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   4951篇
  免费   1351篇
  国内免费   1901篇
测绘学   797篇
大气科学   625篇
地球物理   1054篇
地质学   3395篇
海洋学   1075篇
天文学   54篇
综合类   411篇
自然地理   792篇
  2024年   45篇
  2023年   130篇
  2022年   349篇
  2021年   403篇
  2020年   333篇
  2019年   410篇
  2018年   330篇
  2017年   311篇
  2016年   352篇
  2015年   337篇
  2014年   339篇
  2013年   431篇
  2012年   515篇
  2011年   431篇
  2010年   476篇
  2009年   434篇
  2008年   479篇
  2007年   425篇
  2006年   384篇
  2005年   295篇
  2004年   243篇
  2003年   160篇
  2002年   178篇
  2001年   142篇
  2000年   106篇
  1999年   61篇
  1998年   24篇
  1997年   11篇
  1996年   3篇
  1995年   8篇
  1994年   6篇
  1993年   7篇
  1992年   8篇
  1991年   2篇
  1990年   8篇
  1989年   2篇
  1988年   1篇
  1987年   2篇
  1986年   2篇
  1985年   1篇
  1984年   3篇
  1983年   2篇
  1981年   1篇
  1980年   1篇
  1979年   5篇
  1975年   1篇
  1964年   1篇
  1957年   3篇
  1954年   2篇
排序方式: 共有8203条查询结果,搜索用时 375 毫秒
351.
Results of a series of experiments(on the adsorption of divalent metal ions by dried alginic acid, Na and Ca alginates of different composition and block structure) conducted in this systematic study of the effects of the composition and structure of alginates on the static adsorption equilibrium of divalent metal ions indicate that the properties of alginate adsorption to divalent metal ions are highly different, depending not only on the cations used, but also on the form and structure of the alginates. There is close correlation between the adsorption properties and the structure of the alginates. The selectivity coefficient of Na alginate for Cd-Sr ion exchange tends to increase with the increase of the M/G ratio in alginate, whereas the adsorption capacity of Ca alginate for Cu~(2+) ion decrease with the increase of the G-block or the average length of the G-block ((?)_G) and the total adsorption capacity of alginic acid is found to vary in the same order as the F_(MM)(diad frequency) in alginate in  相似文献   
352.
Water samples were collected in 120 stations in the Bohai Sea of China to analyze the distribution of dissolved nutrients and assess the degree of eutrophication in August 2002. The result shows that the average concentration of DIN increased and the PO4-P concentration sharply decreased compared to the previous data of corresponding period. The high concentrations of DIN and PO4-P occurred in coastal waters, especially in the bays and some river estuaries, while the high concentrations of SiO3-Si in the surface and middle depth occurred in the central area of the Bohai Sea. The average ratio of DIN/ PO4-P was much higher than the Redfield Ratio (16:1). Apparently, PO4-P was one of the limiting nutrient for phytoplankton growing in the sea. The average concentrations of DON and DOP were higher than their inorganic forms. The results of eutrophication assessment show that 22.1% of all stations were classified as violating the concentration levels of the National Seawater Quality Standard (GB 3097-1997) for DIN and only 3.9% for PO4-P. The average eutrophication index in the overall area was 0.21±0.22 and the high values occurred in Bohai Bay, Liaodong Bay and near the Yellow River estuary. This means that the state of eutrophication was generally mesotrophic in the Bohai Sea, but relatively worse in the bays, especially some river estuaries.  相似文献   
353.
依据野外露头、岩芯、测井及相关测试资料,对鄂尔多斯盆地苏里格南部地区上古生界中二叠统石盒子组盒8沉积期的沉积类型、沉积微相特征及沉积相对天然气成藏富集的影响进行了分析。结果表明:鄂尔多斯盆地苏里格南部地区盒8沉积期自北向南依次发育缓坡浅水辫状河三角洲和滨浅湖沉积相,亚相主要为三角洲平原、三角洲前缘、滨湖和浅湖,微相包括分流河道、分流间洼地、水下分流河道、水下天然堤、分流间湾、滨湖砂坝、滨湖泥湾和浅湖泥湾;各微相在空间上相互叠置,复合加积;各微相水动力条件的差异造成微相在沉积物成分和组构上各异,导致其成岩作用不同;分流河道和水下分流河道微相发育处密集形成相互叠置的大面积骨架砂体,构成区内主砂带,成为天然气优质储集层,是有利勘探相带;沉积相明显控制优质储集层的分布,进而影响着天然气的富集成藏。总之,水下分流河道和分流河道微相是砂岩优质储集层分布和发育的最有利相带,也是今后勘探开发的方向。  相似文献   
354.
脂肪水平对厚唇弱棘鯻生长及肠道脂肪酶活性的影响   总被引:1,自引:0,他引:1  
以鱼油为脂肪源,配制成脂肪水平(质量分数,下同)分别为3.1%、5.8%、9.4%、11.9%和15.1%的5种等氮饲料,探讨饲料脂肪水平对初始体重为(4.05±0.11)g的厚唇弱棘鯻(Hephaestus fuliginosus)生长及肠道脂肪酶活性的影响。结果表明,随着饲料脂肪含量提高,实验用鱼脏体比、肥满度及肌肉粗蛋白含量也随着提高,但各组间差异不显著(P>0.05)。饲料脂肪水平从3.1%增加到9.4%,厚唇弱棘鯻的饲料效率呈升高趋势,相对增重率值由76.42%±5.88%升至102.09%±5.31%;而当饲料脂肪水平从9.4%提高至15.1%,饲料效率呈降低趋势(P<0.05),相对增重率值降为70.54%±2.06%。随着饲料脂肪水平提高,厚唇弱棘鯻肝体比、肌肉粗脂肪含量、肠道脂肪酶活性均呈上升趋势,且各组间差异显著(P<0.05)。表明本实验条件下饲料脂肪含量超过9.4%可导致厚唇弱棘鯻生长和饲料利用下降,对其形体特征、肌肉粗脂肪含量以及肠道脂肪酶活性均有显著影响。  相似文献   
355.
Three ship-based observational campaigns were conducted to survey sea ice and snow in Prydz Bay and the surrounding waters(64.40°S–69.40°S, 76.11°E–81.29°E) from 28 November 2012 to 3 February 2013. In this paper, we present the sea ice extent and its variation, and the ice and snow thickness distributions and their variations with time in the observed zone. In the pack ice zone, the southern edge of the pack ice changed little, whereas the northern edge retreated significantly during the two earlier observation periods. Compared with the pack ice, the fast ice exhibited a significantly slower variation in extent with its northernmost edge retreating southwards by 6.7 km at a rate of 0.37 km?d-1. Generally, ice showed an increment in thickness with increasing latitude from the end of November to the middle of December. Ice and snow thickness followed an approximate normal distribution during the two earlier observations(79.7±28.9 cm, 79.1±19.1 cm for ice thickness, and 11.6±6.1 cm, 9.6±3.4 cm for snow thickness, respectively), and the distribution tended to be more concentrated in mid-December than in late November. The expected value of ice thickness decreased by 0.6 cm, whereas that of snow thickness decreased by 2 cm from 28 November to 18 December 2012. Ice thickness distribution showed no obvious regularity between 31 January and 3 February, 2013.  相似文献   
356.
瓮安生物群与宽川铺生物群中球形类化石研究   总被引:2,自引:0,他引:2  
通过对获自瓮安生物群与宽川铺生物群中大量球形类化石的系统研究,建立了便于交流和深入讨论的球形类化石形态分类方案,对各种类型的球形类化石进行了属性探讨,并按形态属描述球形类化石4个新类型(相当于形态属),8个新种类(相当于形态种)。研究表明球形类化石是一个多源的非正式生物类群,包括动物性和植物性球形类化石。具有丰富的研究内涵,对揭示早期生物的起源、演化、多样性及胚胎发育具有深远意义。  相似文献   
357.
为了防止商业综合体建设过剩,形成商业综合体之间的恶性竞争。本文基于微信易出行插件里面的城市热力图数据,以杭州市5个商业综合体为例,获取可以量化的热力值数据,对不同类型的商业综合体活力影响因素进行对比分析研究。研究发现:① 城市商业综合体布局密度、周边用地用途、销售模式、内部功能布局以及内部空间活力5个要素对商业综合体活力的影响最明显;② 商业综合体的高密度布局对其活力存在抑制作用,通过对比各个服务半径内的商业综合体的活力变化,发现商业综合体合理的服务半径应该大于1.0 km。商业综合体周边的居住用地、广场和公园用地对其活力具有增强作用,办公用地对商业综合体活力的影响较弱;③ 针对不同消费对象制定的不同销售模式,也会对商业综合体活力变化产生显著影响;④ 在公共活动空间相对缺乏的区域内,以零售为主且兼具城市公共活动空间的商业综合体比以主力店为主的商业综合体的活力更高,空间利用率越高的商业综合体活力也越高,商业综合体承担部分城市公共活动空间的职能对商业综合体的活力起到了提升作用。  相似文献   
358.
The calibration of sedimentary rock absolute dates is one of the difficulties in sedimentological and stratigraphic research. Since strontium(Sr) resides in seawater much longer(≈106 a) than the seawater intermixing time(≈103 a), the Sr isotopic composition of global seawater is uniform at any time and results in a stable system throughout geological history,based on which a global Sr isotope composition dating database has been established for age-calibration of marine strata.The Permian stratigraphic sections in the northern part of the Upper Yangtze block, southern China, record continuous marine sediments with clear stratigraphic boundaries and is suitable for stratigraphic dating of Sr isotopes. Based on sampling and Sr isotopic compositions of Permian carbonate strata in the northern part of the Upper Yangtze, a Permian Sr isotope evolution curve was established. According to the basic principles of Sr isotope stratigraphy, the global Strontium isotope age database can be used to calibrate the Permian stratigraphic dates in the northern Upper Yangtze. The results show that the Sr isotope evolution curves for the marine carbonate rocks in the Permian stratigraphic section of the Upper Yangtze present a decreasing trend from the mid-Qixia stage(P2) to the mid-Wujiaping stage(P3), and then rise from the middle Wujiaping stage to the end of Changxing stage(P3). When the Permian Sr-isotope evolution curve is compared with the global Sr isotope evolution curve in the northern Upper Yangtze, the two are consistent in their long-term evolutionary trend, indicating that Permian global geological events are important controlling factors for the composition and evolution of Sr isotopes. The 87 Sr/86 Sr value decreased gradually in the background of large-scale regressions at the turn of middle to late Permian period, revealing that the Emeishan basalt eruption occurred near the Maokou/Wujiaping boundary(GLB). Srisotope stratigraphy dating was performed on the boundaries of the Qixia Formation/Maokou Formation, Maokou Formation/Wujiaping Formation(GLB), Wujiaping Formation/Changxing Formation(WCB) and the Permian/Triassic(PTB) using the Global Strontium Isotope Age Database. The results are 270.4 Ma, 261.2 Ma, 254.5 Ma and 249.7 Ma,respectively. Based on this, the eruption age of the Emeishan basalts is defined at about 261.2 Ma., which is more coincident with that acquired from other previous dating methods on the eruption age of the Emeishan basalts, and therefore proves that the application of Sr isotopic stratigraphy to dating marine sedimentary units is an effective method.  相似文献   
359.
SⅡ型海洋重力仪凭借其动态测量精度高、系统可靠性强等优点已日益成为海洋调查中重力测量的重要仪器之一。从SⅡ型海洋重力仪在使用中出现的稳定平台倒台故障着手,分析了平台倒台故障的原因以及处理方法,并对仪器使用中的注意事项以及改进建议作了介绍。  相似文献   
360.
SPOT5与ERS-2卫星影像的联合立体定位   总被引:3,自引:3,他引:0  
本文将SPOT5与ERS-2卫星影像联立构成一组立体像对,并通过立体定位的方式获取了地面点的三维坐标。根据本文立体像对前方交会的特殊几何图形,推导了立体定位理论精度的计算公式,并利用实验验证了公式的正确性,同时分析了影响立体定位精度的主要因素。实验结果表明,联合具有一定重叠度的SPOT5和ERS-2影像进行立体定位具有很强的可行性,其精度已可以满足部分测绘任务的要求。  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