全文获取类型
收费全文 | 3099篇 |
免费 | 429篇 |
国内免费 | 658篇 |
专业分类
测绘学 | 347篇 |
大气科学 | 622篇 |
地球物理 | 561篇 |
地质学 | 731篇 |
海洋学 | 618篇 |
天文学 | 835篇 |
综合类 | 271篇 |
自然地理 | 201篇 |
出版年
2024年 | 16篇 |
2023年 | 34篇 |
2022年 | 70篇 |
2021年 | 102篇 |
2020年 | 102篇 |
2019年 | 146篇 |
2018年 | 101篇 |
2017年 | 136篇 |
2016年 | 108篇 |
2015年 | 127篇 |
2014年 | 155篇 |
2013年 | 192篇 |
2012年 | 164篇 |
2011年 | 158篇 |
2010年 | 139篇 |
2009年 | 229篇 |
2008年 | 183篇 |
2007年 | 247篇 |
2006年 | 206篇 |
2005年 | 193篇 |
2004年 | 210篇 |
2003年 | 161篇 |
2002年 | 107篇 |
2001年 | 140篇 |
2000年 | 120篇 |
1999年 | 102篇 |
1998年 | 115篇 |
1997年 | 59篇 |
1996年 | 57篇 |
1995年 | 58篇 |
1994年 | 47篇 |
1993年 | 46篇 |
1992年 | 38篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 25篇 |
1988年 | 14篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有4186条查询结果,搜索用时 31 毫秒
21.
Synthetic SH seismograms in a laterally varying medium by the discrete wavenumber method 总被引:1,自引:0,他引:1
Summary. We present a new method to calculate the SH wavefield produced by a seismic source in a half-space with an irregular buried interface. The diffracting interface is represented by a distribution of body forces. The Green's functions needed to solve the boundary conditions are evaluated using the discrete wavenumber method. Our approach relies on the introduction of a periodicity in the source-medium configuration and on the discretization of the interface at regular spacing. The technique developed is applicable to boundaries of arbitrary shapes and is valid at all frequencies. Some examples of calculation in simple configurations are presented showing the capabilities of the method. 相似文献
22.
23.
应用地壳波浪与镶嵌构造学说对富氏谱分析法提取地壳垂直形变信息的科学性做了地质学意义上的阐释 ,并提出了根据多期形变资料提取特定波段上构造策应力的数学模型 相似文献
24.
25.
26.
C. Tadhunter K. Wills R. Morganti T. Oosterloo R. Dickson 《Monthly notices of the Royal Astronomical Society》2001,327(1):227-232
We present new spectroscopic observations of the southern radio galaxy . Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O iii ] λλ 5007, 4959, are unusually broad for a narrow-line radio galaxy , and are blueshifted by 600 km s−1 relative to the low-ionization lines such as [O ii ] λλ 3726,3729. The [O ii ] lines are also considerably narrower than the [O iii ] lines, and have a redshift consistent with that of the recently detected H i 21-cm absorption-line system. Whereas the kinematics of the [O iii ] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O ii ] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H i absorption-line system. The unusually broad [O iii ] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that is a radio source in an early stage of evolution. 相似文献
27.
L. K. Erastova 《Astrophysics》2004,47(4):547-554
Some features of a sample of galaxies from the Second Byurakan Survey are discussed. Most of them are small galaxies with star formation. It is shown that different types of galaxies are in the Survey, with high, medium, and low levels of excitation and continua ranging from blue to relatively red. Many of the galaxies have absorption lines in their spectra; their continua are formed by stars in later spectral classes. 相似文献
28.
D. Prandle 《Estuarine, Coastal and Shelf Science》1985,20(5):615-635
An examination is made of the circulation in narrow estuaries subject to a predominant tidal forcing. Velocity structures are derived separately for residual flow components associated with (a) river flow, (b) wind stress, (c) a well-mixed longitudinal density gradient and (d) a fully stratified saline wedge. Dimensionless parameters are introduced to indicate the magnitude of each component and these parameters are evaluated for 9 major estuaries, thereby revealing their sensitivity to each component.For a channel of constant breadth and depth, formulae are deduced for the length of saline intrusion, L. Comparisons with observed data show that such formulae may be used with confidence to predict changes in L arising from variations in river flow, tidal range or channel depths.The level of stratification is shown to be related to a product of two parameters, one associated with velocity structure and a second involving the square of the ‘flow ratio’ (i.e. residual velocity/amplitude of the tidal velocity). This relationship provides a simple classification system for estuarine stratification which can be used to indicate the sensitivity of any particular estuary to changing conditions. 相似文献
29.
Sunetra Giridhar N. Kameswara Rao David L. Lambert 《Journal of Astrophysics and Astronomy》1996,17(1-2):23-40
Two high resolution spectra of the hot RCrB star DY Cen in the red region are compared. The photospheric absorption lines
show a radial velocity variation of 12 kms-1 between 1989 July and 1992 May. Emission components to some CII lines present
in 1989 are almost entirely absent in 1992. Nebular forbidden lines of [OI], [NII] and [SII] appear unchanged from 1989 to
1992 相似文献
30.