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361.
Non-stationary spatial covariance structure estimation in oversampled domains by cluster differences scaling with spatial constraints 总被引:2,自引:2,他引:0
J. F. Vera R. Macías J. M. Angulo 《Stochastic Environmental Research and Risk Assessment (SERRA)》2008,22(1):95-106
In the analysis of spatiotemporal processes underlying environmental studies, the estimation of the non-stationary spatial
covariance structure is a well known issue in which multidimensional scaling (MDS) provides an important methodological approach
(Sampson and Guttorp in J Am Stat Assoc 87:108–119, 1992). It is also well known that approximating dispersion by a non-metric
MDS procedure offers, in general, low precision when accurate differences in spatial dispersion are needed for interpolation
purposes, specially if a low dimensional configuration is employed besides a high number of stations in oversampled domains.
This paper presents a modification, consisting of including geographical spatial constraints, of Heiser and Groenen’s (Psychometrika
62:63–83, 1997) cluster differences scaling algorithm by which not the original stations but the cluster centres can be represented,
while the stations and clusters retain their spatial relationships. A decomposition of the sum of squared dissimilarities
into contributions from several sources of variation can be employed for an exploratory diagnosis of the model. Real data
are analyzed and differences between several cluster-MDS strategies are discussed. 相似文献
362.
A new Silurian palaeolatitude for eastern Avalonia and evidence for crustal rotations in the Avalonian margin of southwestern Ireland 总被引:1,自引:0,他引:1
Conall Mac Niocaill 《Geophysical Journal International》2000,141(3):661-671
Palaeomagnetic data are presented from Mid-Silurian (Homerian, Upper Wenlock, ~425 Ma) sediments from the Dingle Peninsula, SW Ireland, which forms part of the northern margin of the Palaeozoic microcontinent of Avalonia. Three remanence components were recognized. After removal of a low-temperature component ('L'), oriented parallel to the present Earth field at the sampling area, two higher-stability components were isolated: an intermediate-unblocking-temperature component ('I') with mean in situ D = 196.9°, I = 11.0°, α 95 = 10.8, with a corresponding palaeopole at 330.0°E, 30.6°S ( dp = 5.6, dm = 11.0), and a high-unblocking-temperature component ('H') with mean tilt-corrected D = 218.6°, I = 22.1°, α 95 = 7.9, with a corresponding palaeopole at 309.5°E, 18.3°S ( dp = 4.4, dm = 8.4). A primary (Wenlock) age is indicated for the 'H'-component by a positive intraformational conglomerate test, whereas the 'I'-component is thought to be a secondary mid-Carboniferous partial remagnetization.
These data confirm that the sector of the Iapetus Ocean between Avalonia and Laurentia was essentially closed, within the limits of palaeomagnetic resolution, by the Wenlock. There is still, however, a discrepancy between the declinations recorded by similar-aged sequences to the north and south of the Iapetus Suture. These point to either an approximately 30° clockwise rotation of the entire Avalonian microcontinent relative to Laurentia during closure, or local vertical axis rotations of the sampling sites in southern Britain. 相似文献
These data confirm that the sector of the Iapetus Ocean between Avalonia and Laurentia was essentially closed, within the limits of palaeomagnetic resolution, by the Wenlock. There is still, however, a discrepancy between the declinations recorded by similar-aged sequences to the north and south of the Iapetus Suture. These point to either an approximately 30° clockwise rotation of the entire Avalonian microcontinent relative to Laurentia during closure, or local vertical axis rotations of the sampling sites in southern Britain. 相似文献
363.
364.
Gas hydrate decomposition recorded by authigenic barite at pockmark sites of the northern Congo Fan 总被引:1,自引:0,他引:1
Sabine Kasten Kerstin N?then Christian Hensen Volkhard Spie? Martin Blumenberg Ralph R. Schneider 《Geo-Marine Letters》2012,32(5-6):515-524
The geochemical cycling of barium was investigated in sediments of pockmarks of the northern Congo Fan, characterized by surface and subsurface gas hydrates, chemosynthetic fauna, and authigenic carbonates. Two gravity cores retrieved from the so-called Hydrate Hole and Worm Hole pockmarks were examined using high-resolution pore-water and solid-phase analyses. The results indicate that, although gas hydrates in the study area are stable with respect to pressure and temperature, they are and have been subject to dissolution due to methane-undersaturated pore waters. The process significantly driving dissolution is the anaerobic oxidation of methane (AOM) above the shallowest hydrate-bearing sediment layer. It is suggested that episodic seep events temporarily increase the upward flux of methane, and induce hydrate formation close to the sediment surface. AOM establishes at a sediment depth where the upward flux of methane from the uppermost hydrate layer counterbalances the downward flux of seawater sulfate. After seepage ceases, AOM continues to consume methane at the sulfate/methane transition (SMT) above the hydrates, thereby driving the progressive dissolution of the hydrates “from above”. As a result the SMT migrates downward, leaving behind enrichments of authigenic barite and carbonates that typically precipitate at this biogeochemical reaction front. Calculation of the time needed to produce the observed solid-phase barium enrichments above the present-day depths of the SMT served to track the net downward migration of the SMT and to estimate the total time of hydrate dissolution in the recovered sediments. Methane fluxes were higher, and the SMT was located closer to the sediment surface in the past at both sites. Active seepage and hydrate formation are inferred to have occurred only a few thousands of years ago at the Hydrate Hole site. By contrast, AOM-driven hydrate dissolution as a consequence of an overall net decrease in upward methane flux seems to have persisted for a considerably longer time at the Worm Hole site, amounting to a few tens of thousands of years. 相似文献
365.
Alexander Hubbard Mordecai-Mark Mac Low Denton S. Ebel 《Meteoritics & planetary science》2018,53(7):1507-1515
Meteoritical and astrophysical models of planet formation make contradictory predictions for dust concentration factors in chondrule-forming regions of the solar nebula. Meteoritical and cosmochemical models strongly suggest that chondrules, a key component of the meteoritical record, formed in regions with solids-to-gas mass ratios orders above the solar nebula average. However, models of dust grain dynamics in protoplanetary disks struggle to surpass concentration factors of a few except during very short-lived stages in a dust grain's life. Worse, those models do not predict significant concentration factors for dust grains the size of chondrule precursors. We briefly develop the difficulty in concentrating dust particles in the context of nebular chondrule formation and show that the disagreement is sufficiently stark that cosmochemists should explore ideas that might revise the concentration factor requirements downward. 相似文献
366.
Luciana Bianchi Graziela R. Keller Ralph Bohlin Martin Barstow Sarah Casewell 《Astrophysics and Space Science》2018,363(8):166
Hot WDs in binary systems with a less evolved star are particularly invaluable astrophysical probes, the unevolved companion enabling better derivation of distance and age than is usually possible for post-AGB objects, and therefore also of their radius and luminosity. But hot white dwarfs (WD) are elusive at all wavelengths except the UV (Bianchi et al. 2011a). From our GALEX UV source catalogs (Bianchi et al. 2011a,b, 2014, 2017) matched to SDSS, we identified thousands of candidate hot WDs including WDs in binary systems consisting of a hot WD and a companion of spectral type from A to M. The identification and preliminary characterization of the stellar parameters is based on the analysis of the photometric SED from far-UV to z-band.We have observed subsamples of the UV-selected WDs with the Hubble Space Telescope (HST) to better characterize their stellar parameters. We obtained (1) UV spectroscopy with STIS and analyzed the UV spectra together with optical SDSS spectra, and (2) multi-band imaging with WFC3 (\(0.04^{\prime\prime}/\mbox{pixel}\)) to measure angular separation and individual SEDs of the pair’s components in binary systems. In our HST/WFC3 sample of 59 hot-WD binaries with late-type companions, we found that at least a dozen have possibly evolved without exchanging mass. The UV STIS spectroscopy led to the revision of previous results based on optical spectra only, because of the often undetectable or unquantifiable contribution of the hot component to the optical fluxes. 相似文献
367.
Towards a more ecologically relevant assessment of the impact of heavy metals on the photosynthesis of the seagrass,Zostera capricorni 总被引:5,自引:0,他引:5
This in situ study used photosynthetic activity (measured as chlorophyll a fluorescence) and photosynthetic pigment concentrations to assess the effect of copper, cadmium, lead and zinc on the seagrass Zostera capricorni. Custom-made portable in situ exposure (PIE) chambers were developed so seagrasses could be dosed within the meadow. Z. capricorni was exposed to 0.1 and 1 mg l−1 of metal solutions for 10 h. During this time and for the subsequent four-day recovery period, the effective quantum yield of photosystem II (PS II) (ΔF/Fm′) was measured. While the results were variable, copper and zinc exposed samples had a depressed ΔF/Fm′ during the exposure period. Samples exposed to zinc recovered to pre-exposure levels but those exposed to copper did not. Cadmium and lead did not impact on the chlorophyll a fluorescence and the chlorophyll pigment data supported these findings. This study presents an innovative new application of chlorophyll a fluorescence stress assessment. 相似文献
368.
Ralph A.KAHN 《中国科学D辑(英文版)》2009,52(1):132-144
The Multiangle Imaging SpectroRadiometer (MISR) launched by NASA in late 1999 has a unique multiangle design, which points
nine cameras at fixed angles along the satellite flight track and collects reflected solar radiation simultaneously. This
design allows the retrieval of a rich dataset of particle abundance, shape and composition over both land and ocean. Some
of its capabilities have not been seen by any currently operating satellite aerosol sensors. Since MISR is sensitive to fine
particles, it provides a new data source to study the spatial and temporal characteristics of air quality over large geographical
regions. We first briefly introduce the MISR instrument, the retrieval and structure of MISR aerosol data, and then review
the applications of MISR aerosol data in various aspects of air quality research since its launch. These include the spatial
distributions of particle pollution events such as dust storms, wild fires, and urban pollution. Because of the high quality
of MISR aerosol data, they can be used as quantitative indicators of particle pollution levels. We review the current modeling
studies of surface level particle concentrations. Next, we introduce research results using MISR’s advanced data such as the
plume heights, and particle microphysical properties. In the discussion, we compare MISR research with current MODIS research
to the best of our ability as MODIS data have been more extensively explored by the Chinese scientific community. Finally,
we summarize the advantages and disadvantages of MISR data related to its applications to the air quality research. Given
the highly quantitative measurements and comprehensive aerosol information MISR can provide, we believe that it will provide
great values to advance our understanding of the particle air pollution in China.
Supported by Harvard-EPA Center on Particle Health Effects (Grant Nos. R-827353 and R-832416), NASA’s Climate and Radiation
Research and Analysis Program, the EOS-MISR Instrument Project and the National High Technology Research and Development Program
of China (Grant No. 2006AA06A305). 相似文献
369.
C. Ortega-Obregón L. Solari A. Gómez-Tuena M. Elías-Herrera F. Ortega-Gutiérrez C. Macías-Romo 《International Journal of Earth Sciences》2014,103(5):1287-1300
Undeformed felsic to mafic igneous rocks, dated by U–Pb zircon geochronology between 311 and 255 Ma, intrude different units of the Oaxacan and Acatlán metamorphic complexes in southwestern Mexico. Rare earth element concentrations on zircons from most of these magmatic rocks have a typical igneous character, with fractionated heavy rare earths and negative Eu anomalies. Only inherited Precambrian zircons are depleted in heavy rare earth elements, which suggest contemporaneous crystallization in equilibrium with metamorphic garnet during granulite facies metamorphism. Hf isotopic signatures are, however, different among these magmatic units. For example, zircons from two of these magmatic units (Cuanana pluton and Honduras batholith) have positive εHf values (+3.8–+8.5) and depleted mantle model ages (using a mean crustal value of 176Lu/177Hf = 0.015) (T DMC) ranging between 756 and 1,057 Ma, whereas zircons from the rest of the magmatic units (Etla granite, Zaniza batholith, Carbonera stock and Sosola rhyolite) have negative εHf values (?1 to ?14) and model ages between 1,330 and 2,160 Ma. This suggests either recycling of different crustal sources or, more likely, different extents of crustal contamination of arc-related mafic magmas in which the Oaxacan Complex acted as the main contaminant. These plutons thus represent the magmatic expression of the initial stages of eastward subduction of the Pacific plate beneath the western margin of Gondwana, and confirm the existence of a Late Carboniferous–Permian magmatic arc that extended from southern North America to Central America. 相似文献
370.
Al-Saedi Hasan N. Flori Ralph E. Alkhamis Mohammed Brady Patrick V. 《Natural Resources Research》2019,28(1):213-221
Natural Resources Research - In this study, we combined low-salinity (LS) water and steam as a novel enhanced oil recovery (EOR) method that can provide additional oil recovery up to 63% of... 相似文献