全文获取类型
收费全文 | 77206篇 |
免费 | 1327篇 |
国内免费 | 695篇 |
专业分类
测绘学 | 1926篇 |
大气科学 | 5418篇 |
地球物理 | 16315篇 |
地质学 | 27199篇 |
海洋学 | 6500篇 |
天文学 | 17236篇 |
综合类 | 241篇 |
自然地理 | 4393篇 |
出版年
2022年 | 447篇 |
2021年 | 816篇 |
2020年 | 895篇 |
2019年 | 926篇 |
2018年 | 2137篇 |
2017年 | 1970篇 |
2016年 | 2557篇 |
2015年 | 1546篇 |
2014年 | 2443篇 |
2013年 | 4114篇 |
2012年 | 2572篇 |
2011年 | 3409篇 |
2010年 | 2860篇 |
2009年 | 3788篇 |
2008年 | 3498篇 |
2007年 | 3219篇 |
2006年 | 3057篇 |
2005年 | 2527篇 |
2004年 | 2398篇 |
2003年 | 2262篇 |
2002年 | 2050篇 |
2001年 | 1898篇 |
2000年 | 1824篇 |
1999年 | 1449篇 |
1998年 | 1562篇 |
1997年 | 1462篇 |
1996年 | 1152篇 |
1995年 | 1201篇 |
1994年 | 1046篇 |
1993年 | 918篇 |
1992年 | 908篇 |
1991年 | 798篇 |
1990年 | 898篇 |
1989年 | 754篇 |
1988年 | 691篇 |
1987年 | 850篇 |
1986年 | 701篇 |
1985年 | 903篇 |
1984年 | 960篇 |
1983年 | 917篇 |
1982年 | 882篇 |
1981年 | 748篇 |
1980年 | 709篇 |
1979年 | 654篇 |
1978年 | 651篇 |
1977年 | 591篇 |
1976年 | 564篇 |
1975年 | 542篇 |
1974年 | 540篇 |
1973年 | 512篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
M.?B.?BellEmail author L.?W.?Avery J.?M.?MacLeod J.?P.?Vallée 《Astrophysics and Space Science》2011,333(2):377-388
Sensitive spectral observations made in two frequency bands near 6.0 and 17.6 GHz are described for Orion and W51. Using frequency
switching we were able to achieve a dynamic range in excess of 10,000 without fitting sinusoidal or polynomial baselines.
This enabled us to detect lines as weak as T
A
∼1 mK in these strong continuum sources. Hydrogen recombination lines with Δn as high as 25 have been detected in Orion. In the Orion data, where the lines are stronger, we have also detected a systematic
shift in the line center frequencies proportional to linewidth that cannot be explained by normal optical depth effects. 相似文献
192.
193.
194.
The first small active regions of the last two new cycles seem to be formed, in its very first, preparatory stage, close to the equator with uncertain magnetic orientation, and as secondary products at the periphery of equatorial `magnetic bubbles' of the new magnetic flux still belonging to the old cycle. At the real beginning of the new cycle, at the periphery of its higher-latitude `magnetic bubbles', the opposite applies to the secondary regions of the ending cycle. It seems that two modes of magnetic flux supply the visible layers of the photosphere during the cycle transition phase: the emergence of the magnetic flux from the depth of the convective zone, and the mutual interaction of the earlier emerging fluxes, or the action of the `local dynamo', giving rise to the new local magnetic field concentrations. 相似文献
195.
The seismic response of light secondary systems in a building is dependent on the response of the primary structural system to the seismic ground motion with the result that very high accelerations can be induced in such secondary systems. This response can be reduced through the use of aseismic base isolation which is a design strategy whereby the entire building can be decoupled from the damaging horizontal components of seismic ground motion by the use of some form of isolation system. The paper presents a theoretical analysis of the response of light equipment in isolated structures and a parallel experimental programme both of which show that the use of base isolation can not only attenuate the response of the primary structural system but also reduce the response of secondary systems. Thus, the design of equipment and piping in a base-isolated building is very much simpler than that for a conventionally founded structure: inelastic response and equipment-structure interaction need not be considered and multiple support response analysis is rendered unnecessary. Although an isolation system with linear elastic bearings can reduce the acceleration of the structure, it may be accompanied by large relative displacements between the structure and the ground. A system using lead-rubber hysteretic bearings, having a force-displacement relation which is approximately a bilinear loop, can reduce these displacements. A parallel experimental programme was carried out to investigate the response of light equipment in structures isolated using lead-rubber bearings. The experimental results show that these bearings can dissipate energy and limit the displacement and acceleration of the structure but are less effective in reducing the accelerations in the internal equipment. The results of both the analysis and the tests show that base isolation is a very effective method for the seismic protection of light equipment items in buildings. 相似文献
196.
We present here, for the O and B type stars in the Catalogue of Stellar Ultraviolet Fluxes, an approach which does not require
a precise knowledge of spectral type and luminosity class for derivingE(B-V) colour excesses. The method is based on the use of an UV-visual two-colour diagram; galactic variations in the interstellar
extinction law are analyzed and fully taken into account. Our results have been compared with those derived by using the differences
between observed and intrinsic colours for stars with known spectral classification. The very good agreement in a large number
of cases (94 per cent) demonstrates that our approach permits the derivation of reliable colour excess values for early type
stars even if only a rough spectral classification is available. 相似文献
197.
Three‐dimensional (3D) numerical modelling of fault displacement enables the building of geological models to represent the complex 3D geometry and geological properties of faulted sedimentary basins. Using these models, cross‐fault juxtaposition relationships are predicted in 3D space and through time, based on the geometries of strata that are cut by faults. Forward modelling of fault development allows a 3D prediction of fault‐zone argillaceous smear using a 3D application of the Shale Gouge Ratio. Numerical models of the Artemis Field, Southern North Sea, UK and the Moab Fault, Utah, USA are used to demonstrate the developed techniques and compare them to traditional one‐ and two‐dimensional solutions. These examples demonstrate that a 3D analysis leads to significant improvements in the prediction of fault seal, the analysis of the interaction of the sealing properties of multiple faults, and the interpretation of fault seal within the context of sedimentary basin geometry. 相似文献
198.
199.
M. D. Sharma 《Geophysical Journal International》2008,174(3):971-977
Out of the four waves in an anisotropic poroelastic medium, two are termed as quasi-transverse waves. The prefix 'quasi' refers to their polarizations being nearly, but not exactly, perpendicular to direction of propagation. In this composite medium, unlike perfectly elastic medium, the propagation of a longitudinal wave along a phase direction may not be accompanied by transverse waves. The existence of a transverse wave in anisotropic poroelastic media is ensured by the two equations restricting the choice of elastic coefficients of porous aggregate as well as fluid–solid coupling. Necessary and sufficient conditions for the existence of transverse waves along the coordinate axes and in the coordinate planes for general anisotropy are discussed. The discussion is extended to the case of orthotropic materials and existence for few specific phase directions is also explored. The conditions for the transverse waves decided on the basis of their apparent polarizations, that is, particle motion being perpendicular to ray direction, are also discussed. For a particular numerical model, the existence of these apparent transverse waves is solved numerically for phase directions in coordinate planes. For general directions of phase propagation, the existence of these transverse waves is checked graphically for the chosen numerical model. 相似文献
200.
Mohammad M. M. Alsahli Ahmed M. AlHasem 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2016,116(1):56-70
Assessing coastal vulnerability to sea level rise (SLR) at local and regional scales is a fundamental step for designing successful long-term coastal management plans. This study was thus designed to assess Kuwait coastal vulnerability to SLR at four scenarios (.5, 1, 1.5 and 2 m). Potential inundated areas and the number of people at risk were estimated based on these SLR scenarios. A coastal vulnerability index (CVI) map of Kuwait was then computed based on the lowest scenario using eight parameters: elevation, coastal slope, geomorphology, distance to 20-m isobath, population, land use, cultural heritage and transportation. The geographic distribution of inundated areas at an SLR of .5 m revealed that the northern islands of Kuwait and coastal areas along Kuwait Bay would be highly impacted, whereas the coastal area near Shuaibah Port was the most influenced among the southern coasts. Most of the coastal area exhibited a moderate vulnerability to SLR, especially the northern islands. This study presented an initial vulnerability assessment for Kuwait coasts to SLR, which can be extended with more variables. The integrated remote sensing and geographic information system methodology demonstrated in this study can be applied in similar studies elsewhere. 相似文献