全文获取类型
收费全文 | 57860篇 |
免费 | 901篇 |
国内免费 | 790篇 |
专业分类
测绘学 | 1478篇 |
大气科学 | 4620篇 |
地球物理 | 11979篇 |
地质学 | 19655篇 |
海洋学 | 5037篇 |
天文学 | 12787篇 |
综合类 | 150篇 |
自然地理 | 3845篇 |
出版年
2020年 | 374篇 |
2019年 | 361篇 |
2018年 | 846篇 |
2017年 | 776篇 |
2016年 | 1216篇 |
2015年 | 860篇 |
2014年 | 1183篇 |
2013年 | 2849篇 |
2012年 | 1423篇 |
2011年 | 2067篇 |
2010年 | 1703篇 |
2009年 | 2460篇 |
2008年 | 2199篇 |
2007年 | 2103篇 |
2006年 | 2053篇 |
2005年 | 1845篇 |
2004年 | 1796篇 |
2003年 | 1726篇 |
2002年 | 1649篇 |
2001年 | 1435篇 |
2000年 | 1452篇 |
1999年 | 1377篇 |
1998年 | 1271篇 |
1997年 | 1289篇 |
1996年 | 1101篇 |
1995年 | 1019篇 |
1994年 | 932篇 |
1993年 | 853篇 |
1992年 | 826篇 |
1991年 | 753篇 |
1990年 | 797篇 |
1989年 | 666篇 |
1988年 | 693篇 |
1987年 | 768篇 |
1986年 | 717篇 |
1985年 | 943篇 |
1984年 | 1042篇 |
1983年 | 1013篇 |
1982年 | 874篇 |
1981年 | 837篇 |
1980年 | 817篇 |
1979年 | 732篇 |
1978年 | 766篇 |
1977年 | 661篇 |
1976年 | 662篇 |
1975年 | 643篇 |
1974年 | 670篇 |
1973年 | 673篇 |
1972年 | 406篇 |
1971年 | 341篇 |
排序方式: 共有10000条查询结果,搜索用时 69 毫秒
991.
A case study of a modified gravity type cage and mooring system using numerical and physical models 总被引:2,自引:0,他引:2
DeCew J. Fredriksson D.W. Bugrov L. Swift M.R. Eroshkin O. Celikkol B. 《Oceanic Engineering, IEEE Journal of》2005,30(1):47-58
A modified gravity-type cage, developed by SADCO Shelf Ltd., was examined using numerical and physical models to determine if the cage and mooring system is suitable for an exposed site south of the Isles of Shoals, NH. The 3000-m/sup 3/ SADCO Shelf Submersible Fish Cage has angled stays between the upper framework and the ballasted bottom rim (in addition to net) to resist the horizontal shear deformation. The mooring system consists of three legs-each made up of a taut vertical chain and an angled rope, both leading to deadweight anchors. Normalized response amplitudes (response amplitude operators) were found for motion response in heave, surge and pitch, and load response in the anchor and bridle lines, in regular (single frequency) waves. In addition, a stochastic approach was taken to determine the motion and load transfer functions in random waves using a spectrum representative of seas at the selected site. In general, the system motion had a highly damped response, with no resonant peaks within the wave excitation range of 0.05 to 0.45 Hz. The anchor line force response was at all frequencies below 5 kN per meter of wave amplitude. The physical model tests showed consistently more conservative (larger) results compared to those for the numerical model. 相似文献
992.
993.
Green water overtopping analyzed with a SPH model 总被引:3,自引:0,他引:3
Wave overtopping on the decks of offshore platforms and ships can cause severe damage due to the high forces generated by the water. This phenomenon is analyzed within the framework of the Smoothed Particle Hydrodynamics (SPH) method. The presence of a fixed horizontal deck above the mean water level modifies strongly the wave kinematics. In particular, the flow in the wave crest is split into two, showing a different behavior above and below the deck. Numerical results generated by the SPH method are compared to laboratory experiments. The formation of a jet in the rear of the deck after overtopping is observed under extreme conditions. 相似文献
994.
Carbonate-rich, SiO2-poor residua are developed in some kimberlites solidifying as ocelli, layers, or discrete dikes which satisfy petrographic definitions of carbonatite. Arguments that these rocks have mineralogies, antecedents, and comagmatic rocks differing from those of the carbonatites in alkaline rock complexes, including the specific observation that kimberlites and carbonatites contain ilmenites and spinels of different composition, have been used to refute the alleged kimberlite-carbonatite relationship. New microprobe analyses of ilmenites and spinels from carbonate-rich rocks associated with kimberlites in three South African localities correspond to spinels and ilmenites of carbonatites from alkalic complexes, or have characteristics intermediate between those of carbonatites and kimberlites. The ilmenites are distinguished from kimberlite ilmenites by higher MnO, FeTiO3, and Nb2O5, and by negligible Cr2O3. The spinels are distinguished from kimberlite spinels by their Al2O3 and Cr2O3 contents. There is clearly a genetic relationship between the kimberlites and the carbonate-rich rocks, despite the observation that their ilmenites and spinels are distinctly different, which indicates that the same observation is not a valid argument against a petrogenetic relationship between kimberlites and carbonatites. These rocks are among the diverse products from mantle processes influenced by CO2, and we believe that the petrogenetic links among them are forged in the upper mantle. We see insufficient justification to deny the name carbonatite to carbonate-rich rocks associated with kimberlites if they satisfy the petrographic definition in terms of major mineralogy. 相似文献
995.
Improved form of wind wave frequency spectrum 总被引:5,自引:0,他引:5
Wen Shengchang 《海洋学报(英文版)》1989,8(4):467-483
The lower frequency part of the theoretical wind wave spectrum proposed by the authors (Wen et al. , 1988a, b,c) has been improved and the form of spectrum is appreciably simplified. In addition to the field data collected in the Bohai Sea region and used in the previous papers, those obtained in the Huanghai Sea, the East China Sea and the South China Sea have been employed so that the improved spectra can be verified on a more extensive observational basis. Computed results agree with the observations well. Further comparisons have been made between the proposed spectra and the JONSWAP spectrum. Though the two types of spectrum are close to each other in form, the former shows, as a whole, better agreement with the observation than the latter. By introducing an improved relation between the peak-ness factor and significant wave steepness, the spectrum contains only significant wave height and period as parameters. For spectra given in this form, the computed peak frequencies coincide approximately wit 相似文献
996.
The importance of the diet as a source of tributyltin (TBT) in Nucella lapillus was studied using [14C]tributyltin chloride. In N. lapillus provided with prelabelled mussels, Mytilus edulis, in labelled water (mean 20·5 ng/litre TBT) the rate of accumulation of total 14C was 2–3 times that in unfed animals. Owing to its degradation in the tissues of both fed and unfed animals, concentrations of [14C]TBT tended to reach a plateau after only 28 days. However, total concentrations of 14C were still increasing after 49 days. Under experimental conditions (15°C, ample food, no disturbance) the diet accounted for about 50% of the body burden of TBT in N. lapillus after 49 days exposure: concentration factors (dry tissue/water) for [14C]TBT in both male and female N. lapillus were similar at about 60 000 in fed and 30 000 in unfed animals. It is concluded that the diet may contribute less than half of the body burden of TBT found in natural populations subjected to life-long exposure. 相似文献
997.
Charles W. Holmes 《Geo-Marine Letters》1981,1(3-4):237-241
The plot of210Pb activity against depth in carbonate sands on the Virgin Island Bank is a negative asymmetric hyperbolic curve. As depth
increases, an initial rapid decrease in210Pb activity caused by the decay of unsupported210Pb and226Ra is followed by increasing activity as a result of210Pb achieving equilibrium with ingrowing230Th. As this curve is time dependent, an estimate of the relative ages in carbonate sequences and the rates of net carbonate
accumulation can be made. The ease of210Pb activity determinations makes this procedure an attractive method in obtaining carbonate sand accumulation rates. 相似文献
998.
The mineralogy of eight gravity cores recovered between Sierra Leone and 25°W longitude has been examined in an investigation
of the provenance of the recent sediments in the eastern Equatorial Atlantic. X-ray analyses show that the principal mineral
components are calcite, quartz, feldspar, kaolinite, illite and montmorillonite. One core taken from the upper continental
slope off Freetown also contains gibbsite, a product of intense tropical weathering. Biogenic material forms the dominant
component of most sections of the cores but it is clear from the abundance of quartz, kaolinite and freshwater diatoms that
an aeolian supply of continental detritus has been important in the formation of the recent sediment cover. A significant
contribution from volcanic sources can be recognised to the west of the Sierra Leone Rise. The occurrence of calcareous turbidites
in two cores from the St. Paul's Fracture Zone and from the basin west of the Sierra Leone Rise appears to be a reflection
of recent tectonic activity. 相似文献
999.
The process of Differential Roughness Secondary Flow (DRSF), hitherto little recognized, recently was demonstrated in a wind
tunnel and in a flume. It is suggested here the DRSF may, in the natural environment, play an important part in the persistence
and sharp definition of sand ribbons, previously initiated by other processes.
In 1980 Karl described a series of sand ribbons, alternating with exposed substrate, which he attributed to the effect of
Langmuir circulations. Although Langmuir Circulation Secondary Flow (LCSF) may indeed, in some cases, initiate sand ribbons,
the sharply-defined edges typical of these features are explained better by DRSF. 相似文献
1000.
A mechanically-actuated locking-compass was designed for mounting on corers to record equipment orientation during the sampling process, thereby providing magnetic orientation data on the recovered deep-sea core. Locking occurs upon impact by the corer with the sea-floor; after retrieval, the device is easily unlocked and reset for another station. Compass construction makes use of commercially available parts, requires no welding and little machining, and thus allows inexpensive and easy assembly at sea. 相似文献