全文获取类型
收费全文 | 26093篇 |
免费 | 3831篇 |
国内免费 | 5614篇 |
专业分类
测绘学 | 3283篇 |
大气科学 | 3972篇 |
地球物理 | 6498篇 |
地质学 | 11702篇 |
海洋学 | 3290篇 |
天文学 | 1090篇 |
综合类 | 1772篇 |
自然地理 | 3931篇 |
出版年
2024年 | 138篇 |
2023年 | 421篇 |
2022年 | 794篇 |
2021年 | 1002篇 |
2020年 | 1160篇 |
2019年 | 1378篇 |
2018年 | 968篇 |
2017年 | 1264篇 |
2016年 | 1299篇 |
2015年 | 1321篇 |
2014年 | 1673篇 |
2013年 | 1955篇 |
2012年 | 1656篇 |
2011年 | 1741篇 |
2010年 | 1392篇 |
2009年 | 1778篇 |
2008年 | 1743篇 |
2007年 | 1764篇 |
2006年 | 1737篇 |
2005年 | 1361篇 |
2004年 | 1265篇 |
2003年 | 1075篇 |
2002年 | 840篇 |
2001年 | 749篇 |
2000年 | 656篇 |
1999年 | 613篇 |
1998年 | 615篇 |
1997年 | 530篇 |
1996年 | 442篇 |
1995年 | 387篇 |
1994年 | 328篇 |
1993年 | 318篇 |
1992年 | 222篇 |
1991年 | 178篇 |
1990年 | 138篇 |
1989年 | 131篇 |
1988年 | 108篇 |
1987年 | 60篇 |
1986年 | 53篇 |
1985年 | 54篇 |
1984年 | 29篇 |
1983年 | 13篇 |
1982年 | 23篇 |
1981年 | 19篇 |
1980年 | 21篇 |
1978年 | 13篇 |
1977年 | 19篇 |
1976年 | 25篇 |
1973年 | 17篇 |
1971年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
张敏先 《成都信息工程学院学报》2001,16(3):209-212
文献[1][2]认为M-PN空间是局部凸的。通过定理说明这个结论对t-模T(a,b)≥min(a,b)时成立。又通过反例表明,当t-模型T(a,b)=max(a b-1,0)时无论邻域Nθ(ε,λ)概率有界还是概率半有界结论均不成立。还讨论了Takahashi凸性并对S-凸和K-凸作了比较。 相似文献
12.
PROPAGATION OF 30—60 DAY LOW FREQUENCY OSCILLATIONS AND THEIR INFLUENCE UPON THE SUBTROPICAL WESTERLIES JET STREAM DURING NORTHERN HEMISPHERE WINTER 下载免费PDF全文
Based on daily ECMWF gridpoint data of two winters during 1981—1983 including an ENSOyear,propagation of low frequency oscillations(LFO)during Northern Hemisphere winters andtheir influences upon 30—60 day oscillations of the subtropical jet stream are studied with the sta-tistical methods as complex empirical orthogonal function(CEOF)and so on.Results show that inthe winter of a normal year(1981—1982),30—60 day oscillations in the subtropical zone aremainly in the northern and southern flanks of exit region of jet stream.In the ENSO year(1982—1983),they are mainly in the vicinity of entrance and exit regions of jet stream.Intraseasonalchanges of subtropical jet stream manifested themselves as latitudinal fluctuation or longitudinalprogression or regression of about 40 day period.There are marked differences between propagat-ing passages of low frequency modes responsible for changes of subtropical jet stream in the normalyear(1981—1982)and in the ENSO year(1982—1983).Changes of oscillation amplitude showobvious phases.In general,the one in late winter is stronger than that in early winter,strongestone occurs in February. 相似文献
13.
14.
Measurements of salinity perturbations in a partially mixed estuary have been used to evaluate the usefulness of an inductive salinometer and to determine some of the characteristics of the salinity perturbations. The salinometer performed satisfactorily under most conditions. Although internal wave like effects were present, the turbulence fluctuations were dominant. The salinity fluctuations and the turbulent fluxes and were found to behave in a manner similar to the density fluctuations in a thermally stratified atmospheric boundary layer and a laboratory open channel flow. A quadrant analysis suggested that the contribution of each quadrant to the turbulent flux changed with Ri. The turbulence parameters ν and cγ were found to decrease and increase respectively as Ri increases. 相似文献
15.
16.
17.
案例推理的地学应用背景和方法 总被引:3,自引:1,他引:2
案例推理 (CBR)研究 ,是人类认知的一种合理性模型 ,接近于人类认识、解决问题最原始的思维方式 ,具有在无法获取机理模型、确定规则或统计模型时 ,采用简单的历史相似性推理实现问题的定量求解和预测 ,成为人工智能领域近年来的研究热点。 相似文献
18.
Dan Gill 《Mathematical Geology》1993,25(4):471-482
The ability of association analysis to discriminate sedimentary facies was tested on Purdy's modal analyses of modern sediments of the Great Bahama Bank. Purdy's data set has served in the past as a standard reference for evaluating various multivariate classification algorithms. In order to adapt Purdy's data to association analysis, the percent abundance of the 12 constituents was converted to binary form by dichotomizing each variable on its mean value. The results obtained by association analysis are virtually identical to those obtained by Purdy and other authors. The same four main sedimentary facies were discriminated; 86% of the samples were identically classified (97% when misclassified borderline cases are counted as matches); the total partition variance of the classification is only negligibly greater (4%); and the grouping of the variables yielded the same four groups. The rank order of the three division-attributes responsible for the sample classification is fines, oolites, and corals. Association analysis has been employed by other authors to differentiate meaningful facies groups in studies of ancient reef carbonates, modern reef sediments, and heavy minerals in stream sediments. In all these studies, the results were found to be compatible with those obtained by using the continuous quantitative measurements, indicating that qualitative binary data may often be sufficient for the purpose of facies discrimination in many branches of geology and that association analysis is an effective method for this purpose. 相似文献
19.
Qin Chengzhi 《中国地震研究》2004,18(2):178-187
The migration of strong earthquakes is an important research topic because the migration phenomena reflect partly the seismic mechanism and involve the prediction of tendency of seismic activity. Research on migration of strong earthquakes has mostly focused on finding the phenomena. Some attempts on getting regularity were comparatively subjective. This paper suggests that there should be indices of migration in earthquake dataset and the indexes should have statistical meaning if there is regularity in the migration of strong earthquakes. In this study, three derivative attributes of migration, i.e., migration orientation, migration distance and migration time interval, were statistically analyzed. Results in the North China region show that the migration of strong earthquakes has statistical meaning. There is a dominant migration orientation (W by S to E by N), a dominant distance (≤100km and on the confines of 300~700km), and a dominant time interval (≤1a and on the confines of 3~4a). The results also show that the migration will differ slightly with different magnitude range or earthquake activity phase. 相似文献
20.
APPLICATION OF GEOGRAPHICAL PARAMETER DATABASE TO ESTABLISHMENT OF UNIT POPULATION DATABASE 总被引:1,自引:1,他引:0
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of 相似文献