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   1687篇
  免费   90篇
  国内免费   457篇
测绘学   58篇
大气科学   182篇
地球物理   690篇
地质学   701篇
海洋学   358篇
天文学   60篇
综合类   63篇
自然地理   122篇
  2023年   9篇
  2022年   13篇
  2021年   12篇
  2020年   17篇
  2019年   10篇
  2018年   25篇
  2016年   25篇
  2015年   16篇
  2014年   29篇
  2013年   27篇
  2012年   91篇
  2011年   122篇
  2010年   39篇
  2009年   41篇
  2008年   97篇
  2007年   96篇
  2006年   98篇
  2005年   89篇
  2004年   104篇
  2003年   59篇
  2002年   77篇
  2001年   57篇
  2000年   66篇
  1999年   35篇
  1998年   41篇
  1997年   35篇
  1996年   39篇
  1995年   55篇
  1994年   44篇
  1993年   54篇
  1992年   42篇
  1991年   63篇
  1990年   54篇
  1989年   58篇
  1988年   65篇
  1987年   41篇
  1986年   33篇
  1985年   40篇
  1984年   24篇
  1983年   24篇
  1982年   17篇
  1981年   13篇
  1980年   17篇
  1964年   14篇
  1943年   8篇
  1942年   9篇
  1937年   14篇
  1935年   9篇
  1934年   10篇
  1923年   10篇
排序方式: 共有2234条查询结果,搜索用时 46 毫秒
991.
During the past several months, we have been considering "Special Issues" on a variety of topics for the journal of Earthquake Engineering and Engineering Vibration. We are pleased to announce that the December 2008 issue will be  相似文献   
992.
INTRODUCTION Studies on vertebrate palaeontology in China were begun in the lastone and a half decade.Before the year 1920 fossil bones such as thosedescribed by Owen,Lydekker,Koken,Schlosser and others were mostlycollected in an indirect way,that is to say,from Chinese pharmacy  相似文献   
993.
The prevailing ’consensus’ explanation for Phanerozoic development of eastern Australia envisages a ’retreating accretionary orogen’ in which episodic slab retreat resulted in development of offshore island arcs in front of marginal basins.Periodically  相似文献   
994.
We present an efficient,robust computational method for modeling the Newtonian dynamics for rotation curve analysis of thin-disk galaxies.With appropriate mathematical treatments,the apparent numerical difficulties associated with singularities in computing elliptic integrals are completely removed.Using a boundary element discretization procedure,the governing equations are transformed into a linear algebra matrix equation that can be solved by straightforward Gauss elimination in one step without further ...  相似文献   
995.
北黄海中部晚更新世末以来低海面时期发育的硬质粘土层是古环境变化信息的重要载体。文章运用沉积学、孢粉学和年代学等方法,从样品中提取出了可指示北黄海硬质粘土层成因的孢粉、藻类化石,并揭示了其记录的古环境和古气候特征。研究表明,该硬质粘土层的形成年龄介于12602~10357cal.a B.P.之间,年代上与发生在12.9~11.6ka期间的末次冰消新仙女木气候回冷事件相吻合,与不同沉积环境中形成的北黄海泥炭层为同一时期的沉积,可作为新仙女木事件在北黄海陆架响应的一个重要证据。高含量淡水藻类(环纹藻、盘星藻)与香蒲的存在表明北黄海硬质粘土层的形成环境主要为淡水水域,而硬质粘土层中藜科-蒿属的存在指示其形成期间间或受到海水的影响,其上下层段海生沟鞭藻含量的增加说明硬质粘土层沉积初期及形成后期海水作用的增强,淡水藻类与陆生植物孢粉此消彼长的变化规律反映了硬质粘土层形成的阶段性特征。硬质粘土层中蒿属-香蒲-松-禾本科-单缝孢和松-蒿属-禾本科-单缝孢-香蒲孢粉组合表明研究区当时处于寒冷而湿润的环境,低地为以河流湿地为主的平原草甸,周边山地有针阔叶混交林分布。北黄海晚更新世末期形成的硬质粘土层与长江三角洲地区同期形成的硬质粘土层孢粉藻类组合中均出现高含量的淡水藻类化石,而差异性体现在前者存在一定比例的蒿属藜科植物花粉,主要由两者所处的纬度不同所致。  相似文献   
996.
考虑到在这些缺乏技术经验的国家,在实施和利用地下基础设施技术方面,教育是基本的方法,作者针对隧道及非开挖技术在国际上的使用设计了一个精品课程。该课程将覆盖传统隧道的主要设计原则,随后向学生介绍非开挖技术的基本设计原则。在专门设计练习中,作者通过使用MathCad and MS Excel来补充课程的最后一章节。最后的结果是:为在地下基础设施建设富有经验和参与不同地下工程规划、设计的工程师量身定做的一节48小时的密集课程。本课程将提供给超过五个土木工程系的麦德林、哥伦比亚的几所大学。  相似文献   
997.
The AravallieDelhi and Satpura Mobile Belts(ADMB and SMB)and the Eastern Ghat Mobile Belt(EGMB)in India form major Proterozoic mobile belts with adjoining cratons and contemporary basins.The most convincing features of the ADMB and the SMB have been the crustal layers dipping from both sides in opposite directions,crustal thickening(w45 km)and high density and high conductivity rocks in upper/lower crust associated with faults/thrusts.These observations indicate convergence while domal type refectors in the lower crust suggest an extensional rifting phase.In case of the SMB,even the remnant of the subducting slab characterized by high conductive and low density slab in lithospheric mantle up to w120 km across the PurnaeGodavari river faults has been traced which may be caused by fuids due to metamorphism.Subduction related intrusives of the SMB south of it and the ADMB west of it suggest NeS and EeW directed convergence and subduction during MesoeNeoproterozoic convergence.The simultaneous EeW convergence between the Bundelkhand craton and Marwar craton(Western Rajasthan)across the ADMB and the NeS convergence between the Bundelkhand craton and the Bhandara and Dharwar cratons across the SMB suggest that the forces of convergence might have been in a NEeSW direction with EeW and NeS components in the two cases,respectively.This explains the arcuate shaped collision zone of the ADMB and the SMB which are connected in their western part.The Eastern Ghat Mobile Belt(EGMB)also shows signatures of E eW directed MesoeNeoproterozoic convergence with East Antarctica similar to ADMB in north India.Foreland basins such as Vindhyan(ADMBeSMB),and Kurnool(EGMB)Supergroups of rocks were formed during this convergence.Older rocks such as Aravalli(ADMB),MahakoshaleBijawar(SMB),and Cuddapah(EGMB)Supergroups of rocks with several basic/ultrabasic intrusives along these mobile belts,plausibly formed during an earlier episode of rifting during PaleoeMesoproterozoic period.They are highly disturbed and deformed due to subsequent MesoeNeoproterozoic convergence.As these Paleoproterozoic basins are characterized by large scale basic/ultrabasic intrusives that are considerably wide spread,it is suggested that a plume/superplume might have existed under the Indian cratons at that time which was responsible for the breakup of these cratons.Further,the presence of older intrusives in these mobile belts suggests that there might have been some form of convergence also during Paleoproterozoic period.  相似文献   
998.
Cearadactylus atrox is one of the first pterosaurs unearthed from the Araripe Basin (Northeast Brazil) (Leonardi and Borgomanero 1983, 1985) (Fig.1). The fossil was preserved in a calcareous concretion typical of the Romualdo Formation and comprised an almost complete skull with lower jaw, but lacked the occipital region.  相似文献   
999.
Reducing long-term remedial costs by transport modeling optimization   总被引:1,自引:0,他引:1  
The Department of Defense (DoD) Environmental Security Technology Certification Program and the Environmental Protection Agency sponsored a project to evaluate the benefits and utility of contaminant transport simulation-optimization algorithms against traditional (trial and error) modeling approaches. Three pump-and-treat facilities operated by the DoD were selected for inclusion in the project. Three optimization formulations were developed for each facility and solved independently by three modeling teams (two using simulation-optimization algorithms and one applying trial-and-error methods). The results clearly indicate that simulation-optimization methods are able to search a wider range of well locations and flow rates and identify better solutions than current trial-and-error approaches. The solutions found were 5% to 50% better than those obtained using trial-and-error (measured using optimal objective function values), with an average improvement of approximately 20%. This translated into potential savings ranging from 600,000 dollars to 10,000,000 dollars for the three sites. In nearly all cases, the cost savings easily outweighed the costs of the optimization. To reduce computational requirements, in some cases the simulation-optimization groups applied multiple mathematical algorithms, solved a series of modified subproblems, and/or fit "meta-models" such as neural networks or regression models to replace time-consuming simulation models in the optimization algorithm. The optimal solutions did not account for the uncertainties inherent in the modeling process. This project illustrates that transport simulation-optimization techniques are practical for real problems. However, applying the techniques in an efficient manner requires expertise and should involve iterative modification to the formulations based on interim results.  相似文献   
1000.
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