全文获取类型
收费全文 | 2372篇 |
免费 | 396篇 |
国内免费 | 558篇 |
专业分类
测绘学 | 115篇 |
大气科学 | 584篇 |
地球物理 | 648篇 |
地质学 | 1034篇 |
海洋学 | 340篇 |
天文学 | 9篇 |
综合类 | 141篇 |
自然地理 | 455篇 |
出版年
2024年 | 12篇 |
2023年 | 45篇 |
2022年 | 66篇 |
2021年 | 101篇 |
2020年 | 86篇 |
2019年 | 113篇 |
2018年 | 116篇 |
2017年 | 129篇 |
2016年 | 117篇 |
2015年 | 136篇 |
2014年 | 163篇 |
2013年 | 218篇 |
2012年 | 168篇 |
2011年 | 169篇 |
2010年 | 154篇 |
2009年 | 171篇 |
2008年 | 140篇 |
2007年 | 182篇 |
2006年 | 151篇 |
2005年 | 133篇 |
2004年 | 120篇 |
2003年 | 96篇 |
2002年 | 56篇 |
2001年 | 51篇 |
2000年 | 62篇 |
1999年 | 61篇 |
1998年 | 56篇 |
1997年 | 44篇 |
1996年 | 43篇 |
1995年 | 37篇 |
1994年 | 20篇 |
1993年 | 17篇 |
1992年 | 14篇 |
1991年 | 8篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1984年 | 4篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1973年 | 4篇 |
1972年 | 7篇 |
1971年 | 4篇 |
排序方式: 共有3326条查询结果,搜索用时 15 毫秒
21.
宁夏盐池县土地沙质荒漠化的发展趋势及其防治 总被引:5,自引:3,他引:5
宁夏盐池县是我国北方农牧交错带沙质荒漠化强烈发展的地区之一。通过分析该区土地沙质荒漠化的发展趋势及其成因,认为该区土地沙质荒漠化有日趋扩大、日益严重的发展趋势;在潜在自然因素的基础上,人为因素是造成该区土地沙质荒漠化的主要因素。针对该区土地沙质荒漠化的发展趋势及其成因,结合沙质荒漠化土地的农林牧综合开发,提出了防治土地沙质荒漠化的目标、实施步骤及基本对策。 相似文献
22.
23.
24.
The temporal and spatial characteristics of the surface air temperature variations over the Antarctic and its surrounding area 总被引:1,自引:0,他引:1
The characteristics of the spatial distribution, temporal variations trend and oscillation for the surface air temperature
variations during 1957—1993 in the Antarctic and its surrounding area were analyzed. The results show that the short-time
climate change in the Antarctic is complex both temporally and spatially. The Antarctic is by no means the strongest responding
region to the global greenhouse effect. There is a distinguished difference in the trends of the temperature changes for the
Antarctic and global mean, which could not be explained simply by the global greenhouse effect.
Project supported by the National Antarctic Key Project of China (85-905). 相似文献
25.
26.
A Physical and Numerical Investigation on the Stability of Shallow Tunnels in Strain Softening Media 总被引:11,自引:0,他引:11
Summary The behaviour up to failure of shallow underground openings is discussed on the basis of some laboratory, small-scale model tests and of finite element simulation. The experimental results are first illustrated. They were obtained from two-dimensional (plane strain) and three-dimensional tunnel models tested under standard gravity conditions. Then, the phenomenon of strain localisation that characterizes the medium surrounding the model tunnels is discussed, recalling two alternative approaches for its numerical interpretation. On this basis, a finite element procedure for strain softening analyses is outlined and applied to the simulation of the tests in both two- and three-dimensional conditions. The comparison between experimental and numerical results leads to some conclusions on the influence of strain localisation on the overall behaviour of shallow tunnels and on the stability of their headings. 相似文献
27.
Transformation of Residuals to Avoid Artifacts in Geostatistical Modelling with a Trend 总被引:1,自引:0,他引:1
Trend modelling is an important part of natural resource characterization. A common approach to account for a variable with a trend is to decompose it into a relatively smoothly varying trend and a more variable residual component. Then, the residuals are stochastically modelled independent of the trend. This decomposition can result in values outside the plausible range of variability, such as grades below zero or ratios that exceed 1.0. We transform the residuals conditional to the trend component to explicitly remove these complex features prior to geostatistical modelling. Back transformation of the modelled residual values allows the complex relations to be reproduced. A petroleum-related application shows the robustness of the proposed transformation. Furthermore, a mining application shows that when this conditional transformation is applied to the original variable, instead of the residual, simulated values are assured to be nonnegative. 相似文献
28.
29.
T. Wichtmann A. Niemunis Th. Triantafyllidis 《Soil Dynamics and Earthquake Engineering》2005,25(12):967-979
Our aim is the prediction of the accumulation of strain and/or stress under cyclic loading with many (thousands to millions) cycles and relatively small amplitudes. A high-cycle constitutive model is used for this purpose. Its formulas are based on numerous cyclic tests. This paper describes drained tests with triaxial compression and uniaxial stress cycles. The influence of the strain amplitude, the average stress, the density, the cyclic preloading history and the grain size distribution on the direction and the intensity of strain accumulation is discussed. 相似文献
30.
Determining the maximum degree of harmonic coefficients in geopotential models by Monte Carlo methods 总被引:1,自引:0,他引:1
K. R. Koch r 《Studia Geophysica et Geodaetica》2005,49(3):259-275
Random errors for the harmonic coefficients of a geopotential model are generated from the matrix of normal equations by a parallel computer applying the Gibbs sampler. This leads to random values for the harmonic coefficients. They are transformed by nonlinear, quadratic transformations to random values for the square roots of degree variances, of mean squares of geoid undulations and gravity anomalies. The expected values of these quantities are not equal to the values of these quantities computed by the estimated harmonic coefficients, due to correlations and errors in the estimation. By hypothesis tests estimated harmonic coefficients distorted by correlations and errors are detected. Applying the tests to the geopotential model ITG-CHAMP01 of the Institute of Theoretical Geodesy in Bonn it is concluded that above the degree 62 the harmonic coefficients cannot add any information to the geopotential model. 相似文献