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   76720篇
  免费   26590篇
  国内免费   52033篇
测绘学   11526篇
大气科学   25237篇
地球物理   19461篇
地质学   58611篇
海洋学   22032篇
天文学   1363篇
综合类   7697篇
自然地理   9416篇
  2024年   556篇
  2023年   1283篇
  2022年   3338篇
  2021年   4297篇
  2020年   4311篇
  2019年   7653篇
  2018年   7205篇
  2017年   6803篇
  2016年   6854篇
  2015年   7056篇
  2014年   6591篇
  2013年   7773篇
  2012年   8287篇
  2011年   8168篇
  2010年   8173篇
  2009年   7342篇
  2008年   6636篇
  2007年   6469篇
  2006年   5991篇
  2005年   5128篇
  2004年   4686篇
  2003年   3757篇
  2002年   3578篇
  2001年   3321篇
  2000年   2728篇
  1999年   2066篇
  1998年   1860篇
  1997年   1878篇
  1996年   1440篇
  1995年   1380篇
  1994年   1237篇
  1993年   1208篇
  1992年   1043篇
  1991年   755篇
  1990年   720篇
  1989年   593篇
  1988年   513篇
  1987年   395篇
  1986年   335篇
  1985年   251篇
  1984年   290篇
  1983年   169篇
  1982年   213篇
  1981年   147篇
  1980年   110篇
  1979年   138篇
  1978年   53篇
  1977年   46篇
  1971年   53篇
  1970年   44篇
排序方式: 共有10000条查询结果,搜索用时 890 毫秒
921.
林峰  许清辉 《台湾海峡》1990,9(3):251-255
利用潮输沙量的计算方法,估算了闽江口入海口内3个断面所包围区域溶解态镉、铅和铜的收支平衡,从而研究了这些重金属的河口行为。  相似文献   
922.
-The tectonic types of the Zhujiang (Pearl) River Mouth Basin in the South China Sea are epicontinental rift-depression basins. Prior to Early Cretaceous time, the Dongsha Uplift arid its surrounding depressions had been combined with the Eurasia Plate in a single unit. Many ENE-trending narrow rifted basins were formed in the third episode of Yanshan orogeny (Late Laramide). The rifted basins in the Paleocene and Eocene were stretched and extended, forming Zhu 1, Zhu 2 and Zhu 3 depressions. The Dongsha Uplift is located between Zhu 1 and Zhu 2 depressions . covering an area of 28 000 Km2. Its geologic evolution can be divided into four stages:(1) Late Cretaceous - Paleocene block-faulting stage.(2) Eocene -Oligocene uplifting and eroding stage.(3) Late Oligocene - Early Miocene sustained subsiding stage.(4) Middle Miocene -Recent noncompensated subsiding stage.The Dongsha Uplift is a structural zone favourable for oil-gas accumulation.  相似文献   
923.
TECTONIC FRAMEWORKS OF THE BOHAI GULF AND COASTAL, BASINS   总被引:2,自引:0,他引:2  
The Bobai Gulf and its coastal basin is an important oil and gas province of China that covers an area of 200,000 sq. km.It is a polycylic superimposed basin. Its history of development can be subdivided into four stages: (i) Late Proterozoic parageosydine stage (Sinian); (ii) Early palaeozoic cratonic stage (Lower Cambrian-Middle Ordovician): (iii) Late palaeozoic intracratonic coal bearing sequences stage (Middle Carboniferous-Permian); (iv) Mesozoic and Cenozoic taphrogenic rifting and subsidence stages.During the Paieogene rifting stage, three NNE trough systems and three nearly E-W trough systems were initialed as the results of regional extensional stresses. Central rift valleys, low angle gravity faults and a series of asymmetrical half-grabens were developed surrounding the "Bohai Mantle Plume ". Th- riftitip troughs were egtended ard widened gradually. During the Neocene, a large saucer-shaped sedimentary basins was developed into a thick blanket covering the whole areas of either the Paleogene  相似文献   
924.
在风的直接作用下产生的风浪,由于复杂的生成和发展条件,使其具有外观上的不规则性和随机性,内在的非线性性.早在五十年代初期,人们就针对风浪的随机性,运用统计方法,对风浪的统计性质进行了分析研究.就其外观而言,这种统计性质,主要由波面纵座标、波要素等的概率分布来描述,在把风浪作为一线性流体动力学模式来描述时,获得波面纵座标遵循正态分布律.  相似文献   
925.
秦曾灏 《海洋学报》1980,2(3):24-37
近十余年来,海洋和大气相互作用这一课题日益受到人们的关注。从海-气边界层结构以及通过海面的物理量和化学量输送机制的研究到大尺度海洋和大气相互作用的研究都取得了可喜的成果。此外,还从海洋和大气相互作用的观点探索了风暴潮预报的新的可能途径[2]。然而,由于问题的复杂性,通过海面以及海-气边界层的物理量的小尺度输送机制迄未得到澄清,企图从本质上改善大尺度海洋和大气相互作用的理论是不现实的。  相似文献   
926.
In this paper, a comparison viral made between the two results of determining Pb, Cd, Cu and Zn in seawater by direct current (tripping voltammetry (DCS) with rotating glass carbon electrode and by differential pulse stripping voltammetry (DPS) with hanging mercury drop electrode (HMDE). By DCS, the range of linear calibration curves obtained was 4×10-3 M-2×10-7M for Cu, Zn and Cd, and 4× 10-9M-2×10-3M for Pb. By using DPS, the range of linear calibration curve was as follows: Cu 0.65-1.9 ppb; Pb 1.0-10 ppb; Zn 0.65-2.0 ppb; Cd 0.02-0.14 ppb. It was found that DCS could be used for determining Pb, Cu, Zn in coastal waters, but it is necessary to add gallium ion to it to eliminate the interference of Cu-Zn inter-metallic compound for determining Zn. The DPS is better for determinig Cd.  相似文献   
927.
The present study is based on the materials collected from the middle waters of the South China Sea between 12°-15°N and 111°-118°E during May to July in 1978.The biomass of the phytoplankton (Y, dependent variable) is established to give the biomass of each Species and the environmental factors (i. e. water temperature, salinity, oxygen, silicate and phosphate contents), X1,X2...,X139tt are independent variables that are used to stepwisc regression analysis. Accordicg to the mathematical tests and the practical experience by which a lot of species are deleted and added to the equation, the authors consider that the 4th equation as shown below is optimal one for the model of the biomass on the phytoplankton from this waters.The optimal equation refers to text.  相似文献   
928.
南麂列岛海洋自然保护区潮间带贝类资源时空分布   总被引:9,自引:0,他引:9       下载免费PDF全文
为了解南麂列岛海洋自然保护区潮间带贝类十多年来的动态变化,在1992年研究基础上,于2003年度再赴南麂列岛对岩、沙相贝类进行了研究.本次调查鉴定出贝类105种,其中岩相93种、沙相12种,6种为南麂列岛新记录种,对岩相、沙相贝类组合的生态特征进行了分析,岩相多样性和均匀度指数(1.25~2.20和0.54~0.70)高于沙相的(0.23~0.95和0.17~0.94).岩相贝类数量为3025.70g/m2和2660个/m2,生物量垂直分布为低潮区最高(6471.99g/m2),中潮区居中(3772.09g/m2),高潮区最小(78.35g/m2),栖息密度为中潮区最大(4245个/m2),低潮区居中(2973个/m2),高潮区最小(439个/m2).沙相贝类数量为16.81g/m2和11个/m2,低潮区(29.43g/m2和11个/m2)大于中潮区(14.01g/m2和10个/m2).通过与以往资料比对发现,岩礁贝类数量有所增加,沙滩贝类数量则明显下降,监控区贝类数量明显多于其他区域,对贝类资源变化产生影响的相关因子作了分析探讨.  相似文献   
929.
Sequence variation of the first internal transcribed spacer of ribosomal DNA ( ITS - 1 ) was examined and its application to the study of genetic variation was explored in four populations of farter' s scallop Chlamys farreri. ITS - 1 fragments, with a length of about 300 bp,of 78 individuals collected from Dalian, Qingdao, Yantai in China and Korea respectively were amplified via PCR, cloned and sequenced. Intra-genomic variation was examined by sequencing several clones of single individuals. Alignment and polymorphism analysis detected 44 haplotypes and 50 polymorphic sites which consist of 30 substitutions and 20 indels, indicating a high level of polymorphisms. Sequence analysis also showed a very low level of intra-individual variation. All these features validated the feasibility of application of ITS - 1 fragment to population analysis. Polymorphism analysis showed that the Korea sample has the richest genetic variation, followed by Yantai and Qingdao samples. AMOVA (analysis of molecular variance) showed that the majority (96.26%) of genetic variation was distributed within populations and 3.74% resulted from among populations, but with P 〈 0.05 ( = 0.042), indicating that the populations in this study have significant divergence. This output was basically concordant with the result arising from RAPD data and different from that from mitochondrial 16S rDNA sequence data. Discussion on this inconsistency was made accordingly.  相似文献   
930.
渤海湾天津近岸海域水环境空间变异分析   总被引:6,自引:0,他引:6       下载免费PDF全文
朱琳  聂红涛  陶建华 《海洋学报》2007,29(6):134-140
由于海岸带的地理条件和海洋动力特性的复杂性以及人类开发活动对海岸带的影响,近岸海域的生态环境系统在空间分布上有很大的变异性.基于现场实测资料,建立系统的空间变异分析模型,对海岸带生态环境空间变化规律进行分析,可以充分了解不同区域的污染特征和变化趋势.应用所构建的空间变异分析模型对渤海湾天津近岸海域的水环境特性进行分析,结合对应年份的入海流量及其变化,分析了天津渤海湾海岸带污染的主因子和区域水环境的分布特征,结果可以对海洋功能区的划分和水环境综合评价结果进行验证和补充.  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