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   962篇
  免费   171篇
  国内免费   51篇
测绘学   59篇
大气科学   18篇
地球物理   616篇
地质学   148篇
海洋学   63篇
天文学   31篇
综合类   60篇
自然地理   189篇
  2023年   4篇
  2022年   14篇
  2021年   33篇
  2020年   26篇
  2019年   36篇
  2018年   22篇
  2017年   28篇
  2016年   25篇
  2015年   28篇
  2014年   34篇
  2013年   40篇
  2012年   36篇
  2011年   52篇
  2010年   33篇
  2009年   61篇
  2008年   45篇
  2007年   63篇
  2006年   55篇
  2005年   33篇
  2004年   42篇
  2003年   33篇
  2002年   32篇
  2001年   21篇
  2000年   35篇
  1999年   45篇
  1998年   30篇
  1997年   44篇
  1996年   36篇
  1995年   41篇
  1994年   41篇
  1993年   29篇
  1992年   24篇
  1991年   8篇
  1990年   13篇
  1989年   11篇
  1988年   7篇
  1987年   11篇
  1986年   6篇
  1985年   2篇
  1984年   1篇
  1982年   1篇
  1980年   1篇
  1954年   2篇
排序方式: 共有1184条查询结果,搜索用时 16 毫秒
121.
罗葆荣  段长春 《天文学报》1997,38(4):386-393
本文用1970-1995年的冕洞资料,分析了冕洞的分布规律,磁场极性的演化特征和冕洞的地磁效应,以及它们与太阳黑子周期的演化关系,得到了一些有意义的结论。特别指出赤道冕洞和极区冕洞具有相反的演化规律和不同的特征。  相似文献   
122.
利用2016—2018年3期华北地区流动地磁矢量原始测量资料, 经数据计算获得2期华北地区和张家口—渤海地震活动带及邻区岩石圈磁场时空变化模型。 研究结果显示: 张家口—渤海地震带岩石圈磁场变化空间分布不均匀, 具有明显的分区特征, 在张家口段(西段)与北京段(中西段)分界处和北京段(中西段)与唐山段(中东段)分界处岩石圈磁场各要素具有明显的异常变化, 如水平矢量存在转向和幅值变化, 磁偏角与磁倾角具有正负异常高梯度带的特征, 这与张家口—渤海地震带构造分段性特征密切相关。 张家口—渤海地震带位于燕山块体与华北平原块体之间, 两者运动的平动速率之差是张家口—渤海地震带左旋走滑的直接动力来源, 而各断裂带左旋走滑速率之差很可能是岩石圈磁场空间变化分段性分布的主要原因。  相似文献   
123.
http://www.sciencedirect.com/science/article/pii/S1674987114001121   总被引:1,自引:0,他引:1  
The Chinese Tianshan belt of the southern Altaids has undergone a complicated geological evolution.Different theories have been proposed to explain its evolution and these are still hotly debated.The major subduction polarity and the way of accretion are the main problems.Southward,northward subduction and multiple subduction models have been proposed.This study focuses on the structural geology of two of the main faults in the region,the South Tianshan Fault and the Nikolaev Line.The dip direction in the Muzhaerte valley is southward and lineations all point towards the NW.Two shear sense motions have been observed within both of these fault zones,a sinistral one,and a dextral one,the latter with an age of 236-251 Ma.Structural analyses on the fault zones show that subduction has been northward rather than southward.The two shear sense directions indicate that the Yili block was first dragged along towards the east due to the clockwise rotation of the Tarim block.After the Tarim block stopped rotating,the Yili block still kept going eastward,inducing the dextral shear senses within the fault zones.  相似文献   
124.
125.
126.
A new and simple method based on a nonlinearly mathematical optimization concept has been proposed in this research to interpret magnetic anomalies due to vertical faults and thin dikes. This proposed interpretative method consists of three main steps. The first step is to formulate nonlinearly constrained optimization problems to describe the geophysical problems related to the studied structures. The second step is to suggest an interior penalty function in order to convert these nonlinearly constrained optimization problems into nonlinearly unconstrained optimization ones. The third step is to solve the converted nonlinearly unconstrained optimization problems by using the famous Hooke and Jeevess algorithm in order to estimate the geophysical parameters of the studied structures such as: depth, amplitude coefficient, and index parameter. The Hooke and Jeevess algorithm is purposely chosen for being robust and also its application to magnetic data converges rapidly towards the optimal estimation of parameters. This method was first tested on theoretical models with different random noise, where a very close agreement was obtained between the assumed and evaluated parameters. The validity of this new method was also tested on practical field examples taken from Australia, India, United States, and Brazil, where available magnetic data existed and was previously analyzed by different interpretative methods. The agreement between the results obtained by our developed method and those obtained by the other geophysical methods is good. The advantages of this newly proposed method, compared with the other published interpretative methods, also have been discussed and demonstrated.  相似文献   
127.
基于对子午链台站地震地磁前兆异常以及地球空间环境变化等研究方向的需求分析,利用网络、大数据、云计算等新技术,结合地震数据专家(DatistEQ)软件,设计子午链台站地磁指数报告自动产出系统。该系统通过低代码、敏捷式开发工作流程,连接不同功能节点,可以实现用户定制化、数据标准化、功能智能化的系统报告产出。通过本地或者云端一键流程化工作,将不同类型的数据发送到科研工作者或其他需求者,简化工作流程,优化数据处理,实现高效率产品产出。  相似文献   
128.
在"京津冀一体化"的发展目标下,随着新区的建设,政府项目存在着规模大、周期紧、关注度高、管理难度大等特点.利用互联网+的理念提高建筑行业的管理精度是解决问题的主流方向,也符合住建部《2016—2020年建筑业信息化发展纲要》提出的"构建基于BIM、大数据、智能化、移动通信、云计算等技术的工程质量、安全监管模式与机制".本文以某一重大政府项目工程为例,介绍如何将住建部要求的技术要素通过BIM+GIS指挥调度系统应用到项目中.  相似文献   
129.
Knowledge of the age of undated events is not null if a time-order relationship can be found among these events. The knowledge of such a time-ordered sequence can be formalized by using non-informative (uniform) prior probability densities for the ages of undated events and Bayes' theorem to introduce the time-order relationship condition. We show that the conditional probability densities of the ages of events of unknown age are given by various forms of Euler's beta distribution. These distributions yield an estimate of the probability for an undated event to occur in a given age interval.
  We use this method to propose appropriate probabilistic representations of our actual knowledge of the dating of the magnetic polarity reversals during the Cenozoic. These representations take into account the uncertainties arising from irregularities in accretion process and from the quality of a few calibration points. Both types of uncertainties generate large ambiguities in the age of magnetic reversals, which should be taken into consideration when the geomagnetic polarity timescale is used for dating purposes. We propose to use the entropy function to quantify these ambiguities.  相似文献   
130.
A palaeomagnetic study comprising the directional results from 289 individual lava flows, sampled along eight sections in the Palaeocene basalts of West Greenland, is reported. The eight individual sections are correlated using lithostratigraphical marker horizons to form a single composite profile. Generally, the lithological correlation is in good agreement with the record of geomagnetic secular variation.
  The total composite palaeomagnetic profile represents a stratigraphic thickness of 1.6  km through the Vaigat Formation, which is the lowermost of the two volcanic formations formed during the main stage of plateau volcanism. Only two polarity zones are found in the composite profile, suggesting a very short duration for the West Greenland main plateau-building volcanism. 40Ar/39Ar dates support a high extrusion rate and also indicate that the lower normal polarity zone is Chron C27n and that the upper reverse polarity zone is Chron C26r.
  The C27n–C26r transition is fully recorded along one of the sections (Nuusap Qaqqarsua), with intermediate directions covering a 200  m thick succession of lavas. A combined palaeomagnetic, field and geochemical study along this profile showed good agreement; that is, geochemically and geologically derived single magmatic events show groupings of the palaeomagnetic directions. Supposing a duration for the geomagnetic transition of 5000 years, the eruption frequency during this period was as high as one flow every 80 years.  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