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   2044篇
  免费   265篇
  国内免费   724篇
测绘学   310篇
大气科学   63篇
地球物理   451篇
地质学   1555篇
海洋学   169篇
天文学   88篇
综合类   232篇
自然地理   165篇
  2024年   14篇
  2023年   32篇
  2022年   69篇
  2021年   87篇
  2020年   78篇
  2019年   96篇
  2018年   76篇
  2017年   88篇
  2016年   96篇
  2015年   85篇
  2014年   113篇
  2013年   190篇
  2012年   109篇
  2011年   120篇
  2010年   102篇
  2009年   140篇
  2008年   135篇
  2007年   148篇
  2006年   130篇
  2005年   112篇
  2004年   120篇
  2003年   115篇
  2002年   97篇
  2001年   98篇
  2000年   92篇
  1999年   76篇
  1998年   51篇
  1997年   53篇
  1996年   51篇
  1995年   42篇
  1994年   50篇
  1993年   42篇
  1992年   27篇
  1991年   19篇
  1990年   22篇
  1989年   18篇
  1988年   13篇
  1987年   13篇
  1986年   4篇
  1985年   3篇
  1983年   1篇
  1982年   1篇
  1981年   2篇
  1980年   1篇
  1978年   1篇
  1977年   1篇
排序方式: 共有3033条查询结果,搜索用时 46 毫秒
181.
182.
对于氟碳钡铈矿(Cordylite—Ce)成分与结构的质疑   总被引:2,自引:0,他引:2  
本文讨论了氟碳钡铈矿的成分和结构问题。发现氟碳钡铈矿的成分、密度、折射率三者间的一致性参数很差,晶体结构与光性和空间群相矛盾,单个大阳离子的平均占有体积远大于同类矿物的平均占有体积,它的晶体结构与同类矿物不可类比。由于受当时条件的限制,在以往有关氟碳钡铈矿的成分和结构的资料中存在着一些明显的错误,有必要对其进行重新定义。其结构化学式应改为:(Na_(1-x),Ca_(0·5x))BaCe_2(CO_3)_4F。  相似文献   
183.
软土深基坑支护技术中的若干土力学问题   总被引:7,自引:3,他引:7  
评述了软土深基坑支护结构的现有设计方法,分析了墙的刚度和位移对土压力分布的影响,讨论了地下水对侧压力计算的影响、设计参数的测定与选取以及有限元计算中的一些问题,最后对支护结构的安全评价提出了建议。  相似文献   
184.
通过对红井子地区石炭纪人山岩岩石学、岩石化学、稀土元素、微量元素等特征分析;认为该区石炭纪火山岩具岛弧火山岩和裂谷火山岩两种不同属性,其形成构造环境也存在明显差异。该区火山岩分别形成于三个不同的构造环境,即准噶尔岛弧带、博格达裂谷带及喀尔力克岛弧带。  相似文献   
185.
The occurrence of shock metamorphic features substantiates an impact origin for the 1.85 Ga old Sudbury Structure, but this has not been universally accepted. Recent improvements in knowledge of large-scale impact processes, combined with new petrographic, geochemical, geophysical (LITHOPROBE) and structural data, allow the Sudbury Structure to be interpreted as a multi-ring impact structure. The structure consists of the following lithologies: Sudbury Breccia —dike breccias occurring up to 80 km from the Sudbury Igneous Complex (SIC); Footwall rocks and Footwall Breccia — brecciated, shocked crater floor materials, in part thermally metamorphosed by the overlying SIC; Sublayer and Offset Dikes, Main Mass of the SIC and Basal Member of the Onaping Formation (OF) — geochemically heterogeneous coherent impact melt complex ranging from inclusion-rich basal unit through a dominantly inclusion-free to a capping inclusion-rich impact melt rock; Grey Member of OF — melt-rich impact breccia (suevite); Green Member of OF — thin layer of fall back ejecta; Black Member of OF — reworked and redeposited breccia material; Onwatin and Chelmsford Formations — post-impact sediments. Observational and analytical data support an integrated step-by-step impact model for the genesis of these units. Analysis of the present spatial distribution of various impact-related lithologies and shock metamorphic effects result in an estimated original rim-to-rim diameter of the final crater of 200 or even 280 km for the Sudbury Structure, prior to tectonic thrusting and deformation during the Penokean orogeny.  相似文献   
186.
本文研制了适用于地质样品中15个稀土元素分析的快速方法。样品经碱熔沉淀分离和743阳离子交换树脂分离富集将稀土元素制备成溶液用等离子体光谱法(ICP-AES)进行测量。方法检出限为0.002-0.3μg/g,当稀土元素含量为0.5-80,μg/g时,方法相对标准偏差为15%-2%。本方法经过多年的样品分析考验以及不断的改进和完善,证明方法简便快速,分析数据稳定可靠,能很好地满足地质样品中15个稀土分量测定的要求。  相似文献   
187.
The SSQ-1 Digital Tape Recording Horizontal Pendulum Tiltmeter is an instrument of high sensitivity to detect the changes of ground tilt. It uses a quartz horizontal pendulum held by Z?llner bifilar suspension to sense vertical displacement, the eddy-current transducer on the pendulum converts its displacements into electric signals. Then a microcomputer is used for data acquisition and printing as well as digit tape recording. And at the same time, a filtering pen recorder is used for visible recording. The scale value of the instrument is calibrated by computer using the known tilt angle of the bulging plate — mercury cup. The scale value is 0.3–0.5 m(″)/mV. The following are the testing results for the earth tide observation obtained from the east-west component at Baijiatan Seismic Station, Beijing: r(O1): 0.6490±0.0179 α(O1): −3.83°±1.58° r(K1): 0.8049±0.0128 α(K1): 1.40°±0.90° r(M2): 0.6699±0.0040 α(M2): −0.27°±0.34° r(S2): 0.7316±0.0075 α(S2): 2.83°±0.58° r(M3): 0.8497±0.0964 α(M3): 1.61°±6.51° The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 353–359, 1993. In this work also participated Mr. Huai-Wen PAN and Wei-Jin ZHANG. And we here give special thanks to Yun-Zao XI, Qin-Wen XI, Bo-Xong TANG, Yi-Hui CHEN and An-Xu WU for their kind help in the development and test of the instrument. This paper is translated by Mr. Zhong ZHENG.  相似文献   
188.
本文提供了一个事实:同纬度不同经度台站的S_q曲线可以有很大的差异。对拉萨台的S_q异常作了一些初步的、定性的分析与解释。对八丈台的S_q异常则认为难以解释,值得深入研究。  相似文献   
189.
Chol-qeshlaghi altered area lies in the northwestern part of the post-collisional Urumieh-Dokhtar magmatic arc, NW Iran. Pervasive silicic, argillic, phyllic and propylitic altered zones appears to be intimately affiliated to the fluids derivative of upper Oligocene Khankandi granodiorite. This paper is dedicated to the identification of geochemical characteristics of hydrothermal alterations, focusing on the determination of the mass gains and losses of REEs, to gain significant insights regarding the chemical exchanges prevailed between the host rocks and hydrothermal fluids. The low pH and high activity of SO_4~(-2) ligands in silicic alteration fluids, resulting in depletion of entire REEs. Decreasing of LREEs appeared in argillic zone may attributed to reduce in adsorption ability of clay minerals in low pH; whereas HREEs enrichment in phyllic zone was inclined to put it down to the abundance of sericite(± Fe oxides). A significant reduction of Eu/Eu* ratio in silicic zone can be attributed to negligible sulfides and clay minerals as some effective agents in adsorption of released Eu~(+2). Factors such as changes in pH, the abundance of absorptive neomorph mineral phases, activity of ligands play an important role in controlling the distribution and concentration of REEs in Chol-qeshlaghi alteration system.  相似文献   
190.
ABSTRACT

Turning Earth observation (EO) data consistently and systematically into valuable global information layers is an ongoing challenge for the EO community. Recently, the term ‘big Earth data’ emerged to describe massive EO datasets that confronts analysts and their traditional workflows with a range of challenges. We argue that the altered circumstances must be actively intercepted by an evolution of EO to revolutionise their application in various domains. The disruptive element is that analysts and end-users increasingly rely on Web-based workflows. In this contribution we study selected systems and portals, put them in the context of challenges and opportunities and highlight selected shortcomings and possible future developments that we consider relevant for the imminent uptake of big Earth data.  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