全文获取类型
收费全文 | 14681篇 |
免费 | 2860篇 |
国内免费 | 2895篇 |
专业分类
测绘学 | 671篇 |
大气科学 | 1012篇 |
地球物理 | 4592篇 |
地质学 | 7116篇 |
海洋学 | 2068篇 |
天文学 | 2018篇 |
综合类 | 718篇 |
自然地理 | 2241篇 |
出版年
2024年 | 74篇 |
2023年 | 264篇 |
2022年 | 498篇 |
2021年 | 522篇 |
2020年 | 581篇 |
2019年 | 634篇 |
2018年 | 538篇 |
2017年 | 557篇 |
2016年 | 588篇 |
2015年 | 635篇 |
2014年 | 754篇 |
2013年 | 682篇 |
2012年 | 828篇 |
2011年 | 877篇 |
2010年 | 716篇 |
2009年 | 1022篇 |
2008年 | 935篇 |
2007年 | 1008篇 |
2006年 | 981篇 |
2005年 | 857篇 |
2004年 | 852篇 |
2003年 | 852篇 |
2002年 | 677篇 |
2001年 | 625篇 |
2000年 | 600篇 |
1999年 | 526篇 |
1998年 | 569篇 |
1997年 | 341篇 |
1996年 | 316篇 |
1995年 | 294篇 |
1994年 | 261篇 |
1993年 | 232篇 |
1992年 | 193篇 |
1991年 | 119篇 |
1990年 | 79篇 |
1989年 | 93篇 |
1988年 | 60篇 |
1987年 | 66篇 |
1986年 | 39篇 |
1985年 | 21篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1954年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Refraction data taken from ocean bottom seismograph recordings in the western Arafura Sea indicate a continental‐type structure for the region. This structure is characterised by a thin column (2 km) of sediments, with velocities ranging from about to 2 to 4 km s‐1, overlying an essentially two layer crust. The compressional wave velocities in the upper and lower crust are 5.97 and 6.52 km s‐1, respectively, with the boundary between the layers at a depth of 11 km. Very weak mantle‐refracted arrivals with a velocity of about 8.0 km s‐1 were recorded. Large‐amplitude, later arrivals, beginning at distances near 100 and 150 km, have been interpreted to be part of the retrograde branches from the 8.0 and 7.33 km s‐1 layers, respectively. Model studies indicate that a small positive velocity gradient is required between 17 and 30 km, and that the Moho is at a depth of 34 km. A third set of large amplitude, later arrivals starting at a distance near 250 km has been interpreted as most probably multiple refraction‐reflection arrivals from the 5.97 and 6.52 km s‐1 layers. Correlation of this structure with the stratigraphic logs from exploratory oil wells in the Arafura Sea using layer velocities indicates that rocks younger than Jurassic appear to thin towards the east. 相似文献
992.
993.
本文利用攀西地区通过攀枝花的东西向剖面爆炸地震资料,进行了震相的重新识别和二维射线追踪与理论图计算.结果表明,沿剖面地表附近有4个低速区和若干高速带,它们与地质和构造有很好的对应关系.渡口附近的高速岩体一直延伸到了上地壳的底部,形成一个统一的地垒状构造,该高速体与攀枝花成矿岩体相关,并推断华坪及其以西地带也是找矿的有利地区.中地壳下部有一厚度约9km的低速层,它可能是壳内的韧性剪切带.低速层顶部深度为27.0—29.5km,与研究地区的居里面深度及天然地震震源深度的分布基本符合.剖面东段中地壳顶部还有一层很薄的低速层,反映了构造带两侧运动的不对称性.地壳厚度为53—56km,构造带中部的Moho界面没有明显的上隆. 相似文献
994.
Ewa L. okas 《Monthly notices of the Royal Astronomical Society》1998,296(3):491-501
Using the two-point Edgeworth series up to second order in the linear rms density fluctuation we construct the weakly non-linear conditional probability distribution function for the density field around an overdense region. This requires calculating the two-point analogues of the skewness parameter S 3 . We test the dependence of the two-point skewness on distance from the peak for scale-free power spectra and Gaussian smoothing. The statistical features of such a conditional distribution are given as the values obtained within linear theory corrected by the terms that arise as a result of weakly non-linear evolution. The expected density around the peak is found to be always below the linear prediction while its dispersion is always larger than in the linear case. For large enough overdensities the weakly non-linear corrections can be more significant than the peak constraint introduced by Bardeen et al. We apply these results to the spherical model of collapse as developed by Hoffman & Shaham and find that in general the effect of weakly non-linear interactions is to decrease the scale from which a peak gathers mass and therefore also the mass itself. In the case of an open universe this results in steepening of the final profile of the virialized proto-object. 相似文献
995.
L. Clewley S. J. Warren P. C. Hewett John. E. Norris M. I. Wilkinson N. W. Evans 《Monthly notices of the Royal Astronomical Society》2005,362(1):349-360
We present FOcal Reducer/low dispersion Spectrograph-1 spectra (from the European Southern Observatory's Very Large Telescope) of a sample of 34 faint 20.0 < g * < 21.1 A-type stars selected from the Sloan Digital Sky Survey Early Data Release, with the goal of measuring the velocity dispersion of blue horizontal branch (BHB) stars in the remote Galactic halo, R ∼ 80 kpc . We show that colour selection with 1.08 < u *− g * < 1.40 and −0.2 < g *− r * < −0.04 minimizes contamination of the sample by less luminous blue stragglers. In classifying the stars we confine our attention to the 20 stars with spectra of signal-to-noise ratio >15 Å−1 . Classification produces a sample of eight BHB stars at distances 65–102 kpc from the Sun (mean 80 kpc), which represents the most distant sample of Galactic stars with measured radial velocities. The dispersion of the measured radial component of the velocity with respect to the centre of the Galaxy is 58 ± 15 km s−1 . This value is anomalously low in comparison with measured values for stars at smaller distances, as well as for satellites at similar distances. Seeking an explanation for the low measured velocity dispersion, further analysis reveals that six of the eight remote BHB stars are plausibly associated with a single orbit. Three previously known outer halo carbon stars also appear to belong to this stream. The velocity dispersion of all nine stars relative to the orbit is only 15 ± 4 km s−1 . Further observations along the orbit are required to trace the full extent of this structure on the sky. 相似文献
996.
997.
GIS专业人才的素质特征、知识结构与培养方案研究 总被引:21,自引:0,他引:21
GIS学科的工具性、工程性及横断性等特征促使GIS的应用格局以及社会对GIS专业人才的需求朝着多层次、多元化的方向发展。该文在调查分析国内外GIS专业人才培养现状的基础上,指出GIS专业人才应具备以下素质特征:1)融厚实的数理、空间知识基础与精湛的GIS专业知识于一体;2)融扎实的GIS基本技能与迫切的GIS求知、创新欲望于一体;3)融GIS研究、技术开发本领与GIS项目管理能力于一体。同时,GIS专业人才应掌握与之相对应的、立体的知识结构,即以数理基础和空间基础为宽度,以应用背景与应用领域为广度,以专业知识及专业技能为深度的知识结构。笔者还对GIS专业人才培养模型与GIS高、中级人才的培养方案进行探讨。 相似文献
998.
大跨度切眼锚网锚索支护研究 总被引:5,自引:0,他引:5
全煤大跨度切眼传统的支护方式是钢棚支护,为解决无轨胶轮车直接进入工作面,提高工作面设备安装效率,课题组进行锚网锚索支护试验与研究。该项目的试验成功,为矿井高产高效创造了条件。 相似文献
999.
G. S. Dubey U. S. Pandey J. P. Singh L. K. Singh S. N. Singh 《Journal of Astrophysics and Astronomy》1990,11(1):23-28
The equation governing the equilibrium of a centrally-symmetric, self-gravitating distribution of matter is obtained by assuming
that matter is described by magnetofluid and it is shown that the equation of stellar structure described by an ideal fluid
is recovered when magnetic fieldh is vanishing 相似文献
1000.