全文获取类型
收费全文 | 1586篇 |
免费 | 38篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 36篇 |
大气科学 | 157篇 |
地球物理 | 291篇 |
地质学 | 497篇 |
海洋学 | 132篇 |
天文学 | 381篇 |
自然地理 | 137篇 |
出版年
2020年 | 12篇 |
2019年 | 25篇 |
2018年 | 36篇 |
2017年 | 23篇 |
2016年 | 27篇 |
2015年 | 22篇 |
2014年 | 45篇 |
2013年 | 110篇 |
2012年 | 57篇 |
2011年 | 66篇 |
2010年 | 51篇 |
2009年 | 53篇 |
2008年 | 67篇 |
2007年 | 57篇 |
2006年 | 50篇 |
2005年 | 54篇 |
2004年 | 75篇 |
2003年 | 46篇 |
2002年 | 48篇 |
2001年 | 29篇 |
2000年 | 27篇 |
1999年 | 30篇 |
1998年 | 28篇 |
1997年 | 14篇 |
1996年 | 16篇 |
1995年 | 15篇 |
1994年 | 15篇 |
1993年 | 15篇 |
1992年 | 17篇 |
1991年 | 17篇 |
1990年 | 15篇 |
1989年 | 14篇 |
1988年 | 19篇 |
1987年 | 21篇 |
1986年 | 22篇 |
1985年 | 33篇 |
1984年 | 28篇 |
1983年 | 27篇 |
1982年 | 21篇 |
1981年 | 23篇 |
1980年 | 28篇 |
1979年 | 28篇 |
1978年 | 29篇 |
1977年 | 30篇 |
1976年 | 14篇 |
1975年 | 14篇 |
1974年 | 11篇 |
1973年 | 21篇 |
1972年 | 17篇 |
1971年 | 19篇 |
排序方式: 共有1631条查询结果,搜索用时 15 毫秒
101.
Fundamental evidence from world-class stratiform copper deposits repeatedly indicates that they originated by a two-stage mineralization process: a syn-diagenetic enrichment of the host sediment in iron sulfide, followed by a sub-surface addition of copper during early diagenesis of the sediment. In contrast to the previously favored syngenetic hypothesis, which suggested an open equilibration between atmosphere, hydrosphere, biosphere and upper lithosphere during mineralization, the two-stage model implies that only the initial deposition of iron sulfides should take place with open exchanges between the upper “spheres”. The sub-surface emplacement of copper should occur in relative isolation from surface environments and, therefore, the occurrence and distribution of stratiform copper deposits through time and space should not be used to make direct interpretations of contemporaneous conditions in the upper “spheres”. 相似文献
102.
103.
W. David Rust Donald R. MacGorman Eric C. Bruning Stephanie A. Weiss Paul R. Krehbiel Ronald J. Thomas William Rison Timothy Hamlin Jeremiah Harlin 《Atmospheric Research》2005,76(1-4):247
Balloon-borne electric field soundings and lightning mapping data have been analyzed for three of the storms that occurred in the Severe Thunderstorm Electrification and Precipitation Study field program in 2000 to determine if the storms had inverted-polarity electrical structures. The polarities of all or some of the vertically stacked charge regions in such storms are opposite to the polarities observed at comparable heights in normal storms. Analyses compared the charge structures inferred from electric field soundings in the storms with charges inferred from three-dimensional lightning mapping data. Charge structures were inferred from electric field profiles by combining the one-dimensional approximation of Gauss's law with additional information from three-dimensional patterns in the electric field vectors. The three different ways of inferring the charge structure in the storms were found to complement each other and to be consistent overall. Charge deposition by lightning possibly occurred and increased the charge complexity of one of the storms.Many of the cloud flashes in each case were inverted-polarity flashes. Two storms produced ground flash activity comprised predominantly of positive ground flashes. One storm, which was an isolated thunderstorm, produced inverted-polarity cloud flashes, but no flashes to ground. The positive and negative thunderstorm charge regions were found at altitudes where, respectively, negative and positive charge would be found in normal-polarity storms. Thus, we conclude that these storms had anomalous and inverted-polarity electrical structures. Collectively, these three cases (along with the limited cases in the refereed literature) provide additional evidence that thunderstorms can have inverted-polarity electrical structures. 相似文献
104.
105.
106.
Three cores from two connected lakes in Central Ireland (Lough Kinale and Derragh Lough) were investigated using diatom analysis
to establish the Holocene development of the lacustrine system, any local variations within the lakes and any anthropogenic
influences. The study area was situated in a lowland location and the lakes were shallow, unstratified and interconnected.
Litho-and bio-stratigraphical analyses of the lake cores and deposits beneath a mire separating the two lakes showed the changing
spatial configuration of the lake system in the early Holocene and the separation of the initial lake into three basins (cf.
lacustrine cells) and finally into two interlinked lakes. The evolution of the lake system is conceptualised as the development
of distinct lacustrine cells, and its sediments have recorded changes in the physical (geography, depth and sedimentation)
and chemical (water chemistry) properties of the lakes inferred through diatom analyses. The longest sequence, from the early
Holocene, records fluctuating lake levels and these are correlated with geomorphological mapping and surveying of palaeoshorelines.
The diatom assemblages of the upper 2 m of the three cores, covering approximately the last 2000–3000 radiocarbon years show
considerable difference in trophic status and life-form categories. This is related to the location of the cores in the lake
and also the distance from human settlement with particular reference to proximity to crannog (artificial island) construction
and use. The most central core from the deepest part of Lough Kinale has the least representation of the human settlement
and agricultural activity in the catchment and on the fringes of the lake, whereas the core taken from the edge of a crannog
is able to identify when construction and use of the crannog occurred. The local nature of the palaeoecological response to
human activity due to incomplete water mixing has the advantage of allowing the lake sediment cores to be used to determine
spatially discrete settlement patterns. 相似文献
107.
108.
The requirements for a credible large-eddy simulation of neutral, turbulent flow over hills with an aerodynamically rough surface are discussed, in order to select a suitable case for simulation. As well as providing adequate resolution within the dynamically important inner region, obtaining a realistic upstream or undisturbed mean velocity profile is also of critical importance. A distributed drag canopy formulation has been introduced to the model to allow it to obtain such a profile while resolving very close to the rough surface. Simulations have then been performed of flow over ridges of varying heights. The results from the steepest case, which is just on the verge of separation, are compared with wind-tunnel observations. It is shown that the large-eddy simulation results are in much better agreement with the experimental data than are the results from a simple first-order mixing-length closure model. An encouraging lack of sensitivity of the simulation results to numerical resolution is also demonstrated. 相似文献
109.
Acacia nilotica is a tree of international significance both as a beneficial plant and as a species prone to thicket formation and negative impacts on savannas throughout much of its range. While fire has been identified as a useful tool for controlling negative impacts of some Acacia species in Africa, A. nilotica adult trees are apparently fire tolerant. The effects of fire on seed bank mortality, viability and germination, and on juvenile plants of this species are unknown. We undertook a study to investigate the impacts of fire on seeds and first and second year seedlings in Australian Astrebla savannas. The primary objective was to determine the effects of control burns applied during the early stages of invasion and thicket formation. Because of high rates of seedling dessication post fire in 1997, supplementary watering was undertaken until fire mortality could be measured the following year. High seed mortality occurred for seeds located either in cattle dung (100%) or on the soil surface (c. 80%). Germination of those seeds that survived was enhanced following exposure to fire. Very high fire mortality (99%) was observed among first year seedlings and moderate mortality (60%) for second year seedlings even after a low intensity burn. Negative impacts on major pasture species were minimal. The implications of these findings are discussed in relation to A. nilotica fire ecology and to the efficacy of fire as a tool for managing this species. 相似文献
110.
Refraction effects of optical beams are generally caused by an inhomogeneous propagation medium and are a major source of systematic errors in the precise optical determination of angles and distances in the atmospheric surface layer. In this contribution a method for deriving vertical temperature and refractive index gradients from optical scintillation is presented. Knowledge of these gradients is required for the compensation of atmospherically induced errors for highly precise terrestrial geodetic measurements, like direct transfer and levelling. The advantage of the present optical method is, that temperature and refractive index gradients can be derived as line-averaged values over the propagation path, which is not possible by meteorological point measurements. Field observations have been carried out with a displaced-beam scintillometer over flat terrain and under different atmospheric conditions in order to verify this method. The experiments show, that this method allows to derive accurate correction values for precise terrestrial geodetic measurements. 相似文献