全文获取类型
收费全文 | 1730篇 |
免费 | 225篇 |
国内免费 | 279篇 |
专业分类
测绘学 | 459篇 |
大气科学 | 162篇 |
地球物理 | 408篇 |
地质学 | 498篇 |
海洋学 | 162篇 |
天文学 | 141篇 |
综合类 | 160篇 |
自然地理 | 244篇 |
出版年
2024年 | 1篇 |
2023年 | 17篇 |
2022年 | 46篇 |
2021年 | 64篇 |
2020年 | 57篇 |
2019年 | 88篇 |
2018年 | 60篇 |
2017年 | 94篇 |
2016年 | 76篇 |
2015年 | 88篇 |
2014年 | 93篇 |
2013年 | 137篇 |
2012年 | 105篇 |
2011年 | 118篇 |
2010年 | 98篇 |
2009年 | 119篇 |
2008年 | 107篇 |
2007年 | 133篇 |
2006年 | 102篇 |
2005年 | 85篇 |
2004年 | 67篇 |
2003年 | 68篇 |
2002年 | 61篇 |
2001年 | 34篇 |
2000年 | 47篇 |
1999年 | 40篇 |
1998年 | 29篇 |
1997年 | 32篇 |
1996年 | 25篇 |
1995年 | 23篇 |
1994年 | 18篇 |
1993年 | 10篇 |
1992年 | 14篇 |
1991年 | 12篇 |
1990年 | 6篇 |
1989年 | 18篇 |
1988年 | 13篇 |
1987年 | 11篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有2234条查询结果,搜索用时 562 毫秒
1.
介绍了自动高精度测距系统。该系统能以5×10-7的测距精度测定多个观测点的形变,可作为跨断层动态连续观测的新仪器。 相似文献
2.
3.
用遗传算法解算病态方程 总被引:7,自引:1,他引:6
对应用遗传算法解决病态方程问题进行了探讨。利用拟合法而不是通过法方程求解参数,从而避免了法方程系数求逆,使病态方程的解答有了较好的结果。通过模拟计算并和其他方法进行比较,证明该方法是可行的和有效的。 相似文献
4.
In view of rapid developments in iterative solvers, it is timely to re‐examine the merits of using mixed formulation for incompressible problems. This paper presents extensive numerical studies to compare the accuracy of undrained solutions resulting from the standard displacement formulation with a penalty term and the two‐field mixed formulation. The standard displacement and two‐field mixed formulations are solved using both direct and iterative approaches to assess if it is cost‐effective to achieve more accurate solutions. Numerical studies of a simple footing problem show that the mixed formulation is able to solve the incompressible problem ‘exactly’, does not create pressure and stress instabilities, and obviate the need for an ad hoc penalty number. In addition, for large‐scale problems where it is not possible to perform direct solutions entirely within available random access memory, it turns out that the larger system of equations from mixed formulation also can be solved much more efficiently than the smaller system of equations arising from standard formulation by using the symmetric quasi‐minimal residual (SQMR) method with the generalized Jacobi (GJ) preconditioner. Iterative solution by SQMR with GJ preconditioning also is more elegant, faster, and more accurate than the popular Uzawa method. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
5.
Ralph A. M. J. Wijers 《Astrophysics and Space Science》1995,231(1-2):399-402
Since the discovery that dim gamma-ray bursts last longer on average than bright ones (time dilation) the cosmological origin of this effect has been contested by various researchers. I discuss the current status of this issue and conclude that current models for a non-cosmological time dilation only explain part of the observed phenomenon, and even then betray themselves by distinct signatures in the data. As those signatures have not been seen, the cosmological origin remains the favoured explanation of the time dilation. 相似文献
6.
AbstractThe ground vibrations during pile driving operation have a drastic potential to undermine the surrounding structures both in land and reclaimed land. Particularly, reclaimed land necessitates ample application of pile driving due to the weak land condition. To prevent the structural damage, attenuation of the ground vibrations to an allowable level through active isolation of circular open trench is the scope of this study. In this research, finite element simulations of continuous impact pile driving process from the ground surface was executed with particular attention to the pile-soil interaction, and thereby, the efficiency of open trench application in attenuation of the unsafe distance of different structures was surveyed using the vibration sensitivity degree. Regarding the crucial parameters of an open trench (depth, width, and location), it was concluded that a sufficient high depth can attenuate the unsafe distance up to 68%, the trench width variations are less effective, and an average pile-trench distance is the most efficient option. The excavation volume was also concluded as another crucial parameter in open trench design which takes all three parameters into account. The trench depth equal to the pile’s maximum critical depth of vibration was inferred for an optimum design. 相似文献
7.
Optimizing interpolation of shoot density data from a Posidonia oceanica seagrass bed 总被引:1,自引:0,他引:1
Michele Scardi Lorenzo A. Chessa Eugenio Fresi Antonio Pais & Simone Serra 《Marine Ecology》2006,27(4):339-349
A case study on the optimization of Posidonia oceanica density interpolation, using a data set from a large meadow at Porto Conte Bay (NW Sardinia, Italy), is presented. Ordinary point kriging, cokriging and a weighted average based on inverse square distance were used to interpolate density data measured in 36 sampling stations. The results obtained from different methods were then compared by means of a leave‐one‐out cross‐validation procedure. The scale at which interpolation was carried out was defined on the basis of the Hausdorff dimension of the variogram. Optimizing spatial scale and data points search strategy allowed obtaining more accurate density estimates independently of the interpolation method. 相似文献
8.
二次特征值问题 (QEP)的主要的求解方法之一是转化为广义特征值问题 (GEP) ,然后用求解广义特征值的方法 (比如 QZ方法 )求解。本文研究由此获得的计算解的范数意义下的最佳向后扰动分析 ,所得结果是 Tisseur最近所得结果的加强。 相似文献
9.
10.
一类广义Sine-Gordon方程的概周期解 总被引:4,自引:0,他引:4
用最大值原理和Banach压缩映像原理研究一类广义Sine-Gordon方程的概周期解问题,证明概周期解的存在‖u‖L∞<π2中的唯一性。 相似文献