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521.
Germán Da Costa 《Astrophysics and Space Science》1997,256(1-2):333-336
Nonlinear interactions between solitary waves currently observed in Plasma Physics are simulated in one and two dimensional water channels. 相似文献
522.
Jeffrey E. Hughes Linda A. Deegan Melissa J. Weaver Joseph E. Costa 《Estuaries and Coasts》2002,25(2):250-263
We applied an index of estuarine biotic integrity (EBI) to 36 sites in 16 estuaries on Cape Cod and in Buzzards Bay, Massachusetts,
U.S. Two estuaries were sampled in 6 years, from 1988–1999 (Waquoit and Buttermilk Bays), and a total of 14 others in Buzzards
Bay were sampled in 1993, 1996, and 1998. Habitats at each site were classified as either low or medium quality by density
and biomass of submerged rooted vegetation (eelgrass). The EBI and its metrics (fish abundance, biomass, total species, species
dominance, life history, and proportion by life zone) were successful in classifying habitat quality. Greatest success and
least bias of the EBI and its metrics in classifying habitat quality occurred when eelgrass habitats were least degraded.
The EBI tracked habitat degradation over time in Waquoit and Buttermilk Bays. Average EBI values in medium-quality habitats
of Buzzards Bay estuaries during 1996 and 1998 were less than expected based on earlier EBI values from Waquoit and Buttermilk
Bays, suggesting that many of these sites are in transition from medium to low quality. Our results indicate that the EBI
is sensitive to habitat quality change, and further suggest that low-quality habitats may approach a stable fish community
structure that is well reflected by the EBI. The relationship of the EBI to an independent measure of water quality demonstrated
inherent time lags between the degradation and improvement of water quality, fish habitat, and response of the fish community. 相似文献
523.
Mrcia Cristina Costa de Azevedo Francisco Gerson Araújo Antnio Gomes da Cruz-Filho Andr Luiz Machado Pessanha Mrcio de Araújo Silva Ana Paula Penha Guedes 《Estuarine, Coastal and Shelf Science》2007,75(4):468-480
This study analyzed the factors structuring demersal fish community in a tropical bay in southeastern Brazil. The results were used to quantify the partitioning of ecological variation among the environmental, spatial and temporal components molding the fish community. Three bay zones (inner, middle and outer) were defined according to depth and salinity gradient. Monthly samplings were conducted by bottom trawl tows during daylight hours, between October 1998 and September 1999. In each zone, three replicate samples were taken. Ninety-three fish species from 73 genera and 37 families were recorded in the 108 samples. Two demersal fish assemblages were evidenced, one in the inner and the other in the outer zone. These assemblages were characterized by changes in species composition and relative abundance. Depth, followed by transparency and salinity, influenced spatial pattern of fish assemblages. The largest part of the explained variation occurred as a result of the spatial structure of environmental variables, which means that both species and environmental variables presented similar spatial structure. The spatial effect, not the seasonal, explained the highest part of species variations. The amount of unexplained variation was relatively high (76%), even assuming that part of it is due to nondeterministic fluctuation, which could be due to local effects of unmeasured (biotic and abiotic) controlling variables. Knowing the relative importance of these factors can be of decisive importance when applying casual hypotheses in the framework of some precise ecological theory and should facilitate management, planning, and usage of bay resources. 相似文献
524.
Miranda J.M. Silva P.F. Lourenço N. Henry B. Costa R. Saldanha Team the 《Marine Geophysical Researches》2002,23(4):299-318
We present a study of the magnetic properties of a group of basalt samples from the Saldanha Massif (Mid-Atlantic Ridge – MAR – 36° 33 54 N, 33° 26 W), and we set out to interpret these properties in the tectono-magmatic framework of this sector of the MAR. Most samples have low magnetic anisotropy and magnetic minerals of single domain grain size, typical of rapid cooling. The thermomagnetic study mostly shows two different susceptibility peaks. The high temperature peak is related to mineralogical alteration due to heating. The low temperature peak shows a distinction between three different stages of low temperature oxidation: the presence of titanomagnetite, titanomagnetite and titanomaghemite, and exclusively of titanomaghemite. Based on established empirical relationships between Curie temperature and degree of oxidation, the latter is tentatively deduced for all samples. Finally, swath bathymetry and sidescan sonar data combined with dive observations show that the Saldanha Massif is located over an exposed section of upper mantle rocks interpreted to be the result of detachment tectonics. Basalt samples inside the detachment zone often have higher than expected oxidation rates; this effect can be explained by the higher permeability caused by the detachment fault activity. 相似文献
525.
We present the observation and interpretation of a solar radio burst whose evolution of the source position at 48 GHz has been correlated with microwave spectral observations from 3.1 to 19.6 GHz and H imaging spectrograms. The event of November 19, 1990 showed 4 impulsive peaks in microwaves and 2 H kernels. There exists strong evidence that the impulsive emission has originated from nonthermal electrons including an electron beam during the rising phase of the third microwave peak. The complex evolution of the source position at 48 GHz is attributed to two inhomogeneous and spatially separated sources with changing relative brightness. 相似文献
526.
Pedro Alves Costa José Leitão Borges Manuel Matos Fernandes 《Geotechnical and Geological Engineering》2007,25(6):617-629
The studies of excavations in soft clayey soils are normally based on undrained total stress analyses. A better approach consists
of taking into account the effects of consolidation during the excavation-bracing process and after the completion of the
construction by means of coupled finite element analyses in effective stresses. In this paper, the geotechnical behaviour
of a braced excavation in the soft soils of San Francisco (USA) is analysed, both during and after the construction period.
Numerical analyses are performed with a finite element program, which incorporates the Biot consolidation theory (coupled
formulation of the water flow and equilibrium equations) and soil constitutive relations simulated by the p-q-θ critical state model. Numerical results are compared with field results. 相似文献
527.
Paolo Antonio Pirazzoli Stéphane Costa Uwe Dornbusch Alberto Tomasin 《Ocean Dynamics》2006,56(5-6):498-512
This paper is based on statistical analysis of hourly tide measurements for some 285 equivalent full years from the stations
of Weymouth, Bournemouth, Portsmouth, Newhaven, Dover and Sheerness in the UK, and of Cherbourg, Le Havre, Dieppe, Boulogne,
Calais and Dunkirk in France. For each tidal value, surge heights have been determined and correlated with hourly or three-hourly
wind and air pressure data from nearby meteorological stations. Major surges in the area are generally produced by storms
associated with wind from north-west or south-west that tend to push oceanic water into the Channel. Recent medium-term climate
evolution does not seem to increase the flooding risk at French stations, where surge-related winds tend to decrease in frequency
and speed (Cherbourg, Dieppe and Boulogne) or show little change (Le Havre). However, the long-term risk of flooding will
increase through the loss in land elevation due to a continuation of the local relative sea-level rise, especially if this
effect will be enhanced by an acceleration in the global sea-level rise predicted by climatic models. The northern side of
the Channel (Weymouth, Bournemouth and Portsmouth) is mainly exposed to southerly winds that show variable trends. It is also
apparently affected by strong subsidence trends during the last two decades. If lasting, such trends can only increase long-term
flooding risk. The flooding risk has not increased near the eastern end of the Channel. The duration of significant cyclonic
events tends to decrease near Cherbourg but tends to increase near Weymouth, with no conclusive trends in other stations (Portsmouth,
Calais and Dunkirk), where extreme surges may occur also in relatively high-air-pressure situations. In conclusion, medium-term
coastal flooding risk seems to increase especially at Weymouth, Bournemouth and Portsmouth, and also, but less so, at Le Havre
and Sheerness. In addition, few extreme surges occurred during the last decades at the time of spring high tide, which would
seem to be a fortunate coincidence or, in some cases, an effect of tide–surge interaction. The risk of occurrence of less
favourable random events in the near future is therefore of concern, and flood potential would greatly increase if the global
sea-level rise expected in the near future is also considered. 相似文献
528.
Eduardo Salamuni Hans Dirk Ebert Mauricio da Silva Borges Yociteru Hasui Joo Batista Sena Costa Riad Salamuni 《Journal of South American Earth Sciences》2003,15(8):901-910
The Curitiba Basin, Paraná, lies parallel to the west side of the Serra do Mar range and is part of a continental rift near the Atlantic coast of southeastern Brazil. It bears unconsolidated and poorly consolidated sediments divided in two formations: the lower Guabirotuba Formation and the overlying Tinguis Formation, both developed over Precambrian basement. Field observations, water well drill cores, and interpretations of satellite images lead to the inference that regional tectonic processes were responsible for the origin of the Basin in the continental rift context and for morphotecatonic evolution through block tilting, dissection, and erosion. The structural framework of the sediments and the basement is characterized by NE–SW-trending normal faults (extensional tectonic D1 event) reactivated by NE–SW-trending strike–slip and reverse oblique faults (younger transtensional tectonic D2′ to transpressional tectonic D2″ event). This tectonic event, which started in the Paleogene and controlled the basin geometry, began as a halfgraben and was later reactivated as a pull-apart basin. D2 is a neotectonic event that controls the current morphostructures. The Basin is connected to the structural rearrangement of the South American platform, which underwent a generalized extensional or trantensional process and, in late Oligocene, changed to a compressional to transpressional regime. 相似文献
529.
The launch of ROSAT opened a new era in the study of T Tauri stars. Prompted by the IUE results, the new X-ray data confirms
the existence of extremely high levels of activity in these young stars, very likely of magnetic nature. We address a detailed
analysis of the star TW Hya in the X-ray spectral range. A summary of its properties in the UV will also be included in order
to grasp an overall picture of the activity in this star.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
530.
C.R. Macedo C.V. Costa J.T. Ferreira M.P. Ferreira J.H. Reynolds 《Earth and Planetary Science Letters》1977,34(3):411-418
A double-spike method for KAr dating is described. Use of long-lived39Ar in addition to the usual38Ar in the spike permits argon isotopic discrimination occurring after insertion of the spike to be monitored at the same time as ages are determined. A “bootstrap” (self-contained) method of calibrating the spike for isotopic composition is described which, when used appropriately, gives final ages which are independent of the published isotopic composition of the terrestrial atmospheric argon used as a comparison gas. The method calls for a mass spectrometer which resolves adjacent isotopes to a high degree. We have tested the method nevertheless with a mass spectrometer of relatively low resolving power, the AEI MS-10, and found it to be successful in part and without any disadvantages with respect to conventional techniques in those instances where the method fails because of insufficient resolution. That is, we find the discrimination factor inferred in runs on young rocks and blanks to be valid and useful. That factor changes by as much as 1.3% over the life of a filament in the ion source of the MS-10, proving that close control of the discrimination factor is essential for analytical precision with the instrument. These assertions are documented graphically with a compilation of data which represents more than two years experience with the method. We also report measurements of the mass discrimination of the MS-10 and its linearity with mass which indicate that the latter quantity is acceptable when the repeller voltage for the ion source is close to +1 V and is unacceptable otherwise. The spike material is available, upon request, to those laboratories which decide firmly to undertake further testing of the method. 相似文献