全文获取类型
收费全文 | 7402篇 |
免费 | 1397篇 |
国内免费 | 1784篇 |
专业分类
测绘学 | 351篇 |
大气科学 | 1683篇 |
地球物理 | 1958篇 |
地质学 | 3677篇 |
海洋学 | 863篇 |
天文学 | 340篇 |
综合类 | 761篇 |
自然地理 | 950篇 |
出版年
2024年 | 48篇 |
2023年 | 177篇 |
2022年 | 363篇 |
2021年 | 443篇 |
2020年 | 349篇 |
2019年 | 387篇 |
2018年 | 446篇 |
2017年 | 407篇 |
2016年 | 455篇 |
2015年 | 377篇 |
2014年 | 496篇 |
2013年 | 404篇 |
2012年 | 419篇 |
2011年 | 424篇 |
2010年 | 428篇 |
2009年 | 420篇 |
2008年 | 359篇 |
2007年 | 343篇 |
2006年 | 274篇 |
2005年 | 251篇 |
2004年 | 213篇 |
2003年 | 222篇 |
2002年 | 282篇 |
2001年 | 240篇 |
2000年 | 236篇 |
1999年 | 325篇 |
1998年 | 256篇 |
1997年 | 270篇 |
1996年 | 222篇 |
1995年 | 206篇 |
1994年 | 144篇 |
1993年 | 168篇 |
1992年 | 106篇 |
1991年 | 75篇 |
1990年 | 45篇 |
1989年 | 66篇 |
1988年 | 58篇 |
1987年 | 33篇 |
1986年 | 23篇 |
1985年 | 23篇 |
1984年 | 22篇 |
1983年 | 12篇 |
1982年 | 14篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1958年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Organic petrology is a marginal science that is quite practicable. At present, it has developed into a routine research tool that is widely applied in petroleum exploration and assessment. Based on several years‘ research of the authors, this paper presents the advances in organic petrological studies on immature source rocks, including the classification and characteristics of macerals, the composition of macerals and types of organic matter, the abundance and evolution of organic matter, oil-prone macerals, hydrocarbon generation and expulsion. All these results show that organic petrology is of considerable value pertaining to its application in the assessment of immature oil and gas. The immature source rocks consist of various macerals with obvious heterogeneity, contain different hydrocarbon-generating macerals with different oil thresholds and oil peaks, and show a two-staged evolutionary pattern of organic matter. 相似文献
82.
受地物影响,炮点的理论变观方法(恢复性放炮法)只能是三维地震数据采集过程中的一种补救措施而已,它只保证了设计覆盖次数不变的目的,而不能保证道集内炮检距和方位角不变的目的。一方面,变观后要求所有的炮检距均满足最大炮检距的限定条件显得过于苛刻;另一方面,对地层倾角较大或煤层埋藏较浅的勘探区,一个区的最大炮检距允许值也不一定是一个确定的值,通常的做法是取平均值。据此,提出了三维变观80原则,即在变观炮点不多或不集中时,只要变观炮点记录中有80%的炮检距或不低于80%的道满足最大炮检距要求,就可认为变观是成功的。这种实用的变观方法,对三维地震勘探野外数据采集工作有较明确的指导意义。 相似文献
83.
84.
85.
用远场辐射场反演云闪放电参数 总被引:2,自引:0,他引:2
用地面电磁场遥感闪电放电参数无论是在实际应用还是理论研究中都具有重要意义。文中基于电流传输线模式 ,提出了一种用单站远场VLF/LF辐射场反演云内放电参数的简便方法。当传输线电流从底部传播到顶部的时间只有几微秒时 ,整个传输线的VLF/LF辐射可以近似成偶极子辐射 ,即远场辐射场与电流的时间变化率和传输线长度的乘积成正比。因此 ,对远场辐射场做时间积分就能获得完整的电流矩波形 (上升沿有所加宽 ) ,而且传输线模式中的重要参数 ,电流从底部传播到顶端经历的时间 ,也可以近似地用辐射场时间积分幅度与辐射场幅度之比来估计。这一方法具有估计云闪双极性大脉冲辐射过程的放电参数的潜力。 相似文献
86.
87.
基于对第二松花江流域上游小山、松山、两江水电站中长期水文预报研究成果,论述了综合中长期水文预报的研究思路和定性预报、定量预报的分析研究方法。该研究通过2002年实践检验,具有较高的预报精度。 相似文献
88.
P. Ma K. Aggrey C. Tonzola C. Schnabel P. de Nicola J.T. Wasson L. Brown R. Middleton 《Geochimica et cosmochimica acta》2004,68(19):3883-3896
By using accelerator mass spectrometry we have measured the 10Be concentrations of 86 Australasian tektites. Corrected to the time of tektite production ∼0.8 My ago, the 10Be concentrations (106 atom/g) range from 59 for a layered tektite from Huai Sai, Thailand, to 280 for an australite from New South Wales, Australia. The average value is 143 ± 50. When tektites are sorted by country, their average measured 10Be concentrations increase slowly with increasing distance from Southeast Asia, the probable location of the tektite producing event, from 59 ± 9 for 6 layered tektites from Laos to 136 ± 20 for 20 splash-form tektites from Australia. The lowest 10Be concentrations for tektites fall on or within a contour centered off the shore of Vietnam, south of the Gulf of Tonkin (107°E; 17°N), but also encompassing two other locations in the area of northeastern Thailand previously proposed for the site of a single tektite-producing impact. The 10Be concentrations of layered tektites show only a weak anticorrelation (R ∼ −0.3) with the numbers of relict crystalline inclusions.Loosely consolidated, fine-grained terrestrial sediments or recently consolidated sedimentary rocks are the most likely precursor materials. Dilution of sediments with other kinds of rock raises problems in mixing and is not supported by petrographic data. Sedimentary columns that have the right range of 10Be concentrations occur off the coasts of places where sedimentation rates are high relative to those in the deep sea. A single impact into such a region, 15 to 300 m thick, could have propelled near-surface, high-10Be material farthest—to Australia—while keeping the deeper-lying, low-10Be layers closer to home. We do not rule out, however, other proposed mechanisms for tektite formation. 相似文献
89.
Dissolution of water in magmas significantly affects phase relations and physical properties. To shed new light on the this issue, we have applied 1H and 29Si nuclear magnetic resonance (NMR) spectroscopic techniques to hydrous silicate glasses (quenched melts) in the CaO-MgO-SiO2 (CMS), Na2O-SiO2, Na2O-CaO-SiO2 and Li2O-SiO2 systems. We have also carried out ab initio molecular orbital calculations on representative clusters to gain insight into the experimental results.The most prominent result is the identification of a major peak at ∼1.1 to 1.7 ppm in the 1H MAS NMR spectra for all the hydrous CMS glasses. On the basis of experimental NMR data for crystalline phases and ab initio calculation results, this peak can be unambiguously attributed to (Ca,Mg)OH groups. Such OH groups, like free oxygens, are only linked to metal cations, but not part of the silicate network, and are thus referred to as free hydroxyls in the paper. This represents the first direct evidence for a substantial proportion (∼13∼29%) of the dissolved water as free hydroxyl groups in quenched hydrous silicate melts. We have found that free hydroxyls are favored by (1) more depolymerized melts and (2) network-modifying cations of higher field strength (Z/R2: Z: charge, R: cation-oxygen bond length) in the order Mg > Ca > Na. Their formation is expected to cause an increase in the melt polymerization, contrary to the effect of SiOH formation. The 29Si MAS NMR results are consistent with such an interpretation. This water dissolution mechanism could be particularly important for ultramafic and mafic magmas.The 1H MAS NMR spectra for glasses of all the studied compositions contain peaks in the 4 to 17 ppm region, attributable to SiOH of a range of strength of hydrogen bonding and molecular H2O. The relative population of SiOH with strong hydrogen bonding grows with decreasing field strength of the network-modifying cations. Ab initio calculations confirmed that this trend largely reflects hydrogen bonding with nonbridging oxygens. 相似文献
90.