全文获取类型
收费全文 | 8960篇 |
免费 | 1867篇 |
国内免费 | 2754篇 |
专业分类
测绘学 | 1258篇 |
大气科学 | 1415篇 |
地球物理 | 1808篇 |
地质学 | 5413篇 |
海洋学 | 1541篇 |
天文学 | 172篇 |
综合类 | 725篇 |
自然地理 | 1249篇 |
出版年
2024年 | 72篇 |
2023年 | 231篇 |
2022年 | 394篇 |
2021年 | 529篇 |
2020年 | 396篇 |
2019年 | 535篇 |
2018年 | 497篇 |
2017年 | 449篇 |
2016年 | 542篇 |
2015年 | 515篇 |
2014年 | 553篇 |
2013年 | 609篇 |
2012年 | 730篇 |
2011年 | 697篇 |
2010年 | 651篇 |
2009年 | 650篇 |
2008年 | 623篇 |
2007年 | 630篇 |
2006年 | 548篇 |
2005年 | 510篇 |
2004年 | 443篇 |
2003年 | 359篇 |
2002年 | 500篇 |
2001年 | 433篇 |
2000年 | 350篇 |
1999年 | 251篇 |
1998年 | 133篇 |
1997年 | 122篇 |
1996年 | 92篇 |
1995年 | 99篇 |
1994年 | 91篇 |
1993年 | 48篇 |
1992年 | 53篇 |
1991年 | 46篇 |
1990年 | 44篇 |
1989年 | 25篇 |
1988年 | 26篇 |
1987年 | 23篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 15篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1978年 | 2篇 |
1972年 | 2篇 |
1971年 | 3篇 |
1957年 | 3篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
承压水减压引起的沉降分析 总被引:7,自引:0,他引:7
由下卧承压水层减压引起的固结沉降计算与一般的由堆载或潜水位下降引起的固结沉降计算不同,本文着重研究了在深厚弱透水层下卧强透水承压层的复杂地质条件下,下卧承压水层减压引起的土中应力变化及周围地表沉降的计算方法;在假定一维竖向固结的条件下推导了减压引起的沉降固结度计算公式,该公式与常规的双向排水固结公式相同,表明排水减压固结与常规的双向排水固结有着相同的效果.最后,将此分析方法应用于潮州供水枢纽工程西溪水闸沉降分析中,实践的结果表明,该方法简单可行,能较好地满足工程要求. 相似文献
103.
Claude Boutron Kevin Rosman Carlo Barbante Michael Bolshov Freddy Adams Sungmin Hong Christophe Ferrari 《Comptes Rendus Geoscience》2004,336(10):847-867
The investigation of the occurrence of lead in dated snow and ice from Greenland and Antarctica has played a major role in our understanding of the history of the pollution of the atmosphere of our planet by this metal. Such studies have however proved to be very demanding, mainly because of the extreme purity of polar snow and ice. Reliable measurements can be obtained only if ultra-clean and highly sensitive procedures are used, as pioneered by Clair Patterson. The Greenland data show evidence of large-scale pollution of the atmosphere of the Northern Hemisphere for lead as early as two millennia ago during Greco–Roman times, especially because of mining and smelting activities in southern Spain. It peaked at the end of the 1960s, with lead concentrations in snow about 200 times higher than natural values, before declining during recent times because of the fall in the use of leaded gasoline. Lead pollution in Antarctica was already significant at the end of the 19th century as a consequence of whaling activities, the traffic of coal-powered ships crossing the Cape Horn, and mining activities in South America, South Africa and Australia. After declining because of the opening of the Panama Canal, the great economic depression and World War II, it reached a maximum during the 1980s, with lead concentrations 20 times higher than natural values. Other studies focus on past natural variations of lead in ancient ice dated from the last climatic cycles. To cite this article: C. Boutron et al., C. R. Geoscience 336 (2004). 相似文献
104.
105.
GPS广播星历参数拟合算法 总被引:25,自引:2,他引:25
介绍了GPS广播星历参数的最小二乘估计方法。推导了相应的计算公式。计算结果表明。文中给出的公式是正确的,其拟合精度以用户距离误差(URE)示时,对预报轨道的损失小于1cm。 相似文献
106.
107.
108.
提出一种基于高度和彩色纹理信息的目标识别方法 ,其目的是提取具有相对高度的地物。由于传统二维纹理分析的方法往往不能有效地提取这类目标 ,本文把高度、色彩和纹理信息有机地结合起来 ,提高了识别率。依据影像提取结果 ,探讨了树林对森林覆盖地区自动空中三角测量选点的负面影响 ,同时建议采用数据分组的方案来改善森林覆盖地区自动空中三角测量的精度 ,并用对比实验证明了其有效性。 相似文献
109.
The influence of land use and land cover on ecological environment is a focus of global change research. The paper chooses an industrial city-Shuicheng in Guizhou Province-as a study area because the karst water quality around the city is deteriorating with land use and land cover change. The natural susceptibility of karst water system is an important factor leading to karst water pollution. But land use and land cover change is also a main factor according to the chemical analysis of karst water quality and land use change. So it is a good way to protect karst water through rational planning and managing of land use and land cover. 相似文献
110.
During the last decade of the 20th century, extensive conversion in agricultural land use took place in Northeast China. The goal of this study is to ascertain its spatial distribution and regional differentiation, determine its causes, and analyze its environmental impact. Especially we attempt to elucidate how institutional constraints have facilitated the change at a time of agrarian restructuring when newly emerging free market was hybridized with the former planned economy. Information on six categories of land use was mapped from interpretation of Landsat TM images recorded in 1990, 1995 and 2000. Most of land use changes took place during the first half of the decade, coinciding with abrupt and chaotic changes in government directives. Farmland was changed mainly to woodland, water body and built-up areas while woodland and grassland were converted chiefly to farmland. Spatially, the change from farmland to woodland was restricted to the west of the study area. The change from grassland to farmland took place in the grazing and farming interlocked west. These chaotic and occasionally conflicting changes were largely caused by lack of stability and consistency in agricultural land use policies promulgated. They have exerted adverse impacts on the local environment, including land degradation, increased flooding, and modified climate regime. 相似文献