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   134614篇
  免费   1986篇
  国内免费   1629篇
测绘学   3831篇
大气科学   10021篇
地球物理   27188篇
地质学   50538篇
海洋学   10668篇
天文学   25578篇
综合类   2369篇
自然地理   8036篇
  2021年   840篇
  2020年   1040篇
  2019年   1085篇
  2018年   7777篇
  2017年   7097篇
  2016年   5700篇
  2015年   1931篇
  2014年   2601篇
  2013年   5116篇
  2012年   4233篇
  2011年   7555篇
  2010年   6684篇
  2009年   7953篇
  2008年   6756篇
  2007年   7301篇
  2006年   3852篇
  2005年   3580篇
  2004年   3728篇
  2003年   3558篇
  2002年   3118篇
  2001年   2475篇
  2000年   2423篇
  1999年   2031篇
  1998年   2017篇
  1997年   2010篇
  1996年   1733篇
  1995年   1668篇
  1994年   1471篇
  1993年   1345篇
  1992年   1264篇
  1991年   1161篇
  1990年   1358篇
  1989年   1183篇
  1988年   1046篇
  1987年   1244篇
  1986年   1168篇
  1985年   1433篇
  1984年   1620篇
  1983年   1557篇
  1982年   1416篇
  1981年   1356篇
  1980年   1200篇
  1979年   1146篇
  1978年   1175篇
  1977年   1104篇
  1976年   1032篇
  1975年   985篇
  1974年   980篇
  1973年   1025篇
  1972年   642篇
排序方式: 共有10000条查询结果,搜索用时 0 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.
The first integrals of motion of the restricted planar circular problem of three bodies are constructed as the formal power series in r1/2, r being the distance of a moving particle from the primary. It is shown that the coefficients of these series are trigonometric polynomials of an angular variable. Some particular solutions have been found in a closed form. The proposed method for constructing the formal integrals can be generalized to a spatial problem of three bodies.  相似文献   
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.
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   
78.
79.
Dynamic spectra of Jupiter's decametric emission often display narrow-band features, referred to as events of type N (Carr et al., 1983). The average bandwidth of these emissions is in the vicinity of 200 kHz, their durations are typically in the decasecond range, and their f-t slopes are small and random. Although the N-bursts can be described as narrow-band L-bursts, it seems that they are realted to S-bursts in their area of occurrence in the Io-B region, the durations of the emission envelopes, and their bandwidths. Possible implications are discussed.  相似文献   
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