首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   159750篇
  免费   19766篇
  国内免费   42857篇
测绘学   5612篇
大气科学   29503篇
地球物理   37539篇
地质学   77644篇
海洋学   27284篇
天文学   31411篇
综合类   4292篇
自然地理   9088篇
  2022年   1124篇
  2021年   1945篇
  2020年   3315篇
  2019年   6931篇
  2018年   9003篇
  2017年   8471篇
  2016年   9169篇
  2015年   6692篇
  2014年   7880篇
  2013年   11476篇
  2012年   8498篇
  2011年   9239篇
  2010年   8927篇
  2009年   9651篇
  2008年   8202篇
  2007年   8082篇
  2006年   7340篇
  2005年   6059篇
  2004年   6374篇
  2003年   5911篇
  2002年   5598篇
  2001年   4936篇
  2000年   4420篇
  1999年   3943篇
  1998年   4110篇
  1997年   4069篇
  1996年   3361篇
  1995年   3239篇
  1994年   2956篇
  1993年   2693篇
  1992年   2438篇
  1991年   2218篇
  1990年   2196篇
  1989年   1950篇
  1988年   1757篇
  1987年   1918篇
  1986年   1637篇
  1985年   1883篇
  1984年   2099篇
  1983年   1876篇
  1982年   1843篇
  1981年   1620篇
  1980年   1493篇
  1979年   1409篇
  1978年   1342篇
  1977年   1168篇
  1976年   1095篇
  1975年   1082篇
  1974年   1058篇
  1973年   1159篇
排序方式: 共有10000条查询结果,搜索用时 640 毫秒
91.
地壳对海洋潮汐的响应   总被引:2,自引:2,他引:0  
应用三维动态有限元方法研究了中国北部地区的地壳对邻近的渤海与黄海海平面变化的响应。虽然此应力场过于微弱不足以引发地震,但发现应力集中的位置及应力场变化较大的位置恰好与某些现代地震的震中一致。这一结果表明研究地壳对广泛分布的载荷的响应对研究区域地震构造是有帮助的。  相似文献   
92.
Electron impact excitation rates for transitions in the S v ion, calculated with theR-matrix code, are used to derive the electron temperature sensitive emission line ratiosR 1 =I(854.8 Å)/I(786.9 Å),R 2 =I(852.2 Å)/I(786.9 Å),R 3 =I(849.2 Å)/I(786.9 Å), andR 4 =I(1199.1 Å)/I(786.9 Å), which are found to be significantly different from previous estimates. A comparison of the present results with observational data for a sunspot obtained with the Harvard S-055 spectrometer on boardSkylab reveals generally good agreement between theory and experiment, except in the case ofR 1, which is probably due to blending in the 854.8 Å feature. The possible effects of Lyman continuum absorption on the observed line ratios is briefly discussed.  相似文献   
93.
94.
95.
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   
96.
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   
97.
Using a differential method we have carried out observations of oscillations in six sunspots. Spectral lines Fe i 5434 Å and Fe i 5576 Å were used. Horizontal waves are not observed in the sunspot umbra photosphere. Results obtained indicate that, at least, the sunspot umbra oscillates as a single whole.  相似文献   
98.
Summary. A method of synthetic seismogram computation for teleseismic SV -waves is developed in order to treat quantitatively SV -waves in problems of body wave source inversion and source—receiver structure studies. The method employs WKBJ theory for a generalized ray in a vertically inhomogeneous half-space and the propagator matrix technique for waves in near-surface homogeneous layers. Wavenumber integration is done along the real axis of the wavenumber plane and anelasticity is included by using complex velocity in all regions of the earth model. The near-surface source structure is taken into account in the computation for the case of the shallow source by allowing a point source to be located in the homogeneous layers. Source and receiver area structures are also allowed to differ. A general moment tensor point source is considered.  相似文献   
99.
青藏块体东北缘断层形变与中强地震   总被引:7,自引:4,他引:3  
对祁连山-海原断裂带近期断层形变特征进行了初步研究,发现多场地,大范围的断层活动异常是中等强度地震发生的显著背景,并且往往与大陆地震活动的阶段性总体状况相呼应;区域形变存在明显的特征量,包括特征形态和特征时间,同一场地在不同地震前的异常特征具有重复性,但会受到背景差异显著的不同地震的影响,目前形变状况表明研究区仍具有发生中强地震的地壳运动背景。  相似文献   
100.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号