全文获取类型
收费全文 | 34764篇 |
免费 | 2387篇 |
国内免费 | 4166篇 |
专业分类
测绘学 | 2563篇 |
大气科学 | 3678篇 |
地球物理 | 6705篇 |
地质学 | 18167篇 |
海洋学 | 2712篇 |
天文学 | 2036篇 |
综合类 | 3012篇 |
自然地理 | 2444篇 |
出版年
2024年 | 97篇 |
2023年 | 281篇 |
2022年 | 567篇 |
2021年 | 721篇 |
2020年 | 616篇 |
2019年 | 745篇 |
2018年 | 5346篇 |
2017年 | 4620篇 |
2016年 | 3198篇 |
2015年 | 934篇 |
2014年 | 824篇 |
2013年 | 799篇 |
2012年 | 1773篇 |
2011年 | 3487篇 |
2010年 | 2747篇 |
2009年 | 3011篇 |
2008年 | 2622篇 |
2007年 | 3007篇 |
2006年 | 607篇 |
2005年 | 701篇 |
2004年 | 812篇 |
2003年 | 744篇 |
2002年 | 563篇 |
2001年 | 400篇 |
2000年 | 333篇 |
1999年 | 297篇 |
1998年 | 223篇 |
1997年 | 228篇 |
1996年 | 165篇 |
1995年 | 151篇 |
1994年 | 99篇 |
1993年 | 97篇 |
1992年 | 99篇 |
1991年 | 57篇 |
1990年 | 52篇 |
1989年 | 46篇 |
1988年 | 46篇 |
1987年 | 24篇 |
1986年 | 21篇 |
1985年 | 17篇 |
1984年 | 16篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 28篇 |
1980年 | 24篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 11篇 |
1958年 | 9篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
Amitabha Mukhopadhyay 《Hydrogeology Journal》2003,11(3):343-356
The ability of artificial neural network to differentiate water samples from the two aquifers of Kuwait on the basis of their
major ion chemistry has been demonstrated. The major ion concentration distribution in the groundwater of the Kuwait Group
and the Dammam Formation aquifers of Kuwait appears very similar. Cross-plots, supported by the discriminant function analysis
of the data, however, suggest that there are some subtle differences in the overall composition of the water from the two
aquifers that make it possible to differentiate the water from the two aquifers in almost 80% of the cases. An artificial
neural network improved the differentiation capability to 90% of the cases. It is also possible to estimate the fraction of
Kuwait Group water in the flow stream of dually completed wells with the help of an artificial neural network developed for
this purpose.
Electronic Publication 相似文献
32.
Ya. N. Pavlyuchenkov B. M. Shustov V. I. Shematovich D. S. Wiebe Zhi-Yun Li 《Astronomy Reports》2003,47(3):176-185
We consider radiative transfer in C18O, HCO+, and CS molecular lines in a spherically symmetrical, coupled, dynamical and chemical model of a prestellar core whose evolution is determined by ambipolar diffusion. Theoretical and observed line profiles are compared for the well-studied core L1544, which may be a collapsing protostellar cloud. We study the relationship between the line shapes and model parameters. The structure of the envelope and kinematic parameters of the cloud are the most important factors determining the shape of the lines. Varying the input model parameters for the radiative transfer—the kinetic temperature and microturbulent velocity—within the limits imposed by observations does not result in any substantial variations of the line profiles. The comparison between the model and observed spectra indicates that L1544 displays a flattened structure, and is viewed at an oblique angle. A two-dimensional model is needed to reproduce this structure. 相似文献
33.
34.
35.
ChengZhongmei CaiShuqin 《中国地质大学学报(英文版)》2003,14(4):385-387
INTRODUCTIONThenotionof“learningorganization”hasbecomeanewbuzzwordinthemanagementandbusinesslitera turefromthelate 1980s (WangandAhmed ,2 0 0 3;Stewart,2 0 0 1;Garavan ,1997) .Thereappearstobemoreconsensusaboutthatonesourceofcompetitivead vantageistheabilityandrateatwhichanorganizationcanlearnandreactmorequicklythanitscompetitors(Stewart ,2 0 0 1;Garavan ,1997;Senge ,1990 ) ,soanumberofcompaniesandorganizationsaspiretocreateanddevelopintolearningorganizationsinordertofacili tatecontin… 相似文献
36.
37.
在近2年的前兆观测实践中,我们发现了一种十分有趣的,奇特的现象,在1996年3月19日新疆阿图什6.9级强震发生前和1997年3 ̄4月新疆伽师6级强震群发生前,在远隔2500公里外的四川地区观测到了多种前兆手段的群体突变事件。我们称它为强震的“远场效应”。本文还对它的成因机制作了初步的探讨。 相似文献
38.
Qiuming Cheng 《Mathematical Geosciences》1997,29(2):245-266
The concept of multifractal modeling has been used intensively in various fields of science for characterizing measures with self- similarity. It has been shown that multifractal modeling provides powerful tools for characterizing patterns in the spatial distribution of geological quantities and objects. Existing multifractal models were proposed for the purpose of handling spatially intertwined fractals with continuous fractal spectrum f(α) (or continuous codimension function C(γ)). In this paper, these conventional multifractals are termed “continuous multifractals” whereas multifractals with discrete fractal dimensions are termed “discrete multifractals.” The properties of discrete multifractals are investigated. It is shown by various artificial examples and a case study of stratigraphy of Ocean Drilling Program (ODP) site 645 that spatially intertwined fractals/multifractals indeed can have discrete fractal dimensions. Histogram-and moment-based techniques are proposed for discrete multifractal modeling and illustrated using the artificial examples. The new concept of discrete multifractals and associated multifractal modeling yields not only techniques for characterizing multifractals with discrete fractal dimensions but it also provides insight into the relationships between fractals, bifractals, and multifractals. 相似文献
39.
Results of numerical simulation studies of processes associated with Stimulated Electromagnetic Emission (SEE) produced during ionospheric heating experiments are presented. A one-dimensional magnetized electrostatic Particle-In-Cell (PIC) simulation model with uniform plasma density is used to investigate electrostatic wave generation in the region where the pump frequency ω0 approximately equals the upper hybrid frequency ωuh. In particular, the simulation plasma is driven with a uniform oscillating electric field to represent the long wavelength pump wave and power spectra of the electrostatic waves produced are taken. The pump wave frequency and amplitude are varied to consider the effects on the simulation power spectrum. The upper hybrid frequency in the model is varied through harmonics of the electron cyclotron frequency Ωce to consider the effects of stepping the pump frequency through cyclotron harmonics. The power spectrum from the simulation plasma is richly structured. The resulting power spectra show sidebands upshifted and downshifted from the pump frequency by multiples of the lower hybrid frequency ωlh. The structure of the spectrum is highly sensitive to the proximity of the upper hybrid frequency to the cyclotron harmonic frequencies. 相似文献
40.
Budd—Chiari综合征的CT评价 总被引:2,自引:0,他引:2
回顾性分析了52例Budd-Chiari综合征的CT表现,CT显示肝尾叶增大50例(96%),肝脏呈低度密度或密度不均,42例(81%)肝静脉未明显,22例(42%)见有肝内侧枝血管,49例(94%)有肝外侧枝血管,作者认为CT检查对Budd-Chiari综合征有较高的诊断价值。 相似文献