全文获取类型
收费全文 | 5613篇 |
免费 | 1292篇 |
国内免费 | 883篇 |
专业分类
测绘学 | 858篇 |
大气科学 | 610篇 |
地球物理 | 2416篇 |
地质学 | 2082篇 |
海洋学 | 545篇 |
天文学 | 45篇 |
综合类 | 459篇 |
自然地理 | 773篇 |
出版年
2024年 | 43篇 |
2023年 | 111篇 |
2022年 | 191篇 |
2021年 | 248篇 |
2020年 | 252篇 |
2019年 | 301篇 |
2018年 | 240篇 |
2017年 | 292篇 |
2016年 | 298篇 |
2015年 | 318篇 |
2014年 | 351篇 |
2013年 | 369篇 |
2012年 | 363篇 |
2011年 | 318篇 |
2010年 | 265篇 |
2009年 | 293篇 |
2008年 | 306篇 |
2007年 | 356篇 |
2006年 | 315篇 |
2005年 | 288篇 |
2004年 | 249篇 |
2003年 | 237篇 |
2002年 | 217篇 |
2001年 | 143篇 |
2000年 | 156篇 |
1999年 | 147篇 |
1998年 | 154篇 |
1997年 | 153篇 |
1996年 | 137篇 |
1995年 | 104篇 |
1994年 | 84篇 |
1993年 | 93篇 |
1992年 | 75篇 |
1991年 | 67篇 |
1990年 | 52篇 |
1989年 | 41篇 |
1988年 | 46篇 |
1987年 | 37篇 |
1986年 | 28篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1954年 | 13篇 |
排序方式: 共有7788条查询结果,搜索用时 11 毫秒
971.
Research on vertical motion in mesoscale systems is an extraordinarily challenging effort. Allowing for fewer assumptions, a new form of generalized vertical motion equation and a generalized Omega equation are derived in the Cartesian coordinate system(nonhydrostatic equilibrium) and the isobaric coordinate system(hydrostatic equilibrium),respectively. The terms on the right-hand side of the equations, which comprise the Q vector, are composed of three factors:dynamic, thermodynamic, and mass. ... 相似文献
972.
为解决在建渭武高速木寨岭隧道施工过程中遇到的高地应力环境下软岩大变形问题,基于工程区已有地应力实测数据,利用ANSYS有限元软件建立三维地质模型,反演工程区地应力场,并结合Hoek围岩变形预测公式计算分析隧道围岩的变形量。结果表明:工程区的地应力场主要受断裂控制,其次还受到岩体强度和地形的双重影响,强构造变形区的水平主应力值普遍低于弱构造变形区,沿隧道轴线三向主应力大小关系为最大水平主应力(SH)>最小水平主应力(Sh)>垂直应力(SV),强构造变形区最大水平主应力值在G8区段最大,而在G6区段和G11区段最小;弱构造变形区的水平主应力值自G12区段开始逐渐增大,直至G14中段开始因埋深减小而逐渐降低。沿隧道轴线最大水平主应力方向总体为北东向,而在断裂间挤压构造带多偏转为北东东—近东西向。高速公路隧道围岩变形受岩体强度和地应力场的双重影响,其中,岩体强度占主导作用,围岩变形量主要集中在20~80 cm范围内,变形等级以中等和强烈为主。 相似文献
973.
????????????????й??????????????????????LSQR??WLSQR??MLS?????????????????????????????????????????????沨?????????в??????????????????????????????????????????????? 相似文献
974.
Generalized cross-validation for covariance model selection 总被引:4,自引:0,他引:4
Denis Marcotte 《Mathematical Geology》1995,27(5):659-672
A weighted cross-validation technique known in the spline literature as generalized cross-validation (GCV), is proposed for covariance model selection and parameter estimation. Weights for prediction errors are selected to give more importance to a cluster of points than isolated points. Clustered points are estimated better by their neighbors and are more sensitive to model parameters. This rational weighting scheme also provides a simplifying significantly the computation of the cross-validation mean square error of prediction. With small- to medium-size datasets, GCV is performed in a global neighborhood. Optimization of usual isotropic models requires only a small number of matrix inversions. A small dataset and a simulation are used to compare performances of GCV to ordinary cross-validation (OCV) and least-squares filling (LS). 相似文献
975.
以 CG2 0潜山为例 ,从建立地质模型入手 ,包括地层模型、构造模型、储集模型、储盖组合模型、速度模型等 ,认识到各套地层分布和储层物性的差异均与地震响应密切相关 ,因此可以利用地震波的信息 ,预测潜山储层的发育及分布情况。在对 CG2 0潜山进行精细全三维构造解释的基础上 ,探讨性地应用了测井约束反演、吸收系数、相干分析及三维模式识别等技术 ,对潜山储层进行了预测 ,从而提高了潜山勘探的效益 ,并为类似断阶型潜山带的勘探提供了成功的经验 ,具有一定的指导意义。 相似文献
976.
基于复模态的有限元模型修正算法 总被引:2,自引:0,他引:2
针对地下结构地震响应分析中无限地基辐射阻尼问题,引入复模态情况下的具有非简化的堆积阻尼矩阵的阻尼模型,并针对具有集中质量阵的阻尼模型提出了合并与质量有关的阻尼和堆积阻尼的思想,并据此提出了一种修正此类有限元模型的两步法,首先从复模态参数中提取实模态参数,采用基于模态残余力的识别算法修正刚度矩阵,然后根据复模态参数和已得的刚度矩阵来识别阻尼模型中的刚度参与系数和质量阻尼堆积阻尼联合矩阵。 相似文献
977.
978.
Joon Heo Jung Hwan Kim Jin Woo Kim 《International journal of geographical information science》2013,27(9):1165-1177
Coastline recession is one of the best indicators of coastal erosion. Three methods for computing coastline recession – the baseline approach, the dynamic segmentation approach and the area‐based approach – have been used, each of which has one or more drawbacks. To overcome these problems, a new methodology for measuring coastline recession is proposed, using buffering and non‐linear least squares estimation. The proposed method was compared with the three existing methods with respect to two simulated cases and two real coastlines. Test results confirmed that the new method is more reliable than the three other methods, all of which are susceptible to variability of recession, scale, number of line segments, length of coastlines and direction of the baseline. The proposed method, incorporating two physically meaningful values – magnitude and variability of coastline recession according to the mean and standard deviation of coastline offsets, respectively – presents itself as an effective alternative method of assessing coastline recession. 相似文献
979.
B. Kruijt Y. Malhi J. Lloyd A. D. Norbre A. C. Miranda M. G. P. Pereira A. Culf J. Grace 《Boundary-Layer Meteorology》2000,94(2):297-331
The turbulence structure in two Amazon rain forestswas characterised for a range of above-canopystability conditions, and the results compared withprevious studies in other forest canopies and recenttheory for the generation of turbulent eddies justabove forest canopies. Three-dimensional wind speedand temperature fluctuation data were collectedsimultaneously at up to five levels inside and abovetwo canopies of 30–40 m tall forests, during threeseparate periods. We analysed hourly statistics, jointprobability distributions, length scales, spatialcorrelations and coherence, as well as power spectraof vertical and horizontal wind speed.The daytime results show a sharp attenuation ofturbulence in the top third of the canopies, resultingin very little movement, and almost Gaussianprobability distributions of wind speeds, in the lowercanopy. This contrasts with strongly skewed andkurtotic distributions in the upper canopy. At night,attenuation was even stronger and skewness vanishedeven in the upper canopy. Power spectral peaks in thelower canopy are shifted to lower frequencies relativeto the upper canopy, and spatial correlations andcoherences were low throughout the canopy. Integrallength scales of vertical wind speed at the top of thecanopy were small, about 0.15 h compared to avalue of 0.28 h expected from the shear lengthscale at the canopy top, based on the hypothesis that theupper canopy air behaves as a plane mixing layer. Allthis suggests that, although exchange is not totallyinhibited, tropical rain forest canopies differ from other forests in that rapid, coherentdownward sweeps do not penetrate into the lowercanopy, and that length scales are suppressed. This isassociated with a persistent inversion of stability inthat region compared to above-canopy conditions. Theinversion is likely to be maintained by strong heatabsorption in the leaves concentrated near thecanopy top, with the generally weak turbulence beingunable to destroy the temperature gradients over thelarge canopy depth. 相似文献
980.