全文获取类型
收费全文 | 2798篇 |
免费 | 307篇 |
国内免费 | 411篇 |
专业分类
测绘学 | 1373篇 |
大气科学 | 573篇 |
地球物理 | 393篇 |
地质学 | 410篇 |
海洋学 | 257篇 |
天文学 | 71篇 |
综合类 | 323篇 |
自然地理 | 116篇 |
出版年
2024年 | 10篇 |
2023年 | 31篇 |
2022年 | 43篇 |
2021年 | 88篇 |
2020年 | 99篇 |
2019年 | 125篇 |
2018年 | 72篇 |
2017年 | 119篇 |
2016年 | 149篇 |
2015年 | 166篇 |
2014年 | 167篇 |
2013年 | 215篇 |
2012年 | 192篇 |
2011年 | 218篇 |
2010年 | 159篇 |
2009年 | 173篇 |
2008年 | 162篇 |
2007年 | 183篇 |
2006年 | 172篇 |
2005年 | 132篇 |
2004年 | 102篇 |
2003年 | 88篇 |
2002年 | 83篇 |
2001年 | 72篇 |
2000年 | 46篇 |
1999年 | 45篇 |
1998年 | 84篇 |
1997年 | 52篇 |
1996年 | 48篇 |
1995年 | 38篇 |
1994年 | 34篇 |
1993年 | 26篇 |
1992年 | 28篇 |
1991年 | 16篇 |
1990年 | 16篇 |
1989年 | 19篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有3516条查询结果,搜索用时 15 毫秒
11.
12.
Very high-frequency marine multichannel seismic reflection data generated by small-volume air- or waterguns allow detailed, high-resolution studies of sedimentary structures of the order of one to few metres wavelength. The high-frequency content, however, requires (1) a very exact knowledge of the source and receiver positions, and (2) the development of data processing methods which take this exact geometry into account. Static corrections are crucial for the quality of very high-frequency stacked data because static shifts caused by variations of the source and streamer depths are of the order of half to one dominant wavelength, so that they can lead to destructive interference during stacking of CDP sorted traces. As common surface-consistent residual static correction methods developed for land seismic data require fixed shot and receiver locations two simple and fast techniques have been developed for marine seismic data with moving sources and receivers to correct such static shifts. The first method – called CDP static correction method – is based on a simultaneous recording of Parasound sediment echosounder and multichannel seismic reflection data. It compares the depth information derived from the first arrivals of both data sets to calculate static correction time shifts for each seismic channel relative to the Parasound water depths. The second method – called average static correction method – utilises the fact that the streamer depth is mainly controlled by bird units, which keep the streamer in a predefined depth at certain increments but do not prevent the streamer from being slightly buoyant in-between. In case of calm weather conditions these streamer bendings mainly contribute to the overall static time shifts, whereas depth variations of the source are negligible. Hence, mean static correction time shifts are calculated for each channel by averaging the depth values determined at each geophone group position for several subsequent shots. Application of both methods to data of a high-resolution seismic survey of channel-levee systems on the Bengal Fan shows that the quality of the stacked section can be improved significantly compared to stacking results achieved without preceding static corrections. The optimised records show sedimentary features in great detail, that are not visible without static corrections. Limitations only result from the sea floor topography. The CDP static correction method generally provides more coherent reflections than the average static correction method but can only be applied in areas with rather flat sea floor, where no diffraction hyperbolae occur. In contrast, the average static correction method can also be used in regions with rough morphology, but the coherency of reflections is slightly reduced compared to the results of the CDP static correction method. 相似文献
13.
14.
Yosuke Fujii 《Journal of Oceanography》2005,61(1):167-181
I present the derivation of the Preconditioned Optimizing Utility for Large-dimensional analyses (POpULar), which is developed for adopting a non-diagonal background error covariance matrix in nonlinear variational analyses (i.e., analyses employing a non-quadratic cost function). POpULar is based on the idea of a linear preconditioned conjugate gradient method widely adopted in ocean data assimilation systems. POpULar uses the background error covariance matrix as a preconditioner without any decomposition of the matrix. This preconditioning accelerates the convergence. Moreover, the inverse of the matrix is not required. POpULar therefore allows us easily to handle the correlations among deviations of control variables (i.e., the variables which will be analyzed) from their background in nonlinear problems. In order to demonstrate the usefulness of POpULar, we illustrate two effects which are often neglected in studies of ocean data assimilation before. One is the effect of correlations among the deviations of control variables in an adjoint analysis. The other is the nonlinear effect of sea surface dynamic height calculation required when sea surface height observation is employed in a three-dimensional ocean analysis. As the results, these effects are not so small to neglect. 相似文献
15.
用一种新方法研究了三对角方程组及对称三对角方程组计算解的最佳向后扰动分析。由于这两类方程组的系数矩阵具有特殊结构 ,因而本文结果不能从已有结果直接导出。所用方法亦可用于研究 KKT及 SQD等结构线性系统的最佳向后扰动分析。 相似文献
16.
对煤炭资源勘查资源量与客观物质量之间差值的误差理论探讨,提出误差由真值误差、系统误差和离差3个基本部分组成,其中真值误差是不可消除的观测误差、系统误差是控制网度所决定的准确度标准、而离差是由地质构造及矿床形态的多解性产生的资源量波动。在此基础上,构筑了一个关于误差的数学模型,最后应用误差理论对现行勘查工作中常见的错误进行分析,并就如何避免错误及减少误差提出了见解。 相似文献
17.
18.
19.
针对现有3维地形模型简化方法中的不足,提出了一种基于小波变换与滤波的规则格网简化方法。同时还在大量实验的基础上,给出了一般情况下需要的变换层数,分析了滤波门限与简化效果之间的关系。这里的研究不仅为3维地形的多分辨率合理建模与精度评估提供了理论和方法,而且也为今后开展类似研究及进行建模质量控制提供了科学的依据。 相似文献
20.
高精度的数字地图是正确匹配车辆位置的基础。详细分析了地图数据的地理误差和拓扑误差的误差形式,路网数据模型的常见误差因素和改进策略,最后介绍了不同地图匹配算法对地图质量的敏感性和可行性。根据可能出现的误差对现有数字地图和匹配算法加以改进,弥补了原有数字地图带来的不精确缺陷。跑车实验证明,考虑了数字地图误差影响的匹配算法可以明显提高定位精度,减小车辆定位误差。 相似文献