全文获取类型
收费全文 | 1606篇 |
免费 | 219篇 |
国内免费 | 368篇 |
专业分类
测绘学 | 141篇 |
大气科学 | 26篇 |
地球物理 | 358篇 |
地质学 | 1121篇 |
海洋学 | 141篇 |
天文学 | 9篇 |
综合类 | 87篇 |
自然地理 | 310篇 |
出版年
2024年 | 3篇 |
2023年 | 12篇 |
2022年 | 29篇 |
2021年 | 27篇 |
2020年 | 50篇 |
2019年 | 63篇 |
2018年 | 39篇 |
2017年 | 45篇 |
2016年 | 55篇 |
2015年 | 64篇 |
2014年 | 73篇 |
2013年 | 87篇 |
2012年 | 96篇 |
2011年 | 108篇 |
2010年 | 81篇 |
2009年 | 98篇 |
2008年 | 104篇 |
2007年 | 105篇 |
2006年 | 100篇 |
2005年 | 100篇 |
2004年 | 110篇 |
2003年 | 73篇 |
2002年 | 88篇 |
2001年 | 76篇 |
2000年 | 87篇 |
1999年 | 69篇 |
1998年 | 69篇 |
1997年 | 50篇 |
1996年 | 51篇 |
1995年 | 46篇 |
1994年 | 33篇 |
1993年 | 32篇 |
1992年 | 20篇 |
1991年 | 10篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1978年 | 1篇 |
排序方式: 共有2193条查询结果,搜索用时 15 毫秒
101.
The aim of this paper is to analyse the performance of a finite element formulation usable for predicting the mechanical consequences of frost effects on porous media. It considers the characteristics of porous media and how the frost action can be assessed. The problem is then separated into two parts: thermal and poromechanical calculations. The constitutive equations developed in the framework of poromechanics are presented and the implementation in a usual finite element poroelasticity formulation based on Zuber's method is adopted. An analysis of the time‐step influence on the convergence rate is given and leads us to propose a simple method in order to obtain objectivity of the finite element response and avoid over‐long calculations. Frost effect simulations are carried out on real porous media (two fired clays) as a case study. Although the experimental behaviour of the porous media subjected to frost action is in accordance with some observations, the calculated strains appear to be overestimated compared with measurements. The problem could be largely attributable to the difficulty of assessing permeability evolution during frost development. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
102.
We derive the governing equations for the dynamic response of unsaturated poroelastic solids at finite strain. We obtain simplified governing equations from the complete coupled formulation by neglecting the material time derivative of the relative velocities and the advection terms of the pore fluids relative to the solid skeleton, leading to a so‐called us ? pw ? pa formulation. We impose the weak forms of the momentum and mass balance equations at the current configuration and implement the framework numerically using a mixed finite element formulation. We verify the proposed method through comparison with analytical solutions and experiments of quasi‐static processes. We use a neo‐Hookean hyperelastic constitutive model for the solid matrix and demonstrate, through numerical examples, the impact of large deformation on the dynamic response of unsaturated poroelastic solids under a variety of loading conditions. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
103.
104.
105.
Fractal analysis of long-range paleoclimatic data: Oxygen isotope record of pacific core V28-239 总被引:1,自引:0,他引:1
R/S analysis of the oxygen isotope curve of Pacific core V28-239 yields a fractal dimension of 1.22. This value is considered to characterize global climatic change over the last 2 million years as expressed by changing O18 ratios and confirms that climatic variations are characterized by long-term persistence. The fractal dimension of 1.22 compares favorably with the approximate fractal dimension of 1.26 for annual precipitation records for nine major cities in the United States. Although the precipitation and oxygen isotope data are measured in different physical units and recorded at different time scales, fractal analysis allows for a mathematical comparison of the two phenomena. Additionally, since the fractal dimensions of the oxygen isotope and precipitation records are similar, it is implied that such fractal dimensions are characteristic of climate change over the spectral range of 10 to 106 years. Given this temperature curves based on fractal parameters of long-term O18 data could be constructed which would allow examination of characteristics of temperature variation over tens and hundreds of years. Such studies may allow the establishment of limits on natural temperature variation and document the persistence of temperature trends through time. If these trends and limits can be resolved, long-range climatic prediction is feasible. 相似文献
106.
James G. Berryman 《Pure and Applied Geophysics》1988,128(1-2):423-432
Contrary to the traditional view, seismic attenuation in Biot's theory of fluid-saturated porous media is due to viscous damping of local (not global) pore-fluid motion. Since substantial inhomogeneities in fluid permeability of porous geological materials are to be expected, the regions of highest local permeability contribute most to the wave energy dissipation while those of lowest permeability dominate the fluid flow rate if they are uniformly distributed. This dichotomy can explain some of the observed discrepancies between computed and measured attenuation of compressional and shear waves in porous earth. One unfortunate consequence of this result is the fact that measured seismic wave attenuation in fluid-filled geological materials cannot be used directly as a diagnostic of the global fluid-flow permeability. 相似文献
107.
108.
Dispersion of Rayleigh-type surface wave is studied in a homogeneous transversely isotropic elastic layer overlying a nondissipative liquid-saturated porous solid half-space and lying under a uniform layer of homogeneous liquid. The frequency equation in the form of ninth-order determinant is obtained.Special cases have been deduced by reducing the depth of the layers to zero and by changing the transverse isotropic layer to an isotropic layer. Dispersion curves for the phase velocity have been plotted for a particular model. 相似文献
109.
This paper presents an analysis of Hagen-Poiseulle flow through plane random anisotropic networks of interconnected channels. Macroscopic permeability tensor of the network is expressed in terms of statistico-geometrical characteristics like the degree of anisotropy in channel orientations, average co-ordination number of the network and first two moments of channel length distribution. Analytical results are illustrated and verified using numerical analysis of flow in a simulated random network. The emphasis of the paper is on the effects of anisotropy on distributions of flow rates in channels. It is shown that, due to anisotropy the maximum flow rate generally occurs in channels that are not aligned along the direction of the macroscopic pressure gradient. 相似文献
110.
ResearchofpatterndynamicsparametersofcrustaldeformationfieldinseismogenicprocessShuo-YuZHOU(周硕愚),YunWU(吴云),Ruo-BoWANG(王若柏)and... 相似文献