to attract outside flows of economic activity from elsewhere is increasingly important to economic development. Similarly, global or world cities are characterized as the command and control points through which these global economic flows operate. In response to the heightened mobility of highly-skilled labor across national borders, research has begun to examine the role of international human capital as an economic flow. This paper will examine the role of places in determining where the highly-skilled go in the global economy by viewing global city command and control functions as requiring unique labor flows. By reviewing the evidence found in the literature, we can better understand the potential for urban areas to compete for highly-skilled labor in the global economy. Known indicators of place attractiveness are assessed to examine the relationship between the economic trajectories of cities and their labor requirements. Cities are situated between firms and states as the key place-based actors which influence the mobility of highly-skilled labor. Finally, the paper also examines measurement issues and methodological problems in creating indices of world cities as well as explores possibilities for new research.     

Slowness-backazimuth weighted migration: A new array approach to a high-resolution image

We have developed a new array method combining conventional migration with a slowness-backazimuth deviation weighting scheme. All seismic traces are shifted based on the theoretical traveltime of the scattered wave from specific gridpoints in a 3-D volume. Observed slowness and backazimuth are calculated for each raypath and compared with theoretical values in order to estimate slowness and backazimuth deviations. Subsequently, stacked energy calculated by a conventional migration method is weighted by the slowness and backazimuth deviations to suppress any arrival energy whose slowness and backazimuth are inconsistent with the expected theoretical values. This new method was applied to two P- wave data sets which comprise (1) underside reflections at the 410 and 660 km mantle discontinuities and (2) D" reflections as well as their corresponding synthetic data sets. The results show that the weighting scheme dramatically increases the resolution of the migrated images and enables us to obtain well-constrained, focused images, making upper-mantle discontinuities and D" reflections more distinct by reducing their surrounding energy.     

双平方根单程波动方程叠前τ偏移方法

本文将常规双平方根（DSR）单程波动方程从深度域变换到双程垂直走时（τ）域，由此推导出可从数学上实现“沉降观测”的单程波DSR传播算子. 其递归波场延拓算法包含波数域针对常速背景的相移处理和空间域针对横向速度扰动的相位校正，可以应对上覆地层速度横向变化对构造成像的影响. 结合零炮检距、零时间成像条件，提出了在τ域进行波场延拓与成像的DSR方程叠前偏移新方法. 为了克服其全三维偏移算法在实际应用中可能面临的困难，本文采用稳相近似，在crossline常炮检距偏移理论基础上推导了实用的共方位角叠前τ偏移方法. 数值试验表明，DSR方程叠前τ偏移在强横向非均匀介质中的成像精度与分辨率优于传统的时间域成像技术.     

The numerical solution of the Advection-Dispersion Equation: A review of some basic principles

The simulation of solute transport in rivers is frequently based on numerical models of the Advection-Dispersion Equation. The construction of reliable computational schemes, however, is not necessarily easy. The paper reviews some of the most important issues in this regard, taking the finite volume method as the basis of the simulation, and compares the performance of several types of scheme for a simple case of the transport of a patch of solute along a uniform river. The results illustrate some typical (and well known) deficiencies of explicit schemes and compare the contrasting performance of implicit and semi-Lagrangian versions of the same schemes. It is concluded that the latter have several benefits over the other types of scheme.     

横向各向同性介质中的弹性波方程逆时偏移及其成像条件

作者在本文中导出了在横向各向同性介质情况下，二维弹性波逆时传播的高阶差分格式，实现了弹性波在数值空间中的逆时延拓，从snell定量出发，采用插值法求取网格空间中各点的直达波旅行时，以此作为弹性波逆时偏移的成像条件，实现多波多分量资料的逆时偏移，数值试验表明，逆时偏移能够使地表接收到的波场准确归位，提高资料处理的精度。     

The development of an intervening opportunities model with spatial dominance effects

In this paper an intervening opportunities model with spatial dominance is developed. The usual assumption in spatial theory is that decision makers are influenced not just by the size of a destination or distance but by these two factors in combination, that is, spatial dominance. Decision-makers will have more knowledge about, and clearly perceive destinations that exert the greatest amount of spatial dominance on their origins, just as they would primate cities. Thus destinations are ranked in terms of the spatial dominance calculated for each destination. Empirical verification of the model utilizes state-to-state migration flow data for the US. Calibration results compare favourably with the traditional intervening opportunities model and the production constrained gravity model. Received: 7 October 1999/Accepted: 15 June 2000     

高精度混合法叠前深度偏移及其并行实现

叠前深度偏移是复杂构造成像的重要手段.本文基于波场分裂理论,首先给出了波场延拓的一般方程,即一个上下行波的耦合方程组.通常所用的波场延拓方程就是该耦合方程组的特例.根据平方根算子的近似,推导了一种新的高精度混合法偏移方法,用分裂法即可进行计算.通过对一个横向强烈变速模型的叠后偏移及Marmousi复杂构造模型的叠前偏移计算,说明了方法的有效性,精度较高.采用MPI并行编程实现并行计算,提高了计算效率.该方法可用于对横向强烈变速的复杂构造的精确成像.     

各向异性弹性波有限元叠前逆时偏移

利用有限元法和最小走时射线追踪的界面点法，实现了各向异性弹性波的叠前逆时偏移.理论模型资料的偏移结果清晰准确，证实了该系统的可靠性.通过对各向异性模型资料的各向同性偏移处理，发现常规偏移剖面存在较大误差，地质体的垂向深度和横向位置与实际模型有偏离.     

沉积有机质激光热裂解色谱质谱探针分析技术的尝试及其前景探讨

激光热裂解－色谱－质谱分析方法具有从技术层面上融合分子地球化学方法和有机岩石学方法的潜能。通过抚顺煤镜质组分和树脂体的研究实例，介绍了激光热裂解－色谱－质谱探针分析的基本原理及测试方法。抚顺煤显微组分的微区热裂解产物特征中，镜质组和树脂体热解产物的正烷烃、烷基萘和四氢化萘的总体面貌相似，但树脂体中正烷烃含量较低，而烷基四氢化萘明显较高，这可能反映显微组分形成过程中发生过低分子烃类在各显微组分之间的混合作用。除此之外，还把激光热裂解－色谱－质谱探针技术与传统的分子地球化学方法、有机岩石学方法进行比较。探讨了激光热裂解－色谱－质谱探针技术的应用前景以及尚待解决的技术问题。落射到样品表面的激光束能量（温度）和控制和激光辐射时间的，激光束斑大小、热解产物量、色谱－质谱检验灵敏度极限三者关系等，都是限制新技术应用的技术问题。