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   569篇
  免费   131篇
  国内免费   161篇
测绘学   10篇
大气科学   81篇
地球物理   152篇
地质学   309篇
海洋学   172篇
天文学   10篇
综合类   48篇
自然地理   79篇
  2024年   7篇
  2023年   15篇
  2022年   32篇
  2021年   33篇
  2020年   22篇
  2019年   42篇
  2018年   23篇
  2017年   22篇
  2016年   21篇
  2015年   26篇
  2014年   32篇
  2013年   46篇
  2012年   36篇
  2011年   26篇
  2010年   33篇
  2009年   32篇
  2008年   34篇
  2007年   42篇
  2006年   51篇
  2005年   34篇
  2004年   26篇
  2003年   25篇
  2002年   24篇
  2001年   17篇
  2000年   24篇
  1999年   23篇
  1998年   9篇
  1997年   16篇
  1996年   17篇
  1995年   8篇
  1994年   10篇
  1993年   15篇
  1992年   10篇
  1991年   9篇
  1990年   4篇
  1989年   3篇
  1988年   5篇
  1987年   2篇
  1986年   1篇
  1985年   2篇
  1983年   2篇
排序方式: 共有861条查询结果,搜索用时 31 毫秒
51.
Strong winds are a characteristic feature of UK upland areas. Despite this, understanding of aeolian processes in upland environments of the UK is limited. This paper presents direct measurements and observations of blanket peat erosion by wind action during a two week period of desiccation in the North Pennines, Northern England. A circular configuration of mass flux sediment samplers was used to collect peat eroded by wind action from 16 cardinal compass directions. Meteorological conditions (wind speed, wind direction, precipitation and temperature) were recorded by an automatic weather station set up adjacent to the site. Surface desiccation led to peat crust erosion and dust deflation. During short (≤1 hour) periods of precipitation, wind‐driven rainfall also caused erosion. Typically, dust flux rates were up to two orders of magnitude lower than recorded during periods of sustained wet weather. Measurements demonstrate the hitherto unreported rapid switch in process regime between wind‐driven rainfall and dry blow deflation in blanket peat environments. Dry blow processes of blanket peat erosion may become more important in UK upland areas if climate change promotes more frequent surface desiccation. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
52.
浅述次生孔隙的成因   总被引:9,自引:2,他引:9  
李义军 《西北地质》2002,35(1):65-69
生油岩生成的油气,必须运移到储集层中贮存起来,这样油气才可成藏,而岩层的孔隙度是其储集性好坏的重要标志,次生孔隙在此更是有极重要的意义(因为随着岩层的埋深,成岩作用不断在进行,原生孔隙将减少,次生孔隙将在储油方面扮演重要角色),因此,笔者着重论述次生孔隙的成因及其相关问题。  相似文献   
53.
简要介绍了第13届国际变形机理、流变学和构造学学术会议的概况和特点,综述了地震变形作用的研究现状。  相似文献   
54.
农业生态的化学元素分类及其与作物优质高产的关系   总被引:12,自引:0,他引:12  
曾昭华 《湖南地质》2001,20(4):277-283
利用国家科学技术成果,并根据国内外研究现状,将103个化学元素分成对作物生长发育、增产起显著作用、起一定作用、起有害作用和起未知作用4个农业生态元素类型,进而论述了它们与作物优质高产的关系。  相似文献   
55.
论述了水文地质工作者正在应用的几种数学插值方法的特点,针对不足之处,提出了改进方案。  相似文献   
56.
Evert Lagrou 《GeoJournal》2000,51(1-2):99-112
The paper describes the different capital city functions that are accomodated in Brussels. There are in fact five different capitals in Brussels in search of a place. Their locations are conditioned by Brussels' eventful history. The analysis subsequently focuses on the rather negative relation between the local public intellectuals and the city's European capital functions. The location of the European headquarters within the existing urban fabric – unlike Luxemburg and Strasbourg – has caused residential expulsion and further confrontations with the local community during all expansions.Brussels has the most politically decisive institutions. The competition with Luxemburg and Strasbourg has in the past given rise to dubious building processes and hidden agendas regarding the further construction of the necessary space for Europe.Unlike most other capitals Brussels has a low degree of esteem from its citizens. The After-May 68- Movement reacting initially against the CIAM-realisations during the 1970s, continues its activities up until today against two highly symbolic projects: the North Station area called the Manhattan Quarter and the European Headquarters. The movement launched the term Brusselization on the international forum and continues to promote the negative image of its city – notwithstanding the complete change of the official urban planning.The article is a plea for the integration of the actual one-sided advocacy planning within a more comprehensive urban planning approach where strong and soft functions are more in balance. This can also improve the quality of the new European campus now under consideration to accomodate the entry of the East European states in the Union.  相似文献   
57.
地下水浮力作用机理模型试验设计与实施   总被引:4,自引:0,他引:4  
城市空间日趋紧缺,由此引发了地下空间开发的飞速发展,然而各类地下工程的安全运营都无一例外地受地下水动态变化的强烈制约,因地下水影响而引发了一系列新的岩土工程问题,其中尤为突出的是地下结构物的抗浮评价问题。对此,现有的规范尚未给出较为明确的规定,人们对此的认识也相当不统一;理论及概念层面的探讨较多,系统的试验及测试研究较少。详细介绍了用于研究地下水浮力作用机理的模型试验整体构思、试验装置设计及实施过程。  相似文献   
58.
1Introduction SomeSchiffbasecomplexesderivedfromaminoacidsareparticularlyactiveinbiologycatalysisandmaterial.Thestudyofthemisfocusofthestudyof coordinationchemistry.Recently,studiesofsuchmetalcomplexesofmono Schiffbaseshavebeenre ported(Fanetal.,2003a,b;L…  相似文献   
59.
Tidal conditions differently influence inter‐tidal organisms in terms of general physiological and metabolic responses. In this study we investigated the morphological response in shells of Mytilus galloprovincialis native to different micro‐tidal coastal environments in the Northern Adriatic Sea. Our purpose was to highlight the ecophenotypic variability across tidal levels and to elucidate how tidal currents and waves produced by anthropogenic activities may play a part in modulating shell morphology. Three sampling sites were selected: an open‐sea area 15 km off‐shore and two sites within the lagoon of Venice, the first near one of its three inlets, and the other one in the proximity of Venice city centre. At each sampling site, organisms were seasonally collected at different depths within their vertical zonation, either in the inter‐tidal zone – i.e. at both the highest and lowest tide zonation limits, and subtidally. The mussel shells were analysed by investigation of their morphometric relationships (height/length and width/length ratios) and by elliptic Fourier analysis of the shell contours. Shell thickness and condition index were also evaluated for a better comprehension of energy allocation/partitioning. Estimates based on long‐term measurements, visual observation, wind statistics and wave growth laws allowed an evaluation of the forces acting on shells. At the open‐sea site, the observed phenotypic variability of both shell shape and thickness was clearly related to the tidal vertical zonation. At the two lagoon sites, the currents generated by tidal flow through the inlet and the waves caused by the frequent passage of boats influenced both shell shape and thickness. A trade‐off between protection and growth was apparent along the tide gradient, as emphasized by the differences in the partitioning and allocation of energy between shell and flesh production.  相似文献   
60.
为研究青蛤(Cyclina sinensis)对重金属Cu~(2+)的蓄积作用及免疫机能的影响,开展了Cu~(2+)对青蛤的急性毒性实验,观察青蛤在96 h Cu~(2+)半致死浓度和安全浓度胁迫下,不同组织的蓄积趋势,及血淋巴液中SOD、CAT和ACP活性的变化。结果显示:Cu~(2+)的半致死浓度为0.807 mg/L,安全浓度为0.00807 mg/L。鳃和内脏团组织中蓄积的Cu~(2+)浓度与处理时间呈正相关;在半致死和安全浓度胁迫下,鳃组织中Cu~(2+)蓄积速度均快于内脏团;在半致死浓度胁迫下血淋巴液中SOD、CAT和ACP活性呈现先诱导再抑制的趋势,安全浓度下无明显变化,但均高于对照组水平。  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