全文获取类型
收费全文 | 99382篇 |
免费 | 18530篇 |
国内免费 | 42714篇 |
专业分类
测绘学 | 4119篇 |
大气科学 | 26423篇 |
地球物理 | 26381篇 |
地质学 | 55289篇 |
海洋学 | 21418篇 |
天文学 | 16267篇 |
综合类 | 4153篇 |
自然地理 | 6576篇 |
出版年
2021年 | 951篇 |
2020年 | 2299篇 |
2019年 | 5743篇 |
2018年 | 6505篇 |
2017年 | 6208篇 |
2016年 | 6741篇 |
2015年 | 5708篇 |
2014年 | 5627篇 |
2013年 | 8038篇 |
2012年 | 5970篇 |
2011年 | 6467篇 |
2010年 | 6129篇 |
2009年 | 6471篇 |
2008年 | 5484篇 |
2007年 | 5188篇 |
2006年 | 4887篇 |
2005年 | 4468篇 |
2004年 | 4935篇 |
2003年 | 4480篇 |
2002年 | 4127篇 |
2001年 | 3681篇 |
2000年 | 3292篇 |
1999年 | 3063篇 |
1998年 | 3196篇 |
1997年 | 3205篇 |
1996年 | 2642篇 |
1995年 | 2500篇 |
1994年 | 2267篇 |
1993年 | 2126篇 |
1992年 | 1883篇 |
1991年 | 1530篇 |
1990年 | 1691篇 |
1989年 | 1443篇 |
1988年 | 1239篇 |
1987年 | 1349篇 |
1986年 | 1152篇 |
1985年 | 1301篇 |
1984年 | 1486篇 |
1983年 | 1347篇 |
1982年 | 1257篇 |
1981年 | 1165篇 |
1980年 | 979篇 |
1979年 | 965篇 |
1978年 | 956篇 |
1977年 | 859篇 |
1976年 | 818篇 |
1975年 | 744篇 |
1974年 | 752篇 |
1973年 | 771篇 |
1972年 | 489篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
993.
Jürg Hunziker Evert Slob Yuanzhong Fan Roel Snieder Kees Wapenaar 《Geophysical Prospecting》2012,60(5):974-994
We use numerically modelled data sets to investigate the sensitivity of electromagnetic interferometry by multidimensional deconvolution to spatial receiver sampling. Interferometry by multidimensional deconvolution retrieves the reflection response below the receivers after decomposition of the fields into upward and downward decaying fields and deconvolving the upward decaying field by the downward decaying field. Thereby the medium above the receiver level is replaced with a homogeneous half‐space, the sources are redatumed to the receiver level and the direct field is removed. Consequently, in a marine setting the retrieved reflection response is independent of any effect of the water layer and the air above. A drawback of interferometry by multidimensional deconvolution is a possibly unstable matrix inversion, which is necessary to retrieve the reflection response. Additionally, in order to correctly separate the upward and the downward decaying fields, the electromagnetic fields need to be sampled properly. We show that the largest possible receiver spacing depends on two parameters: the vertical distance between the source and the receivers and the length of the source. The receiver spacing should not exceed the larger of these two parameters. Besides these two parameters, the presence of inhomogeneities close to the receivers may also require a dense receiver sampling. We show that by using the synthetic aperture concept, an elongated source can be created from conventionally acquired data in order to overcome these strict sampling criteria. Finally, we show that interferometry may work under real‐world conditions with random noise and receiver orientation and positioning errors. 相似文献
994.
Hans W. Giertz 《Journal of Atmospheric and Solar》2010,72(9-10):767-773
The present paper reveals that the air contains electromagnetic energy of extremely low frequency, low amplitude as well as of a low phase speed. The energy is of great interest because of its impact on certain biological processes. It is created by the interaction of two well-known phenomena. The rotation of the earth generates 24 h periods currents in the magnetosphere, known as the Birkeland currents. The currents generate transverse electromagnetic waves (EM waves) propagating parallel to the geomagnetic field lines. Furthermore, the air and the earth crust contain electrons caused by the global electric circuit. The electric field vectors of the EM waves exert a force on these electrons, causing them to oscillate and thus generate currents of extremely low frequency both in the air and in the earth crust. A theoretical model of the system is presented and measurement techniques are described. Measurements have been performed during a six year period. The results of the performed measurements verified the theoretical model. Impact on biological processes is discussed. 相似文献
995.
996.
With thespectro-coronagraph and themultichannel subtractive double pass spectrograph (MSDP) at the Pic du Midi Observatory two quiescent prominences were observed simultaneously. From the spectro-coronagraph observations 2D maps of Hei 10830 , Fexiii 10798 and 10747 line intensities were obtained. In addition, we obtained 2D maps of the ratioR of the two iron lines. This ratio is used as a diagnostic for determining the density of the hot coronal plasma surrounding prominences. We found that the electron density is higher at the location of the prominences than in the corona, whereas small regions (40) of lower electron density are unevenly distributed around the prominences indicating that the surrounding corona is highly inhomogeneous. The density of the cavity is reduced by a factor 1.5 compared to the density of the prominence environment (5 × 108 cm–3). We discuss the existence of cavities around these prominences according to the orientation of their axes relative to the line of sight and according to the velocity field inside the prominences. Constraints on models for prominence formation are derived. 相似文献
997.
层间氧化带砂岩型铀矿是我国主要的砂岩型铀矿类型之一,属全盲矿,找矿难度大。地电化学方法作为深穿透地球化学方法的分支之一,为隐伏铀矿勘查提供了新的技术手段,并在实际找矿中取得了明显的效果。研究表明,在隐伏砂岩型铀矿上方,存在着地电化学提取U元素异常。总结和分析了层间氧化带砂岩型铀矿形成的物理化学条件以及在实际试验中发现的铀矿上方地电提取铀异常特征,在前人提出的元素由深部向地表迁移机制的基础上,进一步归纳了砂岩型铀矿地电化学异常的形成机理。层间氧化带砂岩型铀矿上方地电提取U元素主要呈现为跳跃的锯齿状高值异常,可以作为深部铀矿找矿的指示标志。 相似文献
998.
滨里海盆地内充填了巨厚的古生代、中生代和新生代沉积物。在剖面上可分为三套地层组合,即盐下层系、含盐层系和盐上层系。盐下层系为下古生界-下二叠统,包括巨厚的碎屑岩和碳酸盐岩沉积,在泥盆纪-早二叠世,滨里海盆地周缘广大地区普遍发育碳酸盐岩,在许多古隆起上还发育生物礁体,说明这一时期盆地的滨、浅海地带具有浅、清、暖的沉积环境,盆外陆源碎屑供应较少。含盐层系为下二叠统上部孔谷阶。早二叠世中-后期由于持续构造抬升,盆地气候变得干旱,海水变浅,潮上带蒸发环境发育,以致这一时期广泛发育盐类沉积,形成含盐层系,主要由盐岩和硬石膏层构成,并形成许多大小不等的盐丘构造。盐上层系为上二叠统-第四系。晚二叠世-三叠纪盆地又经历了一次大的海侵,为浅海陆棚环境,局部为海陆过渡三角洲相。侏罗纪-白垩纪在总的浅海陆棚环境下,盆地不同地区也形成了湖泊和瀉湖环境。晚二叠世以后形成的盐上层系沉积,主要为碎屑岩,在局部地区有碳酸盐岩。 相似文献
999.
西昆仑山阿卡孜岩体锆石SHRIMP定年及其地球化学特征 总被引:12,自引:3,他引:12
西昆仑山阿卡孜岩体是该区已知的最古老侵入体。岩体侵入于太古代赫罗斯坦群(TTG岩系)中。获得最新的单颗粒锆石SHRIMP年龄为2426±46Ma。岩石地球化学研究表明,岩石具有高碱度富K特征;∑REE为409.94×10~(-6)~787.76×10~(-6),δEu为0.48~0.73;微量元素显示富集大离子元素。综合分析该岩体地质地球化学特征,认为该岩体形成于晚太古代~古元古代初期碰撞后拉张背景,是TTG岩系(赫罗斯坦群)重熔的结果。这对研究西昆仑山早前寒武纪陆壳增生及构造演化提供了重要的地质资料。 相似文献
1000.
云县-景谷火山弧带大中河晚志留世火山岩的发现及其地质意义 总被引:5,自引:4,他引:5
云县-景谷火山弧带大中河地区新识别出一套晚志留世中基性-中酸性火山岩组合,其LA-ICPMS锆石U-Pb年龄为421.2±1.2Ma和417.6±5.1Ma。该套火山岩具有富铝(12.73%~16.63%)、富钠(K2O/Na2O=0.56~0.99)和高Mg#(46.0~50.0)的特征,属于钙碱性系列岩石;同时富集轻稀土,Eu具有不同程度的弱亏损,亏损高场强元素(Nb、Ta、Ti),具有正εNd(t)值(3.86~4.39)和较高的Th/Ta比值(15~17),显示与活动大陆边缘岛弧型火山岩相似的地球化学性质。大中河晚志留世火山岩很可能是俯冲沉积物流体交代地幔楔物质部分熔融的产物,并在岩浆上升过程中经历了一定的分离结晶作用和浅部地壳物质的同化混染;结合区域同期(410~420Ma)岩浆活动及相关的高压变质事件分析,应为原-古特提斯洋在早古生代末期向东俯冲消减作用的产物,从而为扬子陆块西部边缘晚古生代"三江"多岛弧盆系的形成演化提供了前锋弧发育的岩石学证据及其动力学机制。 相似文献