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   1414篇
  免费   231篇
  国内免费   412篇
测绘学   73篇
大气科学   62篇
地球物理   223篇
地质学   1159篇
海洋学   129篇
天文学   28篇
综合类   87篇
自然地理   296篇
  2024年   15篇
  2023年   20篇
  2022年   47篇
  2021年   65篇
  2020年   75篇
  2019年   75篇
  2018年   70篇
  2017年   64篇
  2016年   73篇
  2015年   67篇
  2014年   81篇
  2013年   116篇
  2012年   84篇
  2011年   97篇
  2010年   74篇
  2009年   107篇
  2008年   93篇
  2007年   75篇
  2006年   100篇
  2005年   88篇
  2004年   78篇
  2003年   78篇
  2002年   61篇
  2001年   42篇
  2000年   79篇
  1999年   46篇
  1998年   32篇
  1997年   29篇
  1996年   23篇
  1995年   24篇
  1994年   21篇
  1993年   14篇
  1992年   2篇
  1991年   17篇
  1990年   6篇
  1989年   7篇
  1987年   3篇
  1986年   1篇
  1985年   2篇
  1984年   2篇
  1983年   1篇
  1980年   1篇
  1977年   2篇
排序方式: 共有2057条查询结果,搜索用时 46 毫秒
591.
华北地台沉积岩型金矿床的找矿勘查意义   总被引:1,自引:0,他引:1  
冀东地区沉积岩型金矿床主要赋存在中上元古界长城系和蓟县系巨厚钙-镁碳酸盐岩内,金矿化受层间角砾岩带控制。相比之下,内蒙古中西部沉积岩型金矿床主要产出在上元古界青白不钙质粉砂岩和板岩内,矿父母产状受层间构造破碎带控制。尽管上述金矿床的成矿环境、赋矿围岩和成矿时代存在一定差别,但是它们均以埋藏浅、规模大、物质组分简单和易选冶为特点。为突出这两类金矿床的区域性特征,前者命为“长城式”,后者则确定为“阿拉  相似文献   
592.
张朝晖  陈家宽 《中国岩溶》2007,26(2):170-177
对贵州中部18个喀斯特瀑布水生苔藓植物进行了研究。瀑布苔藓植物区系种类由14 科23 属38 种组成, 藓类( Mosses) 含8 科17 属31 种, 苔类( Liv erw o rts) 5科5属6种,而角苔类( Ho rnwo r t)仅1科1属1种。区系地理成分含世界分布( 15. 79% ) , 北温带分布( 36. 84% ) ,旧世界温带分布( 5. 26% )、泛热带分布( 2. 63% )、热带亚洲分布( 5. 267% ) , 东亚分布( 26. 32% ) 和中国特有( 7. 90% )等7种成分。根据喀斯特瀑布沉积环境和苔藓植物优势种类组成特征,可划分黔中瀑布苔藓沉积生态类型为溪流瀑布沉积、河流瀑布沉积、石灰华坝沉积和瀑布洞穴沉积4种类型及15种生物喀斯特沉积亚类型。   相似文献   
593.
第三体摄动分析解的一种表达式   总被引:2,自引:0,他引:2  
季江徽  刘林  张伟 《天文学报》2000,41(1):79-92
在太阳系中,大行星、小行星和卫星(包括自然卫星和人造卫星)等对应的运动问题,都可以处理成受摄二体问题,而摄动源又多为第三体,作为第三体的摄动天体,有的比运动天体离中心天体近,有的则相反,前者称为内摄内体,全者则称为外摄天体,对一个具体的运动天体,可以同时出现这两个摄动天体,但是,只要运动天体与摄动天体的轨道都建立在以中心天体(质心)为坐标原点的同一坐标系内,那么在一定条件下(即除运动天体与摄动天体  相似文献   
594.
A devastating landslide on 18 August 1998 near Malpa Village in Kali Valley of Higher Kumaun Himalaya killed 221 persons. The landslide was a complex rock fall–debris flow. The mass movement generated around one million cubic metres of debris and partially blocked the Kali River, Malpa Gad (a tributary of Kali) being blocked completely. The rock mass failed primarily due to the near vertical slopes hanging over the valley along joints, the formation of structural wedges along the free face, the sheared rock mass due to the close proximity of major tectonic planes, and the enhanced pore–water pressure due to prolonged heavy precipitation in the preceding days. The mesoscopic shear zone, exhibiting ramp and flat structure in quartzites, shows a southward thrust movement that might have generated shear stress in the rocks. The slide clearly demonstrates the distressed state of the rock mass in the Himalayan region due to the ongoing northward drift of the Indian plate.  相似文献   
595.
The passage of subtropical cold fronts through central Australia produces the only significant mesoscale meteorological features in the region. The interaction of these cold fronts with the surface energy balance strongly affects the local weather and climate. The surface energy balance was measured at a semi-arid site in Alice Springs, central Australia, to determine how it was influenced by the passage of subtropical cold fronts. Both Bowen ratio and eddy correlation methods were used. The daytime energy balance of the site showed high net radiation that was partitioned into 75% sensible heat flux and 25% soil heat flux with little or no latent heat flux. At night there was a large net radiative loss that was balanced primarily by a loss of heat from the soil. The cold fronts predominately passed through Alice Springs at night and showed a strong surface signature. The fronts brought moister air resulting in higher water vapour pressures during their passage. The nocturnal boundary layer was often disturbed as the front passed, resulting in warm, moist air being mixed down toward the land surface. Mixing decreased the soil heat flux and increased latent heat fluxes toward the surface. Moisture that accumulated at the surface at these times was often evaporated after a return to drier conditions. During the daytime, surface signatures in soil and sensible heat fluxes were less distinct due to the strong convective mixing. Latent heat fluxes followed a similar trend to the nocturnal case.  相似文献   
596.
Abstract. To evaluate the effects of beach morphodynamics upon the abundance, tidal movement, population structure and burrowing rate of the crab Emerita analoga (Stimpson) (Anomura, Hippidae) we sampled two beaches in south central Chile (ca. 42° S), Mar Brava and Ahui with dissipative and reflective characteristics, res­pectively. The swash zone at the dissipative beach was 5 – 6 times wider than that of the reflective beach. A at the dissipative beach, upwash speeds were higher and the number of effluent line crossings were lower by more than an order of magnitude. To examine the tidal movement of E. analoga, we collected crabs from 5 to 6 tidal levels of each beach every 2 h across 12 h of the tidal cycle. The intertidal distribution of crabs differed between beaches; i. e., at the dissipative beach they were primarily located at the swash zone, while at the reflective beach they were mostly located at the low tide level and shallow subtidal. The change in position of crabs was pronounced across the tidal cycle at the dissipative beach (Mar Brava), with most of the animals remaining in the active swash zone. Body size data were used to construct size frequency distributions for each population. Crabs from the dissipative beach reached larger sizes than those at the reflective beach. Sediments were coarser at the latter versus the former beach. Crabs burrowed at similar rates in the sand from both beaches, a result which supports the idea that E. analoga is a “sediment generalist” capable of burrowing successfully in a wide range of sediment types. This characteristic is likely a key to the broad success of this species on the full range of beach morphodynamic types along the coasts of South and North America.  相似文献   
597.
王益萍 《天文学报》2000,41(4):410-423
在“等级式成团”(hierarchical clustering)宇宙学演化框架下,早型星系(E/SO)的形成和演化不是如经典的“整体塌缩”(monolithic collapse)演化模式所描述的是由高红移处的短时间剧烈恒星形成过程一次性完成的;相反,它们可能是在大尺度结构形成过程中由盘星系间的相互合并演化而来,特别是对于质量相当的盘星系间的剧烈合并过程。目前的数值模拟和高分辨成像观测都已证实了这种可能性,而且红移巡天的结果也更多地支持这种演化模式。在此提出一个星系合并过程中核区星暴和中心黑洞共生演化的模型,来解释由空间望远镜(HST)和地面CCD高分辨测光所得的早型星系“黑洞/核球”统计线性相关,也即中心黑洞质量约为其核球体质量的0.6%;同时,该演化模型还可以进一步合理地解释在类星体的近红外成像观测中统计得出的相似的线性相关性,即类星体中心亮度与其母星系质量成正比。并给出此模型计算的极限情况和模型的解析估计。  相似文献   
598.
南岭中段主要锡矿床He、Ar同位素组成及其意义   总被引:8,自引:0,他引:8  
对南岭中段主要锡矿床的黄铁矿等进行了流体包裹体的He、Ar同位素研究。结果表明,成矿流体的40Ar/36Ar≈288~371,3He/4He≈0.09~28.58Ra ,不同地区流体包裹体He、Ar浓度变化较大,而在同一地区40Ar/36Ar、3He/4He值比较一致;40Ar/36Ar值介于大气饱和水(包括大气降水和海水)的同位素组成(40Ar/36Ar≈295.5)和夏威夷热点(40Ar/36Ar=350~360)之间,3He/4He值大大高于大陆地壳特征值(3He/4He≈0.03Ra),介于地壳特征值与原始地幔特征值(3He/4He≈8.57Ra)和夏威夷热点特征值(3He/4He≈27.14Ra)之间,表明成矿流体与地幔柱的活动有关,为地幔、地壳和大气水的混合产物,以地幔流体为主。这一结果为南岭地区锡多金属矿床为壳幔相互作用的产物及区域地幔柱的存在提供了新证据。  相似文献   
599.
According to the measured data of typhoons going over the Chinese coasts in 1949-2002,a statistic relative equation showing the relation between the central atmospheric pressure of typhoons in a certain region at a certain period of time and their accumulation of frequency is established, and the concept of recurrence interval of typhoons is put forward, which is of actual significance for typhoon disaster reduction along the coastal area.  相似文献   
600.
旅游中心地研究及其规划应用   总被引:35,自引:4,他引:31  
在对已有旅游中心地相关研究回顾的基础上,探讨了旅游中心地的概念与界定方法,分析了旅游中心地理论在旅游空间规划中的指导意义,并以吉林省旅游发展总体规划为例,介绍如何将旅游中心地的理论运用到实际的旅游规划当中。  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