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   1080篇
  免费   76篇
  国内免费   332篇
测绘学   73篇
大气科学   125篇
地球物理   465篇
地质学   514篇
海洋学   141篇
天文学   44篇
综合类   38篇
自然地理   88篇
  2023年   5篇
  2021年   13篇
  2020年   13篇
  2019年   14篇
  2018年   20篇
  2017年   6篇
  2016年   16篇
  2015年   12篇
  2014年   16篇
  2013年   28篇
  2012年   35篇
  2011年   65篇
  2010年   41篇
  2009年   29篇
  2008年   58篇
  2007年   63篇
  2006年   73篇
  2005年   57篇
  2004年   53篇
  2003年   43篇
  2002年   60篇
  2001年   36篇
  2000年   36篇
  1999年   30篇
  1998年   36篇
  1997年   13篇
  1996年   26篇
  1995年   31篇
  1994年   21篇
  1993年   35篇
  1992年   43篇
  1991年   42篇
  1990年   50篇
  1989年   42篇
  1988年   39篇
  1987年   37篇
  1986年   26篇
  1985年   28篇
  1984年   36篇
  1983年   18篇
  1982年   13篇
  1981年   21篇
  1980年   27篇
  1978年   7篇
  1936年   4篇
  1935年   10篇
  1934年   4篇
  1933年   4篇
  1932年   4篇
  1924年   5篇
排序方式: 共有1488条查询结果,搜索用时 31 毫秒
21.
美国西南干旱半干旱地区人口增长非常快。水不仅要满足基本的需求(饮用水、污水系统等),而且还要维持乡村潮湿地区生活方式的改变(郊区的草地、高尔夫球场、庭院的游泳池等)。这些水的使用,既能满足供水系统所在区域地表水供水的需求,又可以满足最基本的地下水开采。然而,地下水的开采对沉积含水层有不利的影响,这些含水层是特有的沙漠区地下水流域的承载。这些影响包括含水层不同程度的压实、旧断层的重新复活、地表的裂缝,还能对人为的基础设施有相当大的影响。  相似文献   
22.
Natural uranium has three isotopes, ^238U, ^235U and ^234U, with natural abundances of 99.27 atom %, 0.72% and 0.0055%, respectively. Only ^235U is fissile and the production of nuclear fuel and nuclear weapons involves enrichment of uranium in ^235U. This process also results in separation of ^234U from ^238U, leaving depleted uranium (DU), with typical ^234U/^238U and ^235U/^238U activity ratios of about 0.19 and 0.013, respectively, as a waste product. The high density, high melting and boiling points and chemical stability of uranium and the availability of DU in relatively pure form mean that DU has many uses, including armour-piercing munitions. Such munitions have been developed in the UK since the 1960s and testing has been carried out by the Ministry of Defence (MoD) at firing ranges such as Dundrennan, SW Scotland and Eskmeals, NW England. The firing of DU munitions can result in the dispersion of DU and its combustion products (oxides) as aerosols or as larger fragments, with the potential for human exposure either directly at the site of detonation or via post-depositional migration in the environment. The aim of this work was to investigate the potential environmental mobility of DU by characterizing the associations of U in soil porewaters with increasing distance from a firing site. To this end, several soil cores located down-wind of the firing site at Dundrennan, near Kirkcudbright, SW Scotland, were collected in May 2006. These were sectioned on-site into 1- or 2-cm depth intervals and porewaters were isolated by centfifugation (10 minutes; 8873 g) on return to the laboratory. Following filtration through 0.2-micron cellulose nitrate filters, the porewaters were analyzed by ICP-QMS (U concentration) and ICP-OES (Fe, Al, Ca, Mg, Mn concentrations). Sub-samples were also subjected to centrifugal ultrafiltration (100, 30, and 3 kD) and to gel electrophoretic fractionation (agarose; 0.045 M Tris-borate; 20 mA, 30 minutes). Results showed that U was present at up to 4 μg/L in the soil porewater and that the associations of U varied with sample location and soil depth.  相似文献   
23.
The gross environmental condition of the Payatas Disposal Facility in Metro Manila, Philippines, implores the need for sustainable technologies to control and abate toxic municipal solid waste leachate and protect public health. A team of environmental engineers recognized this need by selecting the site as a design project for the Mondialogo Worldwide Engineering Award, sponsored by the United Nations and Daimler Chrysler. The project culminated in a technical proposal that highlighted the following design features: an impermeable dumpsite cap to control leachate generation; vertical impermeable barriers for containment and isolation of leachate; a drainage system to divert clean, surface water from the cap and prevent its contamination prior to discharge; a leachate recirculation system to enhance stabilisation of the waste deposit; and a facultative lagoon and constructed wetland system to treat excess leachate.  相似文献   
24.
The regional survey of groundwater used as a small water supply system was performed to know the effect of geology, soil properties and land use on groundwater quality at Nonsan City, Korea. A total of 126 groundwater samples were collected and analyzed at the study area. The multivariate statistical methods, principal components analysis and discriminant analysis, and GIS technique were used for the quantitative interpretation of groundwater quality. The study area is mainly composed of Precambrian gneiss, Jurassic granite, and Cretaceous volcanics, and metasedimentary rocks of the Ogcheon zone. The land use was grouped as paddy, upland, grassland, resident, point source, industrial area, and water system. The soil properties were classified as 4 major groups, Entisols, Alfisols, Inceptisols, and Ultisols, by the degree of development, and reclassified as 11 subgroups. The modified and simplified geologic map, soil map, and land use map were made by using ARCGIS soft-ware. The area of geology, soil property, and land use affecting the groundwater quality for each well were also calculated by ARCGIS soft-ware to acquire the quantitative parameters for multivariate statistical analysis. The monitoring results of groundwater in the study area showed that 13%-21% of the groundwater samples exceeded the portable water guideline and the main causes were turbidity, bacteria, arsenic and nitrate-N. The spatial distribution of each component showed the close relationship between groundwater quality and geology reflecting the topography, land use.  相似文献   
25.
Flood and desiccation are perceived as two most critical and influential disasters which contradict between causes and consequences. Flood occurs when the water surface could no longer flow the whole water flow, thus, the water flooded. Contradicted to that, desiccation occurs when the water flow contains a low volume of water deposit, thus, the water requirement exceeds the available potential. That condition was caused by land utilization as the consequences due to the increased land requirement for housing or industrial needs. An attempt to overcome the flood, nowadays, is implemented mostly in a structural way, through building canals, implementing rivers normalization, building gateways or building flood control pump which are more directed toward the flood direction in order to increase the surface flow in an immediate maner to the sea. However, the effort to overcome the flood its self, could be more effective if followed by an effort to increase the soil ability to absorb natural recharge or artificial recharge or by refilling the water into the earth surface. Absorption reservoir used as one of technology alternatives (artificial recharge) could also be used to support the attempt to overcome flood and desiccation. Absorption reservoir is a dam which was designed according to the basic principles, such as the bottom surface of the reservoir that has a high permeability surface; the surface of the reservoir water is higher (higher aquifer) along with a high permeability, considering the availability of water source that has been absorbed and its quality; considering the aquifer category on the water absorption dam;  相似文献   
26.
This research aims to establish the risks of hurnan exposure to modeling approach. The study site was located on the River Nura 1990s, an acetaldehyde production plant located in the industrial high environmental mercury levels, based on a quantitative risk in central Kazakhstan. From the 1950s until its closure in the mid city of Temirtau released mercury-containing wastewater into the river. River sediments are highly contaminated with mercury up to a distance of 25 km and beyond. In addition, a local power station released an estimated 6 million tonnes of fly-ash into the water and the mercury has become quite tightly associated with the ash deposits. River water, fish and agricultural land in the floodplain are also contaminated with mercury, yet the risks posed to the local population have not been evaluated to date. In June and July 2005, we took samples of soil, interior and exterior dust, drinking water, and food from individual households, communal areas and markets. Additionally, water and sediment samples and fish were taken from the river. Interviews were conducted with householders to establish their age and body weight, general living conditions and sources of irrigation and drinking water. A food frequency questionnaire (FFQ) was designed to investigate the frequencies of consumption of several common regional food items, including fish from the fiver and/or local market. Human hair samples were also collected to estimate the mercury bioburden and to enable the validity of the modeling approach to be established. The paper expands on the main pathways of contamination and looks at linkages between exposure pathways and mercury concentrations found in human hair. Uncertainties inherent in risk analysis as well as their influence on the relative importance of different exposure routes are also discussed.  相似文献   
27.
地下圆形衬砌隧道对沿线地震动的影响(II):数值结果   总被引:5,自引:3,他引:5  
利用地下圆形衬砌隧道对入射平面P波和SV波散射级数解答,定量分析了入射波长、入射角度、隧道直径、衬砌刚度等因素对沿线地表位移放大作用的影响。计算结果表明,当入射频率较低时,地下隧道的存在对地表位移幅值影响不大;而当入射频率较高时,地表位移幅值可以达到无隧道情况的4.5倍以上;衬砌情况下地表位移幅值可以高出非衬砌情况的87.8%。建议在地铁的规划设计中考虑地下隧道的建设对隧道沿线设计地震动的影响。  相似文献   
28.
湖泊沉积物中胶黄铁矿的鉴出及其磁学意义   总被引:6,自引:1,他引:6  
对青藏高原东部若尔盖盆地钻孔获取的湖泊沉积物进行了详细的岩石磁学调查及矿物学鉴定, 发现大量胶黄铁矿存在, 并认定其为主要的磁载体. 该矿物颗粒细小, 粒度均匀, 可能为生物化学成因. 与大量文献报道的相反, 在长期暴露于空气后, 样品中的胶黄铁矿仍未被氧化. 这可能与钻孔内该矿物被硅质胶结物包裹有关. 研究结果初步揭示了湖泊沉积物磁学研究的复杂性, 同时也为环境磁学机理的研究提供了新的线索.  相似文献   
29.
美国地震区划图为编制和修订美国建筑物抗震规范设计提供了所需资料。为了能反映最新的认识水平,科学家经常修订地震区划图。满足现代抗震设计规范的建筑具有较好的抗震性能,不仅能减少人员伤亡,而且能使如生命线工程等正常运转。  相似文献   
30.
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