全文获取类型
收费全文 | 14149篇 |
免费 | 3468篇 |
国内免费 | 4567篇 |
专业分类
测绘学 | 2081篇 |
大气科学 | 2262篇 |
地球物理 | 2945篇 |
地质学 | 8743篇 |
海洋学 | 2430篇 |
天文学 | 299篇 |
综合类 | 1160篇 |
自然地理 | 2264篇 |
出版年
2024年 | 168篇 |
2023年 | 493篇 |
2022年 | 911篇 |
2021年 | 1105篇 |
2020年 | 912篇 |
2019年 | 996篇 |
2018年 | 927篇 |
2017年 | 822篇 |
2016年 | 929篇 |
2015年 | 969篇 |
2014年 | 996篇 |
2013年 | 1070篇 |
2012年 | 1118篇 |
2011年 | 1171篇 |
2010年 | 1155篇 |
2009年 | 1070篇 |
2008年 | 1071篇 |
2007年 | 996篇 |
2006年 | 892篇 |
2005年 | 788篇 |
2004年 | 547篇 |
2003年 | 453篇 |
2002年 | 498篇 |
2001年 | 394篇 |
2000年 | 351篇 |
1999年 | 262篇 |
1998年 | 163篇 |
1997年 | 155篇 |
1996年 | 122篇 |
1995年 | 111篇 |
1994年 | 80篇 |
1993年 | 89篇 |
1992年 | 63篇 |
1991年 | 50篇 |
1990年 | 43篇 |
1989年 | 38篇 |
1988年 | 36篇 |
1987年 | 32篇 |
1986年 | 24篇 |
1985年 | 17篇 |
1984年 | 10篇 |
1983年 | 22篇 |
1982年 | 13篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1965年 | 6篇 |
1964年 | 4篇 |
1954年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
1.
We derive the classical Delaunay variables by finding a suitable symmetry action of the three torus T3 on the phase space of the Kepler problem, computing its associated momentum map and using the geometry associated with this structure. A central feature in this derivation is the identification of the mean anomaly as the angle variable for a symplectic S
1 action on the union of the non-degenerate elliptic Kepler orbits. This approach is geometrically more natural than traditional ones such as directly solving Hamilton–Jacobi equations, or employing the Lagrange bracket. As an application of the new derivation, we give a singularity free treatment of the averaged J
2-dynamics (the effect of the bulge of the Earth) in the Cartesian coordinates by making use of the fact that the averaged J
2-Hamiltonian is a collective Hamiltonian of the T3 momentum map. We also use this geometric structure to identify the drifts in satellite orbits due to the J
2 effect as geometric phases. 相似文献
2.
APPLICATION OF GEOGRAPHICAL PARAMETER DATABASE TO ESTABLISHMENT OF UNIT POPULATION DATABASE 总被引:1,自引:1,他引:0
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of 相似文献
3.
1INTRODUCTION Since the first use in human genetic research in1966,allozyme has become the most popular genetic marker for population genetics studies for almost all groups of animals and plants(Lewontin and Hubby,1966;Murphy et al.,1996).In the past seve… 相似文献
4.
Qiang Wang Derek A. Wyman Jifeng Xu Yusheng Wan Chaofeng Li Feng Zi Ziqi Jiang Huaning Qiu Zhuyin Chu Zhenhua Zhao Yanhui Dong 《Contributions to Mineralogy and Petrology》2008,155(4):473-490
New chronological, geochemical, and isotopic data are reported for Triassic (219–236 Ma) adakite-magnesian andesite-Nb-enriched
basaltic rock associations from the Tuotuohe area, central Qiangtang terrane. The adakites and magnesian andesites are characterized
by high Sr/Y (25–45), La/Yb (14–42) and Na2O/K2O (12–49) ratios, high Al2O3 (15.34–18.28 wt%) and moderate to high Sr concentrations (220–498 ppm) and εND (t) (+0.86 to +1.21) values. Low enrichments of Th, Rb relative to Nb, and subequal normalized Nb and La contents, and enrichments
of light rare earth elements combine to distinguish a group of Nb-enriched basaltic rocks (NEBs). They have positive εND (t) (+2.57 to +5.16) values. Positive correlations between Th, La and Nb and an absence of negative Nb anomalies on mantle
normalized plots indicate the NEBs are products of a mantle source metasomatized by a slab melt rather than by hydrous fluids.
A continuous compositional variation between adakites and magnesian andesites confirms slab melt interaction with mantle peridotite.
The spatial association of the NEBs with adakites and magnesian andesites define an “adakitic metasomatic volcanic series”
recognized in many demonstrably subduction-related environments (e.g., Mindanao arc, Philippines; Kamchatka arc, Russia; and
southern Baja California arc, Mexico). The age of the Touhuohe suite, and its correlation with Triassic NEB to the north indicates
that volcanism derived from subduction-modified mantle was abundant prior to 220 Ma in the central Qiangtang terrane. 相似文献
5.
储量计算与勘探线剖面计算机辅助成图一体化 总被引:1,自引:0,他引:1
引入关系教据库和GIS自动编图技术,改进传统的垂直断面法储量计算过程,可实现矿山储量计算和勘探线剖面计算机辅助成图的同步和一体化.在共用的矿山地矿信息点源数据库平台基础上.通过设置本地数据库,形成GIS操作时的外挂属性库,用来保存储量计算和机助成图过程中的动态数据.储量计算中的每一步骤中,用户以GIS为操作界面来同步实现对空间图元和外挂属性的操作,这其中对工程样段组合、矿体边界圈连和剖面制图综合等开发了大量自动化辅助功能.储量计算的各步完成后,勘探线剖面图和储量报表即同步生成.这种改进的工作方式实现了垂直断面法全过程的计算机辅助化,将提高这一传统方法的应用效率. 相似文献
6.
土壤微生物是陆地生态系统中分解者亚系统的主要组成部分, 参与了包括有机质降解、营养转化、 植物生长的促进或抑制以及各种土壤物理过程在内的一系列反应活动. 土壤微生物则是土壤质量重要的生物指标, 可以用来监控土壤质量的变化. 等温微量热法是一种简便、快速地测量微生物活性的方法, 在土壤微生物代谢热效应的研究领域中广泛应用. 就等温微量热法在土壤微生物活性中的研究进展进行综述: 等温微量热法的简介, 微量热方法与传统方法的比较, 等温微量热法在各种外界环境和土壤条件影响下的土壤微生物活性研究中的应用, 并对等温微量热法在土壤微生物和其它方面的研究进行了展望. 相似文献
7.
8.
扎赉诺尔坳陷带内诸凹陷具有不同的沉积特征和砂体特点,查干诺尔和克鲁伦凹陷砂体形成于粗粒冲积扇和扇三角洲沉积体系,砂体厚度变化大,相变快,有机质含量低,顶底板均不发育;呼伦湖凹陷以小型三角洲平原相砂体为主,具有较稳定的泥砂岩结构,有机质含量高,砂体厚度和横向延伸较稳定,但砂体铀含量较低;胡列也吐凹陷砂体以辫状河三角洲体系产出为主,砂体厚度大,延伸稳定,底板发育,顶板发育不完全,砂体铀含量高.扎赉诺尔坳陷带各凹陷砂体的不同特点,决定了它们具有不同的找矿潜力.通过砂体综合条件对比分析,笔者认为胡列也吐凹陷和克鲁伦凹陷砂体铀成矿条件较好,呼伦湖凹陷次之,查干诺尔凹陷较差. 相似文献
9.
10.
The vertical migration of diatoms in mud-flats was studied in littoral of Xibian, Longhai County and Xiao'ao, Lianjiang County, the dominant species being Gyrosigma spencerii and Hantzschia virgata respectively. Both light and tide are important factors affecting the vertical migration of diatoms. With the advent of tide, the diatom community disappears from the surface. It reappears gradually when exposed to light. In the dark, the diatoms never appear in great quantity on the surface in laboratory or in littoral. Whether in the light or in the dark, the diatoms aggregate in maximum numbers in the upper 2 mm of the sediment. But when the tide has covered the sediments for 4 hr, Hantz. virgata penetrates the mud down to a depth of 14 mm, Gy. spencerii, 12 mm. We have found that Gy. spencerii posseses phototaxis, but no tide rhythm and no day-night rhythm in our laboratory. 相似文献