全文获取类型
收费全文 | 64067篇 |
免费 | 1007篇 |
国内免费 | 587篇 |
专业分类
测绘学 | 1646篇 |
大气科学 | 4323篇 |
地球物理 | 12795篇 |
地质学 | 22326篇 |
海洋学 | 5567篇 |
天文学 | 15578篇 |
综合类 | 144篇 |
自然地理 | 3282篇 |
出版年
2021年 | 555篇 |
2020年 | 640篇 |
2019年 | 678篇 |
2018年 | 1518篇 |
2017年 | 1417篇 |
2016年 | 1758篇 |
2015年 | 978篇 |
2014年 | 1686篇 |
2013年 | 3181篇 |
2012年 | 1900篇 |
2011年 | 2520篇 |
2010年 | 2281篇 |
2009年 | 3124篇 |
2008年 | 2676篇 |
2007年 | 2708篇 |
2006年 | 2537篇 |
2005年 | 1899篇 |
2004年 | 1939篇 |
2003年 | 1781篇 |
2002年 | 1698篇 |
2001年 | 1504篇 |
2000年 | 1480篇 |
1999年 | 1280篇 |
1998年 | 1285篇 |
1997年 | 1243篇 |
1996年 | 1017篇 |
1995年 | 1028篇 |
1994年 | 963篇 |
1993年 | 794篇 |
1992年 | 761篇 |
1991年 | 719篇 |
1990年 | 847篇 |
1989年 | 721篇 |
1988年 | 676篇 |
1987年 | 812篇 |
1986年 | 691篇 |
1985年 | 868篇 |
1984年 | 978篇 |
1983年 | 927篇 |
1982年 | 845篇 |
1981年 | 815篇 |
1980年 | 731篇 |
1979年 | 658篇 |
1978年 | 649篇 |
1977年 | 622篇 |
1976年 | 575篇 |
1975年 | 579篇 |
1974年 | 545篇 |
1973年 | 573篇 |
1972年 | 364篇 |
排序方式: 共有10000条查询结果,搜索用时 468 毫秒
971.
Experimental investigation of the combustive sound source 总被引:3,自引:0,他引:3
In this paper, we describe a unique low frequency underwater sound source called the combustive sound source (CSS). In this device, a combustible gas mixture is captured in a combustion chamber and ignited with a spark. The ensuing combustion produces expanding gases which in turn produce high intensity, low frequency acoustic pulses. With high-speed motion pictures of the CSS event, we relate the motion of the bubble to the acoustic waveform. We also compare the measured first bubble period in the CSS pressure signature with the predictions of the Rayleigh-Willis equation, including the dependence of the radiated acoustic waveform on the volume and depth of the bubble. Measurements of the first bubble period agree with Rayleigh-Willis theory in trend, but not in absolute value. In addition, we discuss the variation of the acoustic output with the fuel/oxygen mixture. Finally, several other factors that affect the acoustic output of CSS are discussed. These include the shape of the CSS combustion chamber, the type of oxidizer and fuel, and the ignition source 相似文献
972.
The Fauna of Floating Cyanobacterial Mats in the Oligohaline Eulittoral Zone off Hiddensee (South-west Coast of the Baltic Sea) 总被引:2,自引:0,他引:2
Abstract. The fauna — including macrofauna, meiofauna, and large ciliates — of floating cyanobacterial mats in a brackish shallow-water area was studied by analysing six 20 cm2 pieces of mat. Although these microbial aggregations were scarcely 1 cm thick, their total meiofauna abundance was about five times as high as in the uppermost 4 cm of the adjacent sediment. The mat fauna was dominated by harpacticoids, although large ciliates, rotifers, nematodes, and oligochaetes were also markedly more abundant than in the sediment. All species occurring in the mats were members of the surrounding sediment fauna. Out of the 47 species found, only a few, among them predominantly the harpacticoid, Mesochra lilljeborgi B oeck , 1864, and the nematode Daptonema setosum ( bütschli , 1874), accounted for the majority of the individual abundance in the mats. Both are regularly found in sulphidic biotopes near Hiddensee. As SEM micrographs revealed, the oligochaete Paranais litoralis (O. F. M üller , 1788) and the harpacticoid Cletocamptus confluens (Schmeil, 1894) were apparently optimal substrates for dense lawns of cyanobacteria. This indicates possible close interactions between the meiofauna and these microbiota. The frequent predation of histophagous ciliates on nematodes and harpacticoids, which were probably weakened by oxygen deficiency and/or high sulphide concentrations, were not only a sign of a generally neglected pathway in the food chain, but also impressively emphasized the huge variety of interactions taking place between meio- and microfauna within this specific benthic microcosm. Because of their floating character, the mats can play an important role in the dispersal of benthic fauna. 相似文献
973.
An elongated sea bed depression (pit), 80 by 125 m and 10 m deep located on the Grand Banks of Newfoundland was identified on high resolution surficial geophysical survey lines and investigated using a manned submersible. The pit occurs at the termination of a long (greater than 3 km) iceberg scour, 1.5 m deep, and 80 m wide in 91 m of water. It is similar to other depressions in the region that indent the sea bed up to seven times deeper than the surrounding deepest iceberg furrows. The pit is interpreted to be the sea bed response to impact and loading by a rolling and grounding iceberg. The iceberg scour and pit appear to be of recent origin. 相似文献
974.
Dolos concrete units have been used extensively throughout the world for the protection of shorelines and rubble structures. A three-dimensional finite element model is used to determine the states of dynamic stress in dolosse with varying dimensions and concrete properties. An analytical procedure is developed which accurately predicts the tensile stress in the shank and horizontal fluke of dolosse subject to drop test loading conditions Numerical examples are presented which illustrate the application of prediction and iso-stress equations. 相似文献
975.
A. A. Bezborodov S. L. Gokchen V. N. Eremeev I. T. Konuk A. K. Saidam E. M. Tirasin 《Physical Oceanography》1991,2(3):227-231
The hydrological and hydrochemical structures of the upper 300 m water column of the Black Sea in autumn 1988 have been studied. Regularities in the distribution of the physico-chemical characteristics in the zone of interaction between aerobic and anaerobic waters as well as the topography of the H2S zone boundary and its connection with a certain density gradient have been found.Translated by Mikhail M. Trufanov. 相似文献
976.
977.
The determination of trace amounts of inorganic elements by electrothermal atomic absorption spectrophotometry (ETA-AAS) or inductively coupled plasma atomic emission spectrometry (ICP-AES) suffers from some kinds of interference due to the inorganic fraction of the matrix of many environmental samples. The ratio between a major matrix element and the analyzed trace element plays an important role in the spectrochemical behaviour of the latter. The resulting enhancing or depressing effect also gives rise to severe faults when the well known standard addition method (SAM) is used. The present work shows the interactions due to the presence of elements such as calcium, sodium, potassium, magnesium and iron during the spectrochemical determinations of trace amounts of cadmium in marine mussels. A possible liquid anion exchange extraction is suggested for the analysis of cadmium by ETA-AAS. 相似文献
978.
Textural isotopic and microfossil data from two gravity cores obtained in Saguenay Fjord, Quebec, suggest that a distinctive sandy clay bed was deposited as the result of a major landslide in the Saguenay River basin. Pb-210 dating of the cores indicate that the bed is of similar age to the magnitude 7 earthquake of February 5, 1663. The slide involved sensitive marine clays and may have occurred in two stages. Slide sediments carried into the Saguenay River channel were probably reworked and subsequently transported down the Fjord basin as two distinct cohesionless mass flows. Fine clay laminae that overlie the older mass flow bed record the modulation of depositional processes by tidal currents for several weeks after this event. 相似文献
979.
Sediment supply and pre-existing shoreline morphology are crucial factors in controlling coastal changes due to sea-level rise. Using examples from both southeast and northeast Ireland, it can be shown that sea-level change may trigger a sequence of events which leads to both static and dynamic shoreline equilibrium. Cliff erosion and longshore sediment movement in east Co. Wexford has led to injection of sediment onto the shelf, and the growth, under both wave and tide regimes, of linear offshore shoals. These shoals now control the pattern of shoreline erosion and provide a template for possible stepwise evolution of the coast under any future sea-level rise. In contrast, the nearby coast of south Co. Wexford comprises a series of coarse clastic barriers moving monotonously onshore, via overwash processes. Here the behavior of the barrier is conditioned by the antecedent morphology of both the beach face and stream outlet bedforms. Finally, the rock platform coast of Co. Antrim presents a far more resistant shoreline to incident marine processes, yet even here there is strong evidence of present process control over so-called ‘raised’ platforms and embayments. It is concluded that coastal sediment supply and dynamics, together with coastal morphology and its interaction with waves, present a far more complex variety of sea-level indicators than is normally acknowledged. 相似文献
980.
Abstract. The marine plant communities of the littoral zone in different biotopes of the Greek coasts were investigated in 1980-81. Seasonal distribution and variation of marine plant biomass were assessed. The communities of Cystoseira crinita and C. compressa were outstanding with maximum biomass during the summer months. Corallina officinalis and Pterocladia capillacea + Viva rigida communities predominated with maximum biomass in autumn and exhibited a decrease in winter, except in stressed biotopes. The species diversity and productivity of seaweeds along the Greek coasts are interpreted in relation to a number of environmental parameters. 相似文献