全文获取类型
收费全文 | 30203篇 |
免费 | 17777篇 |
国内免费 | 42334篇 |
专业分类
测绘学 | 2467篇 |
大气科学 | 20870篇 |
地球物理 | 11718篇 |
地质学 | 32980篇 |
海洋学 | 15647篇 |
天文学 | 759篇 |
综合类 | 4031篇 |
自然地理 | 1842篇 |
出版年
2024年 | 119篇 |
2023年 | 205篇 |
2022年 | 280篇 |
2021年 | 524篇 |
2020年 | 1767篇 |
2019年 | 5213篇 |
2018年 | 5435篇 |
2017年 | 5163篇 |
2016年 | 5197篇 |
2015年 | 4575篇 |
2014年 | 4012篇 |
2013年 | 4564篇 |
2012年 | 4304篇 |
2011年 | 4273篇 |
2010年 | 4164篇 |
2009年 | 3518篇 |
2008年 | 2690篇 |
2007年 | 2714篇 |
2006年 | 2411篇 |
2005年 | 2336篇 |
2004年 | 2769篇 |
2003年 | 2446篇 |
2002年 | 2166篇 |
2001年 | 1944篇 |
2000年 | 1568篇 |
1999年 | 1563篇 |
1998年 | 1663篇 |
1997年 | 1744篇 |
1996年 | 1339篇 |
1995年 | 1301篇 |
1994年 | 1170篇 |
1993年 | 1167篇 |
1992年 | 996篇 |
1991年 | 742篇 |
1990年 | 697篇 |
1989年 | 592篇 |
1988年 | 490篇 |
1987年 | 387篇 |
1986年 | 310篇 |
1985年 | 242篇 |
1984年 | 274篇 |
1983年 | 160篇 |
1982年 | 201篇 |
1981年 | 141篇 |
1980年 | 109篇 |
1979年 | 110篇 |
1978年 | 49篇 |
1977年 | 44篇 |
1971年 | 53篇 |
1970年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
Axial surveys were performed in the two river tributaries of the Cochin estuary, SW India during November 1988. Surficial sediments were subjected to sequential chemical extractions to delineate five metal fractions, namely, exchangeable, carbonate bound, easily reducible, organic/sulfide bound, and residual. The results indicated selective accumulation of Mn and Ni in carbonate bound and organic/sulfide forms, along with marginal amounts of Co in the exchangeable fraction. Large portions of Fe and Cr occurred in the residual fraction, whereas composite fractionation of Zn species was noticed. The exchangeable fractions of Fe and Cr as well as of easily reducible cobalt were below detection limits. The levels of Cr and Zn indicate anthropogenic inputs in this estuary, whereas Co and Ni show regional contamination exceeding natural levels. The analytical speciation procedure helps to deduce the sedimental diagenetic processes in the estuarine environment. 相似文献
63.
DEKORP Research Group 《Geophysical Journal International》1987,89(1):353-360
Summary. In order to investigate the target area of the Continental Deep Drilling (KTB) in the Oberpfalz a network of six seismic reflection lines was acquired in 1985 using the Vibroseis technique. The average length of these lines was 50 km. In addition, the 185 km long NW/SE striking line DEKORP 4 with its short appendix line 4-Q of 40 km length was acquired with the same technique. The results reveal a strongly structured upper crust. This is in contrast with previous surveys in the German Variscides which show a poorly reflective upper crust and a strongly reflective lower crust. Except for the S part of DEKORP 4 in the Oberpfalz area the Mono is only weakly reflective. In addition to the Vibroseis survey 96 shots along line DEKORP 4 were recorded by conventional reflection techniques and by portable reflection and refraction stations from university institutes and geological surveys in order to obtain wide-angle reflection and expanding spread data. 相似文献
64.
65.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献
66.
67.
68.
The purpose of this work was to reinvestigate the existing hydrogeological conceptual model of the basin of Madrid, Spain. A cumulative chemical isotopic diagram which enabled the distinction between different groups of water as well as calculation of the mode of their blending was used for this investigation. It was found that the groups of discharge were lighter in their isotopic composition than that of recharge. The previous explanation of this fact, backed by carbon-14 dating, was the long residence time due to flow lines going down to depths of more than 1000 m. This flow model assumes homogenous conditions to these depths. This assumption can not be supported by evidence from deep wells. Thus a modified model is suggested which maintains homogenous conditions only to about 300 m and a deep confined aquifer below containing paleowater. The higher degree of depletion of this water has been explained by a colder climate on top of an altitude effect. Another interesting observation was the correlation between the isotopic composition of the rains, the month of the rain event and the composition of the recharge group groundwater. It could be seen that the winter rains resemble the groundwater composition, which shows that practically all the spring and summer rains were evapotranspirated. 相似文献
69.
介绍一种自行设计和加工的直接雾化石墨炉进样装置的结构、工作原理和操作方法。该装置由微机控制与石墨炉加热程序同步工作,可以实现自动控制和自动测量。对雾化进样石墨炉分析的仪器参数和实验条件进行了研究和选择。结果表明,该装置与石墨炉结合具有自动化程度高、重现性好、样品利用率高和分析速度快等特点。 相似文献
70.