全文获取类型
收费全文 | 1532篇 |
免费 | 191篇 |
国内免费 | 343篇 |
专业分类
测绘学 | 146篇 |
大气科学 | 194篇 |
地球物理 | 502篇 |
地质学 | 716篇 |
海洋学 | 236篇 |
天文学 | 84篇 |
综合类 | 58篇 |
自然地理 | 130篇 |
出版年
2024年 | 10篇 |
2023年 | 23篇 |
2022年 | 58篇 |
2021年 | 56篇 |
2020年 | 66篇 |
2019年 | 83篇 |
2018年 | 59篇 |
2017年 | 55篇 |
2016年 | 75篇 |
2015年 | 67篇 |
2014年 | 90篇 |
2013年 | 126篇 |
2012年 | 74篇 |
2011年 | 116篇 |
2010年 | 93篇 |
2009年 | 111篇 |
2008年 | 99篇 |
2007年 | 107篇 |
2006年 | 122篇 |
2005年 | 77篇 |
2004年 | 64篇 |
2003年 | 60篇 |
2002年 | 49篇 |
2001年 | 46篇 |
2000年 | 45篇 |
1999年 | 34篇 |
1998年 | 41篇 |
1997年 | 22篇 |
1996年 | 30篇 |
1995年 | 25篇 |
1994年 | 16篇 |
1993年 | 20篇 |
1992年 | 15篇 |
1991年 | 8篇 |
1990年 | 7篇 |
1989年 | 1篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1979年 | 1篇 |
1973年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有2066条查询结果,搜索用时 15 毫秒
101.
A row of rigid piles is addressed as the countermeasures for isolating Rayleigh waves in a poroelastic half‐space. The complex characteristic equations for Rayleigh waves are derived via Biot's theory and their existence conditions are given. The piles are modeled as Euler–Bernoulli beams with longitudinal displacements and the diffracted field by each pile is constructed only with Rayleigh waves. Six infinite linear systems of algebraic equations are obtained in terms of the equilibrium of forces and continuity of displacements at the pile–soil interfaces. The systems are subsequently solved in the complex least‐squares sense. The influence of certain pile and soil characteristics such as the permeability of poroelastic soil, spacing between the piles and length of the piles on the isolating performance of a pile barrier is investigated. Computed results show that the permeability of poroelastic soil displays a significant effect on the vertical amplitude reduction of Rayleigh waves. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
102.
Precise hydrogeochemical modeling of early diagenesis is a key in the reconstruction of sedimentary basin models. This determines the mineralogical evolution of the sediment and consequently the porosity of the rock. During early diagenesis also part of the initial organic matter is converted into biogenic gas: CH4 CO2, and H2S. These processes are part of complex reaction chains during sedimentation, and biogeochemical reactions leave different signals that can be observed today. In this work, we reproduce the early diagenetic processes as integrated signals over geological times in sediments of the Demerara Rise by applying chemical thermodynamics using the PHREEQC (version 2) computer code. The investigated sediments are characterized by the presence of black shales in 410–490 mbsf and by a diagenetic barite layer above in 300–350 mbsf at depth of sulfate-methane transition (SMT). We determine the parameters that influence the location of diagenetic barite peaks in sediments overlying black shales by means of a novel modeling approach. Crucial parameters are the amount of bacterial organic matter mineralization, sedimentation rates and bottom water sulfate concentrations. All parameters are intertwining and influence the sulfate-methane cycle. They affect the location of the SMT visualized by diagenetic barite peaks. However, our model approach opens a wide field in exploring early diagenetic reactions, processes and products (such as biogenic methane) over geological times mirrored by diagenetic minerals and pore water concentration profiles that can be detected in present-day sediments. 相似文献
103.
We present a novel method of constructing streamlines to derive wind speeds within jovian vortices and demonstrate its application to Oval BA for 2001 pre-reddened Cassini flyby data, 2007 post-reddened New Horizons flyby data, and 1998 Galileo data of precursor Oval DE. Our method, while automated, attempts to combine the advantages of both automated and manual cloud tracking methods. The southern maximum wind speed of Oval BA does not show significant changes between these data sets to within our measurement uncertainty. The northern maximum does appear to have increased in strength during this time interval, which likely correlates with the oval’s return to a symmetric shape. We demonstrate how the use of closed streamlines can provide measurements of vorticity averaged over the encircled area with no a priori assumptions concerning oval shape. We find increased averaged interior vorticity between pre- and post-reddened Oval BA, with the precursor Oval DE occupying a middle value of vorticity between these two. 相似文献
104.
Common bivalve larvae from New Zealand: Leptonacea 总被引:1,自引:1,他引:0
J. D. Booth 《新西兰海洋与淡水研究杂志》2013,47(2):241-254
The late stage larvae of three erycinid bivalves (Mollusca: Pelecypoda: Leptonacea) taken from the brood chamber of the adult are described (Kellia cycladiformis, Lasaea rubra hinemoa, and L. maoria), and the provisionally identified late stage larva of the erycinid Arthritica bijurca taken from the plankton is described. Also, the D‐shaped larvae of the erycinid bivalves Kellia cycladiformis, Borniola reniformis, Arlhritica crassijormis, and A. bifurca taken from the parent brood chamber are described. The seasonal occurrence of each late stage larva in the plankton at the Bay of Islands (35° 15'S, 174° 10'E), Wellington Harbour (41° 16'S, 174° 51'E), and Raumati Beach (40° 56'S, 174° 58'E), New Zealand is described. Aspects of the reproductive cycles of Lasaea rubra hinemoa and Arthritica bifurca are presented. 相似文献
105.
Drag partition measurements were made in the atmospheric inertial sublayer for six roughness configurations made up of solid
elements in staggered arrays of different roughness densities. The roughness was in the form of a patch within a large open
area and in the shape of an equilateral triangle with 60 m long sides. Measurements were obtained of the total shear stress
(τ) acting on the surfaces, the surface shear stress on the ground between the elements (τS) and the drag force on the elements for each roughness array. The measurements indicated that τS quickly reduced near the leading edge of the roughness compared with τ, and a τS minimum occurs at a normalized distance (x/h, where h is element height) of
(downwind of the roughness leading edge is negative), then recovers to a relatively stable value. The location of the minimum
appears to scale with element height and not roughness density. The force on the elements decreases exponentially with normalized
downwind distance and this rate of change scales with the roughness density, with the rate of change increasing as roughness
density increases. Average τS : τ values for the six roughness surfaces scale predictably as a function of roughness density and in accordance with a shear
stress partitioning model. The shear stress partitioning model performed very well in predicting the amount of surface shear
stress, given knowledge of the stated input parameters for these patches of roughness. As the shear stress partitioning relationship
within the roughness appears to come into equilibrium faster for smaller roughness element sizes it would also appear the
shear stress partitioning model can be applied with confidence for smaller patches of smaller roughness elements than those
used in this experiment. 相似文献
106.
107.
石油天然气钻井肩负着油气资源勘探开发的国家使命和社会责任,同时又是能耗大户,节能潜力和责任很大。企业的科学发展当责无旁贷的把节能减排看作转变经济发展方式、节能挖潜、降本增效的重要抓手。只有坚持低碳生产,坚持节约发展、清洁发展、安全发展,才能实现企业又好又快地发展。 相似文献
108.
109.
非线性Hoek-Brown强度折减技术 总被引:1,自引:1,他引:0
为精确执行非线性Hoek-Brown(HB)强度折减,首先在p-q空间分析了HB曲线任意点切线与Mohr-Coulomb(MC)准则的对应关系,推求了HB屈服函数材料折减系数和强度折减系数的关系,提出了潜入强度折减系数的HB弹塑性分析模型,介绍了在Flac3D平台上二次开发实现非线性HB强度折减的基本思路。对一边坡算例用本文方法和简化Bishop法进行了对比分析,计算结果表明,潜入强度折减系数的HB弹塑性模型计算获得的边坡潜在滑动面形态、位置和相应的安全系数,与简化Bishop法计算结果很接近。 相似文献
110.
As a widely used approach for feature extraction and data reduction, Principal Components Analysis (PCA) suffers from high computational cost, large memory requirement and low efficacy in dealing with large dimensional datasets such as Hyperspectral Imaging (HSI). Consequently, a novel Folded-PCA is proposed, where the spectral vector is folded into a matrix to allow the covariance matrix to be determined more efficiently. With this matrix-based representation, both global and local structures are extracted to provide additional information for data classification. Moreover, both the computational cost and the memory requirement have been significantly reduced. Using Support Vector Machine (SVM) for classification on two well-known HSI datasets and one Synthetic Aperture Radar (SAR) dataset in remote sensing, quantitative results are generated for objective evaluations. Comprehensive results have indicated that the proposed Folded-PCA approach not only outperforms the conventional PCA but also the baseline approach where the whole feature sets are used. 相似文献