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41.
Hana Grison Eduard Petrovský Neli Jordanova Aleš Kapička 《Studia Geophysica et Geodaetica》2011,55(4):697-716
The enhanced magnetic susceptibility of modern soils is assumed to have several reasons including, e.g., weathering of an
iron-rich geological basement, natural fires, bacterial processes and atmospheric deposition of anthropogenic particles. We
report on a case where none of the above sources of magnetic enhancement is evident: a modern soil with high magnetic susceptibility
over the whole soil profile, developed on nonmagnetic limestones, in an area with no industrial activities. The surface magnetic
susceptibility varies from 60 to 110×10−5 SI, while that of the rock basement is nearly zero. Moreover, significant frequency-dependent magnetic susceptibility (>
12%) suggests that superparamagnetic secondary magnetite/maghemite plays an important role. Possible mechanisms, responsible
for this magnetic enhancement, are discussed. 相似文献
42.
We compared measurements of 126 501 spiral galaxies to test whether the photometry of galaxies that rotate clockwise is different from the photometry of galaxies that rotate counterclockwise for the purpose of testing whether there is a link between photometry and spin direction of galaxies. The rotation directionality of the galaxies was determined by converting the galaxy image to its radial intensity plot, and then the galaxies in each 30° RA sector were separated into clockwise and counterclockwise rotating galaxies. The mean and standard deviation of SDSS DR7 photometric attributes of clockwise and counterclockwise rotating galaxies were then compared. The results show no significant difference between galaxies that rotate clockwise and galaxies that rotate counterclockwise. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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44.
In this paper, quasistatic models are developed for the slow flow of compressible fluids through porous solids, where the solid exhibits fading memory viscoelasticity. Problems of this type are important in practical geomechanics contexts, for example, in the context of fluid flow through unconsolidated reservoir sands and of wellbore deformation behaviour in gas and oil shale reservoirs, all of which have been studied extensively. For slow viscous fluid flow in the poro-viscoelastic media we are able to neglect the dynamic effects related to inertia forces, as well as the dissipation associated with the viscous flows. This is in contrast to the vast body of work in the poro-elastic context, where much faster flow of the viscous fluids may give rise to memory effects associated with microflows in pores of the solid media. Such problems have been treated extensively in both the dynamic and quasistatic cases. We are taking a specific type of the porous medium subject to slow deformation processes possibly inducing moderate pressure gradients and flow rates characterised by negligible inertia effects. As the result of homogenisation of such a two-phase medium, we observe the fading memory behaviour in the Biot modulus which controls the pressure increase due to skeleton macroscopic deformation and pore fluid content. Although our derivation leads to a result which is consistent with the formal phenomenological approach proposed by Biot (J Appl Phys 23:1482–1498, 1962), we offer the reader more insight into the structure of the poro-viscoelastic constitutive relations obtained; in particular, we can show that the Biot compressibility evolves in time according to the creep function while the skeleton stiffness is driven by the relaxation function. 相似文献
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46.
Solar flare accelerated electrons escaping into the interplanetary space and seen as type III solar radio bursts are often detected near the Earth. Using numerical simulations we consider the evolution of energetic electron spectrum in the inner heliosphere and near the Earth. The role of Langmuir wave generation, heliospheric plasma density fluctuations, and expansion of magnetic field lines on the electron peak flux and fluence spectra is studied to predict the electron properties as could be observed by Solar Orbiter and Solar Probe Plus. Considering various energy loss mechanisms we show that the substantial part of the initial energetic electron energy is lost via wave–plasma processes due to plasma inhomogeneity. For the parameters adopted, the results show that the electron spectrum changes mostly at the distances before ~?20 R ⊙. Further into the heliosphere, the electron flux spectrum of electrons forms a broken power law relatively similar to what is observed at 1 AU. 相似文献
47.
Jeremy J. Gabriel Eduard G. Reinhardt Matthew C. Peros Dawn E. Davidson Peter J. van Hengstum Patricia A. Beddows 《Journal of Paleolimnology》2009,42(2):199-213
A 61-cm core was obtained from 4 m below the water table in Cenote Aktun Ha, on the Yucatan Peninsula, Mexico. The cenote
is 8.6 km from the Caribbean coast and its formation and evolution have been largely affected by sea-level change. The base
of the core dates to 6,940–6,740 cal year BP and overlying sediments were deposited rapidly over the subsequent ~200 years.
The pollen record shows that the cenote evolved from a marsh dominated by red mangrove (Rhizophora mangle) and fern (Polypodiaceae) to an open-water system. These vegetation changes were controlled by water level and salinity and
are thus useful indicators of past sea level. At the base, the δ13Corg isotopic ratios reveal the influence of terrestrial vegetation (−29‰ VPDB), but shift to more negative values up-core (−33‰),
indicating an influence from particulate matter in the flooded cenote pool. Although microfossil populations were nearly absent
through most of the core, the microfossil assemblage in the upper 6 cm of the core is dominated by the juvenile foraminifer
Ammonia tepida and the thecamoebian genus Centropyxis. These populations indicate open-water conditions in the cenote and a major environmental shift around 6,600 cal year BP,
which is related to sea-level rise in the Caribbean basin. These data fit well with previously established sea-level curves
for the Caribbean Sea. Our reconstruction of the environmental history of Cenote Aktun Ha helps elucidate the floral and hydrological
history of the region, and highlights the utility of cenote sediments for studying the Holocene sea-level history of the Caribbean
Sea. 相似文献
48.
Stéphane Homke Jaume Vergés Peter Van Der Beek Manel Fernàndez Eduard Saura Luis Barbero Balazs Badics Erika Labrin 《Basin Research》2010,22(5):659-680
We present the first fission‐track (FT) thermochronology results for the NW Zagros Belt (SW Iran) in order to identify denudation episodes that occurred during the protracted Zagros orogeny. Samples were collected from the two main detrital successions of the NW Zagros foreland basin: the Palaeocene–early Eocene Amiran–Kashkan succession and the Miocene Agha Jari and Bakhtyari Formations. In situ bedrock samples were furthermore collected in the Sanandaj‐Sirjan Zone. Only apatite fission‐track (AFT) data have been successfully obtained, including 26 ages and 11 track‐length distributions. Five families of AFT ages have been documented from analyses of in situ bedrock and detrital samples: pre‐middle Jurassic at ~171 and ~225 Ma, early–late Cretaceous at ~91 Ma, Maastrichtian at ~66 Ma, middle–late Eocene at ~38 Ma and Oligocene–early Miocene at ~22 Ma. The most widespread middle–late Eocene cooling phase, around ~38 Ma, is documented by a predominant grain‐age population in Agha Jari sediments and by cooling ages of a granitic boulder sample. AFT ages document at least three cooling/denudation periods linked to major geodynamic events related to the Zagros orogeny, during the late Cretaceous oceanic obduction event, during the middle and late Eocene and during the early Miocene. Both late Cretaceous and early Miocene orogenic processes produced bending of the Arabian plate and concomitant foreland deposition. Between the two major flexural foreland episodes, the middle–late Eocene phase mostly produced a long‐lasting slow‐ or nondepositional episode in the inner part of the foreland basin, whereas deposition and tectonics migrated to the NE along the Sanandaj‐Sirjan domain and its Gaveh Rud fore‐arc basin. As evidenced in this study, the Zagros orogeny was long‐lived and multi‐episodic, implying that the timing of accretion of the different tectonic domains that form the Zagros Mountains requires cautious interpretation. 相似文献
49.
Approximate Analytical Theory of Satellite Orbit Prediction Presented in Spherical Coordinates Frame
Gennadii Alimov Alexhander Greb Eduard Kuznetsov Elena Polyakhova 《Celestial Mechanics and Dynamical Astronomy》2001,81(3):219-234
A comparative review of analytic theories for the motion of Earth satellites in quasi-circular orbits written in the spherical coordinate frame is presented. The theory of motion is developed for satellites in quasi-circular and quasi-equatorial orbits subjected to geopotential, luni-solar and solar radiation pressure force perturbations. The intermediate orbit is Keplerian and the equations of motion are solved by the Lyapunov–Poincaré small parameter method. Both resonant and non-resonant cases are considered. The results can be useful for the development of a complete theory of weakly eccentric orbits. 相似文献
50.