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51.
Raúl R. Castro Giancarlo Monachesi Luca Trojani Marco Mucciarelli Massimo Frapiccini 《Journal of Seismology》2002,6(1):43-59
We studied spatial and temporal characteristics of seismic attenuation inCentral Italy using S- and coda- waves recorded by the MarchesanSeismograph Network from earthquakes located in the epicentral area ofthe 1997 Umbria-Marche sequence. The amplitude decay of the S waveswith distance was defined calculating empirical attenuation functions at 15frequencies between 1 and 25 Hz. We analyzed separately foreshocks andaftershocks and we found the same attenuation functions, suggesting thatthe possible temporal variations could be confined in a small area. Thefrequency dependence of Q
S was approximated by the equation Q
S=18 · f
2.0between 1 and 10 Hz. At higher frequencies (10–25 Hz), the frequencydependence of Q
s weakens, having an average value of Q
S=990. We also estimated Q from coda waves (Q
C) using the single-scattering models of Aki andChouet (1975) and Sato (1977). We found that Q
C=77 · f
0.6, (between 2 and 20Hz) at the western side of the mountain chain, using either foreshocks oraftershocks. This relation is consistent with previous estimates of Q
Creported for the Central Apennines. For a volume sampling the Colfioritobasin, the Apennines and the Marche region we found that Q
C=55 · f
0.8,indicating highattenuation below the mountain belt. To detect small temporal changes ofQ, we calculated spectral ratios of 5 temporal doublets located in theepicentral area and recorded at the closest station. We found temporalchanges of Q that vary from 27% to 56%, depending on the locationof the doublets. This variability suggests that the temporal change ofattenuation may depend on the spatial variation of Q and perhaps on thespatial distribution of tectonic stress in the epicentral area. 相似文献
52.
Giancarlo Serri 《Earth and Planetary Science Letters》1981,52(1):203-212
Ophiolites have been divided into two groups: high-Ti and low-Ti types. These can be discriminated by studying the fractionation trends of both gabbroic complexes (this work) and lavas and dykes [16], particularly in the TiO2/M.I. diagram. The first type typically shows MORB-like magmas whereas in the second the magma types have a spectrum of composition from mid-ocean ridge basalts to island arc tholeiites and boninite-like magmas often occur.High-Ti ophiolites are petrologically and geochemically similar to major oceanic and ensialic back-arc basin crusts as well as oceanic crust generated during the intermediate and late-stage opening of intraoceanic back-arc basins.Parental magmas and fractionation processes of low-Ti ophiolites fit with an hypothesis of their formation in the early stage of opening of intraoceanic back-arc basins. 相似文献
53.
54.
Lateral flow of African mantle below the nearby Tyrrhenian plate: geochemical evidence 总被引:1,自引:0,他引:1
The Quaternary magmatism of the Southern Tyrrhenian basin represents a rare example of an active volcanic arc system where ocean island basalt (OIB) and island arc basalt (IAB) magmas coexist. Although there is general agreement on the importance of the Ionian oceanic lithosphere subduction in the genesis of the IAB magmatism, the tectono‐magmatic processes producing the coexisting OIB magmas are still poorly understood. Here we show that the geochemistry of the Quaternary Southern Tyrrhenian OIB‐type magmatism (i.e. Ustica island and Prometeo, a previously unknown submarine lava field) is very similar to that of OIB‐type volcanoes situated on the nearby African plate (i.e. Etna and Hyblean Plateau). Among the possible geodynamic scenarios proposed to explain the coexistence of OIB and IAB magmas in arc settings, we consider the development of a tear at the edge of the Ionian plate as the more likely mechanism to favour the flow of African asthenospheric mantle below the Tyrrhenian plate. 相似文献
55.
We evaluated nekton habitat quality at 5 shallow-water sites in 2 Rhode Island systems by comparing nekton densities and biomass,
number of species, prey availability and feeding, and abundance of winter flounderPseudopleuronectes americanus. Nekton density and biomass were compared with a 1.75-m2 drop ring at 3 sites (marsh, intertidal, and subtidal) in Coggeshall Cove in Narragansett Bay and two subtidal sites (eelgrass
and macroalgae) in Ninigret Pond, a coastal lagoon. We collected benthic core samples and examined nekton stomach contents
in Coggeshall Cove. We identified 16 species of fish, 16 species of crabs, and 3 species of shrimp in our drop ring samples.
A multivariate analysis of variance indicated differences in total nekton, invertebrates, fish, and winter flounder across
the five sites. Relative abundance of benthic invertebrate taxa did not match relative abundance of prey taxa identified in
the stomachs. Nonmetric multidimensional scaling plots showed groupings in nekton and benthic invertebrate prey assemblages
among subtidal, intertidal, and marsh sites in Coggeshall Cove. Stepwise multiple regression indicated that biomass of macroalgae
was the most important variable predicting abundance of nekton in Coggeshall Cove, followed by elevation and depth. In Rhode
Island systems that do not experience chronic hypoxia, macroalgae adds structure to unvegetated areas and provides refuge
for small nekton. All sites sampled were characterized by high abundance and diversity of nekton pointing to the importance
of shallow inshore areas for production of fishes and decapods. Measurements of habitat quality should include assessment
of the functional significance of a habitat (this can be done by comparing nekton numbers and biomass), some measure of habitat
diversity, and a consideration of how habitat quality varies in time and space. 相似文献
56.
Giancarlo Della Ventura Fabio Bellatreccia Paolo Rossi 《Physics and Chemistry of Minerals》2007,34(10):727-731
We report here a single-crystal polarized-light study of stoppaniite, ideally (Fe,Al,Mg)4(Be6Si12O36)(H2O)2(Na,□), from Capranica (Viterbo). Polarized-light FTIR spectra were collected on an oriented (hk0) section, doubly polished
to 15 μm. The spectrum shows two main bands at 3,660 and 3,595 cm−1; the former is strongly polarized for E ⊥c, while the latter is polarized for E //c. A sharp and very intense band at 1,620 cm−1, plus minor features at 4,000 and 3,228 cm−1 are also polarized for E //c. On the basis of literature data and considering the pleochroic behavior of the absorptions, the 3,660 cm−1 band is assigned to the ν3 stretching mode and the 1,620 cm−1 (associated with an overtone 2*ν2 at 3,230 cm−1) band to the ν2 bending mode of “type II” water molecules within the structural channels of the studied beryl. The sharp band at 3,595 cm−1 is not associated with a corresponding ν2 bending mode; thus it is assigned to the stretching vibration of O–H groups in the sample. The minor 4,000 cm−1 feature can be assigned to the combination of the O–H bond parallel to c with a low-frequency metal-oxygen mode such as the Na–O stretching mode. The present results suggest that the interpretation
of the FTIR spectrum of Na-rich beryl needs to be carefully reconsidered. 相似文献
57.
On the basis of the correspondence principle of visco-elasticity, an application is discussed of the boundary integral equation method to the solution of time-dependent stress analysis problems. The adopted solution technique, in addition to the time-dependent stress and strain distributions, enables the determination of a measure of the error affecting the numerical results. The governing equations for the plane strain visco-elastic problem are derived with reference to a generalized Kelvin model consisting of the series of any number of simple kelvin elements, having both volumetric and deviatoric components. To get some insight into the overall performance of the technique, the results obtained in the solution of some test examples are discussed and compared with those dering from the available closed from solutions. 相似文献
58.
Francesco Soldovieri Giancarlo Prisco Raffaele Persico 《Journal of Applied Geophysics》2009,67(4):288-295
In this paper we deal with an indirect measure of the dielectric permittivity of the soil starting from GPR surface data collected on a buried “cooperative” target, meant as an object buried on purpose and whose extent is known a-priori. This target is exploited in order to achieve, from its image obtained from a suitable GPR data processing, an indirect measure of the dielectric permittivity of the embedding soil. GPR data processing is based on a linear microwave tomographic approach funded on the Born Approximation. Using this Born approach on two-dimensional inversion tests, we investigate the effect of the soil's electrical conductivity and permittivity on this indirect measure and demonstrate that the electrical field scattered by a spot-like buried object permits an accurate estimation of the soil permittivity even when no information of the soil conductivity is available. 相似文献
59.
Monica Montefalcone Giancarlo Albertelli Carla Morri Valeriano Parravicini Carlo Nike Bianchi 《Marine pollution bulletin》2009,58(4):515-262
Using the Conservation Index, which measures the proportional amount of dead matte relative to live Posidonia oceanica, we assessed the health of 15 P. oceanica meadows at a regional scale along the coast of Liguria (NW Mediterranean). These areas were characterized by different degrees of anthropization, from highly urbanized sites to marine protected areas. Two different scenarios were identified according to depth: in shallow zones, the health of P. oceanica meadows was related to the degree of anthropization along the coastline. In contrast, in deep zones, most meadows exhibited poor health, independent of both the degree of disturbance and the legal measures protecting the area. Working synergistically with the regional impact of increased water turbidity, local impacts from the coast were recognized as the main causes of the severe regression of most Ligurian P. oceanica meadows. We conclude that marine protected areas alone are not sufficient to guarantee the protection of P. oceanica meadows. We emphasize the need for a management network involving the Sites of Community Interest (SCIs) containing P. oceanica meadows. 相似文献
60.
Microstructures associated with static and dynamic recrystallization of Carrara marble (Alpi Apuane,NW Tuscany,Italy) 总被引:2,自引:0,他引:2
The present contribution summarizes the first results of a study focusing on microstructures from Alpi Apuane marbles. Its aim is both an analysis of the evolution of the metamorphic complex recorded in marbles and the supply of basic material for process-oriented studies on calcite microstructures due to natural deformation. Quantitative analysis of the variations of statically recrystallized microstructures suggest a relationship with the peak metamorphic temperatures. Previously unrecognized post-thermal peak shear zones, showing overprint microstructures typical of grain-boundary migration and dynamic recrystallization, are described; they document the natural deformation of Carrara marble. 相似文献