全文获取类型
收费全文 | 4468篇 |
免费 | 970篇 |
国内免费 | 1856篇 |
专业分类
测绘学 | 90篇 |
大气科学 | 154篇 |
地球物理 | 2225篇 |
地质学 | 3313篇 |
海洋学 | 744篇 |
天文学 | 10篇 |
综合类 | 297篇 |
自然地理 | 461篇 |
出版年
2024年 | 26篇 |
2023年 | 83篇 |
2022年 | 160篇 |
2021年 | 161篇 |
2020年 | 213篇 |
2019年 | 275篇 |
2018年 | 234篇 |
2017年 | 222篇 |
2016年 | 268篇 |
2015年 | 250篇 |
2014年 | 285篇 |
2013年 | 360篇 |
2012年 | 287篇 |
2011年 | 340篇 |
2010年 | 283篇 |
2009年 | 332篇 |
2008年 | 311篇 |
2007年 | 346篇 |
2006年 | 350篇 |
2005年 | 281篇 |
2004年 | 279篇 |
2003年 | 227篇 |
2002年 | 202篇 |
2001年 | 150篇 |
2000年 | 191篇 |
1999年 | 174篇 |
1998年 | 153篇 |
1997年 | 140篇 |
1996年 | 136篇 |
1995年 | 99篇 |
1994年 | 100篇 |
1993年 | 78篇 |
1992年 | 82篇 |
1991年 | 44篇 |
1990年 | 49篇 |
1989年 | 32篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 9篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 6篇 |
1977年 | 8篇 |
1954年 | 6篇 |
排序方式: 共有7294条查询结果,搜索用时 31 毫秒
981.
982.
Ecological and functional roles of mycorrhizas in semi-arid ecosystems of Southeast Spain 总被引:4,自引:0,他引:4
J.M. Barea J. Palenzuela P. Cornejo I. Sánchez-Castro C. Navarro-Fernández A. Lopéz-García B. Estrada R. Azcón N. Ferrol C. Azcón-Aguilar 《Journal of Arid Environments》2011,75(12):1292-1301
Mycorrhizas are worldwide symbiotic associations established between certain soil fungi and most vascular plants and are fundamental in optimizing plant fitness and soil quality. Mycorrhizal symbioses improve the resilience of plant communities against environment stresses, including nutrient deficiency, drought and soil disturbance. Since these stresses are paramount in the degradation of semi-arid ecosystems in the SE Spain, a series of basic, strategic and applied studies have been made to ascertain how the activity and diversity of mycorrhizal fungi affect plant community composition, structure and dynamics in this region. These investigations are reviewed here in terms of: (i) analysing the diversity of mycorrhizal fungi; (ii) assessing the ecological and functional interactions among plant communities and their associated mycorrhizal fungal populations; and (iii) using mycorrhizal inoculation technology for the restoration of degraded semi-arid areas in Southeast Spain. Disturbance of the target semi-arid ecosystems decreases the density and diversity of mycorrhizal fungust populations. Nevertheless, the mycorrhizal propagules do not disappear completely suggesting a certain degree of stress adaptation, and these remaining, resilient ecotypes are being used as plant inoculants. Numerous field experiments, using plant species from the natural succession inoculated with a community of indigenous mycorrhizal fungi, have been carried out in revegetation projects in the semi-arid Iberian Southeast. This management strategy improved both plant development and soil quality, and is a successful biotechnological tool to aid the restoration of self-sustaining ecosystems. However, despite a 20-year history of this work, we lack a comprehensive view of the mycorrhizal potential to improve the composition, diversity, structure and functionality of drought-adapted plant communities in the Region. 相似文献
983.
Stephen Hill 《New Zealand geographer》2011,67(1):16-20
Geographers and psychologists both seek to understand people's behaviour and their cognitive and emotional lives so it is somewhat surprising that there has not been more theoretical cross‐fertilisation of the two disciplines. ‘On time, place and happiness’ is a rare but welcome exception, showing how research in social‐cognitive psychology might usefully clarify thinking about temporality, place and well‐being. This commentary examines several additional human memory phenomena that geographers might like to consider in their theorising about these matters. 相似文献
984.
Dragos Simandan 《New Zealand geographer》2011,67(1):21-24
The first part of the paper develops the argument that geographers should learn to decompose human memory into its constituent parts because then and then alone will we become attuned to the full range of ways in which we incorporate places into our beings. The second part of the paper articulates Stephen Hill's comments on episodic memory with my recent work on wisdom. 相似文献
985.
A tectonics sedimentation evolution has been researched in Southeast Chongqing, and the reasonable Longmaxi shale highstand system tract (HST) and transgressive system tract (TST) geological model were built respectively based on the rock mechanical test and acoustic emission experiment which the samples are from field outcrop and the Yuye-1 well. The Longmaxi shale two-dimension tectonic stress field during the Cenozoic was simulated by the finite element method, and the distribution of fractures was predicted. The research results show that the tectonic stress field and the distribution of fractures were controlled by lithology and structure. As a result of Cretaceous movement, there are trough-like folds (wide spaced synclines), battlement-like folds (similar spaces between synclines and anticlines) and ejective folds (wide spaced anticlines), which are regularly distributed from southeast to northwest in the study area. Since the strain rate and other physical factors such as the viscosity are not taken into account, and the stress intensity is the main factor that determines the tectonic strength. Therefore, the stronger tectonic strength leads the higher stress intensity in the eastern and southeastern study area than in the northwest. The fracture zones are mainly concentrated in the fold axis, transition locations of faults and folds, the regions where are adjacent to faults. The fragile mineral contents (such as siliceous rock, carbonate rock and feldspar) in the shelf facies shale from south of the study area are higher than in the bathyal facies and abyssal facies shale from center of the study area. The shales characterized by low Poisson’s ratio and high elastic modulus from south of the study area are easily broken during Cenozoic orogenic movement. 相似文献
986.
The natural stress state in the lithosphere consists of the vertical load and Poisson ratio, and then additional horizontal compression and extension (denoted by ΔσH and ΔσT, respectively) are assumed to be superimposed upon this gravitational stress field. The resulting stress state is composed of the maximum, medium and minimum stresses denoted by σ1, σ2, and σ3, respectively. The stress ratio is given as Φ = (σ2 − σ3)/(σ1 − σ3). A linear relation is found between Φ or 1/Φ and the vertical load in wrench-faulting and extensional stress regimes, respectively. The slope and intercept of the linear relation result in the additional horizontal stresses and level of (average) paleo-surface, respectively. Stress ratio is also determinable by the stress tensor inversion of fault-slip data. The earliest tectonic event (T_1 Event) in the Cretaceous Gyeongsang Basin consists of coexisting E–W compression and N–S extensional faulting episodes. Plots of Φ or 1/Φ against the burial depth (or vertical load) display several linear trends: two clusters in extensional episodes, and two or three clusters in wrench-faulting episodes. Because ΔσH is assumed to be null or negligible in the extensional regime, ΔσT is determinable from the slopes of two linear clusters as being −2.5 to −4.0 MPa. In wrench-faulting episodes, the values of ΔσH are given to be 61.6–101.4 MPa by applying determined additional horizontal extensions. Determined levels of average paleo-surfaces and those of syndepostional structures illustrate that more than five wrench-faulting or extensional episodes have occurred during the T_1 Event, whose active age, consequently, ranges from the Barremian to the Coniacian. This supports that the coexisting coaxial faulting episodes with the same extension may correspond to the alternation of wrench-faulting and extensional episodes. 相似文献
987.
不同应力条件下断裂带结构发育特征探讨 总被引:1,自引:0,他引:1
脆性断层断裂带内部结构主要包括破碎带和诱导裂缝带两部分。断层性质、断层活动性、埋深、断层两盘(主动盘、被动盘)、岩石力学性质、距离断层远近、断层组合规律等都可以成为影响断裂带内部结构发育的因素。通过讨论不同的应力条件下断裂带内部结构的发育特征,认为正断层形成的破碎带范围较小,裂隙形态不规则,诱导裂缝带中发育张裂隙和剪裂隙;由逆断层作用形成的破碎带范围较大,诱导裂缝带主要是由张裂隙和剪裂隙组成。由平移断层作用形成的破碎带延伸较稳定。 相似文献
988.
临汾区块地应力三维数值模拟研究 总被引:1,自引:0,他引:1
在地质构造分析和岩石力学性质测试的基础上,进行了临汾区块的地应力有限元三维数值模拟研究,探讨了地应力在区域和层域上的分布规律及其对区内煤层气成藏的影响。研究表明,应力强度总体上随深度递增,断层对应力强度分布具有明显的影响;断层附近应力强度的最大值分布在断层与断层的交叉点、断层与底部硬质岩层界面附近,应力性质在断层夹块内和支断层一侧发生转变出现拉应力区,断层与地表的交点附近也呈现局部拉应力区;煤层软弱带呈现应力降低。研究区的地应力分布特征有利于煤层气从深部向浅部运移,并在背斜轴部相对富集。 相似文献
989.
ANSYS三维地应力场数值模拟方法应用研究 总被引:2,自引:1,他引:1
初始地应力场由自重应力场和构造应力场叠加而成,是深部地下工程分析必须考虑的重要因素。针对目前常用的三维地应力场数值模拟方法在维持地应力场分布规律和避免附加位移方面存在的缺陷,进行了ANSYS地应力场模拟方法的比较研究。用java语言编写了针对不同ANSYS版本的初始应力文件合并程序,提出了高版本ANSYS初始应力文件合并的EXCEL简易方法,实现了不需编写任何其他程序、对所有类型单元通用的ANSYS初始应力平衡等效节点反力法,构建了一套实用的ANSYS三维地应力场模拟方法流程。 相似文献
990.