全文获取类型
收费全文 | 2318篇 |
免费 | 344篇 |
国内免费 | 542篇 |
专业分类
测绘学 | 156篇 |
大气科学 | 158篇 |
地球物理 | 1119篇 |
地质学 | 1054篇 |
海洋学 | 399篇 |
天文学 | 84篇 |
综合类 | 98篇 |
自然地理 | 136篇 |
出版年
2024年 | 7篇 |
2023年 | 23篇 |
2022年 | 78篇 |
2021年 | 75篇 |
2020年 | 100篇 |
2019年 | 117篇 |
2018年 | 102篇 |
2017年 | 90篇 |
2016年 | 91篇 |
2015年 | 109篇 |
2014年 | 142篇 |
2013年 | 163篇 |
2012年 | 127篇 |
2011年 | 188篇 |
2010年 | 145篇 |
2009年 | 164篇 |
2008年 | 159篇 |
2007年 | 166篇 |
2006年 | 190篇 |
2005年 | 132篇 |
2004年 | 126篇 |
2003年 | 113篇 |
2002年 | 107篇 |
2001年 | 75篇 |
2000年 | 75篇 |
1999年 | 60篇 |
1998年 | 57篇 |
1997年 | 36篇 |
1996年 | 46篇 |
1995年 | 29篇 |
1994年 | 21篇 |
1993年 | 29篇 |
1992年 | 19篇 |
1991年 | 11篇 |
1990年 | 11篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 3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3204条查询结果,搜索用时 31 毫秒
971.
972.
973.
974.
大长细比柔性杆件涡激振动实验 总被引:3,自引:1,他引:2
涡激振动(vortex-induced vibration,VIV)是导致深海细长柔性立管发生疲劳破坏的重要因素。采用实验观测手段研究了长细比为1 750的柔性立管多模态涡激振动特性。实验中,通过采用拖车拖拉立管模型在水池中匀速行进来模拟均匀流作用下的涡激振动响应。利用光纤光栅传感器测量立管模型在横流向(cross-flow,CF)和顺流向(in-line,IL)的应变,进而通过模态分解的方法,获得立管模型涡激振动的位移。在此基础上,研究了CF以及IL方向的响应频率、位移标准差的平均值和最大值等随流速的变化规律,并分析了立管模型上测点的运动轨迹及其影响因素。 相似文献
975.
相干声纳条带测深系统采用船舷安装时,连接系统水下单元的钢管在水流作用下可能发生共振和频率锁定而剧烈振动,其下端的以换能器为主的系统水下单元也会随之摆动,使数据质量和测量精度受到影响。笔者通过分析在实际工作中遇到的问题发现:连接系统水下单元的钢管越长,钢管越容易在低流速条件下发生共振现象;换能器剧烈摆动会造成大量噪音信号;换能器摆动幅度较大时系统的安装校准会失效。本文通过解析计算得出了在现场操作中不同悬挂长度的钢管发生频率锁定时的控制流速以及换能器最大摆动幅度,认为实际工作中换能器距固定点在1.0m以内是可靠的,并对现场操作中如何减弱和避免涡激振动对系统精度的影响提出建议。 相似文献
976.
非洲信息通信技术应用的地理格局差异研究及对策 总被引:1,自引:1,他引:0
信息技术迅猛发展深刻影响社会经济发展和人们生活方式,而非洲正处于世界经济体系的最边缘,信息化的挑战源自边缘化与依附性并存。非洲面临贫困和数字鸿沟双重困境,使得信息通信技术在非洲的应用尤为重要。本文在综述了非洲信息技术项目进展和学者研究进展之后,从非洲东、西、南、北、中部5个地区分别选择3个信息化程度相对较好的国家,对比2000年、2008年两个时间点的信息通信技术应用情况,分析信息通信技术在非洲的应用特点,并提出发展措施。得出结论:非洲的技术应用明显落后于其他国家、地区,服务成本却远高于其他国家、地区,移动电话应用快于固定电话,互联网发展迅速但起点太低。因此,需要从政策、减贫、基础设施、产业等方面促进信息通信技术在非洲的应用。 相似文献
977.
R. K. Shevgaonkar 《Journal of Astrophysics and Astronomy》1986,7(4):275-292
Minimum-relative-entropy method (MREM) has been presented as a solution to the missing short-baseline problem in the synthesis
observations. It is shown that a measure of distance between the prior model and the image in the plane of pixel brightness
is an adequate measure of relative entropy. The method has been further extended for polarization observations and the potential
of the method against the conventional MEM has been demonstrated by simulated examples. 相似文献
978.
L.A. Sromovsky 《Icarus》2005,173(1):284-294
Solar radiation reflected by the atmospheres of Neptune and Uranus is dominated by Rayleigh scattering at visible wavelengths, and thus subject to the effects of polarization. Ignoring these effects can lead to errors in reflected intensity of more than 9% in a clear atmosphere. But solving the full vector equation of transfer is computationally very costly, forcing approximations with limitations that are not well understood and not generally applicable to spatially resolved observations and complex atmospheric structures. Using accurate vector radiation transfer calculations, it is here shown that differences between vector and scalar results near zero phase angle have systematic dependencies on optical depth, single scattering albedo, and angle, that provide a basis for accurate approximation of the reflected intensities. With little computational cost, it is possible to calculate corrected spatially resolved scalar intensities that closely match vector intensities, with individual errors rarely exceeding 1%, and mean and RMS errors generally within a few tenths of 1%. The correction method accounts for the attenuating effects of clouds and molecular absorption. 相似文献
979.
RESEARCH ON SYSTEM OF FLOOD DISASTER CONTROL AND REDUCTION SUPPORTED BY GIS IN MEDIUM AND SMALL BASINS 总被引:2,自引:0,他引:2
Southeast China coastal areas belong to subtropical monsoon climatic zone,thus easily affected by floods resulted from typhoons and rainstorm.Since the areas of river basins are small,rivers flood regulation capacities are low,and therefore flood hazard is grave.In the paper,taking the Yongjiang basin in southeast China as an example,the approaches and methods of geographic information system(GIS) applied to flood disaster control and reduction research on small basin are explred.On GIS help the rainfall-runoff calculation model and the river dchannel flood routing model are developed.And the evaluating flood submerged are and the damage assessment models are built supported by digit elevation models.Lastly the decision support system on GIS supported for flood control in research basin has been set up.This greatly improves flood-proofing decision-making capacities in river basin,and provides valuable information and a mode for flood prevention and reduction in the medium and small basin .Meanwhile,the research indicates that technologies of GIS provide a powerful tool for flood disaster control. 相似文献
980.
M. J. Booij 《水文研究》2003,17(13):2581-2598
Appropriate spatial scales of dominant variables are determined and integrated into an appropriate model scale. This is done in the context of the impact of climate change on flooding in the River Meuse in Western Europe. The objective is achieved by using observed elevation, soil type, land use type and daily precipitation data from several sources and employing different relationships between scales, variable statistics and outputs. The appropriate spatial scale of a key variable is assumed to be equal to a fraction of the spatial correlation length of that variable. This fraction was determined on the basis of relationships between statistics and scale and an accepted error in the estimation of the statistic of 10%. This procedure resulted in an appropriate spatial scale for precipitation of about 20 km in an earlier study. The application to river basin variables revealed appropriate spatial scales for elevation, soil and land use of respectively 0·1, 5·3 and 3·3 km. The appropriate model scale is determined by multiplying the appropriate variable scales with their associated weights. The weights are based on SCS curve number method relationships between the peak discharge and some specific parameters like slope and curve number. The values of these parameters are dependent on the scale of each key variable. The resulting appropriate model scale is about 10 km, implying 225–250 model cells in an appropriate model of the Meuse basin meant to assess the impact of climate change on river flooding. The usefulness of the appropriateness procedure is in its ability to assess the appropriate scales of the individual key variables before model construction and integrate them in a balanced way into an appropriate model scale. Another use of the procedure is that it provides a framework for decisions about the reduction or expansion of data networks and needs. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献