全文获取类型
收费全文 | 981篇 |
免费 | 159篇 |
国内免费 | 163篇 |
专业分类
测绘学 | 128篇 |
大气科学 | 202篇 |
地球物理 | 310篇 |
地质学 | 285篇 |
海洋学 | 167篇 |
天文学 | 29篇 |
综合类 | 80篇 |
自然地理 | 102篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 22篇 |
2021年 | 37篇 |
2020年 | 33篇 |
2019年 | 36篇 |
2018年 | 33篇 |
2017年 | 30篇 |
2016年 | 39篇 |
2015年 | 46篇 |
2014年 | 46篇 |
2013年 | 87篇 |
2012年 | 65篇 |
2011年 | 59篇 |
2010年 | 48篇 |
2009年 | 61篇 |
2008年 | 67篇 |
2007年 | 60篇 |
2006年 | 60篇 |
2005年 | 38篇 |
2004年 | 51篇 |
2003年 | 47篇 |
2002年 | 37篇 |
2001年 | 25篇 |
2000年 | 36篇 |
1999年 | 30篇 |
1998年 | 24篇 |
1997年 | 21篇 |
1996年 | 23篇 |
1995年 | 19篇 |
1994年 | 26篇 |
1993年 | 22篇 |
1992年 | 13篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1973年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1303条查询结果,搜索用时 9 毫秒
701.
山西成金中酸性侵入体研究 总被引:1,自引:0,他引:1
山西金矿与燕山期中酸性侵入岩密切相关。为从三维角度全面研究岩体,应用综合信息解译,全省圈定隐伏中酸性岩体66个,这些岩体出露面积小,隐伏范围大,表明岩体剥蚀程度浅,成矿潜力大,据其本身及分布特点,可划分为5个岩带,14个亚岩带。据综合信息标志,判别全省有44个成金岩体,并举实例阐述其综合信息特征. 相似文献
702.
北京相当暴雨日数的气候特征 总被引:9,自引:0,他引:9
根据北京99年6~8月的逐日降水资料和274年6~8月总降水量资料, 分析了相当暴雨日数与总降水量及旱涝等级的相关性,给出相当暴雨日数与总降 水量的定量关系式,建立了北京274年汛期相当暴雨日数资料序列。分析表明: 相当暴雨日数概念的引入,可以把汛期总降水量中暴雨过程降水与非暴雨过程降 水分开,证实汛期旱涝变化只取决于暴雨过程的总次数和强度;相当暴雨日数是 一个气候统计量,有与总降水量一致的周期变化,但其年际变率和3.5年周期比 总降水量更显著,其概率分布满足泊松分布;与旱涝等级比较,相当暴雨日数与 总降水量的相关性更好,且有利于研究形成汛期旱涝灾害的暴雨过程特征。 相似文献
703.
G. V. Gibbs K. M. Rosso D. F. Cox M. B. Boisen Jr. 《Physics and Chemistry of Minerals》2003,30(5):317-320
The average strength, s, of the bonded interactions comprising a cation containing oxide anion coordination polyhedron and the value of the electron
density, ρ(r
c
), at the bond-critical points are inversely correlated with bond length. In each case, the observed bond lengths, R, were modeled with power-law expressions defined in terms of s/r and ρ(r
c
)/r, respectively, where r is the Periodic Table row number of the cation involved in the bonded interaction. On the basis of the close connection between
bond strength and the value of the electron density at the bond-critical point, we conclude that bond strength is a direct
measure of bond type; the greater its value, the greater the localization of electron density in the binding region and the
greater the shared–electron covalent character of the bonded interaction.
Received: 15 October 2002 / Accepted: 17 February 2003
Present address:G. V. Gibbs in care of M. Spackman Department of Chemistry, University of New England, Armidale 2351, Australia
Acknowledgements The NSF is thanked for supporting this study with grant EAR–9627458. The paper was written while GVG was a Visiting NSF Scholar
at The University of Arizona. The faculty and graduate students of the Department of Geosciences and Bob Downs and Marelina
Stimpf in particular are thanked for making the visit great fun. 相似文献
704.
The probaility of crane living in reedy wetlands can reach 100%, at the same time,the area of reed ,the water level and adjacent water area are main factors which control the crane‘‘‘‘‘‘‘‘s habitat selection.We all know that all these factors are spatially heterogeneous.For the Xianghai wetland safety and to protect the Xianghai wetland habitat of crane,this paper has mainly identified a solution to these problems.The wetland in formation is extracted from the TM images,which reflect the whole wetland landscape and are very important for both quantitative analysis of remote sensing observation of the earth system and positioning analysis in GIS database that is automatically extracted from DEM.The DEM for Xianghai characteristics of topography is created.On the basis of the GRID SUBMODULE,applying the GIS spatial overlay analysis,the relationship between the water level and the reed area below the water level and the rating distribution maps of reed area above water level is established ,When the water level reaches the altitude of 165 n ,the reed area,981.2 ha is maximum,i.e., the water level of 165 m is the optimal. 相似文献
705.
三江平原沼泽湿地时空动态特征 总被引:3,自引:0,他引:3
1IntroductionThe wetland ecosystem is the latest one understood by humans and it is the most seriously damaged ecosystem (Williams, 1991). The Sanjiang Plain, located in northeast China, is the largest concentrated area of freshwater wetland in China. Since the 1950s, large-scale reclamation of the marsh in the Sanjiang Plain has been started. According to statistics, the natural marsh has lost about 80% of its total area. The marsh resources in the Sanjiang Plain decreased greatly and the… 相似文献
706.
多媒体CAI试题库的选题算法分析 总被引:1,自引:0,他引:1
通过对多媒体教学软件中试题库的总体总结和选题算法的分析,比较,提出了三层结构的选题思想和素数选题算法,因其进行复杂度和空间复杂度都比较好,可适用于各种多媒体教学测试软件系统中。 相似文献
707.
本文运用有限元法计算分析研究了多节扩孔钻孔灌注桩(DX桩)不同支盘数和支盘间距时土中应力场及其工作性状的变化规律,并从应力角度探讨了该桩对沉降的控制作用。 相似文献
708.
缺位型铬尖晶石的存在是20世纪50年代才被认识的,因其高反射率并具磁性;长期以来被误认为磁铁矿,本文对比了它与正常型铬尖晶石化学结构式的差异;论证了其化学结构式的推导计算方法和鉴别特征,铬尖晶石的组成和结构是地质地球化学的重要信息。 相似文献
709.
Plant allometric relationships were studied at the end of the 1999–2000 growing season for eight grasses. Logarithmic regressions were developed relating above-ground biomass to dimensional measurements or to tiller density. Basal diameters (the longest and the greatest perpendicular to the first) and plant height (defined as that reached by vegetative tillers) were recorded on individual plants of tussock grasses. The number of tillers per 1 m2 plot was counted for a rhizomatous grass. Our study proved that regression models including basal area or a combination of basal area and height as independent variables gave a good fit to the biomass data for tussock grasses. Density of tillers proved to be a good predictor of biomass for a rhizomatous species. A validation test using 20% of the data not used for estimating the regression equations indicated that these models made accurate prediction of grass biomass. Further work is needed to prove if there are year-to-year differences between models. 相似文献
710.
本文从矿床地球化学特征和原生晕特征研究入手,查明了山门银矿床在三度空间上指示元素的分带规律,确定了元素的分带序列,以此为依据进行了深部隐伏矿预测。 相似文献