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91.
N. H. S. Oliver P. J. Pearson R. J. Holcombe A. Ord 《Australian Journal of Earth Sciences》2013,60(3):467-484
The Mary Kathleen U‐REE orebody of the Proterozoic Mt Isa Block was the product of chemical and physical interaction between regional metamorphic/hydrothermal fluids and preexisting calcic skarns. The deposit provides excellent examples of mechanical control on ore localisation and of the complexity of ores in rocks with protracted thermal histories. Host skarns were produced by contact metasomatism around the 1740 Ma Burstall Granite, whereas the allanite‐uraninite ore formed under amphibolite‐facies conditions, late during the D2 phase of the ca 1550–1500 Ma Isan orogeny. Observations of ore geometry are consistent with previous geochronologic data demonstrating a large time gap between skarn formation and ore genesis. Numerical modelling of coupled deformation and fluid flow suggests that veins at the core of ore shoots may have formed as tensile or shear fractures during coupling of the competent skarn host with the late‐D2 Mary Kathleen Shear Zone, allowing a change of orientation of ore shoots with distance from the shear zone. Mineral chemistry and petrographic observations suggest the possible role of a redox control on chemical localisation of ore by conversion of Fe2+‐rich clinopyroxene‐rich skarn host to Fe3+‐rich secondary garnet ‘skarn’ and uraninite‐allanite ore. Alternately, fluid pressure drops as a consequence of fracturing of the host skarn may have triggered fluid unmixing, or fluid mixing, leading to ore precipitation. Available data do not allow clear definition of the ultimate source of the U and REE, nor of the specific chemical ore‐forming mechanism. However, regional constraints, previous Sm–Nd modelling, and our numerical models suggest a combination from proximal skarn hosts and from distal sources accessed by flow of metamorphic and/or late tectonic igneous‐derived fluids. The deposit has some similarities with ironstone‐hosted Cu–Au ± U deposits found in the nearby Cloncurry Belt. 相似文献
92.
Darío Andrés Giagante Salvador Aliotta Silvia Susana Ginsberg Laura Gabriela Vecchi Jorge Osvaldo Spagnuolo 《Quaternary Research》2011,75(3):614-623
The purpose of this research is to analyze the seismostratigraphic and paleoenvironmental features of an ancient fluvial deposit characterized by the presence of paleochannels and sedimentary structures in Bahía Blanca estuary, Argentina. To this end, high-resolution seismic methods were used. Paleochannels exhibiting v-shaped cuts were found at different topographic positions at the base of this deposit. It was observed that channel silting is indicative of the relative change of river base level and the consequent migration of fluvial tributaries. This alluvial deposit is composed of low compacted fine sand and its middle–upper facies is characterized by the presence of horizontal and discontinuous wavy reflectors. The upper boundary of this deposit is an erosive discontinuity resulting from Holocene sea-level rise during which the mouth of old fluvial courses underwent changes. The deposit was subsequently buried by sandy and clayey silt sediments. The paleodrainage at subbottom indicates that this deposit is associated with an ancient river mouth. Based on the seismostratigraphic and lithological characteristics and the paleochannel structures found in the study area, it can be concluded that the deposit analyzed is an alluvial sequence formed in the period from the Middle-Late Pleistocene to Holocene marine transgression. 相似文献
93.
RésuméÀ la suite d’un violent orage, une lave torrentielle d’un volume supérieur à 100 000 m3 s’est répandue sur le cône de déjection du torrent du Bez, provoquant des dégâts considérables aux infrastructures. Les observations réalisées durant l’événement et les investigations menées dans le bassin versant nous permettent de mieux comprendre le déclenchement de cette lave. Le flot boueux marquant le début de l’événement a été en grande partie alimenté par les ravinements affectant le haut du bassin versant. La lave à blocs qui a suivi s’est nourrie quant à elle des dépôts morainiques recouvrant la rive gauche du torrent, dans la partie inférieure du bassin versant. Au lendemain de la crue, on pouvait observer deux grandes entailles d’une profondeur maximale de 15 m. L’instabilité des berges qui en résultait a alors été rapidement enrayée par d’importants travaux de terrassements et d’enrochements. © Elsevier, Paris 相似文献
94.
95.
Rodríguez Juan Sánchez Salvador García Antonio Blasco Manolo Villalonga Miquel A. 《Earth, Moon, and Planets》2002,90(1-4):185-194
The results from the analysis of the extensive CCD photometry that we haveobtained during the long-term monitoring (from its
discovery until its break-up)of comet C/1999 S4 (linear) are presented here. Our observations showthat the comet did not always
behave as expected: it showed activity ahead oftime and in the end became fainter instead of brighter as it approached perihelion.A
sequence of images and a lightcurve are shown. 相似文献
96.
We present a novel experiment with the capacity to independently measure both the electron density and the magnetic field
of the solar corona. We achieve this through measurement of the excess Faraday rotation resulting from propagation of the
polarised emission from a number of pulsars through the magnetic field of the solar corona. This method yields independent
measures of the integrated electron density, via dispersion of the pulsed signal and the magnetic field, via the amount of
Faraday rotation. In principle this allows the determination of the integrated magnetic field through the solar corona along
many lines of sight without any assumptions regarding the electron density distribution. We present a detection of an increase
in the rotation measure of the pulsar J1801-2304 of approximately 170 rad m2 at an elongation of 0.96° from the centre of the solar disc. This corresponds to a lower limit of the magnetic field strength
along this line of sight of >41.8 nT. The lack of precision in the integrated electron density measurement restricts this
result to a limit, but application of coronal plasma models can further constrain this to approximately 0.5 μT, along a path
passing 2.7 solar radii from the solar limb, which is consistent with predictions obtained using extensions to the source
surface models published by the Wilcox Solar Observatory. 相似文献
97.
The Quaternary: its character and definition 总被引:5,自引:0,他引:5
Martin J. Head Philip Gibbard Amos Salvador 《《幕》》2008,31(2):234-238
The Quaternary, is characterised by the development of widespread glaciations in mid-northern latitudes. As a chronostratigraphic term it has attracted vigorous debate. The Quaternao; as accepted by the International Union for Quaternary Research and proposed by the International Commission on Stratigraphy, begins at 2.6 Ma within a 2.8-2.4 Ma interval of profound change in Earth's climate system. 相似文献
98.
José Alfredo Ramos-Leal Janete Morán-Ramírez José Teodoro Silva-García Rosa María Fuentes-Rivas Gustavo Cruz-Cárdenas Salvador Ochoa-Estrada Francisco Estrada-Godoy 《Arabian Journal of Geosciences》2018,11(15):422
The present study characterized the hydrogeochemical processes of the aquifer of Ciénega de Chapala in Michoacán, Mexico. The dominant water families contained bicarbonate magnesium and sodium. In the region, water infiltrates into a fractured volcanic environment exposed in the surrounding mountains and is transmitted to the volcano-sedimentary units of the ciénega, where silicate alteration and ion exchange processes occur in the clays of the subsoil. The Gibb diagrams confirm that the main evolutionary processes in the aquifer are rock-water interactions in the local flow and evaporation in the intermediate and regional flows. The molar ratios of HCO3/Na and Ca/Na are congruent with the type of volcano-sedimentary environment present in the area. Ternary mixing processes associated with three end members were also identified and were related to the flow systems in the area. The local flow contributes 77% of water to the system, and the intermediate and regional flows contribute 16% and 7%, respectively. 相似文献
99.
This paper deals with the theoretical aspects of nonaqueous phase liquid (NAPL)‐dissolution‐induced instability in two‐dimensional fluid‐saturated porous media including solute dispersion effects.After some weaknesses associated with the previous work are analyzed and overcome, a comprehensive dimensionless number, known as the Zhao number, is proposed to represent the main driving force and three controlling mechanisms of an NAPL‐dissolution system that has a finite domain. The linear stability analysis is carried out to derive the critical value of the comprehensive dimensionless number of the NAPL‐dissolution system in a limit case as the ratio of the equilibrium concentration to the density of the NAPL approaches zero. As a result, a theoretical criterion that can be used to assess the instability of planar NAPL‐dissolution fronts in two‐dimensional fluid‐saturated porous media of finite domains has been established. Not only can the present theoretical results be used for the theoretical understanding of the effect of solute dispersion on the instability of an NAPL‐dissolution front in the fluid‐saturated porous medium of either a finite domain or an infinite domain, but also they can be used as benchmark solutions for verifying numerical methods employed to simulate detailed morphological evolution processes of NAPL‐dissolution fronts in two‐dimensional fluid‐saturated porous media. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
100.
This paper aims to assess the instability hazard of a granite boulder. The procedure was first to consider the geological
setting and geomorphologic features of the boulder in relation to typical granite landscape forms. Climatic and seismic data
were next obtained from different sources, and geomechanical parameters were measured in situ and in the laboratory, with
terrestrial laser scanning techniques used to measure shape and volume and to conduct a detailed survey of the boulder. Different
analytical approaches were then applied to the calculation of boulder safety factors against sliding and toppling. Since the
boulder was considered to be unstable in the worst possible scenario, a particle code approach was used to determine its fall
trajectory, calculate the final run out of the block, and assess the risk for houses located at the foot of the slope. Finally,
conclusions were drawn regarding the instability hazard, and recommendations were made in regard to definitively stabilizing
the granite boulder. 相似文献