首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   20487篇
  免费   3246篇
  国内免费   3577篇
测绘学   3368篇
大气科学   2211篇
地球物理   3873篇
地质学   10992篇
海洋学   1999篇
天文学   433篇
综合类   1583篇
自然地理   2851篇
  2024年   56篇
  2023年   145篇
  2022年   467篇
  2021年   664篇
  2020年   651篇
  2019年   784篇
  2018年   607篇
  2017年   762篇
  2016年   730篇
  2015年   844篇
  2014年   1114篇
  2013年   1265篇
  2012年   1188篇
  2011年   1316篇
  2010年   1109篇
  2009年   1216篇
  2008年   1305篇
  2007年   1502篇
  2006年   1456篇
  2005年   1356篇
  2004年   1208篇
  2003年   1041篇
  2002年   961篇
  2001年   810篇
  2000年   763篇
  1999年   676篇
  1998年   613篇
  1997年   527篇
  1996年   448篇
  1995年   380篇
  1994年   337篇
  1993年   242篇
  1992年   208篇
  1991年   147篇
  1990年   112篇
  1989年   98篇
  1988年   66篇
  1987年   52篇
  1986年   28篇
  1985年   19篇
  1984年   11篇
  1982年   4篇
  1981年   3篇
  1980年   1篇
  1979年   6篇
  1978年   3篇
  1977年   2篇
  1976年   1篇
  1973年   2篇
  1954年   3篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Deep dissolution affects great part of soluble rocks (e.g. gypsum and anhydrite) of the Western Italian Alps. The related superficial phenomena (sinkholes, gravity-induced processes and a local worsening of geomechanical rock properties) are not limited to typical karsts landscape and cause slope instability also affecting populated sites and infrastructures. The paper aims to describe general characteristic of dissolution phenomena, to interpret their conditioning factors and evolutionary stages and to assess possible hazards due to their superficial effects.The search for evidences of deep dissolution leads to the selection of representative sites in the central part of the Western Italian Alps (Piemonte and Valle d'Aosta Region). Detailed geological and geomorphological studies have been used to classify the selected sites by type, size and variable state of activity. Very different evolutionary stages of dissolution phenomena have been interpreted by comparison of case-studies: some are early “embryonic”; others are more evolved, up to typical sinkholes, or even remodelled by other phenomena. Some cases show an extreme complexity in the interactions between corrosion phenomena and other geomorphic processes: slope deformations, from one side, and karst, fluvial and glacial phenomena, to the other. A wide range of movement rates on slope instabilities induced by deep dissolution have been estimated by topographic and geomorphic data. Geochemical data on removed rocks by dissolution indicate 0.4 mm/year values for local subsidence. Historical and technical data indicate low frequency of major dissolution-induced collapses, but highlight widespread damages to tunnels, roads and buildings, especially along slopes.  相似文献   
82.
Networking between companies and other regional key actors has grown into a widespread instrument for economic development since the 1990s. Participatory networks have formed the concept for many activities of Local Agenda 21. Simultaneously, economic geography has focused on network theories to explain innovation in and the economic success of regions.This article focuses on the reasons why individual actors participate in environmentally oriented information networks. The questions approached are: Does the concept of learning within networks influence a firm’s development? How do the participants interact with each other? What effects do the networks have on their environment (arenas)?The changing arenas of a firm are analysed with regard to environmental concerns. These changes need to be anticipated for reliable strategies. To gain information and knowledge about current behaviour and activities, the functions of networks as learning platforms are discussed. Drawing upon arguments from different network theories, the motivations of participating in a network are threefold: actor and firm oriented, network internal (inter-firm), and network external. From these, a generic target cube of network motivations and actions is set up.The concept is reviewed by examining 12 ‘regional environmental information-oriented corporation networks’ (RUN) in the Greater Munich Area. The empirical material reveals that this type of network does not provide a guarantee for significant improvements in a firm’s environmental behaviour. However, participants learn about best practices while pursuing explicit or implicit aims as to influence their arenas.  相似文献   
83.
Until recently the traditional spatial configuration of the European geography was based upon the core-periphery model. The ‘pentagon’, broadly defined as lying between London, Paris, Milan, Munich and Hamburg, was seen as the core area characterised by having the highest concentration of economic development in the European Union (EU), with the remainder of the European territory viewed as peripheral, albeit to varying degrees. In a number of cases such peripheral areas equated with clear regional disparities. The elaboration of the European Spatial Development Perspective (ESDP) (CEC, European spatial development perspective, towards balanced and sustainable development in the territory of the European Union, 1999) challenged this core-periphery model. European spatial planning policies, aimed at encouraging social and economic, and with ever increasing importance, territorial cohesion, seek amongst other aspects to encourage the development of a balanced and polycentric urban system. This paper adopts a network analysis approach to the analysis of air passenger flows between some 28 principal European metropolitan urban regions. The evaluation of these flows contributes to an enhanced comprehension of the spatial dynamics of the European metropolitan territory which goes beyond that deriving from the more standard analyses of the individual components of the urban system. Several indicators are used, deriving from gravitational modelling techniques, to analyse the complexity of the air passenger flows. A multidimensional scaling (MDS) technique is introduced in order to interpret and visualise the resulting spatial configuration and positioning of the different metropolitan centres within the conceptual European ‘space of air passenger flows’, thereby contrasting with the more traditional map-based geographical image of Europe, based upon Cartesian coordinates.
Malcolm C. BurnsEmail:
  相似文献   
84.
牛庄洼陷沙河街组超压系统发育特征及其演化   总被引:1,自引:0,他引:1       下载免费PDF全文
李阳  王建伟  赵密福  高侠 《地质科学》2008,43(4):712-726
东营凹陷牛庄洼陷沙河街组发育有超压系统,这对油气运移和聚集过程有着重要的影响。在对超压系统现今发育特征研究的基础上,本文运用约束下数值模拟方法对牛庄洼陷超压系统的演化规律进行研究,并探讨了超压系统的主要形成机制。牛庄洼陷在沙河街组四段、沙河街组三段的下亚段和中亚段存在着超压系统,最大压力系数可以达到1.8,最大剩余压力超过了20MPa。自沙三段上亚段沉积期开始,超压系统开始发育。到东营组沉积期末,超压系统经历了大约10Ma的泄压过程。自新近系馆陶组沉积期,超压系统再次迅速增压,逐渐接近现今发育状况。上覆地层沙三段上亚段高沉积速率导致了超压系统的形成和早期剩余压力的增加,而水热增压和烃类物质大量生成联合造成超压系统晚期迅速增压。超压系统演化规律揭示出在油气主要运移期研究区古异常流体压力的分布状况,这为进一步开展牛庄洼陷油气运移和聚集过程的动力学研究提供了理论依据。  相似文献   
85.
A methodology for the characterization of deep carbonate aquifers has been developed and applied to El Maestrazgo Jurassic aquifer in Castellón, Spain. Characterization of these aquifer formations, located at more than 300 m deep, consisted of a previous phase of compilation, analysis and synthesis of the existing information about the area, followed by a coordinated combination of different speciality studies: geology, stratigraphy, structural analysis, hydrogeology, hydrochemistry, geophysics and remote sensing. Geological studies included geological mapping, definition of stratigraphical units and facies and structural analysis. The aim of the hydrogeology study was to define aquifer formations, recharge area, aquifer points inventory and groundwater flow directions for the establishment of piezometric and water quality observation nets. Special techniques were applied, like thermal infrared aerial images and the evaluation of submarine groundwater discharge by means of natural radium isotopes. Hydrochemical techniques, including majority elements characterization and stable isotopes (18O, 2H and 3H) determination, allowed classifying hydrochemical facies and establishing a renewal pattern for water within the system. Geophysics was useful in determining the aquifer geometry, the features of the basement and the petrophysical characteristics of the geological formations. Preliminary results show an important tectonic complexity and the possibilities for groundwater uses in the area of study.  相似文献   
86.
乌鲁木齐河流域地下水水质监测网设计   总被引:1,自引:0,他引:1  
文章运用地下水易污性编图及污染源分布图法进行了乌鲁木齐河流域地下水水质监测网设计。共设计了130监测孔,现有46个监测孔,另需要84个新的监测孔。按监测类型分为面源监测点22个,点源监测点87个,重点水源地与泉水监测点21个。按监测运行分长期监测点55个,流域普查监测点75个。普查监测点监测频率为1次/5年,长期监测点监测频率为1次/年。首期有针对性地在污染严重的柴窝堡新化厂排污区、乌鲁木齐河谷老排污区、米泉污灌区、米泉工业污染区、老龙河污染区取了25个污染水样测试分析,结果显示地下水已经受到严重污染。  相似文献   
87.
大同盆地孔隙地下水化学场的分带规律性研究   总被引:2,自引:0,他引:2  
本文通过对大同盆地2004年孔隙地下水水化学资料的分析发现,盆地内孔隙地下水的水化学特征在水平方向上具有以盆地中部为中心,呈环状分布的特点,且与盆地水动力分区具有很好的一致性。由山前冲洪积倾斜平原到中部冲湖积平原,地下水依次经历了补给区、径流区和排泄区,相应的主要水化学类型分别为HCO3型、HCO3.SO4型和HCO3.SO4.Cl型。与中深层孔隙水相比,浅层孔隙水由于水位埋深浅,蒸发浓缩强烈,易受人类活动的影响,各组分的含量较高,变化幅度较大,水化学类型也相对复杂。浅层地下水的水质呈好转趋势,深层地下水水质基本保持稳定。  相似文献   
88.
盘山山前地下水与矿泉水污染成因分析   总被引:4,自引:0,他引:4  
文章利用主成分分析方法,结合揭示变量和样品之间在成因或空间上联系的对应分析,通过SPSS统计分析软件,对盘山山前地区地下水和矿泉水污染成因进行分析。研究表明,盘山山前存在地下水和矿泉水污染;局部范围的污染主要是由于食品厂等企业的排污引起的,污染成分主要是Cl-和NO3—N,主要污染地区是塔院地区,其次是西大佛塔至南营,污染程度与至污染源的距离有关;大部分地区存在三氮面状污染,农业氮肥的施用是其主要原因。  相似文献   
89.
卢予北 《探矿工程》2008,35(11):1-6
PVC-U塑料管具有耐腐蚀、不结垢、质量轻、使用寿命长等特点。将其应用于浅层地热能开发和供水管井工程中,不但可以解决金属井管腐蚀破裂、结垢和水源热泵回灌难等问题,而且还减少了水井维修费用、延长其使用寿命。结合河南郑州实施的437、400 m两眼浅层地热能开发和供水管井示范工程实践,详细论述了PVC-U塑料管成井工艺、存在问题及解决方法的研究成果。  相似文献   
90.
泡沫钻进过程中,会产生很多泡沫,如不及时清除,会造成施工现场泡沫的大量堆积,影响正常的生产。因此,选择合适的消泡方法在泡沫钻进过程中是非常重要的环节。选用机械消泡法中的缝隙式消泡器,建立消泡装置室内试验台进行试验,取得了良好的效果,消泡率可达86%。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号