全文获取类型
收费全文 | 1186篇 |
免费 | 53篇 |
国内免费 | 59篇 |
专业分类
测绘学 | 350篇 |
大气科学 | 115篇 |
地球物理 | 92篇 |
地质学 | 85篇 |
海洋学 | 128篇 |
天文学 | 2篇 |
综合类 | 110篇 |
自然地理 | 416篇 |
出版年
2024年 | 8篇 |
2023年 | 14篇 |
2022年 | 51篇 |
2021年 | 58篇 |
2020年 | 50篇 |
2019年 | 58篇 |
2018年 | 42篇 |
2017年 | 65篇 |
2016年 | 73篇 |
2015年 | 60篇 |
2014年 | 75篇 |
2013年 | 123篇 |
2012年 | 67篇 |
2011年 | 92篇 |
2010年 | 59篇 |
2009年 | 55篇 |
2008年 | 65篇 |
2007年 | 57篇 |
2006年 | 46篇 |
2005年 | 38篇 |
2004年 | 28篇 |
2003年 | 26篇 |
2002年 | 21篇 |
2001年 | 14篇 |
2000年 | 10篇 |
1999年 | 11篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 1篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1989年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有1298条查询结果,搜索用时 93 毫秒
61.
13C/12C比值作为海洋生态系统食物网示踪剂的研究──崂山湾水体生物食物网的营养关系 总被引:3,自引:1,他引:3
于1992年9月–1994年5月先后对崂山湾海域作了7个航次调查。由实验室培养试验和现场采样分析两种方法得出食物链一个营养级的13C富集度分别为(1.6±0.2)×10-3和1.7×10-3,两者几乎一致。由7个季度月的现场采集样品的同位素分析结果表明,从处在食物网底部的浮游植物到最高级的肉食性鱼类之间δ13C差值平均为6.9×10-3,相当于5个营养级;并由各类生物的碳同位素数据分别确定了它们在该食物网中的营养位置,其结果与食性分析方法所得结果基本吻合。同位素示踪方法研究的结果还表明,崂山湾水体生物食物网中碳的来源主要是浮游植物碳。 相似文献
62.
海洋中存在着大量的颗粒,包括大型聚合颗粒(即海雪,粒径>500μm)、小型聚合颗粒(1~500μm)和亚微米颗粒粒径(<1μm)等。颗粒在海水中营造了不同于纯海水的小生境,其中生活着与自然海水中不同的生物。异养细菌、蓝细菌、真核藻类、鞭毛虫、纤毛虫等微食物网生物可以黏附在海洋颗粒上,或生活在颗粒内部,其丰度高于周围水体中的自由生活生物,这可能是由于颗粒提供了更适宜生长的营养环境。本文综述了海洋浮游微食物网生物在海洋颗粒形成和沉降中的作用。微食物网生物在颗粒物的形成过程中起到很重要的作用,它们可以直接促进颗粒形成,也可以彼此结合成颗粒,或微型浮游动物排粪形成颗粒。微食物网生物还可以对颗粒进行转化,影响颗粒的大小、沉降速度、或对颗粒及其黏附生物进行摄食。微食物网生物由于本身较小,沉降较慢,但这些生物和颗粒的结合使得微食物网生物在碳通量中发挥重要的作用。 相似文献
63.
64.
Antoine S. Bailly 《The Professional geographer》1995,47(1):70-74
Four directions in producer services research in Europe are identified: the economic role of producer services for urban and regional development; the location of advanced producer services; the role of regional structures and innovative “milieu”; and local development and labor markets. Issues for future research include the role of producer services in the urban structuring and/or restructuring of world cities; the relation between producer services and interregional disparities; the integration of producer services into networks; and the impacts of the liberalization of producer services. 相似文献
65.
Producer Services Research in the United Kingdom 总被引:1,自引:0,他引:1
Peter W. Daniels 《The Professional geographer》1995,47(1):82-87
This paper reviews recent research in the United Kingdom on producer services and suggests directions for further research. A recurrent theme in UK-based geographic research into producer services is the role of service activities, establishments, and trade in uneven development of the UK space economy, a role manifested in uneven provision and quality of services in peripheral regions. During the 1980s, overall growth in demand for producer services and the rapid rise in metropolitan London real estate costs reduced London's dominance, but these trends moderated by the end of the decade. In the absence of adequate government statistics, regular and standardized surveys are needed to compile a time series of service change in the United Kingdom. Directions for continued empirical research include the role of producer services in innovation and technology transfer, the implications of information technology for the location of employment in the sector, and the impacts of productivity increases on employment and wages. 相似文献
66.
Jeff R. Crump 《The Professional geographer》1993,45(3):286-296
The purpose of this research is to analyze the composition and distribution of Department of Defense (DOD) services procurement. The results indicate producer services comprise 53% of DOD services outlays. Defense expenditures for services are particularly important in the Washington, D.C., region and in states that either produce weapons or house defense-related R&D facilities. Defense expenditures are providing impetus to service sector growth in Virginia and, simultaneously, are providing the initial stimulus for development of producer services in states such as New Mexico. 相似文献
67.
Very little is currently known about the globalization of the temporary staffing industry, a strategically significant sector given its role in promulgating wider labor market flexibility. This article starts to rectify this research lacuna in four ways: by conceptualizing the international expansion of temporary staffing and comparing it to other business service sectors, by identifying and mapping the top twenty transnational staffing agencies, by offering a typology of the leading transnational agencies based on their functional and geographic characteristics, and by charting a research agenda for future work on this sector. 相似文献
68.
69.
Current advances in computer hardware, information technology and data collection techniques have produced very large data
sets in a wide variety of scientific and engineering disciplines. We must harness this opportunity to visualize and extract
useful information from geophysical and geological data. We have taken the task of data mining by implementing a map-like
approach over a web server for interrogating the data, using a client-server paradigm. The spatial-data is mapped onto a two-dimensional
grid from which the user (client) can quiz the data with the map-interface as a user extension. The data is stored on the
server, while the computational gateway separating the client and the server can be the front-end of an electronic publication,
electronic classroom, a survey, or an e-business. We have used a combination of Java, Java3D, and Perl for processing the
data and communicating between the client and the server. The user can interrogate the geospatial data over any particular
region with arbitrary dimensions and then receive back relevant statistical analysis, such as the histogram plots and local
statistics. We have applied this method for the following data sets: (1.) distribution of prime numbers (2.) two-dimensional
mantle convection (3.) three-dimensional mantle convection (4.) high-resolution satellite reflectance data over multiple wavelengths
(5.) molecular dynamics describing the flow of blood in narrow vessels. Using the map-interface, one can actually interrogate
this data over the Internet. 相似文献
70.
广州信息密集服务业的空间发展及其对城市地域结构的影响 总被引:27,自引:0,他引:27
采用第三产业普查资料,深入分析了改革开放以来广州信息密集服务业的发展水平,空间发展特征和原因及其对城市地域结构的影响,揭示了信息密集服务业发展与城市地域运动的关系。 相似文献