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   41324篇
  免费   3406篇
  国内免费   4369篇
测绘学   1951篇
大气科学   4803篇
地球物理   8858篇
地质学   17223篇
海洋学   4395篇
天文学   6901篇
综合类   1664篇
自然地理   3304篇
  2023年   441篇
  2022年   1013篇
  2021年   1254篇
  2020年   1137篇
  2019年   1248篇
  2018年   1674篇
  2017年   1508篇
  2016年   1709篇
  2015年   1256篇
  2014年   1759篇
  2013年   2301篇
  2012年   1768篇
  2011年   2030篇
  2010年   1952篇
  2009年   2137篇
  2008年   1931篇
  2007年   1859篇
  2006年   1713篇
  2005年   1391篇
  2004年   1263篇
  2003年   1151篇
  2002年   1082篇
  2001年   1024篇
  2000年   1072篇
  1999年   1176篇
  1998年   1071篇
  1997年   992篇
  1996年   860篇
  1995年   758篇
  1994年   766篇
  1993年   621篇
  1992年   584篇
  1991年   440篇
  1990年   462篇
  1989年   400篇
  1988年   351篇
  1987年   349篇
  1986年   293篇
  1985年   364篇
  1984年   370篇
  1983年   358篇
  1982年   339篇
  1981年   260篇
  1980年   278篇
  1979年   231篇
  1978年   215篇
  1977年   220篇
  1976年   182篇
  1975年   193篇
  1974年   179篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Our current understanding on sedimentary deep-water environments is mainly built of information obtained from tectonic settings such as passive margins and foreland basins. More observations from extensional settings are particularly needed in order to better constrain the role of active tectonics in controlling sediment pathways, depositional style and stratigraphic stacking patterns. This study focuses on the evolution of a Plio-Pleistocene deep-water sedimentary system (Rethi-Dendro Formation) and its relation to structural activity in the Amphithea fault block in the Corinth Rift, Greece. The Corinth Rift is an active extensional basin in the early stages of rift evolution, providing perfect opportunities for the study of early deep-water syn-rift deposits that are usually eroded from the rift shoulders due to erosion in mature basins like the Red Sea, North Sea and the Atlantic rifted margin. The depocentre is located at the exit of a structurally controlled sediment fairway, approximately 15 km from its main sediment source and 12 km basinwards from the basin margin coastline. Fieldwork, augmented by digital outcrop techniques (LiDAR and photogrammetry) and clast-count compositional analysis allowed identification of 16 stratigraphic units that are grouped into six types of depositional elements: A—mudstone-dominated sheets, B—conglomerate-dominated lobes, C—conglomerate channel belts and sandstone sheets, D—sandstone channel belts, E—sandstone-dominated broad shallow lobes, F—sandstone-dominated sheets with broad shallow channels. The formation represents an axial system sourced by a hinterland-fed Mavro delta, with minor contributions from a transverse system of conglomerate-dominated lobes sourced from intrabasinal highs. The results of clast compositional analysis enable precise attribution for the different sediment sources to the deep-water system and their link to other stratigraphic units in the area. Structures in the Amphithea fault block played a major role in controlling the location and orientation of sedimentary systems by modifying basin-floor gradients due to a combination of hangingwall tilt, displacement of faults internal to the depocentre and folding on top of blind growing faults. Fault activity also promoted large-scale subaqueous landslides and eventual uplift of the whole fault block.  相似文献   
952.
齐大山铁矿尾矿矿砂对环境污染的风洞实验   总被引:1,自引:0,他引:1  
杨佐涛 《中国沙漠》1992,12(2):53-57
在开发矿产资源的同时又面临着环境资源的破坏,以及环境的污染问题,如在采矿和冶炼中产生的弃土和尾矿等废弃物,如处理不当在风力的作用下会产生对环境的污染.本文针对齐大山铁矿尾矿进行了有关方面的风洞吹砂实验;1、干尾矿砂(含水率为0.14%),与大于起砂风速的2次方呈正比关系;2、尾矿砂的输砂率随含水量的增加而减少非常明显.因此,给尾矿砂加入一定量的水分,使其保持不低于3-4%的水分,能有效的降低尾矿砂的吹扬量。  相似文献   
953.
954.
This paper outlines a method for explicitly classifying landscape using 4 km2 grid cells, a set of attributes derived from topographic and geologic maps and an agglomerative numerical taxonomic procedure which can accommodate mixed data. The method was used to classify the landscapes of the Hunter Valley, NSW (22 000 km2) and the landscape classes generated are briefly described. These classes are compared with the land systems of the Hunter which were developed by traditional integrated survey procedure. It is concluded that the method could provide a more satisfactory starting-point for integrated land survey than air photo pattern analysis.  相似文献   
955.
This article is an English translation of the work cited in 46: 3092. The authors review the conclusions of Pivovarov (see 44: Title 3118) concerning the nature of the urbanization process and optimal city size in the USSR. The relationship between urbanization and the characteristics of a given socioeconomic system is examined, and the possibility of determining optimal size for cities of different functional types is evaluated  相似文献   
956.
957.
Conclusion The World Data Center-A for Paleoclimatology, located in the NOAA/NGDC Paleoclimatology Program, is committed to providing the scientific community with easy access to all paleoenvironmental data. Efforts to make archived data readily available include international coordination of data acquisition, management, and distribution, sponsoring workshops and data cooperatives to facilitate the compilation of important data sets, development of a browse and visualization software package (PaleoVu), and dispersal of archived data on magnetic media or over ANONYMOUS FTP/INTERNET. The program publishes a semi-annual newsletter that highlights latest developments and accomplishments in the area of paleoenvironmental data for global change research. Contributions to the newsletter are welcome from researchers describing their efforts to coordinate the free flow of paleoclimate data throughout the international scientific community.For information on the program or to be added to the mailing list contact Mrs Mildred England (phone: 303-497-6227; Fax: 303 497-6513; e-mail: MKE@mail.ngdc.noaa.gov), NOAA National Geophysical Data Center, Paleoclimatology Program/World Data Center-A for Paleoclimatology, 325 Broadway, E/GC Boulder, CO 80303 USA  相似文献   
958.
黑龙江省与呼伦贝尔盟土地类型的划分及其分布规律   总被引:1,自引:0,他引:1  
本文就土地研究的几个主要概念,即土地、土地分类、土地分级作了简要讨论,并结合黑龙江省及呼伦贝尔盟的具体条件,对为农、林、牧业服务的土地分类原则进行了初步探讨,还拟订了该区土地分级系统——土地类和土地型,及适合于大比例尺制图的土地组。在此基础上,将全区分为18个土地类、71个土地型。文章还从水平带性、垂直带性和土地结构三个方面,介绍了该区土地类型分布的一般规律。  相似文献   
959.
960.
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