全文获取类型
收费全文 | 8833篇 |
免费 | 1698篇 |
国内免费 | 2523篇 |
专业分类
测绘学 | 999篇 |
大气科学 | 2571篇 |
地球物理 | 2105篇 |
地质学 | 3165篇 |
海洋学 | 1706篇 |
天文学 | 360篇 |
综合类 | 621篇 |
自然地理 | 1527篇 |
出版年
2024年 | 50篇 |
2023年 | 107篇 |
2022年 | 292篇 |
2021年 | 399篇 |
2020年 | 404篇 |
2019年 | 509篇 |
2018年 | 427篇 |
2017年 | 469篇 |
2016年 | 424篇 |
2015年 | 475篇 |
2014年 | 594篇 |
2013年 | 734篇 |
2012年 | 601篇 |
2011年 | 596篇 |
2010年 | 525篇 |
2009年 | 659篇 |
2008年 | 625篇 |
2007年 | 677篇 |
2006年 | 608篇 |
2005年 | 523篇 |
2004年 | 492篇 |
2003年 | 386篇 |
2002年 | 353篇 |
2001年 | 283篇 |
2000年 | 278篇 |
1999年 | 243篇 |
1998年 | 208篇 |
1997年 | 200篇 |
1996年 | 195篇 |
1995年 | 134篇 |
1994年 | 108篇 |
1993年 | 107篇 |
1992年 | 92篇 |
1991年 | 58篇 |
1990年 | 53篇 |
1989年 | 35篇 |
1988年 | 32篇 |
1987年 | 24篇 |
1986年 | 12篇 |
1985年 | 12篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1972年 | 5篇 |
1971年 | 4篇 |
1954年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
151.
152.
153.
Chen Po-fang 《地震学报(英文版)》2002,15(5):519-525
To improve the accuracy of the numerical evaluation through the 3-D finite difference method, the surface boundary conditions
are added to modify the old program. The author has tested the new program by making calculations for the model constructed
by Wanamaker, et al (1984). The comparison between the numerical results obtained from this paper and those by Wannamaker, et al (1984) indicates that a pronounced improvement is realized in the evaluation of the horizontal magnetic components. Moreover,
better calculations for the vertical magnetic components are also obtainable by using the new program. 相似文献
154.
The contravariant components of the wave-propagation metric tensor equal half the second-order partial derivatives of the selected eigenvalue of the Christoffel matrix with respect to the slowness-vector components. The relations of the wave-propagation metric tensor to the curvature matrix and Gaussian curvature of the slowness surface and to the curvature matrix and Gaussian curvature of the ray-velocity surface are demonstrated with the help of ray-centred coordinates. 相似文献
155.
Determination of the influence zone for surface wave paths 总被引:6,自引:1,他引:6
156.
157.
Rachid Ait Akbour Jamaâ Douch Mohamed Hamdani Philippe Schmitz 《Comptes Rendus Geoscience》2002,334(13):981-985
To evaluate the risk of contaminant transport by mobile colloids, it is necessary to understand how colloids and associated pollutants behave during their migration through uncontaminated soil or groundwater. In this study, we investigated the influence of aggregation induced by Ca2+ and trace metals (Pb2+, Cu2+) concentrations on the transport of humic-coated kaolinite colloids through a natural quartz sand at pH=4. Adsorbed divalent cations reduce the colloids surface charge and thereby induce aggregation and deposition in porous media. To cite this article: R. Ait Akbour et al., C. R. Geoscience 334 (2002) 981–985. 相似文献
158.
Velocity profile of a sand cloud blowing over a gravel surface 总被引:2,自引:0,他引:2
Particle dynamic analyzer (PDA) measurement technology was used to study the turbulent characteristics and the variation with height of the mean horizontal (in the downwind direction) and vertical (in the upward direction) particle velocity of a sand cloud blowing over a gravel surface. The results show that the mean horizontal particle velocity of the cloud increases with height, while the mean vertical velocity decreases with height. The variation of the mean horizontal velocity with height is, to some extent, similar to the wind profile that increases logarithmically with height in the turbulent boundary layer. The variation of the mean vertical velocity with height is much more complex than that of the mean horizontal velocity. The increase of the resultant mean velocity with height can be expressed by a modified power function. Particle turbulence in the downwind direction decreases with height, while that in the vertical direction is complex. For fine sands (0.2–0.3 mm and 0.3–0.4 mm), there is a tendency for the particle turbulence to increase with height. In the very near-surface layer (<4 mm), the movement of blown sand particles is very complex due to the rebound of particles on the bed and the interparticle collisions in the air. Wind starts to accelerate particle movement about 4 mm from the surface. The initial rebound on the bed and the interparticle collisions in the air have a profound effect on particle movement below that height, where particle concentration is very high and wind velocity is very low. 相似文献
159.
The karst landforms distributed on the Qinghai-Xizang (Tibet) Plateau can be genetically classed with the Tertiary underground karst, which were gradually exhumed to the surface with the uplift of the plateau during Quaternary period. The relative deposits of the Tertiary palaeokarst processes, such as the residuum and speleothem, were discovered recently in the southern and southeastern fringe areas of the plateau, where has geological-currently been disintegrated by the headward erosion processes of the modern river systems. The major chemical components of the clay portion of the residuum consist mainly of SiO2C, Al2CO3 and Fe2O3. The clay minerals composition of the clay portion belongs to illite-kaolinite pattern for most of the residuum samples, and kaolinite-illite pattern for a few of the samples. It can be judged from the silicic acid index and the clay minerals composition that the formation of the residuum of the Plateau was in its initial phase. However, such a lower chemical weathering index only reflected the weathering degree in the bottom or lower parts of the lateritic weathering crust. The relatively intensive chemical weathering processes of the surface layers of the lateritic weathering crust could be logically speculated. The surface feature textures of quartz grains in the residuum were formed mainly by the chemical erosion, which revealed a long-term humid-tropical environment when the residuum and the palaeokarst formed. 相似文献
160.
中国东部地区耕地土壤肥力变化趋势研究 总被引:57,自引:5,他引:57
随着人口、资源和环境之间的矛盾日趋尖锐,中国耕地土壤肥力及其变化日益为世人所关注。本文利用最近20年东北地区的吉林和黑龙江省,华北地区的北京市和河北省以及长江下游地区的江苏和浙江省的15个县市180个样本点的土壤数据资料,分析了20世纪80年代以来东部地区农业土壤肥力的演变趋势。结果表明,从总体上看,东部地区除土壤速效钾下降和酸碱性有所退化外,农业土壤的有机质、全氮和速效磷平均含量都增加了。在空间分布上,土壤肥力的变化趋势存在明显差异。长江下游和华北地区的平均肥力提高,东北地区下降。另外,除华北地区的土壤酸碱性有改善外,长江下游和东北地区土壤都存在酸化倾向。 相似文献