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61.
The principal seismicity of the Pamirs—Hindu-Kush area is closely related to a high-velocity body in the region. The dimensions of this body, and its relation with the outcrop of a large area of Precambrian shield-like terrain suggest that it may be the rigid downward extension of a piece of old shield, and that the concentration of stress around its eastern extremity is the source of the anomalous intermediate seismic activity of the region. 相似文献
62.
L. P. Vinnik 《Pure and Applied Geophysics》1973,103(1):282-289
Summary Origin ofP waves detected earlier in microseisms of very quiet locations in the USSR is discussed in detail. It appears that the most pronounced sources ofP waves are tropical cyclones over the Pacific. The amplitude of the force in the source for a medium power typhoon is found to be of the order of 1016 dynes. The effective source area is estimated as 104–105 km2 approximately. The shape of the amplitude spectrum ofP wave corrected for the absorption in the mantle does not contradict with the standing wave theory of microseisms generation. Results of observations at various epicentral distances give strong evidences of the predominant attenuation of the fundamental Rayleigh mode as compared with higher Rayleigh modes andP waves in the frequency band of 0.3–0.15 cps. 相似文献
63.
Black shales from the southern Appalachian Basin and the southwest Welsh Basin have anomalous U–Pb and Nd model ages suggesting syn- and post-depositional resetting of the Sm–Nd and U–Pb isotopic systems. This alteration to the primary detrital signature of these two shale sequences is indicative of black shale diagenetic/depositional processes that obscure paleo-environmental and provenance information recorded prior to and during deposition. The trace element and isotopic signatures of these two shale sequences reveal a syn-/post-depositional history that is de-coupled from the coeval orogenic history of the region making it difficult to reconstruct the tectonic and oceanographic conditions present at the time of deposition.Both the Ordovivian Welsh Basin and the Devonian Appalachian Basin sequences host REE- and U-bearing diagenetic phosphate minerals that play a critical role in the whole rock REE and U budgets. In the Welsh Basin shales, early diagenetic apatite and a later monazite phase dominate the REE budget and cause the redistribution of REE early in the basin's history (ca. 460 Ma). This redistribution is recorded by the Sm–Nd system (450 ± 90 Ma) and the Nd model ages that are anomalously old by as much as 20% (TDM > 2.0 Ga). This early history is complicated by a Permo-Triassic fluid event affecting the whole rock U-budget and resetting the U–Pb isotopic system at 193 ± 45 Ma. The Appalachian Basin sequence appears to have a much less complicated history yet still records a significant disturbance in both the Sm–Nd isotopic system (392 ± 76 Ma) and the Pb isotopic system (340 ± 50 Ma) at about the time of deposition (ca. 365 Ma).These two sequences suggest a pattern of diagenetic disturbance common to black shales. These processes are unique to black shales and must be considered when interpreting provenance and paleo-environmental information from the black shale sequences. Although these rocks are susceptible to alteration, the alteration may provide extensive information on the post-depositional history of the basin while still retaining some primary depositional information. If black shale processes are considered during the interpretation of isotopic and trace element signatures from organic-rich shales, it may be possible to recover an extensive basin history. 相似文献
64.
A. KamyshnyJr. M. Zilberbrand I. Ekeltchik T. Voitsekovski J. Gun O. Lev 《Aquatic Geochemistry》2008,14(2):171-192
Zerovalent sulfur and inorganic polysulfides were determined in nine sulfide-rich water wells in central and southern Israel.
Although the two locations belong to the same aquifer, they are characterized by different pH and hydrogen sulfide levels.
Hydrogen sulfide in the central Israel wells ranged between 19 and 32 μM, and the pH was 7.26 ± 0.07. The southern basin is
characterized by lower water circulation, lower pH (around 6.8), and higher hydrogen sulfide levels (>470 μM). Polysulfides
were determined by a rapid single-phase methylation using methyl trifluoromethanesulfonate (methyl triflate) reagent. The
summary polysulfide concentration for S42−–S72− species was found to be around 0.14–0.75 μM in the central region of Israel and substantially higher, 2.3–4.6 μM in the southern
region. The sum of polysulfide zerovalent sulfur and colloidal sulfur was quantitatively detected by cyanide derivatization
and compared to polysulfide sulfur determined by methyl triflate derivatization and to the chloroform extraction of zerovalent
sulfur. A method for the determination of sulfur undersaturation level—the ratio between dissolved elemental sulfur and its
equilibrium concentration in the presence of solid sulfur—based on the observed levels of the major polysulfide species is
described. The observed polysulfide speciation was compared with the predicted speciation under sulfur saturation conditions
taking into account the water temperature, its ionic strength, and pH. Criteria for sulfur saturation versus unsaturated conditions
were established based on (1) the chain length dependence of the ratio between the observed polysulfide concentrations and
their predicted value under sulfur saturated conditions, and (2) the difference between the concentration of zerovalent sulfur,
as determined by cyanolysis, and the total polysulfide sulfur. According to this dual criterion five of the water wells were
classified as being undersaturated with respect to sulfur, though for all the examined water wells the majority of the zerovalent
sulfur was in the form of polysulfide sulfur. 相似文献
65.
Lev I. Dorman 《Astrophysics and Space Science》1998,264(1-4):443-455
Influence of cosmic ray pressure and kinetic stream instability on space plasma dynamics and magnetic structure are considered.
It is shown that in the outer Heliosphere are important dynamics effects of galactic cosmic ray pressure on solar wind and
interplanetary shock wave propagation as well as on the formation of terminal shock wave of the Heliosphere and subsonic region
between Heliosphere and interstellar medium. Kinetic stream instability effects are important on distances more than 40–60
AU from the Sun: formation of great anisotropy of galactic cosmic rays in about spiral interplanetary magnetic field leads
to the Alfven turbulence generation by non isotropic cosmic ray fluxes. Generated Alfven turbulence influences on cosmic ray
propagation, increases the cosmic ray modulation, decreases the cosmic ray anisotropy and increases the cosmic ray pressure
gradient in the outer Heliosphere (the later is also important for terminal shock wave formation).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
66.
Summary Gravitational coagulation of particles in turbulent flow is investigated with respect to the electrostatic forces. As turbulence mixes particle trajectories at large distances, its account reduces the consideration of drawing particles together till some definite distance. Taking into account hydrodynamical forces of interaction, series converging faster thanHocking's one and their more exact values are obtained. For electrostatic forces series with better convergence are also obtained. Equations of moment were solved numerically on electronic computer BESM-2. Results of collection efficiency computation are given. 相似文献
67.
Upper mantle stratification by P and S receiver functions 总被引:11,自引:0,他引:11
68.
Lev N. Neretin Michael E. Böttcher Igor I. Volkov Katharina Hilgenfeldt 《Geochimica et cosmochimica acta》2004,68(9):2081-2093
Pyritization in late Pleistocene sediments of the Black Sea is driven by sulfide formed during anaerobic methane oxidation. A sulfidization front is formed by the opposing gradients of sulfide and dissolved iron. The sulfidization processes are controlled by the diffusion flux of sulfide from above and by the solid reactive iron content. Two processes of diffusion-limited pyrite formation were identified. The first process includes pyrite precipitation with the accumulation of iron sulfide precursors with the average chemical composition of FeSn (n = 1.10-1.29), including greigite. Elemental sulfur and polysulfides, formed from H2S by a reductive dissolution of Fe(III)-containing minerals, serve as intermediates to convert iron sulfides into pyrite. In the second process, a “direct” pyrite precipitation occurs through prolonged exposure of iron-containing minerals to dissolved sulfide. Methane-driven sulfate reduction at depth causes a progressive formation of pyrite with a δ34S of up to +15.0‰. The S-isotopic composition of FeS2 evolves due to contributions of different sulfur pools formed at different times. Steady-state model calculations for the advancement of the sulfidization front showed that the process started at the Pleistocene/Holocene transition between 6360 and 11 600 yr BP. Our study highlights the importance of anaerobic methane oxidation in generating and maintaining S-enriched layers in marine sediments and has paleoenvironmental implications. 相似文献
69.
70.
Satellite gravimetry is a powerful and reliable tool for regional tectono-geodynamic zonation. The studied region contains intricate geodynamical features (high seismological indicators, active rift systems and collision processes), richest structural arrangement (existence of mosaic blocks of oceanic and continental Earth’s crust of various age), and a number of high-amplitude gravity anomalies and complex magnetic pattern. The most hydrocarbon reserves of the world and other important economic deposits occur in this region. Comprehensive analysis of satellite gravity data with application of different approaches was used to develop a sequence of maps specifying crucial properties of the region deep structure. Careful examination of numerous geological sources and their combined examination with satellite gravity (main), magnetic, GPS, seismic, seismological and some other geophysical data enabled to develop a new tectonic map of the Arabian–African region. Integrated analysis of series of gravity map transformations and certain geological indicators allowed to reveal significant geodynamic features of the region. 相似文献