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   69991篇
  免费   946篇
  国内免费   613篇
测绘学   1956篇
大气科学   5261篇
地球物理   13327篇
地质学   23679篇
海洋学   6146篇
天文学   16619篇
综合类   153篇
自然地理   4409篇
  2021年   471篇
  2020年   560篇
  2019年   607篇
  2018年   1338篇
  2017年   1308篇
  2016年   1676篇
  2015年   1043篇
  2014年   1744篇
  2013年   3558篇
  2012年   1779篇
  2011年   2431篇
  2010年   2238篇
  2009年   2929篇
  2008年   2662篇
  2007年   2676篇
  2006年   2586篇
  2005年   2182篇
  2004年   2148篇
  2003年   2020篇
  2002年   1912篇
  2001年   1690篇
  2000年   1636篇
  1999年   1546篇
  1998年   1484篇
  1997年   1462篇
  1996年   1161篇
  1995年   1168篇
  1994年   1123篇
  1993年   983篇
  1992年   965篇
  1991年   890篇
  1990年   1027篇
  1989年   897篇
  1988年   841篇
  1987年   973篇
  1986年   809篇
  1985年   1088篇
  1984年   1216篇
  1983年   1171篇
  1982年   1115篇
  1981年   995篇
  1980年   941篇
  1979年   873篇
  1978年   860篇
  1977年   799篇
  1976年   740篇
  1975年   673篇
  1974年   748篇
  1973年   732篇
  1972年   460篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
881.
882.
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   
883.
884.
Summary. Many geomagnetic variation anomalies are probably caused by the channelling, through small-scale bodies, of electric currents induced in much larger conductors elsewhere. Consequently, the direct interpretation of anomalous magnetic fields by modelling the electromagnetic response of conductive structures may give misleading results. It is suggested that, rather than attempting to proceed directly from the electromagnetic fields to conductivity models, we should instead take the intermediate step of determining the distribution of anomalous current flow.
Maps of the anomalous fields over a conductive structure can be generated from inter-station transfer functions. If it is assumed that the internal currents are concentrated in a thin sheet at a specified depth, the equivalent current system in the sheet can be computed directly from the vertical magnetic field. The most straightforward method of performing this calculation is to compute the Fast Fourier Transform of the magnetic field data, and then to apply a wavenumber filter.
The presence of any vertical currents invalidates the thin sheet model. However, if the spatial distribution of a horizontal component of the anomalous magnetic field is also known, the presence of any vertical currents can be detected directly, and their position determined. The value of the methods is illustrated by applying them to the interpretation of a Geomagnetic Deep Sounding survey of the Kenya rift valley.  相似文献   
885.
Summary. A spherical harmonic model of the second time-derivative of the geomagnetic field is determined, for the first time, directly from measures of the secular acceleration based on observatory annual mean data. The data span the interval 1964.5–1975.5, and 165 observatories are included. The model comprises the 32 coefficients of degree and order up to 6 that are significant at the 5 per cent level. Its primary purpose is to aid in the reduction of data to epoch for the 1980 series of navigational charts. The model is compared with earlier estimates of secular acceleration, derived by less direct methods.  相似文献   
886.
887.
Remote sensing is a geographic analysis tool capable of producing large quantities of data in the spectral, temporal and spatial domains. Techniques for automating the image analysis process would be advanced by the inclusion of artificial intelligence techniques in the design of image processing systems. The remote sensing application which show promise for successful implementation of artificial intelligence techniques are intelligent onboard processing, advanced database interrogation, and the automated analysis of multispectral imagery.  相似文献   
888.
The organizational separation of management and other quaternary activities from production operations in American manufacturing has resulted in a divergent pattern of production and headquarter locations. We examine the external control relationships that arise from this separation between headquarter location and branch plant operations in nonmetropolitan Kentucky. Although corporate headquarters have diffused to the Sunbelt, no shift was found in the corporate control of branch plants in Kentucky. Nonmetropolitan manufacturing in Kentucky was oriented toward the metropolitan centers of the traditional manufacturing belt rather than the newer corporate centers of the South. Also control relationships are more likely to be local among smaller factories. Large plants having more autonomy in production or producing finished goods were more likely to be owned by a distant firm located in a large SMSA.  相似文献   
889.
890.
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