全文获取类型
收费全文 | 1043篇 |
免费 | 88篇 |
国内免费 | 18篇 |
专业分类
测绘学 | 21篇 |
大气科学 | 91篇 |
地球物理 | 312篇 |
地质学 | 387篇 |
海洋学 | 69篇 |
天文学 | 189篇 |
综合类 | 9篇 |
自然地理 | 71篇 |
出版年
2023年 | 4篇 |
2022年 | 11篇 |
2021年 | 16篇 |
2020年 | 35篇 |
2019年 | 36篇 |
2018年 | 54篇 |
2017年 | 75篇 |
2016年 | 59篇 |
2015年 | 54篇 |
2014年 | 58篇 |
2013年 | 68篇 |
2012年 | 55篇 |
2011年 | 72篇 |
2010年 | 59篇 |
2009年 | 54篇 |
2008年 | 65篇 |
2007年 | 49篇 |
2006年 | 51篇 |
2005年 | 41篇 |
2004年 | 39篇 |
2003年 | 41篇 |
2002年 | 28篇 |
2001年 | 15篇 |
2000年 | 13篇 |
1999年 | 14篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 5篇 |
1994年 | 2篇 |
1993年 | 8篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1976年 | 2篇 |
1970年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有1149条查询结果,搜索用时 718 毫秒
991.
AbstractFour ductile shear zones were sampled in the autochthonous Thaya basement and the Upper Bíte? nappe (Moravian unit) at the Eastern margin of the Bohemian massif. In both studied units, the tectono-metamorphic evolution and the chemical mass transfer are different. Two deformational events are recognised: the first deformation stage under amphibolite facies conditions is overprinted by a second event under greenschist facies conditions.The first deformation affected the western margin of the Thaya basement and the whole Bíte? nappe: microstructures are characterised by dynamic recrystallisation of feldspars and quartz, and occurrence of myrmekites and grain-boundary migration of quartz. None or weak chemical mass transfer is related to this medium to high temperature deformation. This deformation corresponds to the thrusting of Moldanubdian units on the Brunovistulian units (Moravian nappes and autochthonous Thaya basement).The second deformation generated shear zones in the until then preserved Thaya basement and reactivated both shear zones of the western margin of the Thaya basement and those of the Bíte? nappe. This deformation is retrograde and mainly associated with chemical mass transfer: a decrease of CaO, FeO, FeO/Fe2O3 and an increase of MgO, K2O and H2O. These chemical changes are related to greenschist metamorphic reactions leading to the destabilisation of feldspars and the crystallisation of white micas and Ca-silicates. The large chemical mass transfer is associated with the circulation of a large volume of fluids. A model of progressive fluid circulation correlated with Variscan prograde and retrograde metamorphism during the collision of Moldanubian and Brunovistulian units is proposed. 相似文献
992.
Nickel-rich metasomatism of the lithospheric mantle by pre-kimberlitic alkali-S–Cl-rich C–O–H fluids
Andrea Giuliani Vadim S. Kamenetsky Mark A. Kendrick David Phillips Karsten Goemann 《Contributions to Mineralogy and Petrology》2013,165(1):155-171
Metasomatism of the lithospheric mantle sometimes produces unusual assemblages containing native metals and alloys, which provide important insight into metasomatic processes in the mantle. In this study, we describe the metasomatic enrichment of a refractory harzburgite xenolith in Ni, Fe and, to a lesser extent, Cu, Co, As and Sb. The xenolith (XM1/422) derives from the Bultfontein kimberlite (Kimberley, South Africa) and hosts Ni mineralisation that includes native nickel (Ni84.5-98.0), heazlewoodite (Ni3S2) and Ni-rich silicates (e.g. up to 37.5 wt % NiO in olivine, and 22.4 wt % NiO in phlogopite). The presence of several mineral phases enriched in alkali and volatile species (e.g. phlogopite, phosphates, carbonates, chlorides, djerfisherite) indicates that the transition metal cations were likely introduced during metasomatism by alkali-rich C–O–H fluids or alkali-carbonate melts. It is postulated that sulphide breakdown and fluid reaction with refractory mantle rocks contributed to the fluid’s enrichment in Ni and other metallic cations. The Ni-rich assemblages of xenolith XM1/422 show local chemical disequilibrium, and modelling of the Ni diffusion profiles adjacent to olivine-native nickel and olivine-heazlewoodite grain boundaries, suggests a close temporal relationship between Ni-rich metasomatism and subsequent entrainment by the kimberlite magma. However, metal-rich metasomatism has also been observed in other lithospheric mantle domains, including orogenic peridotitic massifs and the suboceanic mantle; regions unaffected by kimberlite magmatsim. As micro-scale occurrences of metallic phases are easily overlooked, it is possible that metal-rich metasomatism is more widespread in the Earth’s mantle than previously recognised. 相似文献
993.
The occurrence of roll-waves in mud-flows is investigated based on the formulation of the marginal stability threshold of a linearized onedimensional viscoplastic (shear-thinning) flow model. Since for this kind of non-Newtonian rheological models this threshold may occur in a hypocritical flow, the downstream boundary condition may have a nonnegligible effect on the spatial growth/decay of the perturbation. The paper presents the solution of the 1D linearized flow of a Herschel and Bulkley fluid in a channel of finite length, in the neighbourhood of a hypocritical base uniform flow. Both linearly stable and unstable conditions are considered. The analytical solution is found applying the Laplace transform method and obtaining the first-order analytical expressions of the upstream and downstream channel response functions in the time domain. The effects of both the yield stress and the rheological law exponent are discussed, recovering as particular cases both power-law and Bingham fluids. The theoretical achievements may be used to extend semi-empirical criteria commonly employed for predicting roll waves occurrence in clear water even to mud-flows. 相似文献
994.
Andrea Ballatore Peter Mooney 《International journal of geographical information science》2013,27(12):2310-2327
In crowdsourced cartographic projects, mappers coordinate their efforts through online tools to produce digital geospatial artefacts, such as maps and gazetteers, which were once the exclusive territory of professional surveyors and cartographers. In order to produce meaningful and coherent data, contributors need to negotiate a shared conceptualisation that defines the domain concepts, such as road, building, train station, forest and lake, enabling the communication of geographic knowledge. Considering the OpenStreetMap Wiki website as a case study, this article investigates the nature of this negotiation, driven by a small group of mappers in a context of high contribution inequality. Despite the apparent consensus on the conceptualisation, the negotiation keeps unfolding in a tension between alternative representations, which are often incommensurable, i.e., hard to integrate and reconcile. In this study, we identify six complementary dimensions of incommensurability that recur in the negotiation: (1) ontology, (2) cartography, (3) culture and language, (4) lexical definitions, (5) granularity, and (6) semantic overload and duplication. 相似文献
995.
Nieves R. Brisaboa M. Andrea Rodríguez Rodrigo A. Troncoso 《International journal of geographical information science》2013,27(2):280-304
A spatial data set is consistent if it satisfies a set of integrity constraints. Although consistency is a desirable property of databases, enforcing the satisfaction of integrity constraints might not be always feasible. In such cases, the presence of inconsistent data may have a negative effect on the results of data analysis and processing and, in consequence, there is an important need for data-cleaning tools to detect and remove, if possible, inconsistencies in large data sets. This work proposes strategies to support data cleaning of spatial databases with respect to a set of integrity constraints that impose topological relations between spatial objects. The basic idea is to rank the geometries in a spatial data set that should be modified to improve the quality of the data (in terms of consistency). An experimental evaluation validates the proposal and shows that the order in which geometries are modified affects both the overall quality of the database and the final number of geometries to be processed to restore consistency. 相似文献
996.
997.
Karoly Szego David T. Young Bruce Barraclough Andrew J. Coates Michele K. Dougherty Donald A. Gurnett Andrea Opitz Michelle F. Thomsen 《Planetary and Space Science》2006,54(2):200-211
In this paper we analyze a pre-shock event that we observed in the foot region of the quasi-parallel bow shock (BS) that the Cassini spacecraft crossed on 30 January 2001, at about 1030 UT. Before crossing the BS, the incoming solar wind first decelerated, and then the bulk velocity both of the proton and α components increased, the flow accelerated and decelerated, heated and cooled several times. We characterize the plasma in the foot using the data measured by the magnetometer, the radio and plasma wave science (RPWS) instrument, and the Cassini plasma spectrometer (CAPS) being carried onboard the Cassini spacecraft, and analyze the observations. We argue that the velocity and temperature changes can be caused by firehose instabilities excited by ions reflected from the shock. We investigate another possibility, shocklet formation, to account for the observed features, but conclude that this explanation seems to be less likely. In the foot we also identified both backstreaming electrons and ions and electrostatic waves in the 100-1000 Hz range very likely excited by the backstreaming electrons. 相似文献
998.
Direct measurements of the hydrological conditions for the occurrence of debris flows and of flow behaviour are of the outmost importance for developing effective flow prevention techniques. An automated and remotely controlled monitoring system was installed in Acquabona Creek in the Dolomites, Italian Eastern Alps, where debris flows occur every year. Its present configuration consists of three on‐site stations, located in the debris‐flow initiation area, in the lower channel and in the retention basin. The monitoring system is equipped with sensors for measuring rainfall, pore‐water pressure in the mobile channel bottom, ground vibrations, debris flow depth, total normal stress and fluid pore‐pressure at the base of the flow. Three video cameras take motion pictures of the events at the initiation zone, in the lower channel and in the deposition area. Data from the on‐site stations are radio‐transmitted to an off‐site station and stored in a host PC, from where they are telemetrically downloaded and used by the Padova University for the study of debris flows. The efficiency of the sensors and of the whole monitoring system has been verified by the analysis of data collected so far. Examples of these data are presented and briefly discussed. If implemented at the numerous debris‐flow sites in the Dolomitic Region, the technology used, derived from the development of this system, will provide civil defence and warn residents of impending debris flows. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
999.
Andrea Tiengo Paolo Esposito Sandro Mereghetti Lara Sidoli Diego Götz Marco Feroci Roberto Turolla Silvia Zane Gian Luca Israel Luigi Stella Peter Woods 《Astrophysics and Space Science》2007,308(1-4):33-37
We present a systematic analysis of all the BeppoSAX data of SGR1900+14. The observations spanning five years show that the source was brighter than usual on two occasions: ~20 days after the August 1998 giant flare and during the 105?s long X-ray afterglow following the April 2001 intermediate flare. In the latter case, we explore the possibility of describing the observed short term spectral evolution only with a change of the temperature of the blackbody component. In the only BeppoSAX observation performed before the giant flare, the spectrum of the SGR1900+14 persistent emission was significantly harder and detected also above 10 keV with the PDS instrument. In the last BeppoSAX observation (April 2002) the flux was at least a factor 1.2 below the historical level, suggesting that the source was entering a quiescent period. 相似文献
1000.