全文获取类型
收费全文 | 100722篇 |
免费 | 1396篇 |
国内免费 | 1505篇 |
专业分类
测绘学 | 3010篇 |
大气科学 | 7686篇 |
地球物理 | 19677篇 |
地质学 | 39930篇 |
海洋学 | 7587篇 |
天文学 | 18105篇 |
综合类 | 2305篇 |
自然地理 | 5323篇 |
出版年
2021年 | 552篇 |
2020年 | 652篇 |
2019年 | 744篇 |
2018年 | 7409篇 |
2017年 | 6707篇 |
2016年 | 4943篇 |
2015年 | 1214篇 |
2014年 | 1696篇 |
2013年 | 3085篇 |
2012年 | 3362篇 |
2011年 | 6466篇 |
2010年 | 5632篇 |
2009年 | 6477篇 |
2008年 | 5429篇 |
2007年 | 6208篇 |
2006年 | 2498篇 |
2005年 | 2381篇 |
2004年 | 2436篇 |
2003年 | 2516篇 |
2002年 | 2107篇 |
2001年 | 1536篇 |
2000年 | 1537篇 |
1999年 | 1271篇 |
1998年 | 1267篇 |
1997年 | 1267篇 |
1996年 | 1080篇 |
1995年 | 1041篇 |
1994年 | 916篇 |
1993年 | 833篇 |
1992年 | 766篇 |
1991年 | 793篇 |
1990年 | 810篇 |
1989年 | 736篇 |
1988年 | 693篇 |
1987年 | 807篇 |
1986年 | 799篇 |
1985年 | 904篇 |
1984年 | 1038篇 |
1983年 | 1030篇 |
1982年 | 940篇 |
1981年 | 902篇 |
1980年 | 842篇 |
1979年 | 786篇 |
1978年 | 777篇 |
1977年 | 726篇 |
1976年 | 662篇 |
1975年 | 647篇 |
1974年 | 696篇 |
1973年 | 715篇 |
1972年 | 441篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Summary. Asymptotic ray theory (ART) fails in transition regions near critically reflected, bottom glancing or caustic-forming rays in a vertically inhomogeneous layered earth. These deficiencies are repaired here by replacing the transitional ray fields with guided modes plus truncation remainders. Exact ray-mode equivalences and their high-frequency asymptotic approximations are formulated, and their validity and efficiency are verified by numerical comparisons for SH motion in a two-layer earth model comprised of an inhomogeneous sediment above an homogeneous semi-infinite bedrock. 相似文献
42.
43.
A numerical scheme is developed in order to simulate fluid flow in three dimensional (3‐D) microstructures. The governing equations for steady incompressible flow are solved using the semi‐implicit method for pressure‐linked equations (SIMPLE) finite difference scheme within a non‐staggered grid system that represents the 3‐D microstructure. This system allows solving the governing equations using only one computational cell. The numerical scheme is verified through simulating fluid flow in idealized 3‐D microstructures with known closed form solutions for permeability. The numerical factors affecting the solution in terms of convergence and accuracy are also discussed. These factors include the resolution of the analysed microstructure and the truncation criterion. Fluid flow in 2‐D X‐ray computed tomography (CT) images of real porous media microstructure is also simulated using this numerical model. These real microstructures include field cores of asphalt mixes, laboratory linear kneading compactor (LKC) specimens, and laboratory Superpave gyratory compactor (SGC) specimens. The numerical results for the permeability of the real microstructures are compared with the results from closed form solutions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
44.
Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (~0.1 ms?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators. 相似文献
45.
M.S. Hanner E. Tedesco A.T. Tokunaga G.J. Veeder D.F. Lester F.C. Witteborn J.D. Bregman J. Gradie L. Lebofsky 《Icarus》1985,64(1):11-19
The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ~2 × 105 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ~0.04 at 1.25 μm and ~0.05 at 2.2 μm. 相似文献
46.
Y.-B. WU S. GAO H.-J. GONG H. XIANG W.-F. JIAO S.-H. YANG Y.-S. LIU H.-L. YUAN 《Journal of Metamorphic Geology》2009,27(6):461-477
U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and 176 Lu/177 Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean 207 Pb/206 Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ LuN ≥2213; 14.7 ≤ LuN /SmN ≤ 354) and 176 Lu/177 Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean 207 Pb/206 Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T DM2 ) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust. 相似文献
47.
H. Gerth 《International Journal of Earth Sciences》1941,32(3):321-336
Ohne Zusammenfassung 相似文献
48.
49.
50.
AGGLOMERATION AND RADIATION EFFECT OF THE PULL OF URBANIZATION 总被引:2,自引:1,他引:1
QIJin-li 《中国地理科学(英文版)》2003,13(3):224-227
In order to explore the train of thought for China‘s urbanizing development and coordinated rural eco-nomic development, and to find good ways of solving rural problems through urbanization, this paper absorbs the push-and-pull forces theory and the systematic dynamic theory in the traditional population migration theories, views urbanization as a dynamic system, makes research on the push-and-pull mechanism of urbanization. The pulling power of urbanization is analyzed according to two aspects, the agglomeration effect and the radiation effect of cities. The agglomeration effect provides continuous propelling force for urbanization, and the radiation effect further accelerates the urbanization process by pushing forward the development of rural economy. Of course, the slow de-velopment of urbanization can result in the hindrance to rural economic development. 相似文献