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   98247篇
  免费   1616篇
  国内免费   805篇
测绘学   2310篇
大气科学   6393篇
地球物理   19571篇
地质学   35963篇
海洋学   8668篇
天文学   22472篇
综合类   297篇
自然地理   4994篇
  2022年   692篇
  2021年   1150篇
  2020年   1263篇
  2019年   1357篇
  2018年   3120篇
  2017年   2947篇
  2016年   3373篇
  2015年   1760篇
  2014年   3186篇
  2013年   5169篇
  2012年   3446篇
  2011年   4457篇
  2010年   3809篇
  2009年   4843篇
  2008年   4462篇
  2007年   4445篇
  2006年   4074篇
  2005年   3304篇
  2004年   3296篇
  2003年   2956篇
  2002年   2483篇
  2001年   2332篇
  2000年   2143篇
  1999年   1662篇
  1998年   1750篇
  1997年   1718篇
  1996年   1357篇
  1995年   1403篇
  1994年   1206篇
  1993年   1081篇
  1992年   1056篇
  1991年   983篇
  1990年   1102篇
  1989年   945篇
  1988年   860篇
  1987年   998篇
  1986年   795篇
  1985年   1074篇
  1984年   1153篇
  1983年   1075篇
  1982年   1023篇
  1981年   894篇
  1980年   853篇
  1979年   748篇
  1978年   786篇
  1977年   708篇
  1976年   657篇
  1975年   643篇
  1974年   642篇
  1973年   639篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
71.
72.
Leningrad State University. Translated from Astrofizika, Vol. 32, No. 2, pp. 267–275, March–April, 1990.  相似文献   
73.
Photographic spectra of SN1987A in the LMC have been obtained from 1987 February 25 to 1988 June 30. Microdensitometer tracings of these have been reduced to intensity and corrections for instrumental response have been applied to the spectra. This paper presents these data in an atlas format, discusses the reduction procedures in detail, and presents radial velocity measurements of selected lines in the spectra  相似文献   
74.
Summary. Finite element models for shallow subduction produce realistic behaviour for a wide variety of mechanical strength and density distributions. Characteristic displacements are found to occur even without a discrete low-strength megathrust if there is a high-density subducted plate to localize lithospheric compression. A high-density plate is itself unnecessary in the presence of a low-strength megathrust and regional compression.
Successful finite element models produce an outer arc at the top of the trench slope, and forearc basin with geometry characteristic of natural analogues. These structural features occur by upward inelastic bending of the lithospheric wedge overlying the megathrust. This mechanically unstable behaviour may dissipate significant energy and cause the megathrust to migrate continuously by accretion, tectonic erosion, or abandonment and reinitiation farther offshore. Upward bending in the overriding plate is promoted by low megathrust dip, low megathrust shear strength, and high horizontal compression in the overriding plate.  相似文献   
75.
Byurakan Astrophysical Observatory. Translated from Astrofizika, Vol. 33, No. 2, pp. 271–281, September–October, 1990.  相似文献   
76.
Reconnaissance seismic shot in 1971/72 showed a number of well defined seismic anomalies within the East Sengkang Basin which were interpreted as buried reefs. Subsequent fieldwork revealed that Upper Miocene reefs outcropped along the southern margin of the basin. A drilling programme in 1975 and 1976 proved the presence of shallow, gas-bearing, Upper Miocene reefs in the northern part of the basin. Seismic acquisition and drilling during 1981 confirmed the economic significance of these discoveries, with four separate accumulations containing about 750 × 109 cubic feet of dry gas in place at an average depth of 700 m. Kampung Baru is the largest field and contains over half the total, both reservoir quality and gas deliverability are excellent. Deposition in the East Sengkang Basin probably started during the Early Miocene. A sequence of Lower Miocene mudstones and limestones unconformably overlies acoustic basement which consists of Eocene volcanics. During the tectonically active Middle Miocene, deposition was interrupted by two periods of deformation and erosion. Carbonate deposition became established in the Late Miocene with widespread development of platform limestones throughout the East Sengkang Basin. Thick pinnacle reef complexes developed in the areas where reef growth could keep pace with the relative rise in sea level. Most reef growth ceased at the end of the Miocene and subsequent renewed clastic sedimentation covered the irregular limestone surface. Late Pliocene regression culminated in the Holocene with erosion. The Walanae fault zone, part of a major regional sinistral strike-slip system, separates the East and West Sengkang Basins. Both normal and reverse faulting are inferred from seismic data and post Late Pliocene reverse faulting is seen in outcrop.  相似文献   
77.
Poorly-graphitised carbon particles are formed during manufacture of sample substrates (holey carbon films) for Analytical Electron Microscopy studies of small particles. The particles form during heat treatment of cellulose acetobutyrate at about 975 °C and 1050 °C. In AEM studies of fine-grained carbonaceous extraterrestrial materials, these particles are easily recognised.  相似文献   
78.
The SAS® computer software system, widely used and respected for its capabilities in statistical analysis and data base management, now includes a new set of graphic and cartographic procedures called SAS GRAPH?. We have used these cartographic procedures in research on mapping ethno-cultural census data from metropolitan areas in Ontario and in undergraduate and graduate courses in computer cartography. On the basis of that experience, we describe and evaluate SAS/GRAPH'S cartographic capabilities and illustrate with maps drawn by various devices.  相似文献   
79.
The metasediments in the Chamba region experienced three phases of deformation: DF1, DF2 and DF3.Folded quartz veins are co-folded with the F2 crenulation folds. Their geometric and tectonic significance is studied by microstructures and shortening adjacent to the discrete crenulation cleavage, S2. Microstructures of folded vein segments, their geometric changes and truncation to cleavage (S2) are mainly due to pressure-solution phenomena and the estimated volume loss from reconstructed vein segments range from 16 to 25.5%,which is closely related to volume decrease (26%) calculated from the polydeformed slates of North Wales areas.  相似文献   
80.
In this paper we analyse error-removing techniques used without sufficient theoretical support in a previous paper, where Chandrasekhar's higher-order perturbation theories were developed for either uniformly or differentially-rotating polytropic stars.  相似文献   
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