全文获取类型
收费全文 | 72640篇 |
免费 | 2348篇 |
国内免费 | 2059篇 |
专业分类
测绘学 | 1898篇 |
大气科学 | 6655篇 |
地球物理 | 15449篇 |
地质学 | 24520篇 |
海洋学 | 6695篇 |
天文学 | 15412篇 |
综合类 | 962篇 |
自然地理 | 5456篇 |
出版年
2022年 | 448篇 |
2021年 | 683篇 |
2020年 | 790篇 |
2019年 | 779篇 |
2018年 | 1285篇 |
2017年 | 1257篇 |
2016年 | 1762篇 |
2015年 | 1301篇 |
2014年 | 1787篇 |
2013年 | 3580篇 |
2012年 | 1816篇 |
2011年 | 2620篇 |
2010年 | 2334篇 |
2009年 | 3265篇 |
2008年 | 2949篇 |
2007年 | 2689篇 |
2006年 | 2713篇 |
2005年 | 2328篇 |
2004年 | 2417篇 |
2003年 | 2264篇 |
2002年 | 2160篇 |
2001年 | 1955篇 |
2000年 | 1947篇 |
1999年 | 1810篇 |
1998年 | 1776篇 |
1997年 | 1754篇 |
1996年 | 1501篇 |
1995年 | 1447篇 |
1994年 | 1300篇 |
1993年 | 1163篇 |
1992年 | 1093篇 |
1991年 | 890篇 |
1990年 | 1099篇 |
1989年 | 918篇 |
1988年 | 802篇 |
1987年 | 963篇 |
1986年 | 837篇 |
1985年 | 1043篇 |
1984年 | 1206篇 |
1983年 | 1137篇 |
1982年 | 1034篇 |
1981年 | 993篇 |
1980年 | 845篇 |
1979年 | 827篇 |
1978年 | 870篇 |
1977年 | 790篇 |
1976年 | 751篇 |
1975年 | 697篇 |
1974年 | 703篇 |
1973年 | 708篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Impulsive acoustic waveforms are characterized by a different set of derived quantities than are continuous waveforms. This note presents commonly accepted definitions, units, and symbols used to describe the magnitude of impulsive underwater signals 相似文献
22.
Kunde Yang Yuanliang Ma Chao Sun Miller J.H. Potty G.R. 《Oceanic Engineering, IEEE Journal of》2004,29(4):964-972
This paper discusses the results of geoacoustic inversion carried out using explosive charge data from the Asian Seas International Acoustic Experiment (ASIAEX) East China Sea (ECS) Experiment. A multifrequency incoherent matched-field inversion processor and a genetic algorithm (GA) are used for the inversion. A multistep matched field inversion approach is presented, which makes use of the varying sensitivities of wave fields at various frequencies to reduce the inversion problem into a sequence of smaller inversions with fewer unknowns to estimate at each stage. Different parameters are estimated using data at different frequencies according to their sensitivities. Inversion results for different areas in the ECS region are summarized and compared with core data. 相似文献
23.
J. BERGER O. FEMENIAS J. C. C. MERCIER D. DEMAIFFE 《Journal of Metamorphic Geology》2005,23(9):795-812
The Limousin ophiolite is located at the suture zone between two major thrust sheets in the western French Massif Central. This ophiolitic section comprises mantle‐harzburgite, mantle‐dunite, wehrlites, troctolites and layered gabbros. It has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and/or pargasite‐bearing chloritites. With various thermobarometric methods, it is possible to show that the different varieties of amphibole have registered low‐P (c. 0.2 GPa) conditions with temperature ranging from high‐T, late‐magmatic conditions to greenschist–zeolite metamorphic facies. The abundance of undeformed metamorphic rocks (which is typical of the lower oceanic crust), the occurrence of Ca–Al (–Mg) metasomatism illustrated by the growth of Ca–Al silicates in veins or replacing the primary magmatic minerals, the P–T conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from Ocean Drilling Program and worldwide ophiolites are the main arguments for an ocean‐floor hydrothermal metamorphism in the vicinity of a palaeo‐ridge. Among the West‐European Variscan ophiolites, the Limousin ophiolites constitute an extremely rare occurrence that has not been involved in any HP (subduction‐related) or MP (orogenic) metamorphism as observed in other ophiolite occurrences (i.e. France, Spain and Germany). 相似文献
24.
Longitudinal and transverse dispersion in estuaries may be determined in principle by measuring the change in time of the concentration field of a water soluble dye. In practice, measurements may take the form of several sampling runs crossing the dye patch. The coefficients of dispersion are usually estimated from the second moments of the concentration field; the use of second moments, in particular, introduces errors because measurements at long times or distance are unduly emphasized.A self-consistent method, free of the disadvantages inherent in the use of higher moments, is described in which the distribution of concentration in space is related to the zeroth spatial moment of concentration. In this formulation, based upon sampling runs across the dye patch, the concentration field in the estuary may be directly and simply related to the dispersion coefficients in both the intercept and gradient of a linear graph.The methods are illustrated by the characterization of a series of experiments in the Inner Estuary of Milford Haven. 相似文献
25.
D.J. Curtis C.G. Galbraith J.C. Smyth D.B.A. Thompson 《Estuarine, Coastal and Shelf Science》1985,21(1):75-90
The number of black-headed gulls (Larus ridibundus) in the Clyde Estuary is large. In summer the average density has reached 1350 gulls km?2 and in winter 180 gulls km?2. This paper compares prey selection and feeding efficiency in gulls during summer and winter on tidal flats, and considers how seasonal differences may be adaptations to cope with seasonal changes in prey availability.Gross and net rates of energy intake were highest in summer because gulls captured more of the polychaete N. diversicolor than the amphipod C. volutator. In winter, gulls selected for C. volutator and therefore an energetically less profitable diet. Throughout the year gulls selected more C. volutator relative to N. diversicolor than expected on energetic grounds and so apparently did not maximize potential net rate of energy intake.Gulls used three techniques to capture prey and made most intensive use of the ‘crouch’ technique. Crouching gulls attained a much higher net rate of energy intake than ‘upright’ or ‘paddling’ gulls.A log-linear model showed that (a) season, water depth and gull density determined feeding technique and (b) feeding technique and season independently determined foraging success and prey selection. Thus gull density and water depth acted on prey selection through imposed variations in feeding technique.Reasons for gulls selecting energetically unprofitable C. volutator and for the use of several distinct feeding techniques are discussed. 相似文献
26.
27.
28.
F. J. Ormeling Sr. 《GeoJournal》1986,13(4):413-416
Conclusion The above account is just a selection of Perthes' multifarious activities. As such it is fragmentary and incomplete. Many other publications and collaborators could have been mentioned. Even in its incompleteness the report may justify the statement that the success of Justus Perthes Geographische Verlagsanstalt, was apart from the managing qualities of its governors, undoubtedly due to the long series of scholarly competent collaborators whom they succeeded in engaging. As illustrated above most of theme were among the avant garde of the profession. Apparently they were at ease with Justus Perthes where they were granted enough freedom of action to realise their ambitious projects under the vigilant eye of the management. In fact the history of the firm is a continuous story of their achievements. Their combined effort initiated modern scientific atlas-making, laid down the foundation of methodic school cartography and accelerated the growth of thematic cartography. The Justus Perthes maps and atlasses stand for intellectual honestly and have an authoritative appearance of truth and exactness. For a long time to come the name of the firm will be considered as a classical example of the promotion of geography and cartography by private enterprise. 相似文献
29.
N.J.P. Owens 《Estuarine, Coastal and Shelf Science》1985,20(4):505-510
In this study, the 14N:15N ratio of suspended particulate material collected from the Tamar river estuary, south-west England, is described. Three populations of particles, distinguishable by their 15N content, were observed. This investigation has shown that populations of estuarine particles are generated by biological transformations in situ and that the 15N content of estuarine particles does not merely reflect hydrodynamic mixing of the freshwater and seawater source particulate material. 相似文献
30.
Ocean dynamical processes exist over a wide range of temporal and spatial scales. Remotely stationed acoustic devices are being used to "sense" the interior of the oceans at previously unattainable scales. The method is similar to computer-aided tomography (CAT) scans of the brain, but the measurement difficulties are far more severe. The results of a demonstration experiment are reviewed and some directions for acoustic oceanography are discussed. 相似文献