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391.
William F. Tanner 《Sedimentary Geology》1980,25(3):223-230
Indian mounds, near Careyville, Florida, about 2.0 m high, are located on hillsides and hilltops 10 to 20 m above the floodplain of the nearest river (Choctawhatchee). Each mound is composed largely of quartz sand, with a scattering of artefacts and stream pebbles (not in layers), but with no visible bedding.Probability plots showed 25 Gaussian distributions, 18 having the ‘dune hump’, three having the ‘surf break’ and nine being doubly-truncated or having other patterns of unknown or uncertain origin. The surf breaks probably were inherited from pre-Pleistocene marine terraces in the area. The pebbles and the sand were not introduced by the same agency. The sand probability plots, taken as a set, indicate an eolian origin. The rough symmetry of the mounds, and the lack of cross-bedding, argue against a migrating dune origin.On a variability plot (showing the variability of the means versus the variability of the standard deviations), one suite of samples fell clearly within the ‘dune’ number field, a second suite in the overlap area between ‘dune’ and ‘beach’, and a third suite, taken immediately adjacent to a creek bed, plotted in the overlap area between ‘beach’ and ‘coastal plain stream’.The pebbles, of common Southern Appalachian types, are attributed to the activities of the inhabitants, perhaps children. The sand is thought to have been carried by the wind, perhaps from nearby river sand bars, or from areas burned either by lightning-set wildfires or as part of “slash-and-burn” agriculture. The mounds are thought to represent clearings (for huts), and hence good trapping devices for wind-borne sand. 相似文献
392.
G. Bellieni E. M. Piccirillo R. Petrini V. A. V. Girardi A. B. Menezes Leal W. Teixeira L. R. Bastos Leal A. De Min P. Comin Chiaramonti M. A. F. Tanner de Oliveira 《Contributions to Mineralogy and Petrology》1995,122(3):252-261
: Early Proterozoic (2.0–2.4 Ga), unmetamorphosed mafic dykes intrude Archean (3.1–2.7 Ga) terrains of the northern Sao Francisco
craton (Uaua, NE-Brazil). The dykes are composed of evolved [atomic Mg/(Mg+Fe2)<0.6] two-pyroxene quartz tholeiites, the compositional variations of which are compatible with gabbro fractionation from
different parental melts. The incompatible trace-element patterns indicate that the parental melts derived from partial melting
of chemically heterogeneous garnet peridotite sources. Geochemistry and Sr-Nd isotopes imply some contamination of the Uaua
dyke magma by continental crustal components during emplacement. Sr-isotopes suggest that the “uncontaminated” dykes may be
related to time-integrated depleted mantle materials, while Nd-isotopes suggest mantle sources similar to the bulk Earth composition.
The “uncontaminated” dykes have a positive Nb-anomaly and incompatible element contents which do not support appreciable enrichment
of large ion lithophile and light rare earth elements in the source(s) throughmantle metasomatism and/or crustal components
related to dehydration of a subducting slab, and an anorogenic ensialic emplacement is inferred. The Uaua dykes are chemically
and isotopically distinct from the Early–Middle Proterozoic (1.9–1.7 Ga) unmetamorphosed dykes from W-Uruguay (Rio de La Plata
craton) which are believed to have originated from different garnet peridotite sources. This suggests that Uaua–Uruguay compositional
differences may reflect different Archean mantle differentiation and evolution of the cratonization processes. If this possibility
proves to be correct, then magma genesis from various South American cratons would be by independent cratonic processes and
their imprints on the subcratonic mantle.
Received: 22 November 1994/Accepted: 24 July 1995 相似文献
393.
394.
Nathalie Pisot Jean-Paul Xavier Vronique Miegebielle Dominique Coquelet Pierre Leymarie 《ISPRS Journal of Photogrammetry and Remote Sensing》1993,48(6)
This paper illustrates the benefits of DTMs created from SPOT images for the exploration of sedimentary basins. We chose an example located in the Ebro sedimentary basin in Spain, characterized by good outcropping conditions and slight deformations. The data used consist of a pair of SPOT panchromatic images and a SPOT XS image. The work consists of making up a 3D database, followed by interpretation of stereo pairs computed from orthoimages and the DTM. This interpretation is made on a stereoscopic desk and results in a digital file containing in the form of vectors all the observed faults and lithology. These vectors are then used to make calculations on the geometry of the objects they represent: we demonstrate that we can accurately measure layer directions and dips, sediment thicknesses and fault throws. Synthesis of perspective views made at the same time provide a good understanding of the structures and help to test their geometric consistency. Thus knowledge about relief given by DTMs helps to interpret remote detection images in 3D space, and particularly to accurately quantify the results of this interpretation. 相似文献
395.
396.
397.
Koen de Jong Bo Wang Michel Faure Liangshu Shu Dominique Cluzel Jacques Charvet Gilles Ruffet Yan Chen 《International Journal of Earth Sciences》2009,98(6):1239-1258
Laser-probe dating of mylonite whole-rock samples from the North Tianshan—Main Tianshan fault zone that cross-cuts the North
Tianshan domain’s southern margin yielded 40Ar/39Ar spectra with 255–285 Ma ages. Biotite from an undeformed, Early Carboniferous granite, which cuts the steep mylonitic foliation
in the Proterozoic basement of the Yili arcs’s southern margin, gave a 263.4 ± 0.6 Ma plateau age (1σ). Pre-Carboniferous
metasediments overlying this basement yielded plateau ages (1σ) of 253.3 ± 0.3 (muscovite) and 252.3 ± 0.3 (biotite) Ma. The
Permian ages of mylonites date movement on these ductile, dextral strike-slip shear zones, whereas the mica ages are interpreted
by recrystallisation as a result of fluid flow around such transcurrent faults. We propose that the Tianshan’s Permian syn-tectonic
bimodal magmatism was created in a non-plume-related Yellowstone-like extensional–transtensional tectonic regime. Gold mineralisation,
tracing aqueous flow in the crust, peaked in Permian time and continued locally into the Triassic. The picture is emerging
that a convective fluid system partly driven by magmatic heat, existed in a strongly fractured and weakened crust with an
elevated heat flow, leading to regional-scale isotope resetting. We suggest that surprisingly young isotopic ages in the literature
for early orogenic (ultra)high-pressure metamorphism are similarly due to fluid-mediated recrystallisation. 相似文献
398.
R. Secanell D. Bertil C. Martin X. Goula T. Susagna M. Tapia P. Dominique D. Carbon J. Fleta 《Journal of Seismology》2008,12(3):323-341
A unified probabilistic seismic hazard assessment (PSHA) for the Pyrenean region has been performed by an international team
composed of experts from Spain and France during the Interreg IIIA ISARD project. It is motivated by incoherencies between
the seismic hazard zonations of the design codes of France and Spain and by the need for input data to be used to define earthquake
scenarios. A great effort was invested in the homogenisation of the input data. All existing seismic data are collected in
a database and lead to a unified catalogue using a local magnitude scale. PSHA has been performed using logic trees combined
with Monte Carlo simulations to account for both epistemic and aleatory uncertainties. As an alternative to hazard calculation
based on seismic sources zone models, a zoneless method is also used to produce a hazard map less dependant on zone boundaries.
Two seismogenic source models were defined to take into account the different interpretations existing among specialists.
A new regional ground-motion prediction equation based on regional data has been proposed. It was used in combination with
published ground-motion prediction equations derived using European and Mediterranean data. The application of this methodology
leads to the definition of seismic hazard maps for 475- and 1,975-year return periods for spectral accelerations at periods
of 0 (corresponding to peak ground acceleration), 0.1, 0.3, 0.6, 1 and 2 s. Median and percentiles 15% and 85% acceleration
contour lines are represented. Finally, the seismic catalogue is used to produce a map of the maximum acceleration expected
for comparison with the probabilistic hazard maps. The hazard maps are produced using a grid of 0.1°. The results obtained
may be useful for civil protection and risk prevention purposes in France, Spain and Andorra. 相似文献
399.
Dominique Bockelée-Morvan 《Astrophysics and Space Science》2008,313(1-3):183-189
This paper presents a prospect for the observations of comets with ALMA. Thanks to unprecedented sensitivity, angular resolution
and instantaneous uv-coverage, key measurements on a number of topics related to the chemical and physical properties of the
coma and the nucleus will be obtained. These include (1) the identification of new molecular species and measurements of key
isotopic ratios, (2) measurements of the composition of short-period comets coming from the trans-Neptunian scattered disc,
to investigate chemical diversity within the whole comet population, (3) imaging of gas jets and their relationship with dust
features, (4) the study of extended sources of gas in the coma, and (5) the study of the physical and outgassing properties
of the nucleus. 相似文献
400.