首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2099篇
  免费   273篇
  国内免费   345篇
测绘学   392篇
大气科学   214篇
地球物理   318篇
地质学   768篇
海洋学   145篇
天文学   21篇
综合类   221篇
自然地理   638篇
  2024年   8篇
  2023年   30篇
  2022年   91篇
  2021年   98篇
  2020年   100篇
  2019年   113篇
  2018年   87篇
  2017年   117篇
  2016年   122篇
  2015年   121篇
  2014年   115篇
  2013年   125篇
  2012年   124篇
  2011年   122篇
  2010年   100篇
  2009年   109篇
  2008年   128篇
  2007年   150篇
  2006年   132篇
  2005年   108篇
  2004年   89篇
  2003年   60篇
  2002年   50篇
  2001年   65篇
  2000年   46篇
  1999年   46篇
  1998年   42篇
  1997年   41篇
  1996年   20篇
  1995年   29篇
  1994年   30篇
  1993年   18篇
  1992年   19篇
  1991年   5篇
  1990年   8篇
  1989年   23篇
  1988年   8篇
  1987年   7篇
  1986年   3篇
  1985年   1篇
  1984年   1篇
  1983年   3篇
  1980年   1篇
  1979年   1篇
  1954年   1篇
排序方式: 共有2717条查询结果,搜索用时 328 毫秒
1.
根据印度洋西北部鸢乌贼(Sym plectoteuthis oualaniensis)作业渔场的现场调查数据.分析了作业水深、作业时段、摄食等级和月相等因子对手钓上钩率的影响。统计表明.晚上、午夜和凌晨3个不同时段和不同作业水深对手钓上钩率均有显著性影响。3个时段中,其摄食等级呈先低后高再低的现象。午夜和凌展,上钩率的变化趋势与摄食等级的变化趋势呈显著的负相关。在月相的影响下.离朔日三四天之前,手钓产量呈上升趋势;离望日五六天之前。手钓产量呈明显的下降趋势。  相似文献   
2.
地震前后垂直形变场动态演化的量化指标   总被引:3,自引:2,他引:1  
提出一种表达垂直形变场动态演化过程的量化指标——区域应变率、应变集中度。在此基础上,对南北地震带各水准监测区近30年的垂直形变资料进行了实际计算.并结合具体震例进行了对比研究。结果表明:量化指标在一定程度上能够反映地震前后形变场的动态演化过程和地壳运动状态,对地震的中长期预报有一定的积极意义。  相似文献   
3.
A new technique designed to help quantify the degree of damage to the landscape from one area to another shows a close relationship between population density and the degree of landscape damage. The technique establishes a scale of damage from 0 to 5 (zero = no damage; 5 = severe damage) using data from aerial photographs, land-use maps, and field data. The related formula allows one to compare the relative degree of damage across regions using a combination of an absolute index, a theoretical index, a relative index, and population density. Xing'an County is used to demonstrate the technique.  相似文献   
4.
The composition and distribution of benthic, periphytic and planktonic rotifers in different habitats in Krottensee were investigated. Sixty-five rotifer taxa were identified, their relative abundance estimated. Classification of the data set by cluster analysis allowed the recognition of distinct habitat groups confined to acid bog ponds, macrophytes, sediments and open water. Five groups of rotifer taxa were identified on the basis of their habitat preferences and the occurrence of the taxa. Highest diversities were found on macrophytes and in acid bog ponds.  相似文献   
5.
Abstract

The 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   
6.
SHI  Xiang 《中国海洋工程》2003,17(4):481-494
A three-dimensional fixed offshore platform in deep water modeled by the finite element method is studied in this paper. Analysis of the dynamic response of the MDOF structure is realized taking the non-linearity of the wave drag force and the wave-structure interaction into account. The structural response statistics, which have Gaussian distributions, are used to evaluate the vibration effect of the structure without TMD and with TMD. And an optimal method to design TMD controlling the first mode of the multi-mode structure is proposed. Moreover, the probabilities of occurrence of sea states at the platform site are considered for prediction of the long-term effect of a TMD. Simulation results demonstrate that the long-term effect of a well-designed TMD is good and the practical use is possible due to the good stability of its optimal parameters under different sea states.  相似文献   
7.
根据聚类方法,对闽南-台湾浅滩渔场变性水团进行划分,本海区共有6个变性水团。讨论各个变性水团在不同季节消长变化规律。  相似文献   
8.
CHEN  Shouyu 《中国海洋工程》2001,(4):453-466
In the process of concept design of offshore platforms, it is necessary to select the best from feasible alternatives through comparison and filter. The criterion set, used to evaluate and select the satisfying alternative, consists of many qualitative and quantitative factors. Therefore, the selection is a problem of multicriteria and semi- structural decision- making. Different from traditional methods in semi- structural decision- making, a new framework and methodology is presented in this paper for evaluation of offshore platform alternatives. First, the criterion set is established for the evaluation of alternatives. Next, the approach is studied to construct the relative membership degree matrix, in which both qualitative and quantitative factors are consistent with the uniform calculating standard. And then a new weight-assessing method is developed for calculation of the weights based on the relative membership degree matrix. Finally, a multi-hierarchy fuzzy optimum model is adopted to select t  相似文献   
9.
本文采用《受陆岸影响浅水区台风风浪的推算方案》后报影响汕头近岸区的台风风浪,并以此与Thompson关于波型的判据和合田良实的不同波型能量集中度的规定相结合,计算台风混合浪,经与现场实测资料相比较,效果良好;依44年台风浪的后报结果,可以得到各场台风过程的波高,波能和主波向的变化以及后报台风较大浪时能随方向的分布  相似文献   
10.
研究流形上的聚类分析,针对基于密度的空间聚类引入了流形概念,提出1种基于流形的密度聚类算法,该方法将流形的概念与聚类相结合,可以适用于样本为复杂分布的聚类。文中通过实例证明此算法的有效性。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号