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   136124篇
  免费   2219篇
  国内免费   1485篇
测绘学   3491篇
大气科学   10105篇
地球物理   27883篇
地质学   47242篇
海洋学   11813篇
天文学   29772篇
综合类   399篇
自然地理   9123篇
  2021年   1049篇
  2020年   1252篇
  2019年   1309篇
  2018年   2829篇
  2017年   2655篇
  2016年   3581篇
  2015年   2299篇
  2014年   3475篇
  2013年   7088篇
  2012年   3738篇
  2011年   5254篇
  2010年   4491篇
  2009年   6135篇
  2008年   5614篇
  2007年   5224篇
  2006年   5076篇
  2005年   4265篇
  2004年   4245篇
  2003年   4003篇
  2002年   3770篇
  2001年   3366篇
  2000年   3305篇
  1999年   2819篇
  1998年   2816篇
  1997年   2722篇
  1996年   2372篇
  1995年   2243篇
  1994年   2050篇
  1993年   1886篇
  1992年   1790篇
  1991年   1669篇
  1990年   1824篇
  1989年   1592篇
  1988年   1474篇
  1987年   1758篇
  1986年   1549篇
  1985年   1932篇
  1984年   2161篇
  1983年   2063篇
  1982年   1929篇
  1981年   1783篇
  1980年   1600篇
  1979年   1482篇
  1978年   1545篇
  1977年   1400篇
  1976年   1334篇
  1975年   1263篇
  1974年   1274篇
  1973年   1275篇
  1972年   808篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Theoretical studies have shown the possibility of high-temperature ('high enthalpy') geothermal reservoirs in the pre-Tertiary rocks at 4–5 km depth range within the Pannonian Basin. This expectation was proven by the hotwater/steam blowout of Fábiánsebestyén-4 borehole (16.12.85–31.1.86). Exploration efforts carried out during 1987–88 in the broad vicinity of the borehole proved that reservoirs of this type can be found with the combination of seismic reflection, silica-thermometry and magnetotelluric sounding methods. Deliberate prospection should be continued in all suitable areas within the basin, since high enthalpy reservoirs promise profitable operation of geothermal power stations.  相似文献   
72.
73.
This work deals with the possible solution of the solar neutrino problem in the framework of the resonant neutrino spin-flavor precession scenario. The event rate results from the solar neutrino experiments as well as the recoil electron energy spectrum from SuperKamiokande are used to constrain the free parameters of the neutrino in this model (Δm2 and μν). We consider two kinds of magnetic profiles inside the sun. For both cases, a static and a twisting field are discussed.  相似文献   
74.
75.
The Cumberland Basin, a 118 km2 estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens. Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalley's method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m?2, respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin.  相似文献   
76.
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale.  相似文献   
77.
78.
Progress in the introduction of coulometry for the analysis of total carbon dioxide (TCO2) in marine waters is described. An extractor—stripper removes CO2 that is measured coulometrically by the quantity of electricity (coulombs) used to electrogenerate OH? ions for the titration of the acid formed by the reaction of CO2 and ethanolamine. The equivalence point is detected photometrically with thymolphthalein as the indicator, and Faraday's Law relates coulombs to equivalents of titrant generated and CO2 determined so that there are no standard curves needed or titrants to standardize or store. Accuracy was determined by adding gelatin capsules containing 100–1500 μg C of pure CaCO3 into the stripper, and accuracies of better than ± 1 μg C were achieved. The best precision for natural seawater (± 1 standard error) of ± 0.5 μmol l?1 was found for 17 samples of Bermuda coastal waters having a mean TCO2 of 2007.2 μmol l?1 (0.05% CV). Sources of error and precautions are discussed. This method, which has been used successfully at sea, can be used to study a variety of marine phenomena involving TCO2.  相似文献   
79.
80.
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