中国西部及邻区强震迁移规律与喜马拉雅弧关系研究

在数值模拟的基础上,系统分析研究了中国西部及邻区强震活动的迁移规律,结果表明,印度板块与欧亚板块的碰撞作用控制着中国西部强震活动。 喜马拉雅东部弧顶（缅甸弧顶） 、西部弧顶（兴都库什弧顶） 及喜马拉雅弧中段的强震活动与内陆的强震活动存在密切的联系,与内陆强震活跃期、平静期的存在及强震迁移规律都有重要关系。     

Measuring the environmental dimensions of human migration: The demographer's toolkit

In recent years, the empirical literature linking environmental factors and human migration has grown rapidly and gained increasing visibility among scholars and the policy community. Still, this body of research uses a wide range of methodological approaches for assessing environment–migration relationships. Without comparable data and measures across a range of contexts, it is impossible to make generalizations that would facilitate the development of future migration scenarios. Demographic researchers have a large methodological toolkit for measuring migration as well as modeling its drivers. This toolkit includes population censuses, household surveys, survival analysis and multi-level modeling. This paper's purpose is to introduce climate change researchers to demographic data and methods and to review exemplary studies of the environmental dimensions of human migration. Our intention is to foster interdisciplinary understanding and scholarship, and to promote high quality research on environment and migration that will lead toward broader knowledge of this association.     

Efficient traveltime compression for 3D prestack Kirchhoff migration

Kirchhoff 3D prestack migration, as part of its execution, usually requires repeated access to a large traveltime table data base. Access to this data base implies either a memory intensive or I/O bounded solution to the storage problem. Proper compression of the traveltime table allows efficient 3D prestack migration without relying on the usually slow access to the computer hard drive. Such compression also allows for faster access to desirable parts of the traveltime table. Compression is applied to the traveltime field for each source location on the surface on a regular grid using 3D Chebyshev polynomial or cosine transforms of the traveltime field represented in the spherical coordinates or the Celerity domain. We obtain practical compression levels up to and exceeding 20 to 1. In fact, because of the smaller size traveltime table, we obtain exceptional traveltime extraction speed during migration that exceeds conventional methods. Additional features of the compression include better interpolation of traveltime tables and more stable estimates of amplitudes from traveltime curvatures. Further compression is achieved using bit encoding, by representing compression parameters values with fewer bits.     

Thermodynamic study of the association and separation of copper and gold in the Shizishan ore field, Tongling, Anhui Province, China

The Shizishan ore field is the largest gold–copper ore field in the Tongling ore district of Anhui Province, China. Copper and gold deposits in the district are present as one-commodity deposits or as deposits with both commodities. Copper and gold mineralization are either cogenetic or are temporally and spatially distinct. We present the results of systematic geochemical analysis of fluid inclusions from typical Au–Cu deposits in the Shizishan ore field; these data are used to determine the solubility of Cu and Au in the ore-forming fluids and to ascertain the mechanisms and factors that controlled variations in the association and separation of copper and gold mineralization. Our results indicate that copper in the ore-forming fluids was transported as CuCl₂⁻ and CuCl⁰ complexes and that the solubility of copper was controlled by variations in Cl⁻ concentration. In addition, the precipitation of copper was controlled by changes in temperature, pH, fO₂, and fO₂. In comparison, gold in the ore-forming fluids was transported as Au(HS)₂⁻ and Au₂S(HS)₂²⁻ complexes, and the solubility of gold was controlled by variations in total sulfur concentration; the precipitation of gold was controlled by temperature, pH, fO₂, and fO₂. These differences between the two elements meant that copper and gold in the ore-forming fluids responded in different ways to changes in physicochemical conditions. Copper precipitated under relatively acidic conditions at high temperatures, while gold precipitated under weakly alkaline conditions at relatively low temperatures; this dissociation resulted in the temporal and spatial separation and zonation of copper and gold mineralization in the Shizishan ore field.     

Using a migration systems approach to understand the link between climate change and urbanisation in Malawi

Most scholarship on rural–urban migration in Sub-Saharan Africa demonstrates that migrants tend to move in a “circular” fashion and only spend short periods of time in cities before returning home to rural villages. However, some scholars working on the impacts of climate change on migration suggest that deteriorating environmental conditions may undermine rural livelihoods and lead people to move to cities for longer periods of time. If this is true, then climate change threatens to accelerate urbanisation and lead to renewed stress on urban infrastructure. The purpose of this paper is to explore these positions and we do so by collecting survey [n = 241], in-depth interview [n = 75] and focus group [n = 123 participants] data from rural and urban Malawi. Two key results stand out as significant. The first is that migrants in Malawi's capital city tend to stay in the urban environment for longer periods of time than conventional understandings of migration would predict. The second key result is that climate change may actually lead, in the case of Malawi, to reverse (i.e urban–rural) migration. This is because many of the people in Malawi's cities depend on products produced in rural environments (e.g. food and fuelwood). If climate change undermines rural livelihoods, then many urban residents will find the basis of their livelihoods removed and will likely respond by moving back to rural villages. Overall, our results, therefore, suggest that in at least one case the effect of climate change on migration may not be to increase migration towards cities but to stimulate an exodus from cities and back to the rural countryside.     

Special issue introduction: New research directions and critical perspectives on diaspora strategies

As more countries acknowledge the potential resources represented by their emigrant populations, the diaspora strategies of migrant sending countries are gaining policy and academic attention internationally. ‘Diaspora strategies’ describe initiatives aimed at mobilising emigrants for the purposes of economic development and/or nation building. This special issue in Geoforum identifies new research directions for the study of diaspora strategies. While extant scholarship has focused on state-driven diaspora strategies so far, this special issue introduction suggests that considering a wider range of social actors that engage in diaspora strategising across different spaces and scales will reveal new and productive insights for the study of diaspora strategies. Framing this introduction is an approach that deploys topological analyses as a way of keeping in view the variety of social actors involved in diaspora strategising, their connections to one another, and an evolving constellation of power relations ranging from contestation to collaboration. The special issue introduction draws attention to, first, the subjectivities constituted by diaspora strategies; second, the array of social actors found within webs of diaspora connections; and third, the ethical considerations arising from the power geometries of diaspora engagement. In so doing, it argues for the importance of studying diaspora formations dialogically which means deploying an analytical lens that is attentive to how the actions of different social actors and institutions from one country towards a diaspora population can influence the attitudes and actions of that diaspora towards another country that also claims their loyalty and contributions.     

Moving players,traversing perspectives: Global value chains,production networks and Ghanaian football labour migration

This article examines the production and transnational export of Ghanaian football labour. It does so via a cross-disciplinary approach that utilises perspectives rooted in the sociology of development (global value chains) and economic geography (global production networks). The article is underpinned by two central arguments. Firstly, it contends that the GVC framework is useful in accounting for how Ghanaian players are produced and prepared for the international market, identifying the key agents and agencies involved, mapping the geography of production and export and assessing the institutional context within which the trade operates locally, nationally and internationally. The second draws on the GPN perspective to argue that while Ghanaian football labour migration remains a process contoured by uneven asymmetries of power that favour actors, stakeholders and entities in the global North, there are currently segments of the production–export chain where power is much more diffuse and some benefits are captured in the global South. The paper draws on interview data and observations gleaned from four periods of multi-sited ethnographic fieldwork in Ghana between January 2008 and June 2011.     

地震波干涉偏移及预条件正则化最小二乘偏移成像方法对比

地震波干涉偏移和偏移反演成像是近年来十分活跃的两个研究领域.干涉偏移提供了一个新的地震波数据成像工具,而偏移反演则提供了高逼近度地震成像.二者的共同目的是改善传统直接偏移方法的成像效果,展宽成像区域并提高成像的分辨率.本文研究干涉偏移方法和偏移反演方法对于地震成像效果的影响,探讨二者在提高成像分辨率上的异同.对于偏移反演,通过建立正则化模型,研究了预条件共轭梯度迭代正则化方法及改进措施,并通过绕射点模型数值模拟验证了该方法比直接偏移能够提高振幅的保真度和成像的分辨率.对于干涉偏移和偏移反演这两种方法,对层速度地震模型进行了数值模拟.结果表明干涉偏移和偏移反演成像方法比传统的偏移方法在成像效果上是更加有效的,因而对于实际的地震成像问题很有应用前景.     

检波点与震源能量比值在偏移成像条件中的作用分析

利用检波点能量与震源能量比值构造衰减系数,研究波场各频率成分在自相关与反褶积两类成像条件中偏移成像的作用.分析检波点能量与震源能量比值分布特征,有利于克服反褶积型成像条件中分母趋于零时计算不稳定现象,也有助于平滑检波点波场与震源波场;检波点能量与震源能量比值超出正常范围容易导致偏移结果中出现照明不均匀现象.本文方法有助于改善反褶积型成像条件的偏移计算,也有助于应用照明技术指导偏移成像结果的振幅校正.     

Wide‐azimuth angle gathers for anisotropic wave‐equation migration

Extended common‐image‐point gathers (CIP) constructed by wide‐azimuth TI wave‐equation migration contain all the necessary information for angle decomposition as a function of the reflection and azimuth angles at selected locations in the subsurface. The aperture and azimuth angles are derived from the extended images using analytic relations between the space‐ and time‐lag extensions using information which is already available at the time of migration, i.e. the anisotropic model parameters. CIPs are cheap to compute because they can be distributed in the image at the most relevant positions, as indicated by the geologic structure. If the reflector dip is known at the CIP locations, then the computational cost can be reduced by evaluating only two components of the space‐lag vector. The transformation from extended images to angle gathers is a planar Radon transform which depends on the local medium parameters. This transformation allows us to separate all illumination directions for a given experiment, or between different experiments. We do not need to decompose the reconstructed wavefields or to choose the most energetic directions for decomposition. Applications of the method include illumination studies in complex areas where ray‐based methods fail, and assuming that the subsurface illumination is sufficiently dense, the study of amplitude variation with aperture and azimuth angles.