全文获取类型
收费全文 | 271628篇 |
免费 | 4387篇 |
国内免费 | 3490篇 |
专业分类
测绘学 | 7214篇 |
大气科学 | 19863篇 |
地球物理 | 56770篇 |
地质学 | 95418篇 |
海洋学 | 22708篇 |
天文学 | 58066篇 |
综合类 | 1076篇 |
自然地理 | 18390篇 |
出版年
2021年 | 2287篇 |
2020年 | 2650篇 |
2019年 | 2891篇 |
2018年 | 5007篇 |
2017年 | 4670篇 |
2016年 | 6554篇 |
2015年 | 4293篇 |
2014年 | 6989篇 |
2013年 | 14281篇 |
2012年 | 7110篇 |
2011年 | 9173篇 |
2010年 | 8165篇 |
2009年 | 10706篇 |
2008年 | 9387篇 |
2007年 | 9133篇 |
2006年 | 9811篇 |
2005年 | 8092篇 |
2004年 | 7950篇 |
2003年 | 7456篇 |
2002年 | 6966篇 |
2001年 | 6132篇 |
2000年 | 6037篇 |
1999年 | 5242篇 |
1998年 | 5271篇 |
1997年 | 5068篇 |
1996年 | 4709篇 |
1995年 | 4462篇 |
1994年 | 4136篇 |
1993年 | 3882篇 |
1992年 | 3668篇 |
1991年 | 3615篇 |
1990年 | 3784篇 |
1989年 | 3541篇 |
1988年 | 3316篇 |
1987年 | 3869篇 |
1986年 | 3432篇 |
1985年 | 4249篇 |
1984年 | 4760篇 |
1983年 | 4428篇 |
1982年 | 4329篇 |
1981年 | 3941篇 |
1980年 | 3652篇 |
1979年 | 3515篇 |
1978年 | 3501篇 |
1977年 | 3291篇 |
1976年 | 3062篇 |
1975年 | 2966篇 |
1974年 | 2920篇 |
1973年 | 3085篇 |
1972年 | 2027篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
V. A. Ivanov S. P. Lyubartseva N. Mikhailova N. B. Shapiro 《Physical Oceanography》2001,11(6):509-524
We analyze the results of numerical calculations performed according to the three-dimensional interdisciplinary model of an
ecological system of the Black-Sea shelf zone near the estuary of the Danube. The complete system of equations of hydrothermodynamics
is solved together with transport equations of the advection-diffusion-reaction type used to describe the transformation of
a substance (nitrogen) between the components of the characteristic vectors of the ecosystem: plankton, detritus, and biogenic
elements (nitrates). We describe the distinctive features of the circumcontinental distribution of components obtained as
a result of numerical experiments and present arguments for the conclusion that the ecosystem of the Danube estuary water
area plays the role of a buffer zone between the press of the Danubian biogenic pollutions and the neighbouring areas of the
shelf zone and open sea.
Translated by Peter V. Malyshev and Dmitry V. Malyshev 相似文献
992.
993.
Data collected in three Californian estuaries indicate that hypersaline conditions exist during the dry summers typical of a Mediterranean climate. The generalised seasonal and longitudinal hydrographic structures are described and explained. It is argued that this seasonal hypersalinity is common and that it represents a major class of estuaries. The observed accumulation of salt indicates surprisingly long residence times in small basins which have free exchange with the ocean. This semi-isolation of the inner basin leads to a large build-up or severe depletion of nutrients, pollutants and plankton in these systems. Of concern are the trends to increase pollutant loading in the same systems that are experiencing an increase in residence times owing to freshwater extraction in the watershed. 相似文献
994.
This paper discusses the application of acoustic sounding with the purpose of identifying vortical features in the sea. Analysis
of the results of a multidisciplinary acoustic/hydrological experiment conducted in the Black Sea has revealed a stable recurrent
correlation between the amplitudinal variance of echo signals, on the one hand, and the peculiarities of water temperature
fields, on the other. Given these characteristics, the peripheral and the central areas of a mesoscale eddy essentially differed.
This allows us to apply acoustic sounding to identify mesoscale eddies in the sea.
Translated by Vladimir A. Puchkin. 相似文献
995.
996.
A novel self-contained navigation system has been devised for underwater vehicles operating in and around offshore installations. This system matches data from a sector-scanning sonar device to a computer model of the installation. The paper begins by highlighting the existing approaches to subsea navigation before outlining the main features of the proposed system. It then concentrates on a key component of this system which is a method for calculating the position and heading of an underwater vehicle navigating in the vincinity of tubular steel structures. An iterative solution method is presented which incorporates six degree of freedom vehicle motions and this is verified in a series of laboratory experiments with various arrangements of structural members and using a commercial sonar device. The key features, applications and performance of this method are discussed. The main conclusion is that the proposed method for calculating the position and heading of an underwater vehicle contributes towards achieving an accurate and reliable subsea navigation capability. 相似文献
997.
The commonly used formulae like Hudson's [(1959), Laboratory investigations of rubblemound breakwaters. WES report, Vicksburg], Iribarren's or Vander meer's [(1988), Rockslopes and gravel beaches under wave attack. Ph.D. thesis, Delft University of Technology, The Netherlands], do not give us the design cross section of a rubblemound breakwater for varying core porosity values. The paper presents the results of the experimental study carried out to compute the effects of core porosity on the stability and run-up characteristics of rubblemound breakwaters. Regular waves were made to attack the structure, with different core porosity values in a normal direction. The porosity of the armour and the secondary layers was neglected. It was observed that as core porosity increased the stability also increased considerably within the limits of the experimental data values. This may be due to large inflow and energy dissipation within the core of the structure. The run-up on the rubblemound slope was found to decrease with the increase in the porosity for the same reason. 相似文献
998.
999.
Current specification of the ocean wave environment for the design of offshore platforms does not adequately describe the directional nature of a real seaway. The strong wave frequency dependent nature of the directional behavior of observed seas is often over-simplified for design. A general formulation encompassing a wide range of directional sea models is presented. Parameter values used in some of the more popular directional sea models are examined. Approximate expressions for the two frequency dependent parameters in a modified Longuet-Higgins cosine wave spreading model are presented. A general procedure which allows an engineer to estimate parameters for alternate wave spreading models is discussed. To illustrate this procedure an empirically based modified cosine spreading model is used as the basis to estimate frequency dependent parameters for circular normal and wrapped Gaussian wave spreading models. A comparison of the contours of the various directional sea models and the prediction of the root-mean-square velocity distribution is presented. 相似文献
1000.
Pristane (2,6,10,14 -tetramethylpentadecane) occurs ubiquitously in the marine environment. This hydrocarbon may be of biogenic or petrogenic origin.1 Recently it has been shown that residual amounts of this branched alkane increased in marine organisms after an oil spill.2,3 The lack of data on the fate of pristane in fish, added with the fact that this compound was considered by some authors as a non-metabolisable substance in vertebrates, including man,4 led us to investigate the capability of fish to metabolise pristane. In this study, urinary and fecal excretion, tissue distribution and metabolism of 3H-pristane were analysed in Salmo gairdneri R. after a single intragastric dose. In addition to unchanged hydrocarbon, various labelled compounds have been isolated and identified in liver, bile, faeces, urine and surrounding water, demonstrating that pristane was first oxidised to alcohols (pristanol and pristane-diol) and to acid (pristanic acid). The elimination of these compounds occurred in the form of conjugated products (primarily glucuronides) as well as free metabolites. 相似文献