全文获取类型
收费全文 | 123033篇 |
免费 | 1890篇 |
国内免费 | 1709篇 |
专业分类
测绘学 | 3458篇 |
大气科学 | 8471篇 |
地球物理 | 23563篇 |
地质学 | 46752篇 |
海洋学 | 10303篇 |
天文学 | 25003篇 |
综合类 | 2394篇 |
自然地理 | 6688篇 |
出版年
2022年 | 723篇 |
2021年 | 1214篇 |
2020年 | 1340篇 |
2019年 | 1459篇 |
2018年 | 7320篇 |
2017年 | 6440篇 |
2016年 | 5612篇 |
2015年 | 1876篇 |
2014年 | 3196篇 |
2013年 | 5416篇 |
2012年 | 4250篇 |
2011年 | 6774篇 |
2010年 | 5793篇 |
2009年 | 7072篇 |
2008年 | 6107篇 |
2007年 | 6595篇 |
2006年 | 4225篇 |
2005年 | 3141篇 |
2004年 | 3246篇 |
2003年 | 3065篇 |
2002年 | 2890篇 |
2001年 | 2371篇 |
2000年 | 2324篇 |
1999年 | 1831篇 |
1998年 | 1912篇 |
1997年 | 1749篇 |
1996年 | 1497篇 |
1995年 | 1483篇 |
1994年 | 1313篇 |
1993年 | 1201篇 |
1992年 | 1136篇 |
1991年 | 1158篇 |
1990年 | 1161篇 |
1989年 | 982篇 |
1988年 | 923篇 |
1987年 | 1095篇 |
1986年 | 929篇 |
1985年 | 1184篇 |
1984年 | 1283篇 |
1983年 | 1231篇 |
1982年 | 1142篇 |
1981年 | 1054篇 |
1980年 | 991篇 |
1979年 | 890篇 |
1978年 | 886篇 |
1977年 | 746篇 |
1976年 | 759篇 |
1975年 | 730篇 |
1974年 | 709篇 |
1973年 | 785篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
91.
92.
Observations of the brightness, color, and polarization of the fuor V1057 Cyg over more than 30 years are presented and briefly discussed. Variability of the linear polarization was found. 相似文献
93.
R. Surendiranath N. Kameswara Rao Ram Sagar J. S. Nathan K. K. Ghosh 《Journal of Astrophysics and Astronomy》1990,11(2):151-166
bdAbstract The open cluster NGC 2818 containing a planetary nebula has been observed inVRI bands using the CCD system at prime focus of the 2.3-metre Vainu Bappu Telescope. The study extending to starsV ∼ 21 magnitude establishes the distance modulus as(m-M)
0 = 12.9 ±0.1 for the cluster. Based on the fitting of theoretical isochrones computed for solar metallicity, an age of 5(±1)
× 108 years has been assigned to the cluster. Association of the planetary nebula with the cluster indicates that the progenitor
mass of the planetary nebula on the main sequence is ≥2.5M⊙
Based on observations obtained with the Vainu Bappu Telescope. 相似文献
94.
95.
A. J. Harding 《Geophysical Journal International》1985,80(2):463-492
Summary. The transformation of a set of seismograms to the delay time-slowness, τ—p, domain is presented as a sequence of Fourier and Bessel transforms, For a horizontally layered medium, this sequence gives an exact cylindrical wave decomposition of the response to a point source; correctly compensating for the phase shifting and geometrical spreading associated with transmission through the Earth. The resultant τ—p map or 'slant stack' contains true amplitude and phase information. The spatial aliasing properties of the transformation, when applied to a dataset, are greatly improved by the use of only outgoing waves in the Bessel transform. This is equivalent to using Hankel functions rather than Bessel functions, and is justified by the absence of incoming waves from most datasets. The WKBJ approximation to the medium response enables predictions to be made about the shape and amplitude variation with slowness of truncation effects. Theoretically the τ—p transformation is reversible, thus the τ—p domain is a suitable one in which to perform filtering operations before seismogram reconstruction. 相似文献
96.
Steven N. Ward 《Geophysical Journal International》1985,81(2):409-428
Summary. This paper explores the middle ground between complex thermally-coupled viscous flow models and simple corner flow models of island arc environments. The calculation retains the density-driven nature of convection and relaxes the geometrical constraints of corner flow, yet still provides semianalytical solutions for velocity and stress. A novel aspect of the procedure is its allowance for a coupled elastic lithosphere on top of a Newtonian viscous mantle. Initially, simple box-like density drivers illustrate how vertical and horizontal forces are transmitted through the mantle and how the lithosphere responds by trench formation. The flexural strength of the lithosphere spatially broadens the surface topography and gravity anomalies relative to the functional form of the vertical flow stresses applied to the plate base. I find that drivers in the form of inclined subducting slabs cannot induce self-driven parallel flow; however, the necessary flow can be provided by supplying a basal drag of 1–5 MPa to the mantle from the oceanic lithosphere. These basal drag forces create regional lithospheric stress and they should be quantifiable through seismic observations of the neutral surface. The existence of a shallow elevated phase transition is suggested in two slab models of 300 km length where a maximum excess density of 0.2 g cm−3 was needed to generate an acceptable mantle flow. A North New Hebrides subduction model which satisfies flow requirements and reproduces general features of topography and gravity contains a high shear stress zone (75 MPa) around the upper slab surface to a depth of 150 km and a deviatoric tensional stress in the back arc to a depth of 70 km. The lithospheric stress state of this model suggests that slab detachment is possible through whole plate fracture. 相似文献
97.
CHUDongsheng LIANGMeng SHIXin ZHANGLing 《中国海洋大学学报(英文版)》2004,3(1):106-110
1 Introduction Inrecentyearstherehasbeengrowingresearchin terestinthetwo dimensional (2 D )systemtheory .The 2 Dsystemsmayfindapplicationsinareassuchasmarineseismicdataprocessingandimageprocessing .Althoughmoreandmorevaluableresultshavebeengained ,mos… 相似文献
98.
An elastoplastic model for sands is presented in this paper, which can describe stress–strain behaviour dependent on mean effective stress level and void ratio. The main features of the proposed model are: (a) a new state parameter, which is dependent on the initial void ratio and initial mean stress, is proposed and applied to the yield function in order to predict the plastic deformation for very loose sands; and (b) another new state parameter, which is used to determine the peak strength and describe the critical state behaviour of sands during shearing, is proposed in order to predict simply negative/positive dilatancy and the hardening/softening behaviour of medium or dense sands. In addition, the proposed model can also predict the stress–strain behaviour of sands under three-dimensional stress conditions by using a transformed stress tensor instead of ordinary stress tensor. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
99.
In the frame of 2D-static problems one approaches the problem of elastic-NRT (not-resisting tension) semi-plane loaded on its limit line. This problem is intended to model the stress situation induced in the soil by a foundation structure. The solution, in terms of activated stress field, is searched for in the class of stress fields satisfying equilibrium and admissibility conditions, by applying an energy approach. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
100.
C.A. Mendoza-Briceño 《Astrophysics and Space Science》1997,256(1-2):387-392
The thermal evolution of structures is investigated for different values of the size of structure. A simplified cooling function and a constant heating mechanism are assumed. Analytical criteria for thermal instability are obtained. It is found that the response of the thermal structure not only depends on the amplitude of the disturbances, but also on whether the disturbance increases or decreases the initial steady temperature. Additionally, the evolution of the structure is examined numerically by using a time-dependent code under the assumption that the inertia terms are small. In particular, the analytical criteria obtained for thermal instability are verified. 相似文献