全文获取类型
收费全文 | 2325篇 |
免费 | 401篇 |
国内免费 | 712篇 |
专业分类
测绘学 | 244篇 |
大气科学 | 56篇 |
地球物理 | 694篇 |
地质学 | 1859篇 |
海洋学 | 192篇 |
天文学 | 51篇 |
综合类 | 113篇 |
自然地理 | 229篇 |
出版年
2024年 | 2篇 |
2023年 | 27篇 |
2022年 | 95篇 |
2021年 | 83篇 |
2020年 | 101篇 |
2019年 | 111篇 |
2018年 | 116篇 |
2017年 | 89篇 |
2016年 | 149篇 |
2015年 | 149篇 |
2014年 | 162篇 |
2013年 | 203篇 |
2012年 | 171篇 |
2011年 | 191篇 |
2010年 | 171篇 |
2009年 | 171篇 |
2008年 | 161篇 |
2007年 | 150篇 |
2006年 | 170篇 |
2005年 | 134篇 |
2004年 | 136篇 |
2003年 | 96篇 |
2002年 | 84篇 |
2001年 | 80篇 |
2000年 | 68篇 |
1999年 | 49篇 |
1998年 | 53篇 |
1997年 | 47篇 |
1996年 | 33篇 |
1995年 | 32篇 |
1994年 | 38篇 |
1993年 | 26篇 |
1992年 | 23篇 |
1991年 | 15篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1977年 | 1篇 |
1973年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有3438条查询结果,搜索用时 265 毫秒
161.
A model is suggested to simulate the physical aspect of diagenesis in porous rocks. A bidisperse ballistic deposition model with relaxation of deposited grains is used to generate the porous structure. Sedimentation and erosion are allowed to restructure the pore space as a fluid flows through the rock. The effect of this restructuring of the pore space on permeability is studied. The Navier–Stokes equation is solved numerically by the finite difference method to determine the pressure and velocity distributions in the pore space. We find that though deposition is the dominant process in our model of diagenesis, reducing the porosity, the permeability may increase dramatically in some cases. These are when the erosion takes place at a single narrow constriction in the pore channel. 相似文献
162.
The stiffness degradation and damping ratio evolution of Taipei Silty Clay under cyclic straining 总被引:2,自引:0,他引:2
The non-linear behavior of Taipei Silty Clay under cyclic strain loading was investigated through a series of undrained cyclic strain-controlled tests. The Ramberg–Osgood equation was used with our proposed stiffness degradation model to calculate degraded secant moduli. The proposed degradation model is simple in that it has only one more component than Idriss's model, the modulus ratio for the first cycle, which reflects the effects of the previous cyclic strain history and the current level of the cyclic strain amplitude, and can be used to describe softening and hardening behavior under irregular cyclic straining. It was found that the Ramberg–Osgood equation successfully predicts the damping ratio for small to medium strains. However, it overestimates the damping ratio for larger strains, so we suggest it can be corrected with a damping ratio index. In addition, the proposed equation for describing the evolution of the damping ratio provides the means to assess the variation for Taipei Silty Clay in the measured damping ratio with both the number of cycles and the strain amplitude. 相似文献
163.
The time-domain EM induction response of non-magnetic and magnetic targets can be approximated using a conductive permeable prism composed of six faces of conductive plates, each face being composed of a set of conductive ribbons. The effect of magnetic permeability is included by the use of two “apparent flux gathering” coefficients, and two “effective magnetic permeability” coefficients, in the axial and transverse directions. These four magnetic property coefficients are a function of physical properties and geometry of the target, but are independent of prism orientation relative to a transmitter. The approximation algorithm is computationally fast, allowing inversions for target parameters to be achieved in seconds. The model is tested on profiles acquired with a Geonics EM63 time-domain EM metal detector over a non-magnetic copper pipe target, and a steel artillery shell in horizontal and vertical orientations. Results show that this approximation to a permeable prism has a capability of fitting geometric, conductivity and magnetic parameters at both early and late sample times. The magnetic parameters show strong change from early to late times on the EMI decay curve, indicating that the magnetic properties of the target have non-linear characteristics. It is proposed that these magnetic parameters and the nature of their non-linearity may carry additional discrimination information for distinguishing between intact munitions and scrap in UXO studies. 相似文献
164.
Logic-tree Approach for Probabilistic Tsunami Hazard Analysis and its Applications to the Japanese Coasts 总被引:2,自引:0,他引:2
Tadashi Annaka Kenji Satake Tsutomu Sakakiyama Ken Yanagisawa Nobuo Shuto 《Pure and Applied Geophysics》2007,164(2-3):577-592
For Probabilistic Tsunami Hazard Analysis (PTHA), we propose a logic-tree approach to construct tsunami hazard curves (relationship
between tsunami height and probability of exceedance) and present some examples for Japan for the purpose of quantitative
assessments of tsunami risk for important coastal facilities. A hazard curve is obtained by integration over the aleatory
uncertainties, and numerous hazard curves are obtained for different branches of logic-tree representing epistemic uncertainty.
A PTHA consists of a tsunami source model and coastal tsunami height estimation. We developed the logic-tree models for local
tsunami sources around Japan and for distant tsunami sources along the South American subduction zones. Logic-trees were made
for tsunami source zones, size and frequency of tsunamigenic earthquakes, fault models, and standard error of estimated tsunami
heights. Numerical simulation rather than empirical relation was used for estimating the median tsunami heights. Weights of
discrete branches that represent alternative hypotheses and interpretations were determined by the questionnaire survey for
tsunami and earthquake experts, whereas those representing the error of estimated value were determined on the basis of historical
data. Examples of tsunami hazard curves were illustrated for the coastal sites, and uncertainty in the tsunami hazard was
displayed by 5-, 16-, 50-, 84- and 95-percentile and mean hazard curves. 相似文献
165.
We investigate the estimation of interfacial areas, curvatures, and common curve lengths in multiphase porous medium systems. Algorithms are developed to obtain estimates of these quantities based upon a variety of potential data sources and estimation approaches. The accuracy of the derived approximations are evaluated as a function of the data type and resolution of the data. The methods advanced improve upon standard approaches now in use and show excellent accuracy at resolutions on the order of five lattice points per minimum radius of curvature of the object being resolved. Finally, we suggest a promising class of extensions that could lead to further improvements in the accuracy of such methods. 相似文献
166.
167.
多道瞬态瑞雷波法在探测煤矿采空区中的应用 总被引:7,自引:0,他引:7
采空区探测是工程建筑和煤矿生产中亟待解决的问题。常规物探方法在对落后的“房—柱”式开采技术造成的面积较小、埋深较浅的采空区进行探测时存在漏报或误报的情况。根据采空区的地球物理特征,探讨多道瞬态瑞雷波技术的探测机理,并通过勘探实例阐述了该方法在浅层煤矿采空区探测中的实用性和有效性。 相似文献
168.
本文根据线路平行线间面积计算的统一模型,分别推证了直线,圆曲和缓和曲线段平行线间面积计算的简捷模型,据此归纳出线路平行线间面积计算的统一简捷模型,它在线路土地征及用建材用料计算等方面都有广泛的应用。 相似文献
169.
道路曲线数字化数据的联合平差模型 总被引:6,自引:1,他引:6
在讨论道路曲线基本单元数字化数据处理模型的基础上,进一步建立了道路曲线的联合平差模型,探讨了以线形为特征的交通运输空间数据的平差处理方法,提出了评价道路曲线数字化的精度指标。这些工作为研究交通空间数据的质量和建立动态分段模型以及道路GIS空间基础数据库是有益的。 相似文献
170.
频谱分析在周期拟合中的应用 总被引:5,自引:0,他引:5
在时间序列数据处理中常碰到确定性信息的提取。确定性信息包含趋势项和周期项。拟合周期项需要知道频率,利用频谱分析则可准确地捕捉频率,从而获得良好的计算结果。 相似文献