全文获取类型
收费全文 | 16961篇 |
免费 | 3447篇 |
国内免费 | 4429篇 |
专业分类
测绘学 | 2369篇 |
大气科学 | 1973篇 |
地球物理 | 4799篇 |
地质学 | 9430篇 |
海洋学 | 2457篇 |
天文学 | 818篇 |
综合类 | 1404篇 |
自然地理 | 1587篇 |
出版年
2024年 | 51篇 |
2023年 | 171篇 |
2022年 | 444篇 |
2021年 | 616篇 |
2020年 | 675篇 |
2019年 | 902篇 |
2018年 | 695篇 |
2017年 | 808篇 |
2016年 | 831篇 |
2015年 | 954篇 |
2014年 | 1165篇 |
2013年 | 1124篇 |
2012年 | 1182篇 |
2011年 | 1237篇 |
2010年 | 1089篇 |
2009年 | 1216篇 |
2008年 | 1141篇 |
2007年 | 1265篇 |
2006年 | 1255篇 |
2005年 | 1034篇 |
2004年 | 980篇 |
2003年 | 829篇 |
2002年 | 647篇 |
2001年 | 545篇 |
2000年 | 555篇 |
1999年 | 523篇 |
1998年 | 505篇 |
1997年 | 416篇 |
1996年 | 341篇 |
1995年 | 300篇 |
1994年 | 284篇 |
1993年 | 231篇 |
1992年 | 199篇 |
1991年 | 138篇 |
1990年 | 104篇 |
1989年 | 135篇 |
1988年 | 75篇 |
1987年 | 59篇 |
1986年 | 36篇 |
1985年 | 26篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1976年 | 1篇 |
1972年 | 3篇 |
1954年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
61.
山东省产业重心转移与可持续发展研究 总被引:22,自引:0,他引:22
孙希华 《地球信息科学学报》2001,3(1):27-31
本文利用经济空间结构的重心研究方法,分析了山东省近八年来产业重心的转移,提出了山东省产业可持续发展战略 相似文献
62.
J. P. Bernstein † Suketu P. Bhavsar ‡ 《Monthly notices of the Royal Astronomical Society》2001,322(3):625-630
The brightest, or first-ranked, galaxies (BCGs) in rich clusters show a very small dispersion in luminosity, making them excellent standard candles. This small dispersion raises questions about the nature of BCGs. Are they simply the extremes of normal galaxies formed via a stochastic process, or do they belong to a special class of atypical objects? If they do, are all BCGs special, or do normal galaxies compete for the first rank? To answer these questions, we undertake a statistical study of BCG magnitudes using results from extreme value theory. Two-population models do better than one-population models. A simple scenario where a random boost in the magnitude of a fraction of bright normal galaxies forms a class of atypical galaxies best describes the observed distribution of BCG magnitudes. 相似文献
63.
《The Professional geographer》1987,39(1):85-94
Reviews of geographic software in this article: DEMO-GRAPHICS: WORLD POPULATIONS AND PROJECTIONS. ESP GAUSS. CEMODEL S. Damus LIMDEP. William H. Greene MICROSTAT 4.1 OTIS PCIPS. (Personal Computer Image Processing System) . H.J. Meyers and R. Bernstein. REGRESSION ANALYSIS OF TIME SERIES (RATS) SPSS/PC+ URBAN DATA MANAGEMENT SOFTWARE (UDMS) 相似文献
64.
Results of a single group participating in an international experiment are analyzed. The experiment served to verify computational predictions of the ground-motion variations due to near-surface geological effects at a site established for that purpose by the California Department of Conservation. Based on an acceleration record at a rock location, and geotechnical model of medium, records at the other locations of a nearby sedimentary deposit were predicted. A 2-D finite-difference sensitivity analysis suggested that the lateral wave-propagation effects are negligibly small, and locally 1-D computations are sufficient for the present site. Those computations are compared with observations not available to the authors during the blind prediction. Peak accelerations, peak velocities and RMS accelerations were predicted with errors less than 159%, 114% and 62%, respectively. Maxima of the response spectra were fitted within a factor of 2. The predicted and observed Husid's plots (i.e., the normalized cumulative plots of the acceleration squared) have the correlation coefficients 0.98. The detected misfits do not show any simple relation to the instrument location, component, frequency, or time. 相似文献
65.
On the practice of estimating fractal dimension 总被引:11,自引:0,他引:11
Coastlines epitomize deterministic fractals and fractal (Hausdorff-Besicovitch) dimensions; a divider [compass] method can be used to calculate fractal dimensions for these features. Noise models are used to develop another notion of fractals, a stochastic one. Spectral and variogram methods are used to estimate fractal dimensions for stochastic fractals. When estimating fractal dimension, the objective of the analysis must be consistent with the method chosen for fractal dimension calculation. Spectal and variogram methods yield fractal dimensions which indicate the similarity of the feature under study to noise (e.g., Brownian noise). A divider measurement method yields a fractal dimension which is a measure of complexity of shape. 相似文献
66.
67.
The true potential energy curves for the electronic ground states of astrophysically important AlH and CaH molecules are constructed by the Rydberg-Klein-Rees method. Empirical potential functions, of three-parameters by Lippincott, of five-parameters by Hulburt and Hirsch-felder and, of electronegativity by Szöke and Baitz, are examined for the adequacy to represent the true curve. From the best-fitting function, the dissociation energiesD 0 0 of AlH and CaH molecules are estimated to be 2.99 ± 0.08 and 2.72 ± 0.06 eV respectively. The force constants indicate that these values are of correct magnitude. 相似文献
68.
Statistical analysis of sand grain/bed collision process recorded by high-speed digital camera 总被引:1,自引:0,他引:1
A high‐speed digital camera was employed to record the sand grain/bed collision process. With image processing and a statistical method, a series of parameters of the collision process were obtained. The results show that the collision process of a grain with rebounding can be represented by two parameters: the kinetic energy restitution coefficient and the collision angle. Both parameters satisfy a normal distribution, and they are dependent on one another. With an increase of the collision angle, the distribution of the kinetic energy restitution gradually reduces from a broad to a narrow range with low values. The percentage of vertical velocity restitution coefficients greater than 1 can reach 70% or more, which ensures that the settling time of the sand grains in the air increases and that they receive more energy from the air to progress the saltation movement. 相似文献
69.
70.
Delineation of groundwater protection zones by the backward particle tracking method: theoretical background and GIS-based stochastic analysis 总被引:1,自引:0,他引:1
Konstadinos N. Moutsopoulos Alexandra Gemitzi Vassilios A. Tsihrintzis 《Environmental Geology》2008,54(5):1081-1090
The backward particle tracking method, an effective and powerful tool that can be used to delineate groundwater protection
zones, is presented. The theoretical background and insights on the applicability of this method are provided. Moreover, the
present work enriches the backward particle tracking method with an uncertainty analysis concerning the porosity values, applying
a Monte Carlo (MC) approach, coupled with the use of geographical information systems (GIS). As an application example, a
wellfield in the Komotini area, Greece, is investigated. The present study may serve as a potential guideline for wellfield
delineation, particularly in areas like Greece where lack of data related to the hydrogeological system is often a problem. 相似文献