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   1253篇
  免费   316篇
  国内免费   499篇
测绘学   53篇
大气科学   58篇
地球物理   163篇
地质学   1328篇
海洋学   120篇
天文学   28篇
综合类   53篇
自然地理   265篇
  2024年   7篇
  2023年   17篇
  2022年   44篇
  2021年   60篇
  2020年   78篇
  2019年   71篇
  2018年   73篇
  2017年   60篇
  2016年   76篇
  2015年   82篇
  2014年   103篇
  2013年   100篇
  2012年   100篇
  2011年   92篇
  2010年   86篇
  2009年   108篇
  2008年   101篇
  2007年   88篇
  2006年   117篇
  2005年   79篇
  2004年   77篇
  2003年   81篇
  2002年   64篇
  2001年   42篇
  2000年   80篇
  1999年   43篇
  1998年   26篇
  1997年   22篇
  1996年   16篇
  1995年   21篇
  1994年   12篇
  1993年   11篇
  1992年   1篇
  1991年   11篇
  1990年   6篇
  1989年   2篇
  1987年   3篇
  1986年   1篇
  1985年   2篇
  1984年   2篇
  1983年   1篇
  1977年   1篇
  1954年   1篇
排序方式: 共有2068条查询结果,搜索用时 15 毫秒
41.
目前研究海平面变化的方法尚不能精确确定海平面变化,对记录海平面变化的沉积体或层序物质组成及结构特点的研究尚显薄弱。在分析层序不对称性与海平面变化的关系基础,提出层序不对称系数S0,认为海平面的升降变化对浅水沉积区影响显著,其结果必然造成层序S0的增加;海平面的升降变化对沉水沉积区影响不大,层序S0较为恒定。对进行详细研究中扬子区S0分布特点,运用S0曲线计算了中扬子区海平面变化特点,并与其他方法进行对比,认为可以用S0判断水体相对深浅,分析海平面变化。  相似文献   
42.
鄂尔多斯盆地中部地区下古生界碳酸盐岩生烃潜力探讨   总被引:4,自引:0,他引:4  
下古生界高成熟-过成熟碳酸盐岩的生烃潜力一直是困扰我国海相地层油气评价和勘探的重要问题,至今尚未得到很好的解决。本采用有机岩石学和有机地球化学方法,系统剖析了鄂尔多斯盆地中部地区下古生界碳酸盐岩的基本特征及其生烃潜力。结果表明,鄂尔多斯盆地中部地区发育下古生界海相碳酸盐岩,烃源岩厚度较大,有机质丰度较低,有机质类型主要为I型和Ⅱ型,有机质热演化已达到过成熟阶段,这套烃源岩是该区重要的有效气源岩,具有较好的生烃潜力,可为中部大气田提供良好的气源条件。  相似文献   
43.
利用电子探针和激光探针剥蚀系统(LA-ICP-MS),对北羌塘新第三纪粗面玄武岩中的橄榄石主元素和微量、稀土元素进行了系统分析。结果表明,本区橄榄石Fo平均为88,属贵橄榄石种属。相对富集Ni、Co和重稀土,而强烈亏损轻稀土及Rb、Sr、Ba、Zr等大离子亲石元素。其稀土元素配分型配分型式与粗面玄武岩全岩稀土配分型式呈互补状态。  相似文献   
44.
It has long been debated that the Dabie orogenic belt belongs to the North China or Yangtze craton. In recent years, eastern China has been suggested, based on the Pb isotopic compositions of Phanerozoic ore and Mesozoic granitoid K-feldspar (revealing the crust Pb) in combination with Meso-Cenozoic basalts (revealing the mantle Pb), being divided into the North China and Yangtze Pb isotopic provinces, where the crust and mantle of the Yangtze craton are characterized by more radiogenic Pb. In this sense, previous researchers suggested that the pro-EW-trending Dabie crogenic belt with less radiogenic Pb in the crust was part of the North China craton. In this paper, however, the Late Cretaceous basalts in the central and southern parts of the Dabie orogenic belt are characterized by some more radiogenic Pb (206Pb/204Pb=17.936−18.349,207Pb/204Pb=15.500−15.688,208Pb/204Pb=38.399−38.775) and a unique U-Th-Pb trace element system similar to those of the Yangtze craton, showing that the Mesozoic mantle is of the Yangtze type. In addition, the decoupled Pb isotopic compositions between crust and mantle were considerably derived from their rheological inhomogeneity, implying a complicated evolution of the Dabie orogenic belt. The study was funded by the National Natural Science Foundation of China (No. 49794043) and the Open Laboratory of Constitution, Interaction and Dynamics of the Crust-Mantle System, China.  相似文献   
45.
Apatite fission track analysis was performed on 56 samples from central Spain to unravel the far field effects of the Alpine plate tectonic history of Iberia. The modelled thermal histories reveal complex cooling in the Cenozoic, indicative of intermittent denudation. Accelerated cooling events occurred across the Spanish Central System (SCS) from the Middle Eocene to Recent. These accelerated cooling events resulted in up to 2.8±0.9 km of denudation in the western Sierra de Gredos and 3.6±1.0 km in the central and eastern Gredos (assuming a paleogeothermal gradient of 28±5 °C and a surface temperature of 10 °C). The greatest amount of denudation (5.0±1.6 km) occurred in the Sierra de Guadarrama. Accompanying rock uplift was 4.7±1.0 and 5.9±1.6 km in the eastern Gredos and Guadarrama, respectively. Most denudation in the Gredos occurred from the Middle Eocene to the Early Miocene and can be related to the N–S stress field, induced by the Pyrenean compression. In the Guadarrama, the greatest denudation was Pliocene to Recent of age and seems related to the ongoing NW–SE Betic compression. The fact that the formation of the E–W trending Gredos coincides with the N–S Pyrenean compression and the creation of the present day morphology of the NE–SW trending Guadarrama with the younger NW–SE Betic compression, indicates that they record the far field effects of Alpine plate tectonics on Iberia. The trend of pre-existing lineaments was of major importance in influencing the style and magnitude of these of far field effects.  相似文献   
46.
The mobile food service practice is one of multiple survival strategies adopted by poor urban households in Cameroon to maintain and expand the base of subsistence incomes, especially in the current surge of economic crisis. Though a fast-growing informal sector enterprise, it is still at an artisanal stage in urban Cameroon, creating an urgent need for a supportive policy environment that could have measurable positive impacts on improving the productivity, welfare and income levels of the micro-entrepreneurs. This study looks at the mobile food service practice in Kumba, Cameroon, in terms of its basic characteristics, the locational factors influencing its socio-spatial distribution, the critical success factors (CSFs) determining customer choices, and its impacts on the local environmental resources and quality of urban life. The mobile food service practice creates employment, generates income, and acts as a food energy-support instrument to the urban poor and local economic activities operating in Kumba. The vendors, who are mostly women, can make incomes that are 405 per cent of the national minimum wage and, thus, contribute financially towards the education, health and survival of their families. The paper provides some recommendations on ways to improve the efficiency of this sector so as to achieve sustainable economic and social development and to enhance empowerment thereof.  相似文献   
47.
A greenhouse experiment was conducted to investigate the effects of moderate (14 cm) vs. severe (7 cm) defoliation on tiller recruitment and mortality within the interior and on the perimeter of weeping lovegrass (Eragrostis curvula) tussocks. Tiller number per marked shoot within the interior was significantly greater in plants defoliated at 14-cm stubble height than at 7-cm stubble height (2·37 vs. 0·43, p<0·05) 28 weeks after defoliation; likewise, tiller number per marked shoot was also greater on the perimeter of tussocks in the higher stubble than the lower stubble defoliation treatment (5·10 vs. 2·03, p<0·05). Tiller natality increased significantly 4 weeks after defoliation except for tillers cut to 7-cm stubble height within the interior of tussocks where tiller natality reached a peak at 4·5 weeks after defoliation. During the later stages of canopy development, tiller numbers per marked shoot gradually declined as tiller mortality increased. Tillers began senescing 13 weeks after defoliation, and the senescence rate was 52% greater in the 7-cm stubble height treatment than in the 14-cm stubble height treatment (2·33 vs. 1·53, p<0·0001). Within the interior of tussocks, number of the marked tillers more than doubled under the 14-cm stubble height treatment, whereas the plants lost 57% of the monitored tillers under the 7-cm stubble height treatment. We concluded that the central dieback process in weeping lovegrass is accelerated by severe defoliation.  相似文献   
48.
山东省省际边界地区发展研究   总被引:17,自引:0,他引:17  
省际边界地区有其自身的发展特点,对于区域的发展起着特殊的作用。文章分析了山东省省际边界地区的特点,并根据康维斯(P.D.Converse)断裂点理论,对山东省及周边中心城市的引力范围进行了计算,比较分析了目前山东省省界城市和省内城市吸引范围及竞争力差异,提出了山东省发展省际边界地区的相应对策。  相似文献   
49.
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   
50.
羌塘盆地最大规模沥青脉的发现及其意义   总被引:12,自引:1,他引:12       下载免费PDF全文
在羌塘盆地已经发现了190余处油气显示。近年笔者在盆地北部晚侏罗世地层中发现了迄今最大规模的沥青脉。研究结果表明,该沥青脉产出于上侏罗统索瓦组地层之中,地层下部系与沥青脉的形成有一定关系的中株罗统夏里组局部含油气显示的盐丘。沥青族组分(%)为:饱和烃5.72、芳烃18.46、沥青质52.37、非烃23.45,说明核沥青脉经过了运移、氮化的过程。同时,沥青脉产出于复背斜范围内,利于油气聚集成藏。这一发现对于羌塘盆地的油气地质的作用具有现实意义。  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