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81.
Geochronological data, combined with field and petrological evidence, constrain the timing and rate of near‐isothermal decompression at granulite facies temperatures in rocks from the Lützow‐Holm Complex of East Antarctica. Granulite facies gneisses from Rundvågshetta in Lützow‐Holm Bay experienced a peak metamorphic temperature of over 900 °C at c. 11 kbar, as evidenced by primary orthopyroxene–sillimanite‐bearing assemblages, and secondary cordierite–sapphirine‐bearing assemblages in metapelites. Peak metamorphic assemblages show strong preferred mineral orientation, interpreted to have developed synchronously with pervasive ductile deformation. Zircon from a syndeformational leucosome has a U–Pb age of 517±9 Ma, which is interpreted as a melt crystallization age. This age provides the best estimate of the time of peak metamorphic conditions. The post‐peak metamorphic history is characterized by near‐isothermal decompression, recorded by mineral textures in a variety of rock compositions. Field and textural relations indicate that decompression post‐dated pervasive ductile deformation. K/Ar and 40Ar/39Ar ages from hornblende and biotite represent closure ages during cooling subsequent to decompression, and indicate cooling to temperatures between c. 350 and 300 °C by c. 500 Ma, thus placing a lower time limit on the duration of the high‐temperature isothermal decompression episode. The combination of the zircon age from a syndeformational melt with K/Ar and 40Ar/39Ar closure ages indicates that near‐isothermal decompression from c. 11 to c. 4 kbar at granulite facies temperatures, followed by cooling to c. 300 °C, took place within a time interval of 20±10 Myr. Simple one‐dimensional models for exhumation‐controlled cooling indicate that these data require exhumation rates of the order of c. 3 km Myr?1 for several million years, then cessation of exhumation followed by relatively isobaric cooling during thermal re‐equilibration. 相似文献
82.
Groundwater discharges in the western Canadian oil sands region impact river water quality. Mapping groundwater discharges to rivers in the oil sands region is important to target water quality monitoring efforts and to ensure injected wastewater and steam remain sequestered rather than eventually resurfacing. Saline springs composed of Pleistocene‐aged glacial meltwater exist in the region, but their spatial distribution has not been mapped comprehensively. Here we show that formation waters discharge into 3 major rivers as they flow through the Athabasca Oil Sands Region adjacent to many active oil sands projects. These discharges increase river chloride concentrations from river headwaters to downstream reaches by factors of ~23 in the Christina River, ~4 in the Clearwater River, and ~5 in the Athabasca River. Our survey provides further evidence for the substantial impact of formation water discharges on river water quality, even though they comprise less than ~2% of total streamflow. Geochemical evidence supporting formation water discharges as the leading control on river salinity include increases in river chloride concentrations, Na/(Na + Ca) ratios, Cl/(Cl + SO4) ratios and decreases in 87Sr/86Sr ratios; each mixing trend is consistent with saline groundwater discharges sourced from Cretaceous or Devonian aquifers. These regional subsurface‐to‐surface connections signify that injected wastewater or steam may potentially resurface in the future, emphasizing the critical importance of mapping groundwater flow paths to understand present‐day streamflow quality and to predict the potential for injected fluids to resurface. 相似文献
83.
Zr-in-rutile thermometry in HP/UHP eclogites from Western China 总被引:5,自引:0,他引:5
Guibin Zhang David J. Ellis Andrew G. Christy Lifei Zhang Shuguang Song 《Contributions to Mineralogy and Petrology》2010,160(3):427-439
Four Zr-in-rutile thermometry calibrations are applied to eclogites from Western China. Here, we show that if rutile grows
in equilibrium with Qtz and Zrn, and is isolated inside garnet, it preserves its Zr composition and does not undergo compositional
change due to cation exchange with the host garnet. It thus preserves the composition for the P–T conditions of its formation and the growth zoning of the host garnet. For the HP/UHP metamorphic temperature, the Tomkins
et al. (J Metamorph Geol 25:703–713, 2007) calibration yields temperatures that agree well with previous studies, whereas the other three calibrations (Zack et al.
in Contrib Mineral Petrol 148:471–488, 2004; Watson et al. in Contrib Mineral Petrol 151:413–433, 2006; Ferry and Watson in Contrib Mineral Petrol in 154:429–437, 2007), which do not include a pressure correction, give systematically lower temperatures. Zr contents of rutile inclusions within
garnet show systematic decrease from garnet core to rim. The rutile inclusions in garnet rims contain the lowest Zr content,
similar to that in the matrix. Analyses confirm that the pressure plays a significant role in modifying the primary temperature
dependence of the Zr content of rutile. Rutiles trapped in garnets are unable to re-equilibrate easily during retrogression,
but those in the matrix can do so, providing retrograde P–T path information. 相似文献
84.
We analyze the out-of-plane librations of a tethered satellite system that is nominally rotating in the orbit plane. To isolate
the librational dynamics, the system is modeled as two point masses connected by a rigid rod with the system mass center constrained
to an unperturbed circular orbit. For small out-of-plane librations, the in-plane motion is unaffected by the out-of-plane
librations and a solution for the in-plane motion is determined in terms of Jacobi elliptic functions. This solution is used
in the linearized equation for the out-of-plane librations, resulting in a Hill’s equation. Floquet theory is used to analyze
the Hill’s equation, and we show that the out-of-plane librations are unstable for certain ranges of in-plane spin rate. For
relatively high in-plane spin rates, the out-of-plane librations are stable, and the Hill’s equation can be approximated by
a Mathieu’s equation. Approximate solutions to the Mathieu’s equation are determined, and we analyze the dominant characteristics
of the out-of-plane librations for high in-plane spin rates. The results obtained from the analysis of the linearized equations
of motion are compared to numerical simulations of the nonlinear equations of motion, as well as numerical simulations of
a more realistic system model that accounts for tether flexibility. The instabilities discovered from the linear analysis
are present in both the nonlinear system and the more realistic system model. The approximate solutions for the out-of-plane
librations compare well to the nonlinear system for relatively high in-plane rotation rates, and also capture the significant
qualitative behavior of the flexible system. 相似文献
85.
Bohumil M. Svoma Robert C. Balling Jr. Andrew W. Ellis 《Theoretical and Applied Climatology》2010,102(1-2):159-169
In this investigation, we generate estimates of soil moisture for the Salt River basin on which the expanding city of Phoenix, Arizona relies upon for nearly half of its water supply. While previous empirical studies have produced mixed results concerning recent drought trends in the southwestern USA, our results support the many numerical climate models which predict that this region will become drier in response to the buildup of greenhouse gases. Over the period 1895–2007, we found that soil moisture levels revealed no significant change. However, if we restrict the study to the period 1980–2007, soil moisture levels have decreased at a highly statistically significant rate. We could find little evidence that the trend is associated with variations in El Niño–Southern Oscillation and the Pacific Decadal Oscillation. Increasing surface pressure over our study area during the latter half of the twentieth century may be a catalyst for the drier conditions. 相似文献
86.
Gary A. Robbins Brian E. McAninch Francis M. Gavas Patricia M. Ellis 《Ground Water Monitoring & Remediation》1995,15(1):124-132
A soil-gas survey was conducted at a gasoline service station and a former fire training facility to determine if surveying for hydrogen sulfide could be useful in locating nonaqueous phase hydrocarbon fuel in the subsurface. Relative; to total organic vapor, oxygen, and carbon dioxide distributions, detectable hydrogen sulfide concentrations were much more restricted to the suspected source vicinity at both sites. Appreciable levels of soil-gas hydrogen sulfide. up to 600 Vppb. were observed in areas characterized by anaerobic or microaerophilic conditions having bulk oxygen levels below 4 percent. Based on the hydrogen sulfide distribution, nonaqueous phase hydrocarbon fuel was located at each site. These results suggest that soil-gas surveying for hydrogen sulfide may help locale mobile or residual gasoline and other nonaqueous phase hydrocarbons in the subsurface. 相似文献
87.
Richard S. Ellis 《Astrophysics and Space Science》1999,267(1-4):319-328
I review two observational programs which, together, promise to unravel the detailed astrophysical evolution of normal field
galaxies over the last 5–7 Gyr. Systematic ground-based spectroscopy of faint galaxies have revealed an increasing faint end
slope for the luminosity function with redshift. The trend is strongest for galaxies undergoing intense star-formation. Deep
images taken with the repaired HST can be used to count galaxies as a function of morphological type. Regular "Hubble sequence"
galaxies follow the no-evolution prediction, but irregular/peculiar sources have a steeper count slope and provide the excess
population. Although the overlap between the spectral and HST samples is currently small, plans to merge similar datasets
should reveal the physical explanation for the demise of star formation in faint blue galaxies since z ⋍0.5–1.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
88.
89.
The Acrochordidae consists of three congeneric species of aquatic snakes distributed among fresh water and coastal marine environments in tropical southern Asia. The smallest species,Acrochordus granulatus, is euryhaline and the only acrochordid that permanently inhabits coastal seas and estuaries. The diving and metabolic physiology of this species is highly specialized and reflects the demands of estuarine environments. A capability for prolonged aerobic diving is attributable to low rates of oxygen consumption, high capacity for oxygen storage, nearly complete utilization of the oxygen stores, and cutaneous gas exchange. Recent studies indicate thatA. granulatus is primarily ammonotelic and requires a source of fresh water for elimination of nitrogenous wastes. The requirement for fresh water potentially limits seaward migration of populations due to the dependence of snakes on rivers or coastal rainfall. Adaptations for shallow-water diving conceivably further limit seaward migration, with the result that they have evolved as estuarine specialists that are restricted from deeper waters and the open ocean. 相似文献
90.
Ellis L. Krinitzsky 《Engineering Geology》1993,33(4):257-288
The Lawrence Livermore National Laboratory and the Electric Power Research Institute have developed procedures that statistically merge multiple expert opinions to get probabilistic seismic hazard evaluations. Such methods are intrinsically defective and should not be used for design applications in engineering. 相似文献