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   1100篇
  免费   194篇
  国内免费   238篇
测绘学   72篇
大气科学   32篇
地球物理   281篇
地质学   861篇
海洋学   88篇
天文学   13篇
综合类   36篇
自然地理   149篇
  2023年   6篇
  2022年   11篇
  2021年   33篇
  2020年   40篇
  2019年   47篇
  2018年   38篇
  2017年   51篇
  2016年   58篇
  2015年   50篇
  2014年   75篇
  2013年   97篇
  2012年   71篇
  2011年   65篇
  2010年   57篇
  2009年   67篇
  2008年   89篇
  2007年   87篇
  2006年   94篇
  2005年   58篇
  2004年   50篇
  2003年   51篇
  2002年   43篇
  2001年   45篇
  2000年   46篇
  1999年   35篇
  1998年   38篇
  1997年   27篇
  1996年   19篇
  1995年   14篇
  1994年   12篇
  1993年   9篇
  1992年   7篇
  1991年   8篇
  1990年   7篇
  1989年   4篇
  1988年   1篇
  1987年   1篇
  1986年   5篇
  1985年   1篇
  1984年   4篇
  1982年   1篇
  1981年   3篇
  1980年   1篇
  1979年   3篇
  1978年   1篇
  1976年   1篇
  1971年   1篇
排序方式: 共有1532条查询结果,搜索用时 31 毫秒
91.
以山东淄博煤田富硫煤(w(St>2%)煤矿作为典型研究区,阐述了由于直接排放酸性、高SO4离子、高硬度和较高矿化度(TDS)的矿坑水导致的地表水体及地下水的污染。研究区内地表水体的水质主要受控于矿坑排水的质量,区内主要河流、水库已被污染。引用这种受污染的地表水进行灌溉,会引起浅层地下水的严重污染,并影响到饮用此类地下水的人们的身体健康。为揭示污水灌溉过程中水-岩相互作用及地下水污染的机理,进行了现场土柱模拟实验,分别模拟了污水灌溉和降水入渗过程。实验结果表明,污水灌溉过程中SO4-不会被吸附,也不会产生沉淀,因此,污水灌溉是污灌区地下水受污染的主要原因。降水入渗实验表明,污灌区土壤中的SO4-,Ca2+和Mg2+,也可以通过降水淋滤进入地下水,这是污灌区地下水受污染的另一条途径。就对水环境的影响而言,酸性煤矿矿坑排水的环境效应是负面的,应予以高度重视。  相似文献   
92.
刘旭 《北京测绘》2020,(5):610-613
针对地下管网隐患数据管理的应用需求,本文基于三维平台设计实现了地下管网健康档案管理平台,在三维场景加载地上精细模型和地下管网数据,以三维可视化的手段对管网各类隐患问题进行信息管理,结合二维GIS操作,实现了净距分析、覆土分析、占压分析等管线隐患分析功能,通过本平台建立的地下管网健康档案数据库,方便用户对各类管网隐患数据进行信息管理和统计,为管道病害修复、管网改造提升提供数据支撑,保证地下管网的安全运行。  相似文献   
93.
Enclosure of some portion of one or more natural stream-drainage basins by superposition of a rectangle on a map of drainage network results in fragmentation of the natural basins into a set of disjoint channel networks. Each of these may have some channel links and forks of the natural network plus truncated links intersected by the enclosure boundary. The topological properties of the network elements in the enclosure are used to set up a model of random network patterns, in which the number of disjoint channel networks is expressed as a function of the number of links and forks in the enclosures. This function is shown to be a multiplicative constant times the square root of the number of links or forks. Empirical data on square and rectangular enclosures of several sizes from the Inez (Kentucky)Quadrangle map showed that the predicted multiplicative constants do not agree with observation, but that the square-root relation seems to hold at least to a first approximation. The models thus can be used as a norm against which deviations of real-world enclosures from network pattern randomness can be studied.  相似文献   
94.
95.
Metal cycling via physical and chemical weathering of discrete sources (copper mines) and regional (non-point) sources (sulfide-rich shale) is evaluated by examining the mineralogy and chemistry of weathering products in Great Smoky Mountains National Park, Tennessee, and North Carolina, USA. The elements in copper mine waste, secondary minerals, stream sediments, and waters that are most likely to have negative impacts on aquatic ecosystems are aluminum, copper, zinc, and arsenic because these elements locally exceed toxicity guidelines for surface waters or for stream sediments. Acid-mine drainage has not developed in streams draining inactive copper mines. Acid-rock drainage and chemical weathering processes that accompany debris flows or human disturbances of sulfidic rocks are comparable to processes that develop acid-mine drainage elsewhere. Despite the high rainfall in the mountain range, sheltered areas and intermittent dry spells provide local venues for development of secondary weathering products that can impact aquatic ecosystems.Electronic Supplementary Material  Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.
Jane M. HammarstromEmail: Phone: +1-703-6486165Fax: +1-703-6486252
  相似文献   
96.
东昆仑阿拉克湖地区第四纪水系演化过程及其趋势   总被引:6,自引:0,他引:6  
根据第四系沉积物的沉积时代、沉积类型及盆山耦合与迁移规律,将东昆仑阿拉克湖地区第四纪水系变迁划分了4个阶段:早更新世小型内陆盆地无序水系阶段;中更新世冰川水系与东流水系阶段;晚更新世柴达木盆地内陆水系向南溯源侵蚀阶段;全新世黄河外泄水系形成与发展阶段。根据研究区及相邻地区黄河水系和柴达木盆地内陆水系的溯源侵蚀发展历史及趋势分析,认为研究区未来水系发展趋势不一定是加鲁河袭夺鄂陵湖、扎陵湖两湖及黄河水系,而可能是黄河加快下蚀作用和溯源侵蚀,鄂陵湖和扎陵湖将逐渐消失成为河流,黄河水系最终将袭夺柴达木内陆盆地水系。  相似文献   
97.
Numerical and analytical solutions are presented for the elastic and inelastic response of single‐degree‐of‐freedom yielding oscillators to idealized ground acceleration pulses. These motions are typical of near‐fault earthquake recordings generated by forward rupture directivity and may inflict damage in the absence of substantial structural strength and ductility capacity. Four basic pulse waveforms are examined: (1) triangular; (2) sinusoidal; (3) exponential; and (4) rectangular. In the first part of the article, a numerical study is presented of the effect of oscillator period, strength, damping, post‐yielding stiffness and number of excitation cycles, on inelastic response. Results are presented in the form of dimensionless graphs and regression formulas that elucidate the salient features of the problem. It is shown that conventional Rµ relations may significantly underestimate ductility demand imposed by near‐fault motions. The second part of the article concentrates on elastic‐perfectly plastic oscillators. Closed‐form solutions are derived for post‐yielding response and associated ductility demand. It is shown that all three ground motion histories (i.e. acceleration, velocity, and displacement) control oscillator response—contrary to the widespread view that ground velocity alone is of leading importance. The derived solutions provide insight on the physics of inelastic response, which is often obscured by the complexity of numerical algorithms and actual earthquake motions. The model is evaluated against numerical results from near‐field recordings. A case study is presented. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
98.
王春得  赵忠贤 《地下水》2006,28(6):87-89
渭干河灌区的水土环境及水文地质条件在克孜尔水库建成后发生了一定的变化,本文结合灌区12年来的水盐监测资料,对灌区地下水的埋深变化规律、水质演变趋势、灌区的排水效果及目前存在的问题进行了分析与探讨,并为今后灌区排水措施和保持水土环境的良性发展提出了建议.  相似文献   
99.
Hui-Ping  Zhang  Shao-Feng  Liu  Nong  Yang  Yue-Qiao  Zhang  Guo-Wei  Zhang 《Island Arc》2006,15(2):239-250
Abstract   The Minshan Mountain and adjacent region are the major continental escarpments along the eastern Tibetan Plateau. The Minjiang drainage basin is located within the plateau margin adjacent to the Sichuan Basin. Based on the analysis of the digital elevation model (DEM) acquired by the Shuttle Radar Topography Mission (SRTM), we know that the Minjiang drainage basin has distinct geomorphic characteristics. The regular increasing of local topographic relief from north to south is a result of the Quaternary sediment deposition within the plateau and the holistic uplift of the eastern margin of the Tibetan Plateau versus the Sichuan Basin. Results from DEM-determined Minjiang drainage sub-basins and channel profiles show that the tributaries on the opposite sides are asymmetric. Lower perimeter and area of drainage sub-basins, total channel length and bifurcation ratio within eastern flank along the Minjiang mainstream are the result of the Quaternary differential uplift of the Minshan Mountain region. Shorter stream lengths and lower bifurcation ratio might be the indications of the undergrowth and newborn features of these eastern streams, which are also representative for the eastern uplift of the Minshan Mountain.  相似文献   
100.
目前,在波兰41对主要生产矿井中,有23对高瓦斯矿井进行了瓦斯抽放。2004年瓦斯平均抽放率为30%,平均利用率为39%。根据瓦斯地质条件、瓦斯涌出特点和采区通风方式,着重介绍了波兰煤矿强化煤层、围岩和采空区瓦斯抽放,提高矿井瓦斯抽放效率的工艺技术特点。在工作面的瓦斯排放中,注重开采、通风与瓦斯抽放一体化,通过优化抽放钻孔布置,取得钻孔瓦斯抽放的最佳效果,是波兰煤矿瓦斯治理的一项成功经验,成为煤矿持续安全高效生产的重要技术保障。  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