全文获取类型
收费全文 | 16651篇 |
免费 | 1833篇 |
国内免费 | 2176篇 |
专业分类
测绘学 | 6063篇 |
大气科学 | 2239篇 |
地球物理 | 2295篇 |
地质学 | 3870篇 |
海洋学 | 1400篇 |
天文学 | 1282篇 |
综合类 | 1656篇 |
自然地理 | 1855篇 |
出版年
2024年 | 81篇 |
2023年 | 182篇 |
2022年 | 542篇 |
2021年 | 691篇 |
2020年 | 730篇 |
2019年 | 816篇 |
2018年 | 562篇 |
2017年 | 890篇 |
2016年 | 798篇 |
2015年 | 820篇 |
2014年 | 915篇 |
2013年 | 1179篇 |
2012年 | 1026篇 |
2011年 | 973篇 |
2010年 | 742篇 |
2009年 | 934篇 |
2008年 | 976篇 |
2007年 | 1154篇 |
2006年 | 1051篇 |
2005年 | 844篇 |
2004年 | 799篇 |
2003年 | 621篇 |
2002年 | 534篇 |
2001年 | 450篇 |
2000年 | 378篇 |
1999年 | 335篇 |
1998年 | 277篇 |
1997年 | 203篇 |
1996年 | 193篇 |
1995年 | 174篇 |
1994年 | 165篇 |
1993年 | 143篇 |
1992年 | 92篇 |
1991年 | 74篇 |
1990年 | 57篇 |
1989年 | 54篇 |
1988年 | 38篇 |
1987年 | 26篇 |
1986年 | 26篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1977年 | 12篇 |
1976年 | 7篇 |
1973年 | 10篇 |
1972年 | 5篇 |
1971年 | 7篇 |
1954年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
Hash表技术是流分类的常用方法之一,用Hash表技术实现快速流分类的关键问题是降低冲突率,提高冲突解决的效率。该文通过提出几个新的概念(如发散、最佳流分类比特和相似比特等)来降低冲突率,通过提出查找树方案来提高冲突解决的效率,从而得到了一种能适应进行任意域流分类工作的、高效的流分类哈希(Efficient Packet Classification Hash,EPCH)表技术方案。仿真试验证明:该方案冲突率低、效率高,值得推广。 相似文献
202.
203.
本文扼要阐述公路勘测定界测绘与其土地报批图件资料制作过程,借用现有商业软件南方"CASS成图系统"和办公系统软件微软"Microsoft Office Excel",成图、制表及图件资料涉及内容,提出应注意问题及建议。 相似文献
204.
依托“西部煤炭资源高精度三维地震勘探技术”工程,对晋城矿区进行了旨在提高小断层,小陷落柱探测能力的高密度三维地震勘探。根据面元选择因素及该区地质任务,采用5m×5m网格进行野外数据采集;考虑炮检距、方位角、覆盖次数、排列片横纵比及煤层埋深(350~500m)等因素,采用中点放炮、60道接收,24次覆盖(横向4次,纵向6次)的8线16炮束状观测系统,基岩中激发。原始资料经同一处理流程后,获得5m×5m×1ms、5m×10m×1ms、10m×10m×1ms及2.5m×2.5m×1ms不同单元的三维数据体多个,通过对比可以发现小断层,小陷落柱在其小面元叠加时间剖面、顺层切片及相干切片都有清晰的反映。实例说明,小面元采集技术可以提高对小构造的纵、横向分辨能力,满足山区对三维地震精确勘探的要求。 相似文献
205.
206.
207.
The application of neural networks as classifiers of seismic events is described with the aim of developing an automatic system for the classification of explosion quakes at the Stromboli volcano. The architecture of the network that we trained to identify four different classes of shocks was a Multi-Layer Perceptron, using the Back Error Propagation algorithm. Five different approaches for representing the information embedded in the seismograms, both in the time and in the frequency domain, were considered, and the results compared. The direct use of the time series of the shocks was not satisfactory. The auto-correlation function worked well, but in some cases it was misleading. A better performance was obtained with a frequency domain representation. Finally, the use of the envelope function did not work well. Combining parameters such as the auto-correlation and envelope functions can improve one source of error, but it may introduce new ones. The performance obtained highlights the importance of the data attributes used for the training of the network. Topologies with eight neurons in a single hidden layer gave, on average, the best results among the considered neural network structures. The overall results provide a large number of events (89% with the best performance) correctly classified, indicating that this automatic technique is reliable, and encouraging further applications in the field of volcanic seismology. 相似文献
208.
Erik Rüttener Juan José Egozcue Dieter Mayer-Rosa Stephan Mueller 《Natural Hazards》1996,14(2-3):165-178
Seismic hazard analysis is based on data and models, which both are imprecise and uncertain. Especially the interpretation of historical information into earthquake parameters, e.g. earthquake size and location, yields ambiguous and imprecise data. Models based on probability distributions have been developed in order to quantify and represent these uncertainties. Nevertheless, the majority of the procedures applied in seismic hazard assessment do not take into account these uncertainties, nor do they show the variance of the results. Therefore, a procedure based on Bayesian statistics was developed to estimate return periods for different ground motion intensities (MSK scale).Bayesian techniques provide a mathematical model to estimate the distribution of random variables in presence of uncertainties. The developed method estimates the probability distribution of the number of occurrences in a Poisson process described by the parameter . The input data are the historical occurrences of intensities for a particular site, represented by a discrete probability distribution for each earthquake. The calculation of these historical occurrences requires a careful preparation of all input parameters, i.e. a modelling of their uncertainties. The obtained results show that the variance of the recurrence rate is smaller in regions with higher seismic activity than in less active regions. It can also be demonstrated that long return periods cannot be estimated with confidence, because the time period of observation is too short. This indicates that the long return periods obtained by seismic source methods only reflects the delineated seismic sources and the chosen earthquake size distribution law. 相似文献
209.
华北地台晚古生代含煤地层多重划分 总被引:11,自引:0,他引:11
以60个大体均匀分布于华北地台的地层剖面为基础,以晚古生代聚煤盆地的整体性为依托,对华北晚古生代地层进行多重划分研究,在岩石地层方面,研究了地层分区、划分了18个岩石地层单位“组”,分析了组的穿时性,在生物地层方面,重点介绍了蜒、古植物和孢粉生物地层,划分了各个组合带、延限带和顶峰带,分析了生物演化系列,空间展布和对比,提出了多方面的新认识,在年代地层方面,应用界线层型概念以及与首要门类生物地层单 相似文献
210.
Heinrich Eichhorn 《Celestial Mechanics and Dynamical Astronomy》1996,64(3):273-275
The autodependence, (a special case of the — now quite obsolete — dependences, which had been introduced for very specialized astrometric purposes) is proportional to the parameter variance which is the expectation of the variance of the systematic error of a function evaluated with estimated parameters. 相似文献