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   1897篇
  免费   104篇
  国内免费   504篇
测绘学   90篇
大气科学   169篇
地球物理   772篇
地质学   758篇
海洋学   198篇
天文学   81篇
综合类   308篇
自然地理   129篇
  2024年   7篇
  2023年   19篇
  2022年   18篇
  2021年   15篇
  2020年   15篇
  2019年   24篇
  2018年   31篇
  2017年   13篇
  2016年   24篇
  2015年   24篇
  2014年   46篇
  2013年   25篇
  2012年   72篇
  2011年   101篇
  2010年   39篇
  2009年   62篇
  2008年   87篇
  2007年   108篇
  2006年   114篇
  2005年   117篇
  2004年   111篇
  2003年   102篇
  2002年   108篇
  2001年   131篇
  2000年   137篇
  1999年   57篇
  1998年   51篇
  1997年   82篇
  1996年   32篇
  1995年   43篇
  1994年   50篇
  1993年   61篇
  1992年   55篇
  1991年   71篇
  1990年   48篇
  1989年   62篇
  1988年   48篇
  1987年   30篇
  1986年   24篇
  1985年   44篇
  1984年   31篇
  1983年   13篇
  1982年   17篇
  1981年   13篇
  1980年   29篇
  1979年   5篇
  1947年   7篇
  1939年   7篇
  1926年   5篇
  1924年   4篇
排序方式: 共有2505条查询结果,搜索用时 15 毫秒
41.
42.
一.引 言 赤道太平洋上层海洋环流的主要特征已被建立十多年了。1942年斯维尔德鲁普等人描述了中太平洋的表面流:往西流的北和南赤道流以及往东流的北赤道逆流。随后,南赤道逆流也被用表面动力高度图表示出来(Reia,1961),Cromwell等人(1954)发现了赤道潜流,从而对上述环流的描述进行了主要修正。Hisard和Rual(1970)发现了赤道潜流下面  相似文献   
43.
Man-made defence structures (e.g., breakwaters, jetties) are becoming common features of marine coastal landscapes all around the world. The ecology of assemblages of species associated with such artificial structures is, however, poorly known. In this study, we evaluated the density and size of fish predators of echinoids (i.e., Diplodus sargus, Diplodus vulgaris, Sparus aurata), and the density of sea urchins (i.e., Paracentrotus lividus) at defence structures (i.e., breakwaters) inside and outside the marine protected area of Miramare (northern Adriatic Sea) in order to: (1) assess possible differences in fish predator density and size between protected and fished breakwaters; (2) assess whether fish predation may have the potential to affect sea urchin density in artificial rocky habitats. Surveys were carried out at four random times over a period of two years. Total density, and density of medium- and large-sized individuals of the three predatory fishes were generally greater at the protected than at the fished breakwaters, whereas no differences were detected in the density of small-sized individuals. Density of the sea urchin P. lividus did not show any difference between protected and fished breakwaters. The results of this study suggest that: (1) protection may significantly affect predatory fishes in artificial rocky habitats; (2) differences in predatory fish density, and size may be unrelated with the density of the sea urchin P. lividus; (3) protected artificial structures such as breakwaters, originally planned for other purposes, could represent a potential tool for fish population recovery and enhancement of local fisheries.  相似文献   
44.
1∶25万和龙市幅、汪清县幅等九幅区域地质调查成果显示,华北板块北缘东段残存着数条元古代-古生代不同时期的构造岩片,这些构造岩片均经历了强烈的变形改造及变质作用叠加,记录了增生型造山带在不同演化阶段热-动力学条件。吉林东部华北板块北缘陆缘活动带东段内元古代-古生代构造地层单位的确定,对探讨吉黑地区的大地构造演化具有重要意义。  相似文献   
45.
应用三维地震勘查方法进行煤矿塌陷区调查,既能够准确圈定采空区范围,又能够很好地解释中、浅层塌陷区分布。三维地震能够克服地质雷达、电法、地震横波等传统勘查方法勘探深度浅(一般为30 m,最多不超过50 m)的缺点,其成果对煤层在30 m以下老矿区综合治理、合理开发利用,提供准确地质资料。  相似文献   
46.
印度空间研究组织准备打造“地球”(Bhuvan)工程.发布比“谷歌地球”分辨率更高的网络地图。  相似文献   
47.
引言 减少温室气体排放量的需要 随着地球大气层中二氧化碳和其他温室气体浓度的不断上升,自然温室效应不断增强,这将导致全球气候变化。这些气候变化的特征、程度和时限是不可确定的,但一种主要的气候变化是可预测的,即全球平均气温上升。图1中的观测结果表明,由于外在因素的影响,例如火山尘和太阳辐射输出,全球平均气温逐渐增加且很有可能已超出气候自然变化的范围。  相似文献   
48.
From February to March 1989 the Phobos 2 spacecraft took 37 TV images of Phobos at a distance of 190-1100 km. These images complement Mariner-9 and Viking data by providing higher-resolution coverage of a large region West of the crater Stickney (40-160 degrees W) and by providing disk-resolved measurements of surface brightness at a greater range of wavelengths and additional phase angles. These images have supported updated mapping and characterization of large craters and grooves, and have provided additional observations of craters' and grooves' bright rims. Variations in surface visible/near-infrared color ratio of almost a factor of 2 have been recognized; these variations appear to be associated with the ejecta of specific large impact craters. Updated determinations of satellite mass and volume allow calculation of a more accurate value of bulk density, 1.90 +/- 0.1 g cm-3. This is significantly lower than the density of meteoritic analogs to Phobos' surface, suggesting a porous interior perhaps containing interstitial ice.  相似文献   
49.
"This paper comments upon the role of remittances in a variety of emigrant situations, traces the way remittance patterns change over time and evaluates the importance of remittance payments for personal and national economies. It considers the concept of 'remittance-societies" in the South Pacific and elsewhere, and briefly examines the various mechanisms involved in the emigration-remittance cycle, including the expectation of interactions and interdependence that exists between migrants and their home-based kin, and the sacrifices and hardships often endured by many migrants to meet obligations back home."  相似文献   
50.
Permeable Reactive Barrier (PRB) is an emplacement of inert material (s) in the subsurface, designed to intercept a contaminated plume, provides a preferential flow path through the reactive media, and transforms the contaminant into environmentally acceptable forms to attain concentration remediation goals at the discharge of the barrier. The phenomena, which help in remediation within PRB, are adsorption/sorption, precipitation, oxidation/reduction and biodegradation. Various materials like zero-valent iron, zero-valent bi-metals, natural zeolites, organic carbon, fly ash, zeolites, limestone, activated alumina, apatites, etc. have been tried by many researchers to remove organic and inorganic contaminants. In USA, Canada, and many European countries commercial full-scale and pilot scale PRBs are successfully working. The design and installation of full scale PRBs needs laboratory treatability and dynamic flow column experiments? The concept of PRB is being applied to treat contaminated surface water in the Katedan industrial area, Hyderabad, India. National Geophysical Research Institute (NGRI), Hyderabad, India, conducted systematic studies in collaboration with Norwegian Geotechnical Institute (NGI), Norway, to develop PRB technique to decontaminate the surface water pollution due to industrial effluent. A site assessment study in the Katedan Industrial Area, were carried out and water, soil and sediment from the lakes of the area were found to be polluted with high concentrations of heavy metals like As, Pb, Cr, Cd, Ni, etc. Adsorption studies at NGRI with synthetic samples and in-situ industrial effluent using different reactive media for removing contaminants like arsenic, chromium, cadmium, copper, nickel, lead and zinc have been carried out and yielded satisfactory results. The performance of zero-valent iron and limestone is encouraging in removing As,  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