全文获取类型
收费全文 | 74149篇 |
免费 | 1042篇 |
国内免费 | 1429篇 |
专业分类
测绘学 | 2648篇 |
大气科学 | 5604篇 |
地球物理 | 14282篇 |
地质学 | 29183篇 |
海洋学 | 5545篇 |
天文学 | 13038篇 |
综合类 | 2251篇 |
自然地理 | 4069篇 |
出版年
2021年 | 440篇 |
2020年 | 509篇 |
2019年 | 559篇 |
2018年 | 5742篇 |
2017年 | 5029篇 |
2016年 | 3894篇 |
2015年 | 1032篇 |
2014年 | 1469篇 |
2013年 | 2665篇 |
2012年 | 2508篇 |
2011年 | 4613篇 |
2010年 | 3775篇 |
2009年 | 4571篇 |
2008年 | 3895篇 |
2007年 | 4357篇 |
2006年 | 2147篇 |
2005年 | 1736篇 |
2004年 | 1937篇 |
2003年 | 1808篇 |
2002年 | 1596篇 |
2001年 | 1288篇 |
2000年 | 1304篇 |
1999年 | 1029篇 |
1998年 | 1025篇 |
1997年 | 1038篇 |
1996年 | 808篇 |
1995年 | 832篇 |
1994年 | 821篇 |
1993年 | 659篇 |
1992年 | 659篇 |
1991年 | 584篇 |
1990年 | 656篇 |
1989年 | 577篇 |
1988年 | 570篇 |
1987年 | 622篇 |
1986年 | 544篇 |
1985年 | 708篇 |
1984年 | 720篇 |
1983年 | 733篇 |
1982年 | 698篇 |
1981年 | 599篇 |
1980年 | 646篇 |
1979年 | 524篇 |
1978年 | 494篇 |
1977年 | 482篇 |
1976年 | 431篇 |
1975年 | 430篇 |
1974年 | 429篇 |
1973年 | 408篇 |
1972年 | 266篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
A detailed survey of a 1°×1°-square of seafloor 100 miles south-east of the Azores shows a strong correlation between directions of regional topographic and magnetic lineations. The area is dissected by the East Azores Fracture Zone at 36°55N, identified as the active Eurasian-African plate boundary, and by another large, non-active fracture zone at 36°10N. Both fracture zones strike 265° and are accompanied by large amplitude magnetic anomalies. The general strike in the area in between is 000°–015°. The skewing effect at this magnetic latitude is very sensitive to variations in strike of the magnetic contrasts. This effect was eliminated by a non-linear transformation which also gives the positions of magnetic contrasts. Some N-S contrasts were identified as sea floor spreading polarity contrasts (anomalies 31 and 32). Weak contrasts could be identified as topographic effects and gave a magnetization intensity of 5 A m-1. The identified sea floor spreading anomalies to both sides of the fracture zone at 36°10N agree very well, also quantatively, with a three-dimensional model for the fracture zone anomalies. This model describes the non-linear anomalies as end effects of the magnetic layer which is divided in blocks of alternating polarity. 相似文献
982.
Sand ridges on the inner Atlantic shelf of North America: Morphometric comparisons with Huthnance stability model 总被引:1,自引:0,他引:1
A. G. Figueiredo Jr. D. J. P. Swift Stubblefield T. L. Clarke 《Geo-Marine Letters》1981,1(3-4):187-191
Sand ridge fields on the inner shelf of the Middle Atlantic Bight are generally thought to have formed in response to northeasterly
storm flows as the shoreface underwent erosional retreat with postglacial sea-level rise. However, the hydrodynamic mechanism
is poorly unerstood. Coastal boundary models see the ridges as responses of the seafloor to distortions in the flow induced
by the coastal boundary. Stability models propose that an irregular initial topography will evolve toward an ordered array
of bedforms in response to repeated flow events. The two classes of models are not mutually exclusive, nor are members within
each class mutually exclusive. Results of measurements of ridge spacing on the inner Atlantic shelf of North America agree
with the predictions of stability models. 相似文献
983.
T. R. Parsons 《Journal of Oceanography》1981,37(5):294-298
During the past 20 years, the controlled experimental ecosystem has found wide-spread use in marine sciences. Uses have included the study of natural ecology, environmental pollution and computer model analysis. In all these roles, the experimental ecosystem serves as a model of nature and results can be used to obtain greater insight into the oceanography of the sea. Four different types of experimental apparatus are described, and experimental procedures and results are reviewed and discussed. 相似文献
984.
Measurements of 226Ra and 210Pb in barite samples separated from different depths of a piston core from the eastern equatorial Pacific are reported. 226Ra is enriched in the barite as would be expected from the chemistries of Ra and Ba; the ratio centers around 1.5 · 1014 atoms/mole. The results suggest that barite is concentrating Ra at a rate comparable to its decay rate, viz. 5.78 · 10?3 yr?1. The gross 210Pb depletion with respect to 226Ra is indicative of 222Rn loss due to the smallness of the barite crystals. 相似文献
985.
The distribution of the colour index is considered in the region bounded by 8–11°N and 13°30–18°30W based on the results of measurements made on board vessels of the Marine Hydrophysical Institute of the Ukrainian SSR Academy of Sciences (MHI) in 1977–1985. Mean values and statistical characteristics are calculated for the colour index variability over one-degree squares. A map of its multi-yearly average distribution is plotted.Translated by M. M. Trufanov. 相似文献
986.
The NDRE-AUV flight control system 总被引:1,自引:0,他引:1
The flight control system of an autonomous underwater vehicle (AUV) developed at the Norwegian Defence Research Establishment (NDRE) is presented. A mathematical model of the vehicle is derived and discussed. The system is separated into lightly interacting subsystems, and three autopilots are designed for steering, diving, and speed control. The design of the separate controllers is based on PID techniques. Results from extensive sea testing show robust performance and stability for the autopilot 相似文献
987.
Blubber of harbour seals (Phoca vitulina) sampled from the Strait of Georgia, BC, in 1991 and 1992 contained higher concentrations of polychlorinated dibenzo-p-dioxins and furans (PCDD/F) than did samples from Quatsino Sound on western Vancouver Island. The higher proportions of 1,2,3,6,7,8-hexachlorodibenzodioxin and 2,3,7,8-tetrachlorodibenzofuran in the Strait of Georgia samples probably reflect contamination arising from the discharge of effluents from bleached kraft mills. Higher concentrations of non-ortho- and mono-ortho substituted polychlorinated biphenyls were also found in the Strait of Georgia samples, probably as a result of general industrial activity in the region. Principal components analysis showed that residues were distributed differently between males and females. Within males, concentrations of PCDD, PCDF and PCBs appeared to increase with age. Toxic equivalents (TEQ) to 2,3,7,8-tetrachlorodibenzodioxin in the Quatsino Sd. samples were close to the working guidelines for edible fish used by Health Canada; TEQ calculated for the Strait of Georgia samples exceeded these guidelines by up to a factor of five. 相似文献
988.
A population of killifish (Fundulus heteroclitus) inhabiting a creosote-polluted inlet of the Elizabeth River demonstrates tolerance to the acute toxic effects exerted by contaminated sediments on reference site killifish. Previous data have suggested that upregulated antioxidant defenses contribute to short-term tolerance in killifish exposed to Elizabeth River sediments. This study investigated population differences in antioxidant defenses from wild caught Elizabeth River and reference population killifish in different seasons, and after being held in the laboratory. Parameters measured in the killifish were total glutathione concentrations (GSH(T)), activities of glutathione reductase (GR), glutathione peroxidase (GPx), and lipid peroxidation (LPO), all in adult hepatic tissues. The Elizabeth River population exhibited greater GSH(T), higher GPx activities, and increased LPO as compared to the reference population. Sex specific population differences were also observed in GSH(T) and GPx. Both populations displayed decreased GSH(T) and increased GR from early to late summer, as well as after being held in the laboratory. This study indicates that there are many factors that may contribute to differences in levels of antioxidant defenses in addition to exposure to contaminants, including reproductive status and environmental conditions. 相似文献
989.
A study of suspended sediment concentration in the buoyant plume of the Fraser River, Canada, showed that unstratified flow conditions at the river mouth caused resuspension of sandy bed material and high concentrations of coarse sediment. When flow at the mouth was stratified, sediment was fine-grained and concentrations were low. Application of a multivariate model revealed that suspended sediment concentration along the plume axis was controlled primarily by distance seaward of the river mouth, secondly by tidal height, and least by sediment concentration in the river. 相似文献
990.
R. E. Kirk K. Robertson R. B. Whitmarsh P. R. Miles 《Marine Geophysical Researches》1991,13(2):153-160
A technique has been devised for firing arrays of bottom shots on the ocean bed in depths upto 4000 m or more. Ten kilogram explosive charges are dropped from the surface while the shooting ship is navigated acoustically. They are detonated at preset times by an electronic timer which initiates an electrical detonator, detonating cord and cast PETN/TNT explosive. Ranges to ocean bottom seismographs, and the shot instants, can be calculated from the arrival-time differences of the direct and surface-reflected water waves. The accuracy, which is dependent on water-depth and range, was better than 22 m in range and 14 msec in shot instant for our experiments. 相似文献