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   6531篇
  免费   1187篇
  国内免费   1765篇
测绘学   502篇
大气科学   1562篇
地球物理   1553篇
地质学   3298篇
海洋学   790篇
天文学   292篇
综合类   649篇
自然地理   837篇
  2024年   48篇
  2023年   105篇
  2022年   269篇
  2021年   329篇
  2020年   273篇
  2019年   283篇
  2018年   292篇
  2017年   283篇
  2016年   352篇
  2015年   293篇
  2014年   352篇
  2013年   345篇
  2012年   319篇
  2011年   358篇
  2010年   366篇
  2009年   348篇
  2008年   309篇
  2007年   288篇
  2006年   251篇
  2005年   233篇
  2004年   189篇
  2003年   206篇
  2002年   219篇
  2001年   218篇
  2000年   244篇
  1999年   360篇
  1998年   346篇
  1997年   340篇
  1996年   251篇
  1995年   230篇
  1994年   223篇
  1993年   188篇
  1992年   144篇
  1991年   130篇
  1990年   88篇
  1989年   77篇
  1988年   81篇
  1987年   46篇
  1986年   47篇
  1985年   35篇
  1984年   34篇
  1983年   21篇
  1982年   22篇
  1981年   13篇
  1980年   15篇
  1979年   11篇
  1978年   2篇
  1977年   2篇
  1958年   2篇
  1951年   1篇
排序方式: 共有9483条查询结果,搜索用时 15 毫秒
151.
北京五里营井水温每天都有24次锯齿状畸变,有很好的规律性,经分析认为是气氡观测对水温造成影响.将气氡和水温供电分离,并在水温地线和地网之间增加一个电感,以阻挡其他干扰信号由地线反馈到观测仪器,改造后水温脉冲干扰彻底消除,观测质量明显提高.  相似文献   
152.
Dating data, altitude of Neolithic sites, climatic changes from sedimentary records and previous research results were collected and analyzed to detect possible connections between climatic changes and human activities in the Changjiang River Delta in the Neolithic Age. The results indicated that hydrological changes greatly impacted the human activities in the study region. Low-lying geomorphology made the floods and sea level changes become the important factors affecting human activities, especially the altitude change of human settlements. People usually moved to higher places during the periods characterized by high sea level and frequent floods to escape the negative influences from water body expansion, which resulted in cultural hiatus in certain profiles. However, some higher-altitude settlements were not the results of climatic changes but the results of social factors, such as religious ceremony and social status. Therefore, further research will be necessary for the degree and types of impacts of climatic changes on human activities in the study area at that time.  相似文献   
153.
长河煤矿为地质构造中等复杂区,其矿区地质及水文地质条件表明,地表水、直接充水含水层煤系砂岩裂隙水、老窑水、构造断裂带水都可构成该矿井开采的充水因素,且其涌水量大小与大气降水关系密切。针对矿井开采的具体情况提出了一系列的防治措施,并取得了良好的效果,对煤矿水害的防治有一定的指导意义。  相似文献   
154.
本文提出水库重力测量检测非牛顿引力的实验数据分析新方法。利用近一天两重力仪的重力差值信号,基于同液面高度变化水体重力信号相等这一具有明显物理意义的准则,建立仪器零漂信号多项式系数矢量的矩阵方程,通过极大似然反演比较严格地计算漂移信号,并从中提取检测信号。实例计算结果表明:(1)当数据充分、漂移多项式阶数取值适当时,该方法完全有效;(2)在现有水库实验的设备和技术条件下,测定万有引力常数G对实验室值的偏离精度可达0.15%,优于矿井实验和先前水库实验的分析结果;若进一步改善水面高度测量,检测精度还可明显提高;(3)Hornberg水库实验重力仪G709在0.4%的精度范围内,未发现G对实验室值的偏离。  相似文献   
155.
Root zone soil water content impacts plant water availability, land energy and water balances. Because of unknown hydrological model error, observation errors and the statistical characteristics of the errors, the widely used Kalman filter (KF) and its extensions are challenged to retrieve the root zone soil water content using the surface soil water content. If the soil hydraulic parameters are poorly estimated, the KF and its extensions fail to accurately estimate the root zone soil water. The H‐infinity filter (HF) represents a robust version of the KF. The HF is widely used in data assimilation and is superior to the KF, especially when the performance of the model is not well understood. The objective of this study is to study the impact of uncertain soil hydraulic parameters, initial soil moisture content and observation period on the ability of HF assimilation to predict in situ soil water content. In this article, we study seven cases. The results show that the soil hydraulic parameters hold a critical role in the course of assimilation. When the soil hydraulic parameters are poorly estimated, an accurate estimation of root soil water content cannot be retrieved by the HF assimilation approach. When the estimated soil hydraulic parameters are similar to actual values, the soil water content at various depths can be accurately retrieved by the HF assimilation. The HF assimilation is not very sensitive to the initial soil water content, and the impact of the initial soil water content on the assimilation scheme can be eliminated after about 5–7 days. The observation interval is important for soil water profile distribution retrieval with the HF, and the shorter the observation interval, the shorter the time required to achieve actual soil water content. However, the retrieval results are not very accurate at a depth of 100 cm. Also it is complex to determine the weighting coefficient and the error attenuation parameter in the HF assimilation. In this article, the trial‐and‐error method was used to determine the weighting coefficient and the error attenuation parameter. After the first establishment of limited range of the parameters, ‘the best parameter set’ was selected from the range of values. For the soil conditions investigated, the HF assimilation results are better than the open‐loop results. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   
156.
Some of the tridymite in the monomict Northwest Africa (NWA) 11591 eucrite are found to have sulfide‐rich replacement textures (SRTs) to varying degrees. The SRTs of tridymite in NWA 11591 are characterized by the distribution of loose porous regions with aggregates of quartz and minor troilite grains along the rims and fractures of the tridymite, and we propose a new mechanism for the origin of this texture. According to the volume and density conversion relationship, the quartz in the SRT of tridymite with a hackle fracture pattern was transformed from tridymite. We suggest that the primary tridymite grains are affected by the S‐rich vapors along the rims and fractures, leading to the transformation of tridymite into quartz. In addition, the S‐rich vapors reacted with Fe2+, which was transported from the relict tridymite and/or the adjacent Fe‐rich minerals, and/or the S‐rich vapors react with the exotic metallic Fe to form troilite grains. The sulfurization in NWA 11591 most likely occurred during the prolonged subsolidus thermal metamorphism in the shallow crust of Vesta and might be an open, relatively high temperature (>800 °C) process. Sulfur would be an important component of the metasomatic fluid on Vesta.  相似文献   
157.
The distribution of L absorption lines has been investigated in the fractal scheme. It is found that (1) the L absorption clouds distribute completely different from that of galaxies; (2) the L absorption clouds are anti-associated with galaxies and quasars. These results may imply that there are two kinds of objects formed by different processes of clustering. This is favourable for the cosmic-string theory on the formation of large-scale structure of the Universe. In the string model, the objects can be divided into two kinds according to their clustering with or without string loops as their initial density perturbation.On leave from the Center of Astrophysics, University of Science and Technology of China, Hefei, P.R. China. Y. Chu is a research fellow of Alexander von Humboldt-Foundation.  相似文献   
158.
This study deals with the toxic effects of Zn2+, Cu2+, Cd2+ and NH3 on Chinese prawn (Penaeus orientalis). The median lethal concentrations of the toxic matters on Chinese prawn were determined. Cd2+>Cu2+>Zn2+>NH3-Nm is the toxic order of these chemicals to Chinese prawn. The toxic order of concentrated Zn2+ and Cd2+ in tissues and organs of Chinese prawn is in the order. gill>shell+appendage>viscera>muscle.  相似文献   
159.
对新疆东疆地区某氧化铜矿石进行选别试验,确定该氧化铜矿选别流程为一粗二扫一精:选别的最佳条件为:磨矿细度90%;粗选硫化钠用量2500g/t;BX与丁胺黑药组合使用做捕收剂,用量分别为3500g/t和850g/t;2#油用量106g/t.铜精矿品位25.76%,回收率73.51%.  相似文献   
160.
In this paper, the equilibrium configurations of rapid rotating compact stars and some gravitational effects are studied within the general relativity by use of the Harrison-Wheeler equation of state and by the self-consistent field method. Numerical calculations show that the equilibrium configuration of a rotating star is a spheroid. For large spin velocities, say, ω > 3.0 × 102 sec−1 the eccentricity and mass increase very rapidly as the angular velocity increases, for the critical angular velocity of the rotating star, the eccentricity is about 0.7, the increase in mass is about 10–35%. The difference of the gravitational redshifts at the surface of the star caused by rotation, and the difference of the light bending when the beam moves in the direction of rotation or in the opposite direction are obvious.  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