全文获取类型
收费全文 | 1619篇 |
免费 | 65篇 |
国内免费 | 29篇 |
专业分类
测绘学 | 39篇 |
大气科学 | 177篇 |
地球物理 | 360篇 |
地质学 | 597篇 |
海洋学 | 131篇 |
天文学 | 248篇 |
综合类 | 4篇 |
自然地理 | 157篇 |
出版年
2023年 | 5篇 |
2022年 | 9篇 |
2021年 | 31篇 |
2020年 | 33篇 |
2019年 | 21篇 |
2018年 | 48篇 |
2017年 | 40篇 |
2016年 | 74篇 |
2015年 | 45篇 |
2014年 | 65篇 |
2013年 | 109篇 |
2012年 | 54篇 |
2011年 | 98篇 |
2010年 | 83篇 |
2009年 | 92篇 |
2008年 | 90篇 |
2007年 | 93篇 |
2006年 | 93篇 |
2005年 | 61篇 |
2004年 | 63篇 |
2003年 | 47篇 |
2002年 | 47篇 |
2001年 | 39篇 |
2000年 | 31篇 |
1999年 | 36篇 |
1998年 | 23篇 |
1997年 | 19篇 |
1996年 | 19篇 |
1995年 | 16篇 |
1994年 | 12篇 |
1993年 | 16篇 |
1992年 | 7篇 |
1991年 | 17篇 |
1990年 | 15篇 |
1989年 | 15篇 |
1987年 | 13篇 |
1986年 | 17篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1974年 | 7篇 |
1973年 | 5篇 |
排序方式: 共有1713条查询结果,搜索用时 15 毫秒
751.
Eric G. Blackman George B. Field 《Monthly notices of the Royal Astronomical Society》2000,318(3):724-732
We show that a steady mean-field dynamo in astrophysical rotators leads to an outflow of relative magnetic helicity and thus magnetic energy available for particle and wind acceleration in a corona. The connection between energy and magnetic helicity arises because mean-field generation is linked to an inverse cascade of magnetic helicity. To maintain a steady state in large magnetic Reynolds number rotators, there must then be an escape of relative magnetic helicity associated with the mean field, accompanied by an equal and opposite contribution from the fluctuating field. From the helicity flow, a lower limit on the magnetic energy deposited in the corona can be estimated. Steady coronal activity including the dissipation of magnetic energy, and formation of multi-scale helical structures therefore necessarily accompanies an internal dynamo. This highlights the importance of boundary conditions which allow this to occur for non-linear astrophysical dynamo simulations. Our theoretical estimate of the power delivered by a mean-field dynamo is consistent with that inferred from observations to be delivered to the solar corona, the Galactic corona, and Seyfert 1 AGN coronae. 相似文献
752.
Wang-Hong Yang R. James Kirkpatrick Norma Vergo John McHone Tryggvi I. Emilsson Eric Oldfield 《Meteoritics & planetary science》1986,21(1):117-124
High-resolution solid-state silicon-29 nuclear magnetic resonance spectroscopy using “magic-angle” sample-spinning can readily detect the presence of the high pressure silica polymorphs coesite and stishovite in whole-rock samples from a Meteor Crater, Arizona, impact sample, and yields accurate coesite/stishovite ratios. Such determinations are being carried out by partially suppressing (saturating) intense quartz signals (which have long spin-lattice relaxation times) by means of short experimental recycle-times. This method enhances the signal-to-noise ratios of coesite and stishovite (which have relatively short spin-lattice relaxation times). For the sample examined, the coesite/stishovite ratio is about 27. 相似文献
753.
Complete mantle section of a slow-spreading ridge-derived ophiolite: An example from the Isabela ophiolite in the Philippines 总被引:2,自引:0,他引:2
Abstract The Isabela ophiolite shows a complete ophiolite sequence exposed along the eastern coast of northern Luzon, the Philippines. It forms the Cretaceous basement complex for the northeastern Luzon block. This ophiolite is located at the northern end of a trail of ophiolites and ophiolitic bodies along the eastern margin of the Philippine Mobile Belt. This paper presents new findings regarding the nature and characteristics of the Isabela ophiolite. Peridotites from the Isabela ophiolite are relatively fresh and are composed of spinel lherzolites, clinopyroxene-rich harzburgites, depleted harzburgites and dunites. The modal composition, especially the pyroxene content, defines a northward depletion trend from fertile lherzolite to clinopyroxene-rich harzburgites and more refractory harzburgites. Variation in modal composition is accompanied by petrographic textural variations. The chromium number of spinel, an indicator of the degree of partial melting, concurs with petrographic observations. Furthermore, the Isabela ophiolite peridotites are similar in spinel and olivine major-element geochemistry and clinopyroxene rare earth-element composition to abyssal peridotites from modern mid-oceanic ridges. Petrological and mineral compositions suggest that the Isabela ophiolite is a transitional ophiolite subtype, with the fertile lherzolites representing lower sections of the mantle column that are usually absent in most ophiolitic massifs. The occurrence of the fertile peridotite presents a rare opportunity to document the lower sections of the ophiolitic mantle. The variability in composition of the peridotites in one continuous mantle section may also represent a good analogy of the melting column in the present-day mid-oceanic ridges. 相似文献
754.
Merouane Chott ephemeral lake is an ideal natural system for studying mineral dissolution and precipitation rates because (1) it undergoes annual cycles of filling and complete evaporation, and (2) it has a simple, well-defined hydrology. The major element concentrations of Merouane Chott lake waters have been measured weekly from January to June 2003. These concentrations are used together with estimates of the chott lake volume to calculate the temporal evolution of the total mass of these major elements. Dividing the first derivative of these masses with respect to time by the chott surface area yields precipitation rate estimates for halite, calcite, gypsum, and K–Mg salts during the complete evaporation of the lake. These rates are compared with the saturation indexes of these minerals to deduce the degree to which they are consistent with laboratory measured rates available in the literature. 相似文献
755.
756.
Social vulnerability indices: a comparative assessment using uncertainty and sensitivity analysis 总被引:2,自引:1,他引:2
Eric Tate 《Natural Hazards》2012,63(2):325-347
Social vulnerability indices have emerged over the past decade as quantitative measures of the social dimensions of natural hazards vulnerability. But how reliable are the index rankings? Validation of indices with external reference data has posed a persistent challenge in large part because social vulnerability is multidimensional and not directly observable. This article applies global sensitivity analyses to internally validate the methods used in the most common social vulnerability index designs: deductive, hierarchical, and inductive. Uncertainty analysis is performed to assess the robustness of index ranks when reasonable alternative index configurations are modeled. The hierarchical design was found to be the most accurate, while the inductive model was the most precise. Sensitivity analysis is employed to understand which decisions in the vulnerability index construction process have the greatest influence on the stability of output rankings. The deductive index ranks are found to be the most sensitive to the choice of transformation method, hierarchical models to the selection of weighting scheme, and inductive indices to the indicator set and scale of analysis. Specific recommendations for each stage of index construction are provided so that the next generation of social vulnerability indices can be developed with a greater degree of transparency, robustness, and reliability. 相似文献
757.
758.
Eric Hsueh-Chan Lu Chen-Hao Liao 《International journal of geographical information science》2020,34(9):1873-1901
ABSTRACT Finding a parking space is usually challenging in urban areas. The literature shows that 30% of traffic congestion is caused by searching for parking spaces, which results in unnecessary energy consumption and environmental pollution. With the development of sensor technologies, smart parking guidance systems provide users with a variety of real-time parking space information. However, users cannot know whether the target parking space remains available upon arrival. Moreover, parking resources may be under competition when multiple users target the same open parking space. In this research, we develop a new framework named prediction-based parking allocation (PPA) that provides smart parking services to users. In PPA, we first construct a prediction model of parking occupancy and predict the subsequent parking availabilities. Then, we design a matching-based allocation strategy to assign users to selected parking spaces. To the best of our knowledge, this is the first study that combines occupancy prediction and space allocation simultaneously to address smart parking issues. Finally, we collect a real dataset from the SFPark on-street parking system for performance evaluation. According to experimental results, PPA can effectively increase the parking success rate and reduce costs, fuel consumption, and carbon emissions. 相似文献
759.
Mark A. Green George G. Waldbusser Lane Hubazc Eric Cathcart Joshua Hall 《Estuaries and Coasts》2013,36(1):18-27
After a pelagic larval phase, infaunal bivalves undergo metamorphosis and transition to the underlying sediments to begin the benthic stage of their life history, where they explore and then either accept or reject sediments. Although the settlement cues used by juvenile infaunal bivalves are poorly understood, here we provide evidence that carbonate saturation state is a significant chemical cue in both direct observation laboratory studies and field manipulations. In the laboratory, plantigrade-stage Mercenaria mercenaria (200 μm shell height) showed a significant positive relationship between percent burrowed and Ωaragonite, with an increasing probability of settlement with increasing saturation state. In the field, we increased bivalve recruitment by a factor of three in a 30-day field study by raising the pH (~0.3) and saturation state of surface sediments by buffering sediments with crushed shell (CaCO3). The susceptibility of infaunal bivalves to dissolution mortality and the tight coupling of other sedimentary biogeochemical processes with carbonate dynamics suggest that mineral thermodynamics may be an overarching cue new settlers are responding to. 相似文献
760.
Terry Coppock Eric Anderson 《International journal of geographical information science》2013,27(3):201-202
Abstract With the increase in volume of spatial data now available, more effective ways must be found of storing and processing these data. This paper presents a compacted version of the linear quadtree and a spatially-referenced index method that can significantly reduce the storage requirements of a set of images and the time taken to process spatial queries. The index acts as a high-level summary of a regular-sized portion of the underlying image and so can be used to avoid examining areas of the image where none of the required features is present. Some example results are given. A method for the optimization of spatial searches is presented which takes into account the area and distribution of features within an image. Finally, a method for directly associating the edges of features with the individual nodes of a quadtree is reported. This is important since the edges of objects are no longer explicitly present in linear quadtrees and so must be recalculated when they are required for part of a query. Recalculation of object edges or boundaries is expensive; it is best, therefore, to perform the operation once only, and then save the results. 相似文献