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   10603篇
  免费   2700篇
  国内免费   1897篇
测绘学   594篇
大气科学   2380篇
地球物理   2748篇
地质学   5300篇
海洋学   1348篇
天文学   693篇
综合类   1022篇
自然地理   1115篇
  2025年   6篇
  2024年   183篇
  2023年   246篇
  2022年   435篇
  2021年   489篇
  2020年   460篇
  2019年   474篇
  2018年   475篇
  2017年   480篇
  2016年   598篇
  2015年   460篇
  2014年   604篇
  2013年   553篇
  2012年   528篇
  2011年   532篇
  2010年   633篇
  2009年   533篇
  2008年   527篇
  2007年   490篇
  2006年   374篇
  2005年   378篇
  2004年   290篇
  2003年   302篇
  2002年   277篇
  2001年   334篇
  2000年   341篇
  1999年   537篇
  1998年   479篇
  1997年   432篇
  1996年   445篇
  1995年   378篇
  1994年   317篇
  1993年   311篇
  1992年   258篇
  1991年   201篇
  1990年   163篇
  1989年   130篇
  1988年   132篇
  1987年   69篇
  1986年   63篇
  1985年   50篇
  1984年   34篇
  1983年   27篇
  1982年   37篇
  1981年   32篇
  1980年   22篇
  1979年   20篇
  1978年   7篇
  1976年   9篇
  1958年   18篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
通过耦合布设海底表面和孔内水合物观测系统获取物理、化学、微生物等数据是了解掌握海洋水合物环境动态变化、碳循环规律和资源开发技术的有效手段,据此初步提出了一套海底孔内水合物观测系统,并指出了系统的关键技术难点。而在钻井布设该孔内观测系统时,要注意防止井内安全事故的发生,充分评估安放位置处含水合物地层的地质力学稳定性并采取适当的钻井和完井方式稳定或强化含水合物地层以支撑孔内观察仪器,实现孔内长期监测。最后讨论了在我国南海实施孔内水合物观测系统的必要性,并提出了初步想法。  相似文献   
62.
前陆盆地类型及其沉积动力学研究综述   总被引:15,自引:2,他引:15  
从前陆盆地的特点及类型入手,综述了前陆盆地研究的热点内容,最新方法及其沉积动力学进展状况,揭示了前陆背景下盆地充填序列的构成及内部演化特征,指出了盆地的沉积-地层格架与冲断造山带的幕式活动之间的响应关系。  相似文献   
63.
1INTRODUCTIONEnrichment of3He and heavy ions(i.e.,Ne,Mg,Si and Fe),characteristic of impulsive?ares,have beenstudied for more than three decades.It is found that they are generally associated with nonthermal energeticelectron-rich events(Reames et al.1988;Reames1999and references therein;Ho et al.2001;Wang et al.2006)and are related to the peculiar ratio of charge to mass(Mazur et al.1996;Reames1999).Althoughthe abundance of3He ions is not correlated with the abundance of heavy ions,s…  相似文献   
64.
The feasibility of using fluorescence excitation-emission matrix (EEM) along with parallel factor analysis (PARAFAC) and nonnegative least squares (NNLS) method for the differentiation of phytoplankton taxonomic groups was investigated. Forty-one phytoplankton species belonging to 28 genera of five divisions were studied. First, the PARAFAC model was applied to EEMs, and 15 fluorescence components were generated. Second, 15 fluorescence components were found to have a strong discriminating capability based on Bayesian discriminant analysis (BDA). Third, all spectra of the fluorescence component compositions for the 41 phytoplankton species were spectrographically sorted into 61 reference spectra using hierarchical cluster analysis (HCA), and then, the reference spectra were used to establish a database. Finally, the phytoplankton taxonomic groups was differentiated by the reference spectra database using the NNLS method. The five phytoplankton groups were differentiated with the correct discrimination ratios (CDRs) of 100% for single-species samples at the division level. The CDRs for the mixtures were above 91% for the dominant phytoplankton species and above 73% for the subdominant phytoplankton species. Sixteen of the 85 field samples collected from the Changjiang River estuary were analyzed by both HPLC-CHEMTAX and the fluorometric technique developed. The results of both methods reveal that Bacillariophyta was the dominant algal group in these 16 samples and that the subdominant algal groups comprised Dinophyta, Chlorophyta and Cryptophyta. The differentiation results by the fluorometric technique were in good agreement with those from HPLC-CHEMTAX. The results indicate that the fluorometric technique could differentiate algal taxonomic groups accurately at the division level.  相似文献   
65.
台湾海峡西部海域软质海底多毛类的生态   总被引:2,自引:0,他引:2  
吴启泉 《台湾海峡》1993,12(4):324-334
本文根据1984-1985年4航次底栖生物调查资料,首次全面地研究了台湾海峡西部海域多毛类的种类组成,数量分布及其与环境因子的关系。结果表明,该海域多毛类404种,习见种191种,区系属亚热带性质;平均生物量4.33g/m^2,平均栖息密度68个/m^2;有沿岸性和外洋性两种明显不同的生态类型,前者位于沿岸水系常年控制海域,种类多,数量较大,后者位于南部粗粒沉积物分布区,种类较少;多毛类高数量区与  相似文献   
66.
基于MapXtrme2004的WebGIS系统的研究与开发   总被引:5,自引:1,他引:5  
本文通过介绍.NET框架用于Web开发的主要技术ASP.NET和ADO.NET的技术优点以及MapInfo公司针对.NET环境开发的新组件Mapxtreme2004,通过分析Mapxtreme2004的结构组成总结了Mapxtreme2004用于WebGIS开发的主要方法类的用途,并着重研究了在.Net框架下利用Mapxtreme2004进行WebGIS开发的可行性及主要功能特点。  相似文献   
67.
分别构建广州主建成区垂直比例尺为1﹕2 000、1﹕1 000和1﹕500的3个建筑物模型,利用大型边界层风洞,在西北和东南两风向下,基于中性流模拟分析了复杂城市地形下湍流度随高度的变化及其对宏观地形的依赖。结果表明:风廓线指数α与不同高度的湍流度之间的关系密切,利用现有模型,根据4类粗糙度边界层和不同垂直比例尺,可确定相应的湍流度随高度变化模型的主要系数,预测精度高。城市地形下最大湍流度面发育在0~0.2 h之间狭窄的范围内。用湍流度形态指数β来表征湍流度随高度的变化,无论城市屋脊还是平坦地形,随着风程区的延伸,廓线的指数α升高,湍流度形态指数β降低。表明同一高度湍流度值具有由迎风区、丘顶区向背风区增高,沿风程逐渐增大的规律,对地形部位和风程的依赖性强,与来流翻越简单地形时的特征一致。  相似文献   
68.
New Prediction Models for Mean Particle Size in Rock Blast Fragmentation   总被引:1,自引:1,他引:1  
The paper refers the reader to a blast data base developed in a previous study. The data base consists of blast design parameters, explosive parameters, modulus of elasticity and in situ block size. A hierarchical cluster analysis was used to separate the blast data into two different groups of similarity based on the intact rock stiffness. The group memberships were confirmed by the discriminant analysis. A part of this blast data was used to train a single-hidden layer back propagation neural network model to predict mean particle size resulting from blast fragmentation for each of the obtained similarity groups. The mean particle size was considered to be a function of seven independent parameters. An extensive analysis was performed to estimate the optimum value for the number of units for the hidden layer for each of the obtained similarity groups. The blast data that were not used for training were used to validate the trained neural network models. For the same two similarity groups, multivariate regression models were also developed to predict mean particle size. Capability of the developed neural network models as well as multivariate regression models was determined by comparing predictions with measured mean particle size values and predictions based on one of the most applied fragmentation prediction models appearing in the blasting literature. Prediction capability of the trained neural network models as well as multivariate regression models was found to be strong and better than the existing most applied fragmentation prediction model. Diversity of the blasts data used is one of the most important aspects of the developed models.  相似文献   
69.
对虾暴发性流行病及其预防研究   总被引:1,自引:0,他引:1  
1993~1994年我国沿海养殖对虾发生了特大的暴发性流行病,损失惨重。本文报道该病的特点、症状及危害,并在患病的中国对虾、日本对虾、长毛对虾的中肠、肝胰腺的上皮细胞核和细胞质及腹肌纤维间质中发现大量具有囊膜的杆状病毒。通过病毒引起宿主靶组织细胞的病理变化、无包涵体等特性分析,认为该病毒属于有囊膜杆状病毒科C亚群,不同于已知的对虾所有杆状病毒,为对虾的一类新致病病毒。文中还分析了主要致病原因,通过调查和试验提出8条有针对性的预防措施。  相似文献   
70.
Vertical profiles of chl-a and primary productivity in the middle continental shelf area and eddy area of the East China Sea were studied using data from a cruise in the East China Sea in February to March, 1997 and a cruise in July, 1998. The results showed that chl-a vertical distribution closely related to in situ hydrological and nutrient conditions. Chla-a concentration ranged from 0.22 to 0.35 mg/m3 and 0.93–1.09 mg/m3 in the eddy area and in the middle continental shelf area, respectively. In both areas, chl-a concentrations in deep layers were slightly higher than those in shallow layers, but was of the same order of magnitude. In summer, when a thermocline existed in the water column, highest chl-a concentrations appeared at the base of the thermocline layers in both areas. In the eddy area, chl-a concentration maximized at 31.743 mg/m3, and averaged 1.143 mg/m3 below 30 m depth. In the middle continental shelf area, the highest chl-a concentration was 2.120 mg/m3, the average was 1.168 mg/m3. The primary productivity reached 1418.76 mgC/(m2·d) in summer and 1360.69 mgC/(m2·d) in winter. In the eddy area, the primary productivity was 787.50 mgC/(m2·d) in summer and 159.04 mgC/(m2·d) in winter. Vertical carbon sinking rate from the deep layer to the bottom in both areas is also discussed in this paper. Contribution NO. 4183 from the Institute of Oceanology, Chinese Academy of Sciences. Project No. 49636210 supported by NSFC.  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