全文获取类型
收费全文 | 4221篇 |
免费 | 502篇 |
国内免费 | 636篇 |
专业分类
测绘学 | 533篇 |
大气科学 | 624篇 |
地球物理 | 1054篇 |
地质学 | 1081篇 |
海洋学 | 512篇 |
天文学 | 341篇 |
综合类 | 232篇 |
自然地理 | 982篇 |
出版年
2024年 | 25篇 |
2023年 | 60篇 |
2022年 | 128篇 |
2021年 | 141篇 |
2020年 | 170篇 |
2019年 | 175篇 |
2018年 | 152篇 |
2017年 | 195篇 |
2016年 | 208篇 |
2015年 | 219篇 |
2014年 | 213篇 |
2013年 | 283篇 |
2012年 | 248篇 |
2011年 | 225篇 |
2010年 | 198篇 |
2009年 | 230篇 |
2008年 | 258篇 |
2007年 | 263篇 |
2006年 | 255篇 |
2005年 | 195篇 |
2004年 | 203篇 |
2003年 | 158篇 |
2002年 | 139篇 |
2001年 | 138篇 |
2000年 | 174篇 |
1999年 | 116篇 |
1998年 | 118篇 |
1997年 | 84篇 |
1996年 | 60篇 |
1995年 | 48篇 |
1994年 | 59篇 |
1993年 | 41篇 |
1992年 | 36篇 |
1991年 | 36篇 |
1990年 | 21篇 |
1989年 | 16篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1954年 | 1篇 |
排序方式: 共有5359条查询结果,搜索用时 0 毫秒
1.
Majid T. Manzari 《国际地质力学数值与分析法杂志》2004,28(10):1011-1032
A micropolar elastoplastic model for soils is formulated and a series of finite element analyses are employed to demonstrate the use of a micropolar continuum in overcoming the numerical difficulties encountered in application of finite element method in standard Cauchy–Boltzmann continuum. Three examples of failure analysis involving a deep excavation, shallow foundation, and a retaining wall are presented. In all these cases, it is observed that the length scale introduced in the polar continuum regularizes the incremental boundary value problem and allows the numerical simulation to be continued until a clear collapse mechanism is achieved. The issue of grain size effect is also discussed. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
在文献」1「4 基础上,根据近年文选址实践中提出的问题,先进一步讨论了大气相干直径和时间的的物理和意义,然后强调了自由大气湍流的天文成像效应及选址中进行研究和测量的重要性。最后对差分像运动方法测量视宁度的定标和系统误差等问题作了分析。 相似文献
3.
A new analysis of all 346 published 14C dated Holocene alluvial units in Britain offers a unique insight into the regional impacts of global change and shows how surprisingly sensitive British rivers have been to relatively modest but repeated changes in climate. Fourteen major but probably brief periods of flooding are identified bracketed within the periods 400–1070, 1940–3940, 7520–8100 and at ca. 10 420 cal. yr BP. There is a strong correspondence between climatic deteriorations inferred from mire wet shifts and major periods of flooding, especially at ca. 8000 cal. yr BP and since ca. 4000 cal. yr BP. The unusually long and complete British record also demonstrates that alterations in land cover have resulted in a step change in river basin sensitivity to variations in climate. This has very important implications for assessing and mitigating the impact of increasing severe flooding. In small and medium‐sized river basins land use is likely to play a key role in either moderating or amplifying the climatic signal. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
4.
Ralph A. M. J. Wijers 《Astrophysics and Space Science》1995,231(1-2):399-402
Since the discovery that dim gamma-ray bursts last longer on average than bright ones (time dilation) the cosmological origin of this effect has been contested by various researchers. I discuss the current status of this issue and conclude that current models for a non-cosmological time dilation only explain part of the observed phenomenon, and even then betray themselves by distinct signatures in the data. As those signatures have not been seen, the cosmological origin remains the favoured explanation of the time dilation. 相似文献
5.
Suction dredging for cockles removes large cockles from tidal flats and may also cause mortality of non-target fauna and make the habitat less suitable for some species. This study examines whether suction dredging for cockles on tidal flats of the Dutch Wadden Sea had affected densities of non-target fauna, directly after fishing and one year later. Densities of non-target fauna in two randomly chosen undredged locations were compared to densities at the surrounding heavily commercially dredged area. A significant negative effect of cockle dredging on densities of 0-group Macoma balthica was observed and this effect persisted one year after dredging. The dredged area appeared to be less suitable for settlement of mussels Mytilus edulis. No significant effects of dredging on the mudsnail Hydrobia ulvae and on 0 and 1-group C. edule were found. For the mobile young Macoma balthica it seems unlikely that the effect found after one year was still due to the mortality caused by dredging and this suggests that the habitat was less suitable as a consequence of dredging. Thus, even in the highly dynamic ecosystem of the Wadden Sea, effects of bottom disturbance by cockle dredging may persist after one year. 相似文献
6.
7.
8.
Anna Badosa Dani Boix Sandra Brucet Rocío Lpez-Flores Xavier D. Quintana 《Estuarine, Coastal and Shelf Science》2006,66(3-4):513-522
The role of the hydrological regime in the nutrients and zooplankton composition and dynamics has been analysed in five lagoons of La Pletera salt marshes (NE Iberian Peninsula) during a complete hydrological cycle (2002–2003). Two of the lagoons have their origin in the old river mouths while the other three were recently created in the framework of a Life Restoration project. This fact has also allowed us to study the effect of the lagoon age on nutrient and zooplankton composition and dynamics. The salt marsh hydrology is determined by a prolonged period of confinement without water inputs, irregularly interrupted by sudden water inputs due to flooding events (sea storms or intense rainfalls). While the dynamics of oxidized nitrogen compounds in the lagoons depends on the water inputs variability within each hydrological cycle, the internal load of phosphorus, total nitrogen and organic matter is related more to the cumulative mechanisms during the confinement periods. Accumulation processes may be easily related to lagoon age, since old lagoons have higher content of nutrients and organic matter, suggesting that these lagoons progressively accumulate nutrients during the successive confinement events. This is the usual case for most Mediterranean salt marshes without an artificially manipulated water regime. The zooplankton community in La Pletera integrates the effects of both the hydrological regime and the lagoon age since the former determines the temporal pattern of the main zooplankton species and the latter explains differences in composition and structure between old and new lagoons. 相似文献
9.
10.
Evaluating fishery impacts using metrics of community structure 总被引:8,自引:14,他引:8