全文获取类型
收费全文 | 255738篇 |
免费 | 4312篇 |
国内免费 | 3357篇 |
专业分类
测绘学 | 6856篇 |
大气科学 | 18887篇 |
地球物理 | 53561篇 |
地质学 | 88235篇 |
海洋学 | 21177篇 |
天文学 | 55861篇 |
综合类 | 1006篇 |
自然地理 | 17824篇 |
出版年
2021年 | 2233篇 |
2020年 | 2591篇 |
2019年 | 2836篇 |
2018年 | 3198篇 |
2017年 | 2825篇 |
2016年 | 5542篇 |
2015年 | 4186篇 |
2014年 | 6884篇 |
2013年 | 14205篇 |
2012年 | 6367篇 |
2011年 | 7576篇 |
2010年 | 6579篇 |
2009年 | 9230篇 |
2008年 | 8130篇 |
2007年 | 7494篇 |
2006年 | 9634篇 |
2005年 | 7620篇 |
2004年 | 7570篇 |
2003年 | 7068篇 |
2002年 | 6699篇 |
2001年 | 5988篇 |
2000年 | 5935篇 |
1999年 | 5199篇 |
1998年 | 5224篇 |
1997年 | 5034篇 |
1996年 | 4677篇 |
1995年 | 4427篇 |
1994年 | 4097篇 |
1993年 | 3847篇 |
1992年 | 3632篇 |
1991年 | 3583篇 |
1990年 | 3750篇 |
1989年 | 3509篇 |
1988年 | 3301篇 |
1987年 | 3847篇 |
1986年 | 3405篇 |
1985年 | 4223篇 |
1984年 | 4736篇 |
1983年 | 4401篇 |
1982年 | 4310篇 |
1981年 | 3929篇 |
1980年 | 3636篇 |
1979年 | 3508篇 |
1978年 | 3481篇 |
1977年 | 3278篇 |
1976年 | 3044篇 |
1975年 | 2959篇 |
1974年 | 2914篇 |
1973年 | 3073篇 |
1972年 | 2024篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
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
993.
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
994.
995.
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
996.
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
997.
Low-frequency damping of a moored semisubmersible drilling platform was obtained from numerical extinction tests simulated in still water and in regular waves and from mean wave drift forces calculated at zero forward speed. The influence of drag forces was represented by the modified Morison equation. The platform as used for the 18th ITTC Comparative Mooring Study was analyzed in irregular beam waves. The computed time series of sway as well as the corresponding sway response spectrum compared favourably with model test measurements, demonstrating that this procedure to determine low-frequency damping can be effective. 相似文献
998.
999.
Oysters and mussels exposed to a concentration of 0·7 ppb (μg/liter) tributyltin from painted panels in flowing seawater accumulated tin in the digestive glands to comparable levels. The mussels experienced approximately 50% mortality during the 60-day test period, but the oysters suffered virtually no deaths. There was no evidence from either bivalve of elevated numbers of hemocytes during the test period and no evidence for cellular disruption as detected by increased levels of serum lysosomal hydrolases. Serum protein of exposed mussels relative to controls increased with time of exposure to the toxicant, while oyster serum protein, normally 10 x higher than in mussels, did not. No evidence was found for elevated stress proteins (heat shock proteins) or metallothioneins in the serum hemocytes of either bivalve. Responses by these animals to fatal or near fatal doses of TBT were thus very different from responses to copper that we have reported elsewhere.1,2 相似文献
1000.
James T. Byrd 《Marine Chemistry》1988,25(4)
Arsenic concentrations vary with season on the continental shelf of the South Atlantic Bight. During periods of high winds in the winter and early spring, inorganic arsenic concentrations are reduced to as little as 20% of typical open ocean concentrations by sorption onto resuspended sediments or incorporation into phytoplankton. In the early fall, arsenic sequestered in sediments in the spring is regenerated and returned to the water column, creating elevated arsenic concentrations in the nearshore zone that are up to 50% greater than open ocean concentrations. Arsenic in the adjacent estuaries is nearly conservative over the seasonal cycle, although its distribution in the estuaries is greatly affected by the seasonal changes in arsenic concentrations in the nearshore waters. 相似文献