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   27287篇
  免费   1254篇
  国内免费   1437篇
测绘学   917篇
大气科学   1831篇
地球物理   5588篇
地质学   11645篇
海洋学   2845篇
天文学   5369篇
综合类   462篇
自然地理   1321篇
  2023年   164篇
  2022年   547篇
  2021年   759篇
  2020年   640篇
  2019年   750篇
  2018年   1223篇
  2017年   1125篇
  2016年   1180篇
  2015年   644篇
  2014年   1051篇
  2013年   1579篇
  2012年   1187篇
  2011年   1406篇
  2010年   1368篇
  2009年   1501篇
  2008年   1310篇
  2007年   1466篇
  2006年   1265篇
  2005年   768篇
  2004年   689篇
  2003年   653篇
  2002年   686篇
  2001年   636篇
  2000年   539篇
  1999年   473篇
  1998年   402篇
  1997年   446篇
  1996年   344篇
  1995年   319篇
  1994年   295篇
  1993年   263篇
  1992年   263篇
  1991年   226篇
  1990年   220篇
  1989年   203篇
  1988年   173篇
  1987年   197篇
  1986年   186篇
  1985年   220篇
  1984年   206篇
  1983年   201篇
  1982年   194篇
  1981年   184篇
  1980年   165篇
  1979年   190篇
  1978年   161篇
  1977年   143篇
  1976年   133篇
  1975年   137篇
  1973年   167篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
A collection of brachiopods by the Institute of Oceanology, Academia Sinica (Qingdao), contains eight species from seven genera. Six of the species have been recorded previously from China seas—Lingula adamsi, L. anatina, Discinisca stella, Pelagodiscus atlanticus, Campages mariae, Terebratalia coreanica. Two species (Terebratulina hataiana andFrenulina sanguinolenta) have been described from other parts of the Pacific area. The apparent absence of any endemic species is a noteworthy feature of Chinese in comparison with Japanese faunas.  相似文献   
62.
The solar sources of the magnetic storms of November 8 and 10, 2004, are analyzed. The preliminary results of such an analysis [Yermolaev et al., 2005] are critically compared with the results of the paper [Tsurutani et al., 2008], where solar flares were put in correspondence with these magnetic storms. The method for determining solar sources that cause powerful magnetospheric storms is analyzed. It has been indicated that an optimal approach consists in considering coronal mass ejections (CMEs) as storm sources and accompanying flares as additional information about the location of CME origination.  相似文献   
63.
In previous papers we have published preliminary results on the stellar oxygen-abundance gradient in M33 by using B-type supergiants. In the present study, we include one new B-supergiant and derive effective temperatures and gravities for these stars. The determination of atmospheric parameters is based on analyses of silicon ionisation balance. Detailed non-LTE model atmospheres and non-LTE line formation calculations were used for the abundance determination of Si and O. Careful differential analyses are used to quantify the differences in metal abundances between M33 stars and MK spectral standards in our Galaxy. We compare our results with the abundances expected from studies of HII regions. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
64.
The elastic and density properties of rocks of the Shamakha-Ismailly seismogenic blocks are studied by the ultrasonic pulse method in quasi-hydrostatic high pressure apparatuses. An attempt is made to more accurately determine the upper crustal lithology of these blocks. The observed values of the elastic and density characteristics of the rocks at high pressures suggest that the upper layers in the Ismailly and Shamakha blocks can consist of sedimentary carbonate rocks such as marls, sandstones, mudstones, and limestones. The middle layers apparently consist of volcaniclastic rocks: lithoclastic tuffs, andesites, etc. The lower layer (basement) in both blocks is likely composed of basic rocks such as basalts, trachybasalts, etc. According to the values of elastic wave velocities and densities, gabbroid rocks may compose the fourth layer of the Buinuz intrusion, identified from seismic data.  相似文献   
65.
本文提出一种基于纵横波速度关系的带误差校正的横波速度预测方法,它主要针对在进行横波速度预测时所采用纵波模量M近似变换结果与标准的Gassmann关系(K变换)之间存在的误差提出了校正的方法,并给出了完整的基于纵横波速度关系的带误差校正横波预测技术流程,该技术是一个简单实用的横波速度预测工具.  相似文献   
66.
67.
Gold deposits and occurrences small in reserves and high in Au grade conventionally determine the line of prospecting in terrigenous sequences of the Verkhoyansk–Kolyma region. In this paper, the geological structure of such gold objects is considered with the example of the deposits and prospects making up the Zhdaninsky ore–placer cluster in the Republic of Sakha (Yakuia). From lithological, structural, and mineralogical–geochemical data, the formation conditions of ore-bearing complexes are specified, the geological evolution history of the northern Ol’chan Zone of the Kular–Nera Belt is reconstructed, and the zonal distribution of mineralization within the ore–placer cluster is revealed. The structural–compositional complexes were formed in the following succession: (1) sedimentation at the shelf of the passive margin accompanied by synsedimentation deformations; (2) metagenesis of sediments and the development of bedding-plane intraformational detachments of collision stage D1 under conditions of tangential compression and accompanied by the formation of carbon dioxide–aqueous metamorphic fluid at a temperature of 300°C and under a pressure of 1.4 kbar; (3) folding and faulting of orogenic stage D2 with the formation of synkinematic magmatic bodies, metasomatic alteration, and Au-bearig mineral assemblages. Small Au-bearing objects with veined mineralization and high Au grade are localized in structures of stage D2 transverse to bedding-plane schistosity S1. They form at the collision stage above intraformational detachment surfaces and are controlled by shear structures of the orogenic stage with misalignment of these deformations. The ore zoning is determined by the distribution of Co and Ni minerals and by variations in the anionic composition of ore (S, As, Sb).  相似文献   
68.
It is an objective fact that there exists error in the satellite dynamic model and it will be transferred to satellite orbit determination algorithm, forming a part of the connotative model error. Mixed with the systematic error and random error of the measurements, they form the unitive model error and badly restrict the precision of the orbit determination. We deduce in detail the equations of orbit improvement for a system with dynamic model error, construct the parametric model for the explicit part of the model and nonparametric model for the error that can not be explicitly described. We also construct the partially linear orbit determination model, estimate and fit the model error using a two-stage estimation and a kernel function estimation, and finally make the corresponding compensation in the orbit determination. Beginning from the data depth theory, a data depth weight kernel estimator for model error is proposed for the sake of promoting the steadiness of model error estimation. Simulation experiments of SBSS are performed. The results show clearly that the model error is one of the most important effects that will influence the precision of the orbit determination. The kernel function method can effectively estimate the model error, with the window width as a major restrict parameter. A data depth-weight-kernel estimation, however, can improve largely the robustness of the kernel function and therefore improve the precision of orbit determination.  相似文献   
69.
70.
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