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   57541篇
  免费   659篇
  国内免费   532篇
测绘学   1295篇
大气科学   3197篇
地球物理   10928篇
地质学   21650篇
海洋学   5431篇
天文学   13926篇
综合类   167篇
自然地理   2138篇
  2022年   613篇
  2021年   915篇
  2020年   1031篇
  2019年   1088篇
  2018年   2207篇
  2017年   2049篇
  2016年   2323篇
  2015年   996篇
  2014年   2051篇
  2013年   3199篇
  2012年   2290篇
  2011年   2798篇
  2010年   2548篇
  2009年   2999篇
  2008年   2646篇
  2007年   2821篇
  2006年   2571篇
  2005年   1493篇
  2004年   1385篇
  2003年   1303篇
  2002年   1282篇
  2001年   1233篇
  2000年   1092篇
  1999年   825篇
  1998年   866篇
  1997年   849篇
  1996年   687篇
  1995年   662篇
  1994年   664篇
  1993年   539篇
  1992年   532篇
  1991年   537篇
  1990年   610篇
  1989年   464篇
  1988年   454篇
  1987年   508篇
  1986年   385篇
  1985年   582篇
  1984年   561篇
  1983年   521篇
  1982年   530篇
  1981年   434篇
  1980年   472篇
  1979年   381篇
  1978年   436篇
  1977年   346篇
  1976年   331篇
  1975年   343篇
  1974年   321篇
  1973年   348篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
An ecotoxicological assessment is given to the state of small lakes in specially protected natural areas in Northwestern and Central European Russia, which suffer from aerotechnogenic acidification. The toxicity of water and bottom sediments is shown to increase with decreasing water pH. The most sensitive characteristics of aquatic organisms in different trophic groups are chosen, allowing the state of lakes to be monitored.  相似文献   
212.
This work is based on the results of 2-D and 3-D surveys in the Tuapse Basin and field works in the Crimea-Caucasus region. Seven zones were outlined in the model of sedimentation starting from the eroded mountain structure of the Greater Caucasus down to deep water sediments of the Tuapse Basin. The slope and deep-water sediments of channels and fan complexes are characterized. Volume models of the sedimentation system in the Black Sea are shown for the first time.  相似文献   
213.
214.
Direct correlation have been observed between certain trace element and hydrocarbon anomalies in the near subsurface soils of Vindhyan basin, India. This relationship with hydrocarbon is very useful in hydrocarbon exploration. 52 soil samples from Vindhyan basin were collected from a depth of 2.5m. All the soil samples were analyzed for light hydrocarbon, isotope and trace element concentrations. The adsorbed light gaseous hydrocarbon analyses show the presence of methane (8–328 ppb), ethane (0–27 ppb) and propane (0–11 ppb) respectively and these values indicate the presence of hydrocarbon micro-seepage in the study area. The carbon isotopic values determined for methane and ethane for these soil samples are (?26.41 to ?47.70 ‰ PDB) and (?20.07 to ?35.30 ‰ PDB) respectively and they are thermogenic in nature. The trace element concentrations of nickel (33–220 ppm), vanadium (72–226 ppm), copper (20–131 ppm), chromium (94–205 ppm), zinc (66–561 ppm) and cobalt (9–39 ppm) have higher than the normal concentrations in soils. Trace element concentrations are used to plot with the data obtained from light gaseous hydrocarbon concentrations and carbon isotopic values of soil samples of the Vindhyan basin. Trace element anomalies have been observed around the hydrocarbon anomalies in the study area.  相似文献   
215.
216.
An optimization scheme is considered of the method of instrumental determination of soil moisture used within the Roshydromet system. The scheme enables to reduce the volume of instrumental operations by 2.5–3 times using the empirical statistical dependences between the water content in different layers of the soil profile. Efficiency of the filtration of outliers in the data sample is studied on the basis of the analysis of regression residuals when deriving the regression equations.  相似文献   
217.
A sediment core from a high-elevation bog on Maui in the Hawaiian Islands contains evidence for drier conditions between 9.4–5.8 kyr BP, followed by a wetter interval between 5.8–2.2 kyr BP, and a variable late Holocene. These precipitation changes may be a reflection of vertical displacements of the upper boundary of the mid-Pacific Trade Wind Inversion (TWI) cloud layer. Fires, probably volcanically ignited, occurred in the forests prior to human arrival. Polynesian activity in this high-elevation, remote site was apparently limited, with no pollen, charcoal, or sedimentological evidence for local anthropogenic disturbance. After European contact, grass fires increased and introduced plant species invaded the site. Values for Cd, Cu, Pb, and Zn in sediments throughout the Holocene indicate low trace-metal deposition from atmospheric particulates at the site, even in the twentieth century.This paper is one of a series of papers guest edited by Dr. Mark Brenner on tropical paleolimnology  相似文献   
218.
The mutual gravitational potential ofN solid bodies is expanded without approximation in terms of harmonic coefficients of each body. As an application the Euler dynamical equations for the motion of the axis of figure of the rigid Earth are integrated analytically by the method of variation of parameters.  相似文献   
219.
Current and future wide field surveys in infrared passbands present opportunities for ultra low‐mass stellar sample selection via proper motions, as well as or instead of purely photometric techniques. Furthermore, proper motion measurements yield additional clues as to candidate membership of cluster or field (kinematic) populations. In this paper we review some recent results and speculate as to the possibilities afforded by the next generation of wide‐field infrared surveys. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
220.
Geography at elementary and middle schools in Louisiana, USA., remains a social studies strand along with civics, economics, and history, with no state-required geography course at any level. But because schools may require more geography than the state standard, this research examines the extent to which K–12 students are exposed to geography in Louisiana, using an electronic survey and an interview with state department of education officials. Results suggest that geography remains underemphasized, but the 2011 implementation of new social studies standards, the Advanced Placement Human Geography course, and other initiatives offer promise for more geographically literate Louisiana citizens.  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