全文获取类型
收费全文 | 54744篇 |
免费 | 9908篇 |
国内免费 | 13012篇 |
专业分类
测绘学 | 3632篇 |
大气科学 | 11935篇 |
地球物理 | 13827篇 |
地质学 | 26572篇 |
海洋学 | 6263篇 |
天文学 | 2458篇 |
综合类 | 6134篇 |
自然地理 | 6843篇 |
出版年
2024年 | 144篇 |
2023年 | 711篇 |
2022年 | 2166篇 |
2021年 | 2479篇 |
2020年 | 2129篇 |
2019年 | 2361篇 |
2018年 | 2918篇 |
2017年 | 2651篇 |
2016年 | 3095篇 |
2015年 | 2602篇 |
2014年 | 3186篇 |
2013年 | 3184篇 |
2012年 | 3027篇 |
2011年 | 3090篇 |
2010年 | 3257篇 |
2009年 | 3178篇 |
2008年 | 2924篇 |
2007年 | 2809篇 |
2006年 | 2274篇 |
2005年 | 2064篇 |
2004年 | 1648篇 |
2003年 | 1664篇 |
2002年 | 1578篇 |
2001年 | 1585篇 |
2000年 | 1923篇 |
1999年 | 2754篇 |
1998年 | 2233篇 |
1997年 | 2142篇 |
1996年 | 1979篇 |
1995年 | 1720篇 |
1994年 | 1561篇 |
1993年 | 1371篇 |
1992年 | 1083篇 |
1991年 | 865篇 |
1990年 | 633篇 |
1989年 | 582篇 |
1988年 | 526篇 |
1987年 | 318篇 |
1986年 | 283篇 |
1985年 | 194篇 |
1984年 | 166篇 |
1983年 | 130篇 |
1982年 | 126篇 |
1981年 | 102篇 |
1980年 | 61篇 |
1979年 | 60篇 |
1978年 | 23篇 |
1977年 | 14篇 |
1976年 | 15篇 |
1958年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Environmental data seldom follow normal distributions, so how to calculate their means is a very important problem. Commonly used methods for mean calculation, such as arithmetic mean, geometric mean, and median, were evaluated. Arithmetic means should only be used when datasets follow normal distributions. Geometric means are suitable for datasets which follow log-normal distributions. Medians are a kind of robust treatment. However, their efficiency is very low. Based on the methods described, two new ideas are developed: robust and symmetric, for calculating means. As far as the symmetric feature is concerned, Box-Cox power transformation is better than logarithmic transformation. Robust statistics and Box-Cox transformation are combined to produce the robust-symmetric mean. As environmental data often follow log-normal or skewed distributions, this method is better than the previous ones and also is appropriate. 相似文献
992.
A brief account of the development of the research on mining earthquakes and the general situation of the Mentougou Coal Mine medium-scale experiment field for earthquake prediction and the project of monitor and prediction is given. The differences of waveforms between mining earthquakes and natural earthquakes is discussed. The magnitude-frequency distribution of the 79 000 mining earthquakes of overM l 1. 0 from 1984 to 1995 is summarized. Finally, taking PH and PV, the principal compressive stress components of the focal mechanism of the mining earthquakes, as the criteria, analyses the stress background of the 12 large mining earthquakes. 相似文献
993.
Li Mingsen 《地理学报(英文版)》1996,(4)
ISoil-fo~gEnvironmentandMainSoilTrocsThesoLItheastQinghai-Xizang(Tibetan)PlateauwheretheHengduanMoUnains,eastempartoftheThmalayanandNyainqentanglhamoUnainsarelocatedextendsfrom25'to34"Nand92'to1O5'Ewithanareaof5OO,OOOkm',oraboutone-fiffooftheentireQinghai-XiZangPlateau.ItisatransitionalzoneofdeePdisseCtedmoUntainseXtendingsoutheastWardpenpherallowlandsandwichedin-betWeenSichuanBasinandcentralYunnanPlateau.Whletheterrainofthesmponinclinestosoutheast,thealtitudeofthevaIleysraises… 相似文献
994.
A solar flare with both H and Fe i 5324 emissions was observed in AR 7529 (S13, E65) on 24 June, 1993 at the Bejing Astronomical Observatory. Our calculations show that the Fe i 5324 emission region of the flare was located in the low photosphere at a height of about 180 km above 5000 = 1, which is lower than many previous studies of white-light flares. To study a Fe i 5324 flare, which represents a kind of extreme case in solar flares, would be useful for clarifying some arguments in the researches of white-light flares as well as for understanding the mechanism of solar flares.The synthetic analyses from vairous features of the flare lead to the following possible exciting mechanism of the Fe i 5324 flare: owing to the flow of energetic electrons from the corona and probably also the thermal conduction downward into the lower atmosphere, a condensation with a temperature higher than that below it was formed near the transition region. Then the low photosphere was heated through backwarming. The Fe i 5324 flare occurred as an indicator of the excitation in the low photosphere. 相似文献
995.
996.
997.
通过对胶东牟平—乳山热液脉状金,铅锌矿床成矿动力学控制规律的研究,说明了建立热液成矿反应体系的方法,并得出以下认识:(1)区域断裂构造活动,通过影响成矿反应体系的热力学性质和条件,控制元素的富集和分散。(2)热液成矿过程中容矿断裂活动可划分为两种作用方式:脆性破裂和韧—脆性张开,构成热液成矿的两种构造动力学环境。(3)热液成矿反应体系是一种开放的动态的热力学体系。断裂的脆性破裂阶段,使体系处于强烈的过饱和状态,矿质在远离平衡状态下快速结晶,加剧物质分异;在脆—韧性张开阶段,矿质在接近平衡态体系中缓慢晶出。(4)热液演化晚期,金在残余溶液中富集;当断裂再次发生构造脉动震颤时,早期形成的块状黄铁矿矿石碎裂,富金溶液充填其中,形成含裂隙金和晶隙金的富矿石。成矿体系热力学演化与构造动力学条件有利的匹配控制着富矿石的形成。 相似文献
998.
The densities of 36 water samples from the Huanghe River estuary and Bohai Bay were determinedby a magnetic float densimcter under three temperatures from 15℃ to 25℃.All the measured densities ofsamples were greater than that of the values calculated from the International Equation of State of Seawater.The differences between the measured and calculated densities increased with the decrease of salinities.The dif-ferences appeared exponentially correlated with[Ca~(2+)]/s,[Mg~(2+)]/s and[SO_4~(2-)]/s,and had"s"type curverelationship with the alkalinity in all salinity range.But in the salinity ranging from 25.72 to 31.57,therelationships were all linear.The density difference can be estimated from the equation △ρ(10~3kg·m~(-3))=(-2.79+236.5([Ca~(2+)]/s)/(-9.7464×10~(-3)+[Ca~(2+)]/s).It was the high alkalinity and[Ca~(2+)]/s that resulted in the measured densi-ties of seawaters being higher than the calculated densities in the Huanghe estuary and Bohai Bay. 相似文献
999.
Hongqi Zhang 《Solar physics》1994,154(2):207-214
A set of H chromospheric magnetograms at various wavelengths near the line center, chromospheric Dopplergrams, and photospheric vector magnetograms of a unipolar sunspot region near the solar limb were obtained with the vector video magnetograph at the Huairou Solar Observing Station. The superpenumbral chromospheric magnetic field is almost parallel to the surface at the outside of the sunspot penumbra, where the magnetic lines of force are mainly concentrated in the superpenumbral filaments. In the gaps between the filaments the chromospheric horizontal field is weak. 相似文献
1000.
用褶积滤波处理井水位对固体潮响应的滞后 总被引:2,自引:1,他引:1
本文给出了一种用褶积滤波处理井水位对固体潮响应滞后的新方法。鲁07井水位观测资料用这种方法处理比一般回归方法的平均中误差少4.6mm。文中讨论了褶积滤波积分区间长度与水井含水层渗透系数的关系及褶积滤波比别尔采夫滤波的优越性。 相似文献