全文获取类型
收费全文 | 648篇 |
免费 | 122篇 |
国内免费 | 218篇 |
专业分类
测绘学 | 42篇 |
大气科学 | 14篇 |
地球物理 | 158篇 |
地质学 | 552篇 |
海洋学 | 98篇 |
天文学 | 42篇 |
综合类 | 55篇 |
自然地理 | 27篇 |
出版年
2023年 | 9篇 |
2022年 | 28篇 |
2021年 | 29篇 |
2020年 | 32篇 |
2019年 | 35篇 |
2018年 | 18篇 |
2017年 | 20篇 |
2016年 | 29篇 |
2015年 | 25篇 |
2014年 | 37篇 |
2013年 | 41篇 |
2012年 | 30篇 |
2011年 | 44篇 |
2010年 | 49篇 |
2009年 | 35篇 |
2008年 | 39篇 |
2007年 | 45篇 |
2006年 | 24篇 |
2005年 | 23篇 |
2004年 | 31篇 |
2003年 | 21篇 |
2002年 | 30篇 |
2001年 | 32篇 |
2000年 | 31篇 |
1999年 | 20篇 |
1998年 | 21篇 |
1997年 | 23篇 |
1996年 | 27篇 |
1995年 | 20篇 |
1994年 | 20篇 |
1993年 | 18篇 |
1992年 | 20篇 |
1991年 | 14篇 |
1990年 | 10篇 |
1989年 | 15篇 |
1988年 | 7篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1978年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有988条查询结果,搜索用时 171 毫秒
921.
922.
A. M. Curry P. R. Porter T. D. L. Irvine‐Fynn G. Rees T. B. Sands J. Puttick 《地球表面变化过程与地形》2009,34(1):48-62
Particle size analysis and scanning electron microscopy have been used with some success to differentiate sediments sampled from varying geomorphological environments. In the paraglacial environment, however, discrimination of paraglacially reworked deposits from in situ glacigenic deposits has proven to be problematic. We consider till sediments that have been reworked by paraglacial debris flows and in situ, unreworked parent material, and apply a series of quantitative techniques in an attempt to discriminate samples from each environment. Initial analysis of particle size suggests some eluviation of fines from the debris‐flow‐reworked deposits but is of insufficient significance to allow reliable differentiation of geomorphological environment. Similarly, although slight differences are identified in terms of quartz grain surface microtextures and quartz grain outline shape, subsequent PCA, cluster and Fourier analyses fail to reliably differentiate quartz grains from each geomorphological environment, owing to high levels of intra‐sample heterogeneity. This lack of apparent difference may arise from a lack of paraglacial process operation of significant magnitude or duration for a characteristic process imprint to emerge at this scale of observation. Accordingly, further assessment of particle‐size and microtextural characteristics of glacial and paraglacial sediments may be most fruitful where sediments have been reworked over long distances. Moreover, differentiating glacial from paraglacial deposits is likely to yield the most reliable results where rigorous statistical analyses are combined with a wide range of sedimentological and geomorphological techniques. Copyright © 2008 John Wiley and Sons, Ltd. 相似文献
923.
在略高于古藤堡面的压力和温度条件下,地核物质中的原子结构状态发生了质变.如对于外层能级分布是3d64s2的铁原子而言,Fe原子的最外层和极少一部分Fe原子的次外层将受到破坏,使外层电子发生"公有化"现象,残破的原子(处于离子状态)在自引力作用形成的压力作用下,按原子密度的大小产生分异,"内核"多为显示有正电性能的密度较高的残破原子组成,"外核"(包含过渡层)为富含"公有化"电子而显示负电性能的密度相对较低的残破原子组成,这是一个地核物质量子状态变化的静态过程,从地球内部宏观电性结构看来,地球的"内核"和"外核"组成缮了一个静态的巨型似"原子球","外核"显示负电性,"内核"显示正电性,"内核"所带电量与"外核"所带电量相等,但符号相反,在静态的情况下,似"原子球"并不显示磁性.由于地球自转,"外核"上界面赤道上的自转线速度为252.9m/s,"内核"外界面赤道上的自转线速度为88.9 m/s,二者的速度差为164.0 m/s,这种含有由巨大电量的"外核"电子壳层相对"内核"绕固定的轴向(自转轴)转动,自然会形成它的基本磁场. 相似文献
924.
结合小波分析及交叉小波分析方法,研究了地球同步轨道高能电子动态变化的多时间尺度结构,分析了电子通量在不同周期随着太阳风速、地磁指数Dst变化的具体特点.结果发现:(1)电子通量的长期变化受控于太阳风速,在太阳活动低值年,电子通量值高,具有明显的13.4天,27.4天及187天周期;交叉小波分析表明,电子通量的13.4天及27.4天周期受太阳风速周期变化信号的影响,187天周期变化受Dst指数周期变化信号的影响.(2)电子通量半年变化主要归因于太阳风的驱动作用,在每年的第100天及270左右达到两次峰值,峰值大小不对称,与Dst指数的谷值大小呈反比.(3)由于冕洞形成过程中的太阳风高速流影响,电子通量具有13.4及27.4天的周期,峰值水平受控于太阳风速结构. 相似文献
925.
Selecting three half orbits near the epicenter of Pu’er earthquake, we analyzed the Ne data recorded in their revis-ited orbits during a year before this earthquake, and extracted Ne precursors. The results show that: ① There are significant seasonal variations of ionospheric Ne in night time, which exhibit different shapes respectively in four seasons; ② There are three main shapes of Ne: single-peak, saddle-shaped and even-shaped, all of which may oc-cur in four seasons, but each season with its typical shape relatively; ③ Spatial images of Ne showed high values near the epicenter in 30 days before the earthquake, and there is a good correlation between anomaly and distribu-tion of earthquake in space and time, which reflects that these spatial anomalies were indeed concerned with the earthquake; ④There shows a certain similarity of the Ne curves among revisited orbits, which can provide back-ground information for distinguishing and identification of seismic anomaly. 相似文献
926.
927.
西华山钨矿床晶洞中水晶与黑钨矿流体包裹体显微测温与特征元素测定 总被引:2,自引:0,他引:2
摘要:西华山钨矿床晶洞中水晶与黑钨矿以及晶洞外部块状石英与共生黑钨矿中流体包裹体对比研究结果表明,后期应力作用和流体改造是造成共生石英与黑钨矿包裹体均一温度出现重大差异的主要原因。一般来说,在后期应力作用和流体改造下,黑钨矿有较好的稳定性,能较好地保存原生包裹体和较少出现次生包裹体,而与之共生石英中的原生包裹体几乎破坏殆尽,现在所看到的包裹体绝大部分是后来形成的次生包裹体,且很难确切区别原生与次生关系。当后期应力作用和流体改造很微弱时,石英中可保存较多原生包裹体,并且原生与次生关系较易区分。当完全未受应力作用改造时,共生石英(水晶)与黑钨矿中都只有原生包裹体,其Th值和盐度值基本相同或完全一致,且石英(水晶)流体包裹中富含W、Mn、Fe等成矿元素,表明石英与黑钨矿是由同一流体在相同时间和相同条件下沉淀的。鉴于一般金属矿床皆在一定程度上经受了后期应力作用和流体改造,因此在有关流体包裹体研究中应该结合不透明矿物进行全面对比,并且切实加强岩相学研究。 相似文献
928.
?????????????????ο????????????????????????2011???й???????????????????????????????????????????б???????????????????????ο?????????????????????????????仯????????ο??????????°汾IRI2011??????IRI2007???????????? 相似文献
929.
2012��4�����Ŵ�����������������ܶ��쳣�о� 总被引:1,自引:0,他引:1
????GPS??????????????????????????4??1????15??????????????????????????????????2???????????????????????????????????????????????????????£???????11??UT10:00??13??UT6:00??14??UT6:00-14:00??15??UT14:00-16??00???γ?????Χ???????????????????????????????????????????????4????????????????й?? 相似文献
930.
Auroral intensity ratios at Zhongshan Station in Antarctica on 8 April 1999 are studied, along with variations in pene- trated electron energy. Ratios of/(557.7 nm)/I (427.8 nm) during the quiet period were from 5 to 22, and I (630.0 nm) / I (427.8 rim) ranged from 1 to 2.76. These variations were not caused by changes of atomic oxygen concentration, but rather by penetrated electron energy variability, or other mechanisms. Ratios decreased sharply during the auroral substorm, ranging from 1.66--6.5 and 0.071-1, respectively, mainly because of the increase in penetrated electron energy. At the onset of the substorm, the ratios reached their minima. This means that penetrated electron energy was maximized. When the substorm weakened, the penetrated electron energy returned to the pre-substorm level. 相似文献