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   1303篇
  免费   117篇
  国内免费   85篇
测绘学   22篇
大气科学   19篇
地球物理   215篇
地质学   247篇
海洋学   46篇
天文学   759篇
综合类   18篇
自然地理   179篇
  2024年   1篇
  2023年   7篇
  2022年   2篇
  2021年   8篇
  2020年   6篇
  2019年   11篇
  2018年   10篇
  2017年   16篇
  2016年   18篇
  2015年   29篇
  2014年   28篇
  2013年   32篇
  2012年   28篇
  2011年   30篇
  2010年   17篇
  2009年   85篇
  2008年   92篇
  2007年   110篇
  2006年   112篇
  2005年   95篇
  2004年   85篇
  2003年   87篇
  2002年   85篇
  2001年   74篇
  2000年   74篇
  1999年   63篇
  1998年   70篇
  1997年   30篇
  1996年   29篇
  1995年   29篇
  1994年   23篇
  1993年   20篇
  1992年   17篇
  1991年   10篇
  1990年   18篇
  1989年   24篇
  1988年   8篇
  1987年   11篇
  1986年   3篇
  1985年   2篇
  1984年   3篇
  1983年   1篇
  1981年   1篇
  1980年   1篇
排序方式: 共有1505条查询结果,搜索用时 359 毫秒
41.
紫外线辐射强度预报模型研究   总被引:5,自引:0,他引:5  
以陈万隆的到达地面的紫外辐射的参数化方案、Frederick模式中的高度修正方程和NWS的云量修正方程为基础。建立了一个到达地面紫外总辐射的预测模型。进行紫外线辐射强度预报。结果表明:模型能很好地反映出晴天紫外线辐射强度的变化趋势及其规律。少云、多云和阴天由于天空状况复杂,预报误差相对较大。但基本上能反映出紫外线辐射强度的变化情况。模型考虑因子较全面,可以用于城市紫外线辐射强度预报。  相似文献   
42.
3DstructuralreconstructionofMohointheTangshanearthquakeareabyusinginversionofcurvedinterfaceXIAOLINGLAI(赖晓玲)XIANKANGZHANG...  相似文献   
43.
湖南黄金洞金矿毒砂中Au赋存状态的电子探针研究   总被引:1,自引:0,他引:1  
本文利用JXA-8800M超大型电子探针,以湖南黄金洞金矿含金毒砂为研究对象。利用金特征X-射线扫描方法和微区含金含量,对不可见金赋存状态的研究方法和温试条件进行了探索,先观察背散射电子图像,排除金在含金矿物毒砂中呈显显微颗粒存在的可能性,继而用LiF晶体测定金L=88.76Lal峰位,然后逐渐放大倍数(x300,x1000,x5000,x30000,x200000)进行金特征,X-射线连续测量,,结果发现金元素总 是均匀分布在重砂中,从而证实了金呈品格金形式存在的推测。  相似文献   
44.
45.
本文论述了宇宙中子、反射宇宙中子的形成过程和反射宇宙中子测量装置的研制。对入射宇宙中子流和反射宇宙中子流的测量与研究 ,可为揭示大气环境和解决工程地质问题提供一种新的方法和手段  相似文献   
46.
Permian deep‐water mudstones in the Tanqua Basin, South Africa, have been studied using geochemical and spectral gamma ray techniques. The mudstones occur as thick sequences between sand‐rich submarine fans, but also occur as thinner mud‐rich units within each fan. The interfan mudstones are interpreted to have accumulated during transgression and the consequent period of relatively high sea‐level, while the submarine fans and their intrafan mudstones were deposited during regression and relatively low sea‐level. Geochemical analyses revealed systematic differences between interfan and intrafan mudstones because the two types of mudstones have slightly different source lithologies. Differences between the two types of mudstone suggest that changes in relative sea‐level played a role in controlling exposure of sediment source areas. There are geochemical signals that display systematic stratigraphic trends within both interfan and intrafan mudstones. These are best explained by gradual denudation, exposure and weathering of different lithologies within a single sediment source area. Both interfan and intrafan mudstones have uniform geochemical signals along the flow direction except for the relative amount of uranium. It is most likely that the basinward increase in uranium in the mudstones is the result of reduced clastic dilution of uranium‐bearing pelagic fallout.  相似文献   
47.
48.
Helium concentration and isotopic composition were measured in a suite of samples across the Permian-Triassic boundary at Opal Creek, Canada, to determine whether high extraterrestrial helium concentrations are associated with a possible extinction-inducing impact event at this time. No extraterrestrial 3He was detected, implying that neither fullerene-hosted nor IDP-hosted He is present at or near the boundary. This observation is consistent with similar studies of some Permian-Triassic sections, but contrasts sharply with reports of both fullerene- and IDP-hosted extraterrestrial 3He at other sections.Step-heat experiments indicate rapid diffusion of extraterrestrial helium from sediments heated to temperatures above ∼ 70 °C. Given the likelihood of burial and associated heating in Permian-Triassic age rocks, the initially unexpected absence of IDP-hosted 3He likely indicates thermally induced diffusive loss. Indeed one of the key sections (Graphite Peak, Antarctica) from which extraterrestrial 3He has been reported at and near the Permian-Triassic boundary has been sufficiently heated that the reported preservation of extraterrestrial helium, in both IDPs and fullerenes, is inexplicable. Recent contamination provides a plausible explanation for extraterrestrial 3He in these samples.While no extraterrestrial 3He was detected at Opal Creek, there is a sharp increase in nucleogenic 3He very close to or at the Permian-Triassic boundary. This presumably arises from the major lithologic change at this time, from cherts in the Permian to shales and siltstones in the Triassic. Increased nucleogenic 3He is associated with increases in both lithium and organic carbon content into the Triassic. Either the production rate or the retention of this 3He is higher in the shales and siltstones than in the cherts. Care must be taken to eliminate such artifacts before interpreting changes in 3He concentration in terms of fluctuations in the delivery of 3He from space.  相似文献   
49.
50.
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