全文获取类型
收费全文 | 14986篇 |
免费 | 3207篇 |
国内免费 | 4726篇 |
专业分类
测绘学 | 857篇 |
大气科学 | 1808篇 |
地球物理 | 5799篇 |
地质学 | 9668篇 |
海洋学 | 1930篇 |
天文学 | 497篇 |
综合类 | 1038篇 |
自然地理 | 1322篇 |
出版年
2024年 | 101篇 |
2023年 | 273篇 |
2022年 | 468篇 |
2021年 | 530篇 |
2020年 | 653篇 |
2019年 | 777篇 |
2018年 | 618篇 |
2017年 | 653篇 |
2016年 | 762篇 |
2015年 | 816篇 |
2014年 | 928篇 |
2013年 | 1086篇 |
2012年 | 973篇 |
2011年 | 1037篇 |
2010年 | 863篇 |
2009年 | 1076篇 |
2008年 | 976篇 |
2007年 | 1144篇 |
2006年 | 1091篇 |
2005年 | 841篇 |
2004年 | 812篇 |
2003年 | 735篇 |
2002年 | 645篇 |
2001年 | 531篇 |
2000年 | 596篇 |
1999年 | 544篇 |
1998年 | 480篇 |
1997年 | 476篇 |
1996年 | 461篇 |
1995年 | 357篇 |
1994年 | 337篇 |
1993年 | 292篇 |
1992年 | 230篇 |
1991年 | 158篇 |
1990年 | 159篇 |
1989年 | 128篇 |
1988年 | 102篇 |
1987年 | 64篇 |
1986年 | 28篇 |
1985年 | 26篇 |
1984年 | 23篇 |
1983年 | 6篇 |
1982年 | 10篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1977年 | 13篇 |
1974年 | 1篇 |
1954年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
南宁、百色盆地红土风化壳的地球化学特征及其环境意义* 总被引:4,自引:0,他引:4
本文以广西南宁盆地那桐风化壳,及百色盆地江泽风地壳的大量土体化学分析数据为依据,进行了第四纪红土的确认、形成、分布和土壤肥力状况。以及通过地球化学强度、硅铁系数、硅铝系数等指标进行了两风化壳的对比研究。同时,还探讨了风化壳的发育过程及其与环境的关系。 相似文献
22.
The superimposed basin must have undergone the changes of regional stress field. Study on the nature and switch of regional stress field of superimposed basin is very useful to understanding its stress state and tectonic events during its formation and evolution. As sensitive markers of small stress changes, joint and shear fracture, characterized by consistency of orientation over wide area, can be used to reconstruct paleostress state and its evolution. Detailed observations and analysis on the orientations, geometrical patterns, sequences of joints and shear fractures and their chronological relation to faults and folds show that, the NEE-SWW systematic joints and NNW-SSE systematic joints developed in the Mesozoic and Cenozoic strata are much more prominent than NW-SE systematic joints and shear fractures with different orientations. And the NWW-SEE and NW-SE systematic joints formed later than NEE-SWW systematic joints but earlier than shear fractures with different orientations. According to the relationships between joint and shear fractures and stress, the NEE-SWW systematic joints are inferred to result from lateral weak extension caused by the late Cretaceous regional uplift, while the NNW-SSE and NW-SE systematic joints are interpreted as syn-tectonic deformation relating to strong N-S compression in the Neogene. But some conjugate shear fractures occur probably due to sinistral strike-slip faulting in the Kuqa depression. At the beginning of the Neogene, the stress field changed and the maximal principal stress σ1 switched from vertical to horizontal. 相似文献
23.
Kela 2 Gas Field, with high formation pressure (74.35MPa), high pressure coefficient (2.022) and difficulty of potential test and evaluation, is the largest integrated proved dry gas reservoir in China so far and the principal source for West-East Gas Development Project. In order to correctly evaluate the elastic-plastic deformation of rocks caused by the pressure decline during production, some researches, as the experiment on reservoir sensitivity to stress of gas filed with abnormal high pressure, are made. By testing the rock mechanic properties, porosities and permeabilities at different temperature and pressure of 342 core samples from 5 wells in this area, the variations of petro-physical properties at changing pressure are analyzed, and the applicable inspection relationship is concluded. The average productivity curve with the reservoir sensitivity to stress is plotted on the basis of the research, integrated with the field-wide productivity equation. The knowledge lays a foundation for the gas well productivity evaluation in the field and the gas field development plan, and provides effective techniques and measures for basic research on the development of similar gas fields. 相似文献
24.
25.
26.
27.
28.
西安市城市资源压力与城市化水平的灰色关联分析 总被引:5,自引:3,他引:2
统计结果显示西安城市化水平的提高也伴随着城市资源压力的加大,为了找出它们之间定量的数字特征关系,运用综合评价法对西安市近15年的城市化水平进行了测度.同时,使用资源压力指数法对西安市城市资源压力进行了定量评价,最后,用灰色关联分析方法对城市资源压力指数与城市化水平的各相关因子之间的关系进行研究.结果表明,城市资源压力与最能体现城市化发展水平的"城市现代化水平"因子之间存在着极强的关联,针对这一现状,提出了缓解城市资源压力和协调加快城市化发展进程之间矛盾的几点建议. 相似文献
29.
Timing and amount of solar radiation were examined as factors influencing the distribution of seven perennial plants on a small mountain located in the Chihuahuan Desert. Average direct beam solar radiation fluxes at differing times throughout the day and year were estimated with computer calculations. Principal components analysis was used to reduce the number of solar radiation parameters and include the maximum available information with a manageable number of variables. The remaining solar radiation parameters were compared to plant distributions using redundancy analysis and generalized additive models. Unimodal, bimodal, and monotonic responses were all found depending upon the species and solar radiation parameter. Niche separation at this location depends upon the timing as well as the amount of solar radiation. 相似文献
30.