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   7435篇
  免费   1386篇
  国内免费   1808篇
测绘学   438篇
大气科学   1565篇
地球物理   1884篇
地质学   3549篇
海洋学   1091篇
天文学   227篇
综合类   950篇
自然地理   925篇
  2024年   15篇
  2023年   85篇
  2022年   328篇
  2021年   374篇
  2020年   339篇
  2019年   329篇
  2018年   400篇
  2017年   387篇
  2016年   459篇
  2015年   351篇
  2014年   427篇
  2013年   405篇
  2012年   379篇
  2011年   407篇
  2010年   462篇
  2009年   421篇
  2008年   372篇
  2007年   360篇
  2006年   315篇
  2005年   246篇
  2004年   219篇
  2003年   241篇
  2002年   213篇
  2001年   207篇
  2000年   235篇
  1999年   366篇
  1998年   338篇
  1997年   308篇
  1996年   258篇
  1995年   257篇
  1994年   226篇
  1993年   196篇
  1992年   168篇
  1991年   104篇
  1990年   99篇
  1989年   77篇
  1988年   53篇
  1987年   41篇
  1986年   21篇
  1985年   23篇
  1984年   26篇
  1983年   15篇
  1982年   19篇
  1981年   17篇
  1980年   11篇
  1979年   12篇
  1978年   3篇
  1977年   4篇
  1975年   2篇
  1958年   5篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
241.
Four metallogenic epochs occurred in different tectonic environments during theevolution of the Northern Qilian metallogenic province through the geological time. The Mid-dle Proterozoic metallogenic epoch witnessed the tectonic environment of crustal breakupcaused by mantle diapirism, in which ultramafic-mafic rocks were intruded along beep faultbelts and the superlarge Jinchuan magmatic Cu-Ni sulphide deposit was formed. In theMiddle-Late Proterozoic metallogenic epoch the crust was further broken to form anintracontinental rift, in which the Chenjiamiao style massive Cu-Fe sulphide deposits hosted bybasic volcanic tuff were formed in the lower volcano-sedimentary sequence, while the largesedex type Jingtieshan style Fe-Cu deposits were formed within the upper abyssal carbon-richargillaceous sedimentary sequence. The Early Palaeozoic saw the aulacogen environment, with-in which the Baiyinchang style superlarge massive base and precious metal sulphide depositshosted by quartz keratophyric tuff were formed in the Middle-Late Cambrian rifted island arcand the massive Cu-Zn sulphide deposits and magmatic chromite deposits associated with theophiolite suite were formed in the Early-Middle Ordovician, and the Honggou style massiveCu-Fe sulphide deposits hosted by spilite were formed in the Late Ordovician back-arc basinenvironment. In the Late Palaeozoic-Meso-Cenozoic, the metallogenic province went into anintracontinental orogenic stage characterized by compressive tectonic environment, in whichthere occurred carbonate-quartz vein type and tectono-alteration gold deposits associated withductile-shear structures.  相似文献   
242.
岩体结构力学岩体工程地质力学的新发展   总被引:6,自引:1,他引:6  
本文从岩体工程地质力学角度, 研讨了岩体结构力学的意义, 地位与作用。作者就个人的认识, 将岩体结构力学的理论特点, 概括为五个方面, 即再变形和再破坏, 岩体结构的控制作用、岩体结构控制论、赋存环境以及关于岩体的多种力学介质和力学模型构成的力学体系。  相似文献   
243.
Study on the Impacts of Climate Change on China's Agriculture   总被引:1,自引:0,他引:1  
This paper measures the economic impacts of climate change on China's agriculture based on the Ricardian model. By using county-level cross-sectional data on agricultural net revenue, climate, and other economic and geographical data for 1275 agriculture dominated counties, we find that under most climate change scenarios both higher temperature and more precipitation would have an overall positive impact on China's agriculture. However, the impacts vary seasonally and regionally. Autumn effect is the most positive, but spring effect is the most negative. Applying the model to five climate scenarios in the year 2050 shows that the East, the Central part, the South, the northern part of the Northeast, and the Plateau would benefit from climate change, but the Southwest, the Northwest and the southern part of the Northeast may be negatively affected. In the North, most scenarios show that they may benefit from climate change. In summary, all of China would benefit from climate change in most scenarios.  相似文献   
244.
Thirty-seven Kupferschiefer samples from southwestern Poland were analyzed by microscopy, Rock-Eval approach and instrumental neutron activation analysis to understand the geochemical and morphological characteristics of kerogen present in the samples. The analytical results indicate that there are two different types of kerogens. One type was only subjected to thermal alteration processes, and the other was further oxidized after deposition of the sediment.In the oxidized samples migrabitumen was transformed into pyrobitumen. Rock-Eval analyses show a significant decrease in HI values in the oxidized samples and an increase in OI values in relation to the samples that were not influenced by oxidation. Variations in S2 versus Corg contents indicate a change in kerogen from Type II to Type III with progressing oxidation. The presence of pyrobitumen and the depletion of hydrogen in the altered kerogen allow one to conclude that the kerogen was used as hydrogen donor for thermochemical sulfate reduction(TSR).  相似文献   
245.
本工程场地为欠固结厚层填土现状,为保证承台底不脱空,采用粉喷桩加固承台底欠固结填土,并在粉喷桩内施工静压桩的技术对策,这样既加固了填土,又提高了静压桩单桩承载力,获得了良好的效果。  相似文献   
246.
曹文京  郭延杰  孙兰华 《地下水》2004,26(2):115-116
地下水是聊城市重要的而且是主要的水资源,近年来,地下水的开采和补源引起了人们的广泛关注.本文拟对地下淡水资源的时空分布、运动规律、地下水资源计算和评价及聊城市地下水开发利用问题进行一些探讨,对于引黄(河)灌区合理引用黄河水和开采地下水具有重要的指导作用和应用价值.  相似文献   
247.
The Sanchazi mafic-ultramafic complex in Mianlue tectonic zone, South Qinling can be subdivided into two blocks, i.e. Sanchazi paleo-magmatic arc and Zhuangkegou paleo-oceanic crust fragment (ophiolite). The Sanchazi paleo-magmatic arc is mainly composed of andesite, basaltic and basalt-andesitic gabbro (or diorite), andesitic dyke, plagiogranite and minor ultramafic rocks, which have typical geochemical features of island arc volcanic rocks, such as high field strength element (e.g. Nb, Ti) depletions and lower Cr, Ni contents. The Light rare earth element (LREE) and K enrichments of these rocks and zircon xenocrystals of 900 Ma from plagiogranite suggest that this magmatic arc was developed on the South active continental margin of the South Qinling micro-continent. The U-Pb age of (300 ± 61)Ma for zircons from plagiogranite indicates that the Mianlue paleo-oceanic crust was probably subducted underneath the South Qinling micro-continent in Carboniferous. This is consistent with the formation time (309Ma) of the Huwan eclogite originating from oceanic subduction in Dabie Mountains, suggesting that the Mianlue paleo-ocean probably extended eastward to the Dabie Mountains in Carboniferous. The high-Mg adakitic rocks in Sanchazi paleo-magmatic arc suggest that the subducted oceanic crust was relatively young (<25Ma) and hot.  相似文献   
248.
Kela 2 Gas Field, with high formation pressure (74.35MPa), high pressure coefficient (2.022) and difficulty of potential test and evaluation, is the largest integrated proved dry gas reservoir in China so far and the principal source for West-East Gas Development Project. In order to correctly evaluate the elastic-plastic deformation of rocks caused by the pressure decline during production, some researches, as the experiment on reservoir sensitivity to stress of gas filed with abnormal high pressure, are made. By testing the rock mechanic properties, porosities and permeabilities at different temperature and pressure of 342 core samples from 5 wells in this area, the variations of petro-physical properties at changing pressure are analyzed, and the applicable inspection relationship is concluded. The average productivity curve with the reservoir sensitivity to stress is plotted on the basis of the research, integrated with the field-wide productivity equation. The knowledge lays a foundation for the gas well productivity evaluation in the field and the gas field development plan, and provides effective techniques and measures for basic research on the development of similar gas fields.  相似文献   
249.
介绍了基于“9210工程”和MICAPS系统的资料自动存储、整理、检索和历史天气图分析业务系统——基于MICAPS系统的数字化天气图及常规气象资料检索分析系统,较全面地介绍了该系统高度自动化、设计友好的功能和在业务中的应用。  相似文献   
250.
GPS定位中卫星坐标计算的切比雪夫多项式拟合法   总被引:16,自引:1,他引:16  
余鹏孙学金  赵世军 《气象科技》2004,32(3):198-200,F004
在GPS定位中需要不断地计算卫星的坐标,实时的卫星坐标是根据接收到的广播星历来计算的,而每次计算卫星坐标都需要占用大量内存,影响计算速度。利用切比雪夫多项式拟合广播星历的卫星坐标,以实现卫星轨道的标准化。通过实例分析了用切比雪夫多项式进行卫星坐标拟合的精度及多项式阶数和时间间隔的合理选择等问题,得出了一些有益的结论。  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