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   6062篇
  免费   1320篇
  国内免费   1753篇
测绘学   754篇
大气科学   882篇
地球物理   1348篇
地质学   3497篇
海洋学   966篇
天文学   263篇
综合类   645篇
自然地理   780篇
  2024年   44篇
  2023年   132篇
  2022年   429篇
  2021年   464篇
  2020年   387篇
  2019年   420篇
  2018年   461篇
  2017年   405篇
  2016年   477篇
  2015年   428篇
  2014年   457篇
  2013年   485篇
  2012年   464篇
  2011年   445篇
  2010年   434篇
  2009年   376篇
  2008年   331篇
  2007年   299篇
  2006年   307篇
  2005年   217篇
  2004年   159篇
  2003年   141篇
  2002年   125篇
  2001年   119篇
  2000年   116篇
  1999年   165篇
  1998年   110篇
  1997年   110篇
  1996年   90篇
  1995年   91篇
  1994年   78篇
  1993年   73篇
  1992年   56篇
  1991年   39篇
  1990年   33篇
  1989年   38篇
  1988年   18篇
  1987年   18篇
  1986年   26篇
  1985年   23篇
  1984年   6篇
  1983年   8篇
  1982年   7篇
  1980年   7篇
  1979年   5篇
  1978年   3篇
  1965年   2篇
  1958年   3篇
  1957年   1篇
  1954年   2篇
排序方式: 共有9135条查询结果,搜索用时 562 毫秒
991.
本文指出了"信息确定性"所涉及的信息的不可 修改性、信息"保真性"是认识观念的重大变革。 以取消数量稳定性限制条件,得出了非线性不稳 定具有特殊性信息及其周期性,并可解释翁文波 的"可公度"法是运用了特殊性信息的特殊规律, 在物质演化问题上不能将数量的一般规律拓广至 特殊性信息中;由此涉及了当代科学没有解决物 质演化的"时间"、特殊性、非惯性系及其数量分 析伪造信息等问题。信息的客观存在性和不可修 改性,已实质上表明了将近代的"信息科学"的 信息分析等同于当代科学的数量处理是认识上的 误解。即"信息"不能等同于"数量"。  相似文献   
992.
Multiple cities in a diveloped economic area may consist of a city cluster,and the difusion and mixing of its pollutants result in the effect of pollutants plume between cities and the large-scale regional pollution diffusion phenomenon.The distant transfer and diffusion of pollutants occurs when massive aerosols are affected by the dynamic porcess of large-scale circulations.Research suggesten that the life span of aerosol particles whose diameters are about 1 um is the longest.The longevity …  相似文献   
993.
Since the mid-1980s,Tanyaokou large Zn-Cu-Fe sulfides deposit,located at the southwest end of Langshan-Zhaertaishan-Bayan Obo Mesoproterozoic metallogenic belt in the west section of the northern margin of the North China Platform[1?9](Fig.1),has been confirmed to be submarine volcanic exhalative-sedimentary metamorphosed deposit hosted in the miogeosynclinal mud-carbonaceous formation of the Langshan Group(LG)[1],or submarine volcanic exha-lative-deposition-altered deposit[2]or stratabo…  相似文献   
994.
本文提出OH超脉泽源的一个具体模型,在该模型中OH超脉泽源由非饱和的球型子源组成.在此估算出超脉泽星系致密射电核的尺度和连续谱亮温度,以及OH源距星系核的最小非饱和距离.计算表明,OH超脉泽源所包含的子源数目可能相当大.R_H~log ρ_(25μm)/ρ_(60μm)图、log L_(OH)/L_(IR)~log ρ_(25μm)/ρ_(60μm)图和其它结果显示,有一部分OH超脉泽源很可能是弱饱和的,本文对此作了解释.  相似文献   
995.
Using spectrographic plates obtained on 11 nights during its 1979/1980 eclipse, we have obtained a radial velocity curve of ζ Aur. Our results are in general agreements with previous measurements, but we note that the radial velocity just before the total eclipse showed an appreaciable decrease. A preliminary discussion is made.  相似文献   
996.
    
This paper concerns the reconstruction of a dynamic system based on phase space continuation of monthly mean temperature 1D time series and the assumption that the equation for the time-varying evolution of phase-space state variables contains linear and nonlinear quadratic terms, followed by the fitting of the dataset subjected to continuation so as to get, by the least square method, the coefficients of the terms, of which those of greater variance contribution are retained for use. Results show that the obtained low-order system may be used to describe nonlinear properties of the short range climate variation shown by monthly mean temperature series.  相似文献   
997.
Relatively strongly magnetic fine components (< 30μm, XS-4J and DS-4J) which are most environmentally sensitive were separated from layer S5-1 in the Xifeng and Duanjiapo loess sections and analyzed by MPV-3 for their morphometric characteristics and reflectance, SEM-ESD for their element contents and XRD for their mineral phases, respectively. The results showed that minerals in both samples are dominated by detrial Fe-Ti oxides of aeolian origin. In sample XS-4J the reflectance and iron contents of magnetic minerals are usually high. In addition to magnetite (Fe3O4), maghemite (γFe2O3) and hematite (Fe2O3), some Fe-high oxide (72.25 wt%–86.67 wt%), ilmenite (FeTiO3), and magnetite-ulvöspinel [Fe(FeCr)O4, Fe (FeNi)O4] were also detected. In sample DS-4J obvious negative linear correlations were found between Ti and Fe, and the contents of Mn, Si, Al and Ca are usually high and the minerals are dominated by magnetite (maghemite), goethite (FeOOH) and limonite (containing Si and OH). In addition, the signs of corrosion of magnetic minerals and newly crystallized magnetite (maghemite) were recognized. Differences in the composition and assemblage characteristics of magnetite minerals between XS and DS reflect significant differences in source rocks and preserving conditions.  相似文献   
998.
A 3-D chemical transport model (OSLO CTM2) is used to investigate the impact of the increase of NOx emission over China. The model is capable to reproduce basically the seasonal variation of surface NOx and ozone over eastern China. NOx emission data and observations reveal that NOx over eastern China increases quite quickly with the economic development of China. Model results indicate that NOx concentration over eastern China increasingly rises with the increase of NOx emission over China, and accelerates to increase in winter. When the NOx emission increases from 1995 to its double, the ratio of NO2/NOx abruptly drops in winter over northern China. Ozone at the surface decreases in winter with the continual enhancement of the NOx level over eastern China, but increases over southern China in summertime. It is noticeable that peak ozone over northern China increases in summer although mean ozone changes little. In summer, ozone increases in the free troposphere dominantly below 500 hPa.Moreover, the increases of total ozone over eastern China are proportional to the increases of NOx emission.In a word, the model results suggest that the relationship between NOx and ozone at the surface would change with NOx increase.  相似文献   
999.
This paper presents Nd-Sr-Pb isotope data on scheelite, inclusion fluids and residues of gangue quartz, and sulfides from the W-Sb-Au ore deposits at Woxi and Liaojiaping (LJP) in the Xuefeng Uplift Belt (XUB), Western Hunan, China. Sm and Nd concentrations in scheelite from Woxi are much lower than in scheelite from LJP and can be distinguished by their high 147Sm/144Nd ratios of ~1.25 from the much lower ratios around 0.26 in scheelites from the LJP. Nd values (compared to values at 200 Ma, which is the average timing of granitoid emplacement during the Indosinian-Yanshanian periods in the XUB) are around –10 for the LJP and compare well with the range of –5 to –11 defined by the granitoids, whereas they are around –27 for scheelite from Woxi. This might indicate that REEs in the mineralizing fluids at LJP originated from granitoids that are concentrated along the southern border of the XUB, whereas in the case of Woxi, the original fluids might have been masked by REEs released during intense high-temperature wall rock alteration of unexposed Precambrian basement rocks at depth. Sr isotopes of scheelite from these two deposits show similar relations to host / nearby rocks, in that 87Sr/86Sr (T=200 Ma) ratios of ~0.721 for LJP scheelite agree with values ranging between 0.718 – 0.726 for granitoids, whereas these ratios are much higher (i.e. 0.745) for scheelite from Woxi and correspond to the 87Sr/86Sr (T=200 Ma) ratio range of 0.743 – 0.749 for Precambrian host slates. Crushing experiments to release inclusion fluids from gangue quartz and sulfides deposited during later stages of ore deposition in both deposits failed to provide accurate and geologically meaningful two-point (fluid-residue) tie lines in Rb-Sr isochron diagrams. However, Sr released from fluid inclusions generally reveals lower initial 87Sr/86Sr ratios than the respective residues and shows affinities to 87Sr/86Sr (T=200 Ma) values of Indosinian-Yanshanian granitoids, both at Woxi and LJP. Pb stepwise leaching of scheelite and sulfides did not result in sufficient spreads in Pb isotope diagrams and therefore no information regarding exact mineralization ages in the two deposits could be deduced. Overall, ore Pb isotopes reveal upper crustal signatures and are compatible with Pb isotope signatures of the predominant Precambrian slates in the Woxi area. Steep trajectories through late stage quartz-sulfide mineralization in Pb isotope diagrams may hint at mixing scenarios involving Pb from the host rocks and a component with lower 207Pb/204Pb and 208Pb/204Pb ratios relative to 206Pb/204Pb ratios, which cannot be linked to any known reservoir in the XUB mining district. Sr isotopes of four out of seven residual sulfide samples from Woxi plot along a paleomixing line at an age of 199 ± 8 Ma, supporting a mixing scenario for the fluids indicated by the Pb isotopes and pointing to a possible genetic relationship with the emplacement of Indosinian—Yanshanian granitoids. The budgets of REEs, Rb-Sr and Pb in the original fluids were severely affected by contamination of these elements apparently during intense wall rock alteration but, after sealing of the major pathways, the mineralizing fluids tend to have better preserved their original signatures. These attest a genetic relationship between the metallogeny in the XUB W-Sb-Au province and the emplacement of Indosinian-Yanshanian granitoids during Mesozoic intracontinental tectonic uplift and thrusting.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial handling: B. Lehmann  相似文献   
1000.
一种改进的等高线断点连接方法   总被引:3,自引:0,他引:3  
黄雪莲  胡鹏  白轶多 《测绘科学》2006,31(1):111-114
介绍了几种现有的等高线断点连接方法,并在最小点对法的基础上提出了一种改进的断点连接方法。此方法在判断断点间距离大小之前先对等高线进行预处理,即将各条目标等高线自动赋予不同的颜色值,然后将图幅进行格网划分,在不同网格内搜索断点,根据颜色的差异及距离的远近自动进行等高线连接。此方法连接的准确率高于一般的断点连接方法。  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