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   4069篇
  免费   312篇
  国内免费   936篇
测绘学   212篇
大气科学   394篇
地球物理   1707篇
地质学   1814篇
海洋学   543篇
天文学   179篇
综合类   136篇
自然地理   332篇
  2024年   20篇
  2023年   55篇
  2022年   40篇
  2021年   57篇
  2020年   54篇
  2019年   40篇
  2018年   64篇
  2017年   19篇
  2016年   56篇
  2015年   40篇
  2014年   96篇
  2013年   77篇
  2012年   149篇
  2011年   241篇
  2010年   79篇
  2009年   113篇
  2008年   229篇
  2007年   223篇
  2006年   240篇
  2005年   246篇
  2004年   208篇
  2003年   170篇
  2002年   166篇
  2001年   134篇
  2000年   194篇
  1999年   99篇
  1998年   100篇
  1997年   99篇
  1996年   113篇
  1995年   111篇
  1994年   133篇
  1993年   176篇
  1992年   126篇
  1991年   191篇
  1990年   131篇
  1989年   149篇
  1988年   124篇
  1987年   116篇
  1986年   62篇
  1985年   99篇
  1984年   70篇
  1983年   71篇
  1982年   42篇
  1981年   37篇
  1980年   59篇
  1979年   14篇
  1964年   15篇
  1959年   16篇
  1958年   22篇
  1957年   15篇
排序方式: 共有5317条查询结果,搜索用时 15 毫秒
51.
Through theoretical analysis and computer simulation,this short communication comments on theresidual bilinearization(RBL)method and compares it with non-bilinear rank annihilation(NBRA)forthe treatment of second-order calibration with non-bilinear data.It is found that these two methods aremathematically equivalent but have different noise propagation properties.The second-order advantage,namely quantitation in the presence of unknown interferences,can be carried over to non-bilinear dataonly if there exists a net analyte rank(NAR)for the analyte of interest.  相似文献   
52.
Multivariate outliers in environmental data sets are often caused by atypical measurement error in a singlevariable.From a quality assurance perspective it is important to identify these variables efficiently so thatcorrective actions may be performed.We demonstrate a procedure for using two multivariate tests toidentify which variable‘caused’each outlier.The procedure is tested with simulated data sets that havethe same correlation structure as selected water chemistry variables from a survey of lakes in the WesternUnited States.The success rates are evaluated for three of the variables for sample sizes of 50 and 100,significance levels of 0.01 and 0.05 and various amounts of mean shift.The procedure works best forhighly correlated variables.  相似文献   
53.
This paper investigates the problems in integrating different data sets for social science research. A cross-national analysis of the effects of family migration on labor market participation rates for female partners was used as an example to concretize these problems. The data used in the cross national analysis came from the 1991 British Census Sample of Anonymised Records and the 1990 United States Public Use Microdata Sample. The problems addressed were the following: data collection; manipulation and reliability; question type and definition; and measurement of individual and family variables. As part of the analysis, some empirical findings were also presented. Significant overall differences between the US and Great Britain (GB) samples were observed, particularly concerning graduate qualifications, marital and employment status, and ethnic group. There were higher numbers of married partners and full-time workers in the US sample, while female partners undertaking part-time employment were greater in GB. Furthermore, the US sample had more couples with a female graduate as compared with the GB sample who had a higher proportion of nongraduate couples. Finally, this paper recognized the importance of cross- national research in the evaluation of national ideological and institutional structures.  相似文献   
54.
原型盆地恢复有助于盆地构造—岩相古地理的研究和揭示盆山耦合,对油气勘探与开发具有重要的指导意义。本文利用最新的钻井、测井及露头资料,以沉积相为研究实体,运用盆—山结合的思路,由点—线—面进行分析,重建了鄂尔多斯盆地晚石炭世本溪组沉积时期的构造—沉积环境,研究了原型盆地特征及性质。本溪组沉积期,鄂尔多斯盆地沉积具有东西分异的特征:淳化—庆阳—吴起地区及北部伊盟地区为暴露剥蚀区,东部为克拉通内坳陷盆地,西部为克拉通边缘裂陷盆地。乌达—呼鲁斯台及鄂尔多斯—神木地区发育三角洲相,并向南逐渐推进;鄂西石嘴山、环县及鄂东府谷、延安地区以环带状潮坪—泻湖相为主,受间歇性海侵影响;吴忠、柳林—吉县地区发育低能的泻湖相;中卫—中宁、韩城及以东地区发育不同深度的陆棚及开阔台地。该时期盆地构造环境表现为南北向双向俯冲、贺兰拗拉槽再活化,格局经历了南隆北倾向北隆南倾的转变。受此影响,鄂尔多斯盆地构造古地理格局开始由东西分异逐渐向南北分异转变,受东西两侧海侵影响沉积相带呈环带状展布,但在盆地北部发育潮控三角洲,表现出近缘特征。晚石炭世鄂尔多斯盆地构造—沉积环境与原型盆地特征与其对周缘大地构造运动的响应。  相似文献   
55.
Introduction The eastern margin of the Tibetan Plateau, adjacent to the Sichuan Basin (Figure 1), has become a testing ground for a variety of models that contrast mechanisms of extrusion and crustal thickening associated with the India-Asia collision (Avouac and Tapponnier 1993, England and Houseman 1986),but that also address the extent to which the upper crust and upper mantle are coupled (Royden et al. 1997, Holt 2000). The margin is characterized by topographic relief of over 5 km an…  相似文献   
56.
Fluorescence lifetime provides a third independent dimension of information for the resolution of totalluminescence spectra of multicomponent mixtures.The incorporation of this parameter into theexcitation-emission matrix(EEM)by the phase modulation technique results in a three-dimensionalexcitation-emission-frequency array (EEFA).Multicomponent analysis based on the three-dimensionalEEFA brings a qualitative change for the resolved spectra,i.e.individual spectra can be uniquely resolved,which is impossible with any two-dimensional analysis.In this paper we present a method for analyzingthe EEFA.We show mathematically that with the three-dimensional analysis of the EEFA individualspectra and lifetimes can be obtained.Our algorithm is developed in mathematical detail and isdemonstrated by its application to a two-component mixture.  相似文献   
57.
58.
The Ni-Cu-platinum group element sulfide ore deposits of the Kharaelakh Intrusion,Noril′sk Region,Siberia,represent a large concentration of sulfides associated with a small differentiated intrusion formed at the edge of the Siberian Craton in the roots of the Siberian Trap flood basalt.The deposit is associated with an intrusion that occupies a flanking periclinal structure adjacent to the Noril′sk-Kharaelakh Fault.The intrusion is strongly differentiated and comprises taxitic gabbrodolerites,picritic gabbrodolerites,and gabbrodolerites within the main body which in turn forms a chonolith within a sheet-like intrusion that extends laterally to form extensive undifferentiated sills of gabbrodolerite.The intrusion substantially replaces the stratigraphy of the country rocks,and although it appears to have exploited the axis of structures developed in response to transtension,the intrusion has created space by both mechanical dilation of stratigraphy and magmatic replacement of pre-existing sedimentary rocks.The frontal lobes of the main intrusion have complex apophyses of gabrodolerite on a range of scales that demonstrate replacement of the sedimentary rocks and link to the development of an extensive metamorphic halo in the country rocks.This halo is much narrower over the main body of the intrusion,and these observations have implications for the thermal history of the intrusion.Mg-skarns and breccias are developed in the roof of the main body of the intrusion.Within the intrusion,the taxitic rocks contain vesicles and the blebby sulfides developed in the picritic and taxitic gabbrodolerites appear to have a linkage to volatile phases.Cuprous sulfide mineralization developed at the roof of the Kharaelakh Intrusion is associated with metamorphosed and skarn-bearing country rocks,and appears to have been generated by a combination of sulfide fractionation and associated metasomatism.The geological relationships appear consistent with a chonolith model for the development of the differentiated intrusion and mineralization,but the extent of metasmorphism of the country rocks appears to be related to the unusual thickness of gabbrodolerite apophyses at the flanks of the intrusion rather than metamorphism produced by the passage of mafic magma through the intrusion.Variations in disseminated sulfide compositions and metasomatic textures in the skarns are described,and a model is proposed which balances traditional views on the evolution of the magma conduits with the impact of magmatic fluids transported through the magma column(i.e.transmagmatic fluids).The importance of structures in controlling the nature of the conduit,and the resultant small intrusions with excess sulfide is a feature of many other Ni-Cu sulfide deposits including Voisey′s Bay,and it is suggested that the sulfides are more likely to have beentransported from depth into their final resting place rather than developed by in-situ equilibration of sulfide with fresh magma in the chonolith.  相似文献   
59.
Air entrapment is an important consideration in environments with shallow water tables and sandy soil, like the condition of highly conductive sandy soils and flat topography in Florida, USA. It causes water table rises in soils, which are significantly faster and higher than those in soils without air entrapment. Two numerical models, Integrated Hydrologic Model (IHM) and HYDRUS-1D (a single-phase, one-dimensional Richards′ equation model) were tested at an area of west central Florida to help further understanding the shallow water table behavior during a long term air entrapment. This investigation employed field data with two modeling approaches to quantify the variation of air pressurization values. It was found that the air pressurization effect was responsible at time up to 40 cm of water table rise being recorded by the observation well for these two models. The values of air pressurization calculated from IHM and HYDRUS-1D match the previously published values. Results also indicated that the two numerical models did not consider air entrapment effect (as the predictive parameters remain uncertain) and thus results of depth to water table from these models did not compare to the observations for these selected periods. Incorporating air entrapment in prediction models is critical to reproduce shallow water table observations.  相似文献   
60.
The "mean value concept" in a mono-linear regression of multi-variables is clarified.Its applications to reconstruction of the past 90-year salinity of the sea surface water in Xisha Islands and to tracing the past 80 ka paleo-geomagnetic events from the Luochuan loess 10Be record are introduced in detail,which show the significance and the potential of the "mean value concept" in geoscience research.  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