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   37279篇
  免费   6498篇
  国内免费   7903篇
测绘学   3811篇
大气科学   4487篇
地球物理   6840篇
地质学   21019篇
海洋学   4180篇
天文学   3381篇
综合类   2404篇
自然地理   5558篇
  2024年   167篇
  2023年   447篇
  2022年   1314篇
  2021年   1494篇
  2020年   1419篇
  2019年   1774篇
  2018年   1196篇
  2017年   1383篇
  2016年   1439篇
  2015年   1628篇
  2014年   2078篇
  2013年   2044篇
  2012年   2199篇
  2011年   2362篇
  2010年   2061篇
  2009年   2583篇
  2008年   2504篇
  2007年   2583篇
  2006年   2559篇
  2005年   2420篇
  2004年   2109篇
  2003年   2005篇
  2002年   1724篇
  2001年   1542篇
  2000年   1547篇
  1999年   1368篇
  1998年   1204篇
  1997年   828篇
  1996年   706篇
  1995年   590篇
  1994年   564篇
  1993年   473篇
  1992年   332篇
  1991年   291篇
  1990年   193篇
  1989年   149篇
  1988年   123篇
  1987年   75篇
  1986年   37篇
  1985年   33篇
  1984年   19篇
  1983年   17篇
  1982年   12篇
  1981年   10篇
  1980年   9篇
  1979年   7篇
  1978年   14篇
  1977年   7篇
  1976年   4篇
  1954年   7篇
排序方式: 共有10000条查询结果,搜索用时 858 毫秒
561.
Mark Wise 《Area》2006,38(2):204-212
This paper calls for more analysis of contemporary transnational linguistic movements. Geographers have hitherto focused on linguistic problems within states and paid less attention to issues that arise as languages, notably English, are used internationally across state boundaries. It does this by examining a conflict that arose when forces integrating the supranational European Single Market space came into conflict with a Member State's determination to protect the linguistic 'integrity' of its national territory. This conflict would appear to be a precursor of similar ones to come as languages increasingly compete in a linguistic market within an integrating Europe and a globalizing world.  相似文献   
562.
Stable carbon and nitrogen isotope ratios from bones of contemporaneous Late Atlantic aurochs and early cattle in eastern Denmark are significantly different and provide information on the origin and feeding strategies of the earliest domestic cattle. The data show that the early cattle were feeding on grass right from the beginning 4000 cal. yr BC. In contrast, the youngest aurochs population primarily browsed and grazed from the dense forest floor resulting in rather negative δ13C values measured on bone collagen. The oldest aurochs have similar isotope values to the earlier cattle, whereas the youngest aurochs have similar values to Late Atlantic red deer from the same locality. As eastern Denmark was largely covered by forest, speculations on the origin of the grazing areas are many. The grass may have grown in openings in the forest, at the forest fringe, or more likely on the newly reclaimed coastal land areas exposed by the decreasing rate of eustatic sea‐level rise contemporaneously with isostatic uplift, during the Littorina transgressions. The stable isotope values do not indicate that leaf foddering of the early cattle was of importance.  相似文献   
563.
Remote sensing technologies are increasingly used to monitor landscape change in many parts of the world. While the availability of extensive and timely imagery from various satellite sensors can aid in identifying the rates and patterns of deforestation, modelling techniques can evaluate the socioeconomic and biophysical forces driving deforestation processes. This paper briefly reviews some emerging spatial methodologies aimed at identifying driving forces of land use change and applies one such methodology to understand deforestation in Mexico. Satellite image classification, change analysis and econometric modelling are used to identify the rates, hotspots and drivers of deforestation in a case study of the southern Yucatán peninsular region, an enumerated global hotspot of biodiversity and tropical deforestation. In particular, the relative roles of biophysical and socioeconomic factors in driving regional deforestation rates are evaluated. Such methodological approaches can be applied to other regions of the forested tropics and contribute insights to conservation planning and policy.  相似文献   
564.
The climatic zones where residual soils occur are often characterized by alternate wet and dry seasons. Laboratory studies of earlier workers have established that the alternate wetting and drying process affects the swell-shrink potentials, water content, void ratio and particle cementation of expansive soils. The influence of cyclic wetting and drying on the collapse behaviour of residual soils has not been examined. This paper examines the influence of alternate wetting and drying on the collapse behaviour of compacted residual soil specimens from Bangalore District. Results of such a study are useful in anticipating changes in collapse behaviour of compacted residual soil fills. Experimental results indicated that the cyclic wetting and drying process increased the degree of expansiveness of the residual soils and reduced their collapse tendency. Changes in the swell/collapse behaviour of compacted residual soil specimens from wetting drying effects are attributed to reduction in water content, void ratio and possible growth of cementation bonds.  相似文献   
565.
Palestinian suicide terrorism has been a key feature in the latest phase of the Israeli–Palestinian conflict. During the past decade, and particularly since September 2000, there has been a substantial increase in the use of this type of warfare. Recent studies suggest that, contrary to common belief, suicide terrorism is highly rational and driven by strategic considerations. This article explores the rationality of Palestinian suicide terrorism from a geographical perspective. It is argued that suicide terrorism works along two parallel paths: rationality and randomness. It complies with geographical fundamentals, and target selection is highly rational, subject to spatial considerations such as distance, agglomeration, and accessibility. As the permeability to Israel became more difficult, suicide bombers and their organizers had to adopt more flexible practices which emphasized other spatial considerations. Timing is of importance both for strategic and tactical reasoning. Obstructing negotiations and peace talks has been a salient objective, but the exact timing of suicide bombings has been influenced by tactical considerations, which aim at maximizing casualties.  相似文献   
566.
?zmir is the third largest city in Turkey and has being the centre of art, culture, tourism and trade activities throughout the 5,000 years of its history. Natural stones brought from different parts of the world have been widely used for construction of the prestigious buildings, monuments and roads etc., in the past in the city. Renovation of the street pavements and public gathering areas in the city centre has been undertaken by the Metropolitan Municipality in 2000 and continued through the year of 2001. These renovation activities have mainly been carried out in the streets running parallel to the sea shore. Volcanic rocks brought from the Central Anatolia Ankara-Gölba?? (andesites) and Kayseri-?ncesu (tuffs) have been used in the renovation works. These rocks have shown extensive deteriorations within 4 years of their usage between 2001 and 2005 under the influence of different environmental factors. In this study, the deteriorations developed in the recently placed volcanic rocks used as kerb and pavement stones in the city centre of ?zmir in the light of their mineralogical, chemical, physical and mechanical properties, used locations and the environmental factors are presented.  相似文献   
567.
竖轴向承载力是桩基础设计的重要参数,现场静载试验是目前确定桩基承载力的主要手段之一。本文依托黄陵延安段高速公路洛河特大桥试桩工程,通过对原有工程桩施加预应力,形成了以工程桩为锚桩的锚拔体系。加载过程中试桩应力、锚桩应力及桩顶位移测试,结果表明,结合预应力技术,以工程桩作为锚桩构成桩基础静载试验的锚拔体系是可行的,同时提出了施加预应力大小的原则、方法以及确保后期曾作为锚桩的工程桩有足够承载能力应采取的必要措施。  相似文献   
568.
江西省宜春城区地面塌陷特征及防治建议   总被引:2,自引:0,他引:2  
宜春城区属岗阜低丘地形,分布石炭系-二叠系覆盖型及裸露型碳酸盐岩,占城区面积近70%。浅部岩溶发育,覆盖层厚度一般小于20m,土洞较多,基岩面起伏大,第四系岩性结构复杂,地下水埋藏浅。由于抽采地下水及建筑施工振动等人为因素影响,诱发地面塌陷12处,造成重大经济损失。在城市规划功能布局时应充分考虑地面塌陷的易发程度及场地建设适宜性;应限制诱发塌陷因素的强度,在易发区须严格控制地下水开采,严禁新建开采井;塌陷治理的主要方法有填堵、跨越、灌注、深基础控制抽排水强度、及其它综合治理或避让方法,同时做好灾害的监测预报工作。  相似文献   
569.
邯邢东部平原区温增温率3℃/100m,被任为是不宜开采利用的“非地热异常区”。但近几年来在该区已成功施工了井深1500~1800m的探采结合地热井拾余口,井口出水温度达50℃以上,证实了邯邢东部平原区地热开发利用前景广阔。根据该区域地热条件背景及地热资源赋存特点,结合该区近几年地热探采施工体会,对地热井成井工艺和合理开发利用地热资源的提出了有益见解,对类似平原区地热资源的开发利用具有一定的启发和借鉴意义。  相似文献   
570.
推进煤矸石资源化利用的对策建议   总被引:10,自引:0,他引:10  
煤矸石是我国排放量和堆存量最大的工业固体废弃物之一,但又是可以利用的资源,只要加以资源化利用,就能会变废为宝。在研究我国煤矸石综合利用现状的基础上,指出了煤矸石综合利用中存在的主要问题,从制定规划、完善经济政策、加强法制建设等方面提出了加快推进煤矸石资源化利用的建议。  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