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   44695篇
  免费   760篇
  国内免费   335篇
测绘学   1022篇
大气科学   3213篇
地球物理   8536篇
地质学   15925篇
海洋学   4150篇
天文学   10124篇
综合类   151篇
自然地理   2669篇
  2022年   256篇
  2021年   479篇
  2020年   510篇
  2019年   549篇
  2018年   1105篇
  2017年   1017篇
  2016年   1287篇
  2015年   695篇
  2014年   1196篇
  2013年   2324篇
  2012年   1397篇
  2011年   1879篇
  2010年   1726篇
  2009年   2207篇
  2008年   1861篇
  2007年   1947篇
  2006年   1770篇
  2005年   1402篇
  2004年   1308篇
  2003年   1265篇
  2002年   1216篇
  2001年   1083篇
  2000年   999篇
  1999年   816篇
  1998年   822篇
  1997年   843篇
  1996年   674篇
  1995年   660篇
  1994年   617篇
  1993年   573篇
  1992年   528篇
  1991年   499篇
  1990年   507篇
  1989年   497篇
  1988年   480篇
  1987年   556篇
  1986年   491篇
  1985年   608篇
  1984年   658篇
  1983年   592篇
  1982年   529篇
  1981年   578篇
  1980年   469篇
  1979年   459篇
  1978年   433篇
  1977年   434篇
  1976年   383篇
  1975年   378篇
  1974年   378篇
  1973年   385篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
A procedure for eliciting displacements in tidal waves from seismograms of broadband velocimeters like STS-2 and KSESH-R and the evaluation conversion of these displacements into vertical or volumetric deformation is proposed.  相似文献   
102.
Variations in Cd, Cr, Cu, and Fe concentrations in water and the soft tissues of bivalved mollusks Anodonta anatina and Unio tumidus along the Southern Bug River channel are examined. Fe concentration was found to be somewhat higher than the MAC. Cd, Cr, and Cu concentrations in water met the drinking water standards. The comparison of individual reaches of the channel in terms of heavy metal concentrations in mollusks allowed polluted river reaches to be identified. The highest Fe concentrations in the soft tissues of mollusks were recorded near the river source. The mollusks in the middle and lower reaches of the Southern Bug featured high accumulation of Cd, Cr, and Cu.  相似文献   
103.
The methodological features and results of two soil surveys of the Senezh Reservoir carried out in 1992 and 2006 as a part of multidisciplinary limnological studies of this large recreation water body with the aim to monitor its ecological state and prepare recommendations for water protection measures are analyzed  相似文献   
104.
Due to the increasingly serious ecological degradation of land systems, the land ecological security issues have attracted more and more attention of policy makers, researchers and citizens. Aiming at overcoming the disadvantages in subjectivity and complexity of the currently used assessment methods, an integrative fuzzy set pair model for assessing the land ecological security was developed by integrating fuzzy assessment and set pair analysis (SPA). The approximate degree of land ecological security to the optimal standard set was calculated to describe the secure level by combining multiple indices. The indices and weights were determined by a pressure-state-response model and the fuzzy analytic hierarchy process (AHP), respectively. Aided by a geographic information system, this model was applied to evaluate comprehensively the status of land ecological security in Xiaolangdi Reservoir Region, China, taking the administrative division as the assessment unit. The results showed that 20% of the total area maintained a slightly secure status, while 50% of the study area was of a middle or seriously low grade of land ecological security. The remaining portion (30%) was the most ecologically insecure. From the spatial prospective, obvious variations were observed. The land eco-security gradually decreased from the Xiaolangdi Dam to its surrounding regions. It was concluded that the status of the integral land ecological security of Xiaolangdi Reservoir Region was in the middle level, and increasingly intense human activities speeded up the degradation of regional land ecosystem in recent years and thus induced the crisis of land ecological security.  相似文献   
105.
106.
Two samplings of 65 diamond crystals divided by the intensity of a slow X-ray luminescence component are studied from the Arkhangel’skaya and Karpinskogo-1 pipes. IR and EPR spectroscopies revealed a relationship between the nitrogen A and P2 centers of the diamonds and the presence of a slow X-ray luminescence component. Its absence in most diamonds with high content of P1 (C) centers is explained by the low number of A and P2 centers.  相似文献   
107.
108.
109.
Spectra of three high-redshift quasars discovered on low-dispersion objective prism plates were obtained. We present the emission line identifications and redshifts of the objects. Equivalent widths, line widths and low-resolution line profiles are given characterizing the quasar emission region. The Lyα and CIV equivalent widths indicate that the considered quasars are luminous objects at high redshift. Both these equivalent widths are too low by a factor 4, whereas the N V/Lyα ratio is 5 times larger than predicted by the photoionisation models. The line profiles are compared with logarithmic, electron-scattering and Gaussian profiles. In all the spectra rich absorption line systems are evident.  相似文献   
110.
It has been argued (Gough and McIntyre in Nature 394, 755, 1998) that the only way for the radiative interior of the Sun to be rotating uniformly in the face of the differentially rotating convection zone is for it to be pervaded by a large-scale magnetic field, a field which is responsible also for the thinness of the tachocline. It is most likely that this field is the predominantly dipolar residual component of a tangled primordial field that was present in the interstellar medium from which the Sun condensed (Braithwaite and Spruit in Nature 431, 819, 2004), and that advection by the meridional flow in the tachocline has caused the dipole axis to be inclined from the axis of rotation by about \(60^{\circ}\) (Gough in Geophys. Astrophys. Fluid Dyn., 106, 429, 2012). It is suggested here that, notwithstanding its turbulent passage through the convection zone, a vestige of that field is transmitted by the solar wind to Earth, where it modulates the geomagnetic field in a periodic way. The field variation reflects the inner rotation of the Sun, and, unlike turbulent-dynamo-generated fields, must maintain phase. I report here a new look at an earlier analysis of the geomagnetic field by Svalgaard and Wilcox (Solar Phys. 41, 461, 1975), which reveals evidence for appropriate phase coherence, thereby adding support to the tachocline theory.  相似文献   
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