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431.
C. Desnoyers M. Christophe Michel-Levy I.S. Azevedo R.B. Scorzelli J. Danon E. Galvo da Silva 《Meteoritics & planetary science》1985,20(1):113-124
The Bocaiuva iron contains 10 to 15% by volume of silicate inclusions which are surrounded by kamacite (6.5 wt % Ni). The metal shows a Widmanstätten pattern in metal areas devoid of silicates; taenite evolved in plessite fields. The silicate inclusions occur as nodules, and as irregular or chain-like aggregates in which olivine may be rounded or faceted. The magnesian silicates (forsterite, enstatite, diopside) are similar in composition to those of the group IAB irons, whereas the interstitial plagioclase is much more calcic (An 50) than that usually found. Iron sulfide occurs as pyrrhotite and contains 1–2 wt% Cu. Chromite and euhedral magnetite are accessory phases always associated with pyrrhotite. Some patches of pyrrhotite enclose rounded chromite and small plagioclase crystals displaying compositions different from those of the ground mass of the inclusions. Schreibersite shows a compositional variability. This preliminary study underlines the unusual nature of Ms iron and raises several questions concerning the genetic relations between silicates, sulfide and metal, and the thermal history of the whole material. 相似文献
432.
M. Christophe Michel-Lvy 《Meteoritics & planetary science》1978,13(3):305-309
An eventual correlation between petrologic type and porosity can only be found if measurements are made on unshocked or slightly shocked chondrites. The main advantage of the method (described earlier in this journal) is that it offers the possibility of correlating on a very small scale porosity with structural and petrographical features along with punctual chemical and isotopic data. Moreover, the method is relatively unpolluting and non destructive, and requires very small amounts of matter. Porosity measurements have been carried out on polished thin section of five H chondrites, after sample impregnation with a fluorescent resin. The microscope images were analysed with a Quantimet. Two types of information have been derived. First, the macroporosity of the material ranges up to 13% in Sena (H5) but falls to 4% in samples that have undergone slight shock or compression. Second, approximately 25% by volume of the stones consist of zones of great permeability which were fissured or filled with loose debris. This has been observed in all cases but the well-recrystallised Kernouve (H6) stone. Additional measurements are required to correlate petrology and porosity on a statistically significant basis. 相似文献
433.
Data collected during the french VOVES-1977 experiment are used for the determination of the functionsA() andB() of the stability parameter . It is found that, although pertaining to different geographic conditions, they lead to the same kind of dependency ofA andB upon as found previously considered data. The influence of baroclinicity onA andB is also studied and it is found in that, statistically, consideration of this parameter does not lead either to a decrease of the large scatter of experimental points or to a significant improvement of the similarity theory. It is also shown that a satisfactory estimation of the surface geostrophic wind can be made from the network of meteorological stations, to compute such functions. 相似文献
434.
A basalt and its weathered profile have been analysed for major elements. Graphical examination shows Al. Fe and Ti to have been essentially immobile. Determining mobility sequences for this deposit is complicated by the long pedogenetic history of the area. Three different sequences are presented, two for different stages of weathering integrated over time, and one that reflects the weathering regime of the present day. 相似文献
435.
Abstract This study provides a first evaluation of the biological impact of a marine fishery reserve on the stock of queen conch (Strombus gigas) in the Turks and Caicos Islands. The density and the shell length of the population living in the reserve are compared with those of the individuals living in the surrounding fished areas. The results show that the adult density is six times higher in the reserve than in the fished areas. The shell length analysis shows that both adults and juveniles are significantly smaller in the reserve than in the fished area. This unexpected result suggests the existence of a crowding effect (i. e. a high density‐induced reduction in growth rate) within the reserve. It is hypothesised that this crowding effect is due to the superimposition of two factors leading to very high density values in the reserve: (a) the reduced fishing mortality following the creation of the reserve, (b) the existence of natural barriers that impede the emigration of adults outside the reserve. These results are then discussed in relation to current considerations on marine fisheries reserves. 相似文献
436.
Within a landform, the channelized water path from any point to the corresponding outlet is split into successive components within the Strahler ordering scheme. The probability density functions (pdf) of the length L of the whole channelized path and of the lengths of the components are studied as multi‐level structural functions. We have considered a granitic area and studied both its main basin and the set of its 48 constituent basins. With respect to the main basin, the pdf of the component lengths exhibit a strong scaling property, except for the highest orders, due to a hierarchical constraint; hence, the pdf of sum L has no particular shape. We have nevertheless identified an underlying structural pattern at particular infra‐ and supra‐basin levels, where the hierarchical constraint is weaker. This identification process entails noting structurally emerging patterns based on multi‐level variables and distributions, which satisfy the general self‐similarity of networks. The fairly good fit of an analytical gamma law with most of these emerging patterns can prove to be a positive step towards both a general modelling approach to the geomorphometric functions and a stronger geomorphological basement of hydrological transfer functions. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
437.
The regional climate model (MAR) is used to perform a simulation of the year 1992 over West Africa. It is shown that MAR is
able to simulate the main features of the rainy regime over West Africa and especially the discontinuous seasonal progression
of the West African Monsoon along the year. One particular feature that is reasonably well reproduced is the abrupt shift
of the rain band from 5° to 10°N at the end of June (also called “monsoon jump”). This study suggests that such a phenomenon
is associated with the shift of the Saharan heat low between two favourite positions: one being over the Sahelian area (10–15°N)
and the other over the Saharan area (20–25°N). These two favourite locations of the heat low are linked to the spatial distribution
of surface albedo over West Africa that drives the spatio-temporal location of the surface temperature maxima. A detailed
analysis of this “monsoon jump” is performed and the causes of the strong decrease in precipitation that precedes the northward
shift of the rain band are also investigated. 相似文献
438.
Christophe Messager Hubert Gallée Olivier Brasseur Bernard Cappelaere Christophe Peugeot Luc Séguis Michel Vauclin Romain Ramel Gilles Grasseau Laurent Léger Denis Girou 《Climate Dynamics》2006,27(2-3):199-214
The forcing of a hydrologic model (ABC) by both observed and simulated precipitation from a regional climate model (MAR) has been performed over the Sirba watershed (39,000 km2) located in the Sahelian region. Two aspects have been more specifically examined: the spatial and temporal representations of precipitation. The comparison between simulated and observed discharges—using observed rainfall datasets as forcing of the hydrologic model—has shown that the representation of daily precipitation (which is mainly convective in the Sahelian region) was not sufficiently accurate to correctly simulate the hydrologic response of the watershed. Since this response drives the soil water budget and consequently the amount of evaporation in forthcoming coupling experiments, it is thus necessary to develop more realistic infra-daily precipitation associated with convective events. A new temporal disaggregation scheme has been then developed. Considering observed as well as simulated precipitation fields, this method has significantly improved the simulated discharge at the catchment outlet. The major role played by the temporal component compared to spatial component of the precipitation has been then underlined. In addition, the present study shows the unsuitability of the simulated precipitation from the RCM to directly force a hydrologic model at infra daily timescale even if the cumulative amount and the main features of the precipitation seasonal cycle are well simulated. 相似文献
439.
Mireille Christophe Michel-Levy 《Earth and Planetary Science Letters》1976,30(1):143-150
Finely divided products, making a black carbonaceous crust, have coated the chondrules in space. After their agglomeration, a white silica and alumina rich deposit has accumulated between them. Crystalline debris that are found outside the chondrules must not be confounded with the black and white products, which are the only constituents of the low-temperature matrix. These debris are broken chondrules but do not necessarily result from shock effects. Many of them are indeed very fragile, because they are very porous: they contain primary cavities (voids in the crystals, druses) and secondary ones because of selective leaching of part of the glass. The chemical elements leached out of the chondrules may have been reconcentrated in the matrix. 相似文献
440.
Ralf Jaumann Robert H. Brown Jason W. Barnes Christophe Sotin Roger N. Clark Bonnie J. Buratti Thomas B. McCord Kevin H. Baines Phil D. Nicholson Mirjam Langhans 《Icarus》2008,197(2):526-538
The surface of Titan has been revealed by Cassini observations in the infrared and radar wavelength ranges as well as locally by the Huygens lander instruments. Sand seas, recently discovered lakes, distinct landscapes and dendritic erosion patterns indicate dynamic surface processes. This study focus on erosional and depositional features that can be used to constrain the amount of liquids involved in the erosional process as well as on the compositional characteristics of depositional areas. Fluvial erosion channels on Titan as identified at the Huygens landing site and in RADAR and Visible and Infrared Mapping Spectrometer (VIMS) observations have been compared to analogous channel widths on Earth yielding average discharges of up to 1600 m3/s for short recurrence intervals that are sufficient to move centimeter-sized sediment and significantly higher discharges for long intervals. With respect to the associated drainage areas, this roughly translates to 1-150 cm/day runoff production rates with 10 years recurrence intervals and by assuming precipitation this implies 0.6-60 mm/h rainfall rates. Thus the observed surface erosion fits with the methane convective storm models as well as with the rates needed to transport sediment. During Cassini's T20 fly-by, the VIMS observed an extremely eroded area at 30° W, 7° S with resolutions of up to 500 m/pixel that extends over thousands of square kilometers. The spectral characteristics of this area change systematically, reflecting continuous compositional and/or particle size variations indicative of transported sediment settling out while flow capacities cease. To account for the estimated runoff production and widespread alluvial deposits of fine-grained material, release of area-dependent large fluid volumes are required. Only frequent storms with heavy rainfall or cryovolcanic induced melting can explain these erosional features. 相似文献