Wavefield smoothing and the effect of rough velocity perturbations on arrival times and amplitudes

Geometric ray theory is an extremely efficient tool for modelling wave propagation through heterogeneous media. Its use is, however, only justified when the inhomogeneity satisfies certain smoothness criteria. These criteria are often not satisfied, for example in wave propagation through turbulent media. In this paper, the effect of velocity perturbations on the phase and amplitude of transient wavefields is investigated for the situation that the velocity perturbation is not necessarily smooth enough to justify the use of ray theory. It is shown that the phase and amplitude perturbations of transient arrivals can to first order be written as weighted averages of the velocity perturbation over the first Fresnel zone. The resulting averaging integrals are derived for a homogeneous reference medium as well as for inhomogeneous reference media where the equations of dynamic ray tracing need to be invoked. The use of the averaging integrals is illustrated with a numerical example. This example also shows that the derived averaging integrals form a useful starting point for further approximations. The fact that the delay time due to the velocity perturbation can be expressed as a weighted average over the first Fresnel zone explains the success of tomographic inversions schemes that are based on ray theory in situations where ray theory is strictly not justified; in that situation one merely collapses the true sensitivity function over the first Fresnel zone to a line integral along a geometric ray.     

历史环境变化数据库的建设与应用

阐述了历史环境变化数据库的资料来源及其计算机化的规范 ;介绍了以计算机、数据库等为技术手段的历史环境变化数据库的建设方案及数据库的主要功能 ;展望了历史环境变化数据库的应用前景     

A polygon-based approach for matching OpenStreetMap road networks with regional transit authority data

Matching road networks is an essential step for data enrichment and data quality assessment, among other processes. Conventionally, road networks from two datasets are matched using a line-based approach that checks for the similarity of properties of line segments. In this article, a polygon-based approach is proposed to match the OpenStreetMap road network with authority data. The algorithm first extracts urban blocks that are central elements of urban planning and are represented by polygons surrounded by their surrounding streets, and it then assigns road lines to edges of urban blocks by checking their topologies. In the matching process, polygons of urban blocks are matched in the first step by checking for overlapping areas. In the second step, edges of a matched urban block pair are further matched with each other. Road lines that are assigned to the same matched pair of urban block edges are then matched with each other. The computational cost is substantially reduced because the proposed approach matches polygons instead of road lines, and thus, the process of matching is accelerated. Experiments on Heidelberg and Shanghai datasets show that the proposed approach achieves good and robust matching results, with a precision higher than 96% and a F1-score better than 90%.     

基于高速公路流的区域城市网络空间组织模式——以江苏省为例

在“流空间”理论的影响下,强调要素连接性和空间关联性的城市网络成为指示区域空间关系的核心视角,探讨城市网络空间结构将能为区域发展政策制定提供科学依据。以江苏省2014年日平均的高速公路流数据为依托,刻画城市网络的宏观格局与层级特征,利用社区发现算法对城市网络进行空间剖分并挖掘潜在的关联结构。研究发现：① 江苏城市网络宏观格局的空间构造呈现出类似围棋的“金角、银边、草肚皮”的分散性特征,可大致辨别内部发育出多个不同的子网络系统;② 城市节点和联系轴线具有明显的层级结构,城市等级的重要性与节点本身的社会经济属性、地理区位等具有一定的空间耦合性,轴线层级与地理距离具有明显的空间交互叠加作用;③ 社区发现算法对网络的空间剖分共识别出包括苏锡常社区、宁镇扬泰社区、通盐社区、连云港社区、宿淮社区和徐州社区等6个在空间上相连且边界清晰的“城市社区”,具有一体化效应、邻域效应、行政区效应、袭夺效应和置换效应等五个具有典型性的区域效应;④ 依据社区间的空间交互作用,可凝练出双核—多核交叉型、双核—单核交叉型、单核—多核交叉型、单核—单核集中型、双核—多点分岔型、单核—多点分散型和多核—多点分散型等7类空间关联结构。     

Susceptibility and triggering scenarios at a regional scale for shallow landslides

The work aims at identifying susceptible areas and pluviometric triggering scenarios at a regional scale in Calabria (Italy), with reference to shallow landsliding events. The proposed methodology follows a statistical approach and uses a database linked to a GIS that has been created to support the various steps of spatial data management and manipulation. The shallow landslide predisposing factors taken into account are derived from (i) the 40-m digital terrain model of the region, an  15,075 km² extension; (ii) outcropping lithology; (iii) soils; and (iv) land use. More precisely, a map of the slopes has been drawn from the digital terrain model. Two kinds of covers [prevalently coarse-grained (CG cover) or fine-grained (FG cover)] were identified, referring to the geotechnical characteristics of geomaterial covers and to the lithology map; soilscapes were drawn from soil maps; and finally, the land use map was employed without any prior processing.Subsequently, the inventory maps of some shallow landsliding events, totaling more than 30,000 instabilities of the past and detected by field surveys and photo aerial restitution, were employed to calibrate the relative importance of these predisposing factors.The use of single factors (first level analysis) therefore provides three different susceptibility maps. Second level analysis, however, enables better location of areas susceptible to shallow landsliding events by crossing the single susceptibility maps.On the basis of the susceptibility map obtained by the second level analysis, five different classes of susceptibility to shallow landsliding events have been outlined over the regional territory: 8.9% of the regional territory shows very high susceptibility, 14.3% high susceptibility, 15% moderate susceptibility, 3.6% low susceptibility, and finally, about 58% very low susceptibility.Finally, the maps of two significant shallow landsliding events of the past and their related rainfalls have been utilized to identify the relevant pluviometric triggering scenarios. By using 205 daily rainfall series, different triggering pluviometric scenarios have been identified with reference to CG and FG covers: a value of 365 mm of the total rainfall of the event and/or 170 mm/d of the rainfall maximum intensity and a value of 325 mm of the total rainfall of the event and/or 158 mm/d of the rainfall maximum intensity are able to trigger shallow landsliding events for CG and FG covers, respectively.The results obtained from this study can help administrative authorities to plan future development activities and mitigation measures in shallow landslide-prone areas. In addition, the proposed methodology can be useful in managing emergency situations at a regional scale for shallow landsliding events triggered by intense rainfalls; through this approach, the susceptibility and the pluviometric triggering scenario maps will be improved by means of finer calibration of the involved factors.     

区域尺度土壤固碳量估算方法评述

增加陆地生态系统碳汇已成为一项重要的减排手段,被纳入有关国际气候变化协定。区域尺度土壤固碳量估算成为当前研究的热点与难点。本文综述了目前国内外常用的区域土壤固碳量估算方法,并依据估算精度与方法差异将这些方法总结为四类:简易模型、清查法、经验模型和基于GIS的机理模型。同时对这四类方法的特点和适用性进行了评述,建议根据研究地区及目的不同选择合适的方法。此后,通过分析土壤固碳机理、数据收集、尺度推移、情景分析和土壤有机质机理模型发展等与区域土壤固碳量估算密切相关的各方面中存在的不足,提出今后区域尺度土壤固碳量估算方法将向耦合土壤有机质机理模型、土壤可溶性碳模型、土地利用模型和区域水文模型的基于GIS平台的区域综合模型方向发展。     

1990—2015年中国县市尺度人口收缩的演变特征及影响因素

近年来,区域人口收缩问题在全球范围内引起了广泛关注,并且在中国也日益明显。利用1990年、2000年和2010年人口普查数据以及2015年1%人口抽样调查数据,本文在县市尺度上分析了1990—2000年、2000—2010年和2010—2015年三个时期人口收缩区的空间特征及演化趋势,并定量探讨了其形成与演变的影响因素,主要发现如下：① 人口收缩现象在县市尺度非常明显,三个时期人口收缩区占比均超过20%;② 2000—2010年人口收缩区以大面积扩张为主要特征,东北地区、川黔渝地区、长江中游地区最为明显,而东部地区则主要集中在江苏北部和福建西部等局部范围;③ 2010—2015年人口收缩转缓慢增长成为主要特征,仅东北地区、河南等人口收缩区仍有所扩张;④ 人均GDP和非农就业占比等经济发展水平因素对人口收缩区的形成影响显著,而经济发展速度因素则与人口收缩的演变密切相关;⑤ 除经济因素外,人口自然增长因素对人口收缩区形成及演变的影响均非常显著,且其作用呈增强趋势。     

Benefiting from the regional problem: The politics of 60 years of regional development

The New Zealand government approach to regional development and the economy more broadly has oscillated from the 1970s economic growth and investment focus to 1980s neoliberal neglect, followed by the more recent rediscovery of the regions' importance in national economies, all with tenuous outcomes. This paper explores the changing scope of regional development initiatives by successive New Zealand governments over the last 60 years to show a pattern of opportunism and neglect that suggests regional development is more aligned to electoral priorities than regional success.