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131.
Gabriela Ferracutti Ernesto Bjerg Aberra Mogessie 《International Journal of Earth Sciences》2013,102(3):701-720
The layered mafic–ultramafic rocks in the Sierras de San Luis, Argentina (Las Águilas, Las Higueras and Virorco), constitute a 3–5-km-wide belt that extends over 100 km from NE to SW. They carry a sulphide mineralization consisting of pyrrhotite, pentlandite and chalcopyrite, in veins and as disseminated to massive ore. Disseminated spinels are frequently associated with the sulphide minerals as well as platinum group minerals. A strong correlation between S, Ni, Co, Cu, Cr, Pt and Pd indicates the presence of one to three levels of mineralization within the ultramafic units. The maximum concentration of these elements coincides with the units containing platinum group minerals (PGM) and spinel group minerals. This clear relationship constitutes a good prospecting guide in the search for layers with high-grade ore, probably associated with deeper stratigraphic levels where ultramafic rocks are dominant. The bulk rock chemistry and concentrations of metals and platinum group elements as well as textural evidence suggest that the parental magma was mafic with tholeiitic affinities and MgO rich. The Las Águilas layered mafic–ultramafic body and the remaining bodies in the area bear similar characteristics to well-known stratified complexes developed in extensional tectonic regimes, as it is the case of Jinchuan (China), Kabanga (Tanzania) and Fiambala (Argentina). 相似文献
132.
Rainfall-induced debris flows involving ash-fall pyroclastic deposits that cover steep mountain slopes surrounding the Somma-Vesuvius volcano are natural events and a source of risk for urban settlements located at footslopes in the area. This paper describes experimental methods and modelling results of shallow landslides that occurred on 5–6 May 1998 in selected areas of the Sarno Mountain Range. Stratigraphical surveys carried out in initiation areas show that ash-fall pyroclastic deposits are discontinuously distributed along slopes, with total thicknesses that vary from a maximum value on slopes inclined less than 30° to near zero thickness on slopes inclined greater than 50°. This distribution of cover thickness influences the stratigraphical setting and leads to downward thinning and the pinching out of pyroclastic horizons. Three engineering geological settings were identified, in which most of the initial landslides that triggered debris flows occurred in May 1998 can be classified as (1) knickpoints, characterised by a downward progressive thinning of the pyroclastic mantle; (2) rocky scarps that abruptly interrupt the pyroclastic mantle; and (3) road cuts in the pyroclastic mantle that occur in a critical range of slope angle. Detailed topographic and stratigraphical surveys coupled with field and laboratory tests were conducted to define geometric, hydraulic and mechanical features of pyroclastic soil horizons in the source areas and to carry out hydrological numerical modelling of hillslopes under different rainfall conditions. The slope stability for three representative cases was calculated considering the real sliding surface of the initial landslides and the pore pressures during the infiltration process. The hydrological modelling of hillslopes demonstrated localised increase of pore pressure, up to saturation, where pyroclastic horizons with higher hydraulic conductivity pinch out and the thickness of pyroclastic mantle reduces or is interrupted. These results lead to the identification of a comprehensive hydrogeomorphological model of susceptibility to initial landslides that links morphological, stratigraphical and hydrological conditions. The calculation of intensities and durations of rainfall necessary for slope instability allowed the identification of deterministic hydrological thresholds that account for uncertainty in properties and observed rainfall intensities. 相似文献
133.
Abstract Rome has been plagued by flooding since its foundation, and, in December 2008, the largest flood event over the past 20 years caused a fatality and more than €150 million in economic damage. Meteorological conditions associated with the December 2008 flooding are shown to be typical of flooding in the Tiber. The long record of discharge measurements of the Tiber River at the Ripetta station in downtown Rome was used to examine flood frequency for the Tiber, including assessment of the return interval of the December 2008 flood. Particular attention is given to examination of the stationarity assumption for flood peaks through change-point and trend analyses, quantile regression, and statistical modelling of the flood-peak distribution. Once anthropogenic changes linked to reservoir regulation of the Tiber River have been accounted for, the stationarity assumption holds and can be used for flood frequency analysis. We highlight the difficulties in detecting departures from the stationarity assumption due to climate change. In the current regime, the December 2008 flood event has a return period of the order of 10–20 years. Citation Villarini, G., Smith, J.A., Napolitano, F. & Baeck, M.L. (2011) Hydrometeorological analyses of the December 2008 flood in Rome. Hydrol. Sci. J. 56(7), 1150–1165. 相似文献
134.
V. Ernesto Méndez Christopher M. Bacon Meryl Olson Katlyn S. Morris Annie Shattuck 《The Professional geographer》2013,65(3):357-376
We used households as the primary unit of analysis to synthesize agrobiodiversity research in small-scale coffee farms and cooperatives of Nicaragua and El Salvador. Surveys, focus groups, and plant inventories were used to analyze agrobiodiversity and its contribution to livelihoods. Households managed high levels of agrobiodiversity, including 100 shade tree and epiphyte species, food crops, and medicinals. Small farms contained higher levels of agrobiodiversity than larger, collectively managed cooperatives. Households benefited from agrobiodiversity through consumption and sales. To better support agrobiodiversity conservation, our analysis calls for a hybrid approach integrating bottom-up initiatives with the resources from top-down projects. 相似文献
135.
136.
Mass-to-light ratio gradients in early-type galaxy haloes 总被引:1,自引:0,他引:1
N. R. Napolitano M. Capaccioli A. J. Romanowsky N. G. Douglas M. R. Merrifield K. Kuijken M. Arnaboldi O. Gerhard K. C. Freeman 《Monthly notices of the Royal Astronomical Society》2005,357(2):691-706
Owing to the fact that the near future should see a rapidly expanding set of probes of the halo masses of individual early-type galaxies, we introduce a convenient parameter for characterizing the halo masses from both observational and theoretical results: ∇ℓ ϒ , the logarithmic radial gradient of the mass-to-light ratio. Using halo density profiles from Λ-cold dark matter (CDM) simulations, we derive predictions for this gradient for various galaxy luminosities and star formation efficiencies εSF . As a pilot study, we assemble the available ∇ℓ ϒ data from kinematics in early-type galaxies – representing the first unbiased study of halo masses in a wide range of early-type galaxy luminosities – and find a correlation between luminosity and ∇ℓ ϒ , such that the brightest galaxies appear the most dark-matter dominated. We find that the gradients in most of the brightest galaxies may fit in well with the ΛCDM predictions, but that there is also a population of fainter galaxies whose gradients are so low as to imply an unreasonably high star formation efficiency εSF > 1 . This difficulty is eased if dark haloes are not assumed to have the standard ΛCDM profiles, but lower central concentrations. 相似文献
137.
138.
Summary. Palaeomagnetic results for a sequence of Permocarboniferous rhythmites presented in the previous paper have been submitted to maximum entropy spectral analysis to test whether these palaeomagnetic data could supply information on geomagnetic variations. There is a good correlation between the thickness of the rhythmites and sunspot spectra, suggesting that these sediments are really seasonal. The palaeomagnetic spectra are compared with those of observatory records. Periods of approximately 24.4, 12.4, 8.6, 6.7 and 5.5 found for palaeomagnetic data have corresponding values in the geomagnetic spectrum. Most of these periods, however, are the same as those found in the thickness data, implying that magnetization can be influenced by the sedimentation process as suggested by other investigators. On the other hand, both geomagnetic and climatic (thickness) variations seem to be related to solar activity. Therefore, at least indirectly, palaeomagnetic data may reflect geomagnetic variations. 相似文献
139.
Chlorine atom-initiated photooxidations of CH2FCF3 (HFC-134a) in O2-N2 diluent were carried out to identify the products formed from the \(CF_3 CHF\dot O\) radical reactions and to determine the product yields as a function of temperature, pressure and O2 concentration. CF3C(O)F and HC(O)F were the major ‘first-generation’ products observed, along with smaller yields of C(O)F2 and, as yet, undetermined yields of CF3OOOCF3 and CF3OOC(O)F. The relative importance of the two major \(CF_3 CHF\dot O\) reaction pathways, is expressed by the rate constant ratio $$k_{O_2 } /k_d = 3.2 \times 10^{ - 25} e^{(3510 \pm 470)/T} cm^3 molecule^{ - 1}$$ The decomposition reaction leading to HC(O)F and ?F3 radical products is predicted to be the dominant pathway at the Earth's surface while mainly CF3C(O)F formation will occur at the tropopause. 相似文献
140.
Blanca Mendoza Virginia García-Acosta Victor Velasco Ernesto Jáuregui Rosa Díaz-Sandoval 《Climatic change》2007,83(1-2):151-168
Using unprecedented catalogues of past severe drought data for the Yucatan Peninsula between 1502 and 1900 coming from historical
written documentation, we identified five conspicuous time lapses with no droughts between 1577–1647, 1662–1724, 1728–1764,
1774–1799 and 1855–1880, as well as time epochs with most frequent droughts between 1800 and 1850. Moreover, the most prominent
periodicity of the historical drought time series was that of ∼40 years. Using the Palmer Drought Severity Index for the Yucatan
Peninsula for the period 1921–1987 we found prominent negative phases between ∼1942–1946 and 1949–1952, 1923–1924, 1928–1929,
1935–1936, 1962–1963, 1971–1972 and 1986–1987. Two prominent periodicities clearly appear at ∼5 and 10 years. Most modern
and historical severe droughts lasted 1 year, and share a quasi-decadal frequency. Also, in the first 66 years of the twentieth
century the frequency of occurrence of severe drought has been lower compared with the nineteenth century. Some of the major
effects and impacts of the most severe droughts in the Yucatan region are examined. We also studied the relation between historical
and modern droughts and several large scale climate phenomena represented by the Atlantic Multidecadal Oscillation (AMO) and
the Southern Oscillation Index (SOI). Our results indicate that historical droughts and the cold phase of the AMO coincide,
while the influence of the SOI is less clear. The strongest coherence between historical droughts and AMO occurred at periodicities
of ∼40 years. For modern droughts the coherence of a drought indicator (the Palmer Drought Severity Index) is similar with
AMO and SOI, although it seems more sustained with the AMO. They are strongest at ∼10 years and very clearly with the AMO
cold phase. Concerning the solar activity proxies and historical droughts, the coherence with a record of beryllium isotope
Be10, which is a good proxy of cosmic rays, is higher than with Total Solar Irradiance. We notice that the strongest coherence
between historical droughts and Be10 occurs at periods ∼60–64 years. When studying modern droughts and solar activity, frequencies of ∼8 years appear, and the
coherences are similar for both sunspots and cosmic rays. Comparing natural terrestrial and solar phenomena, we found that
the most sustained and strongest modulation of historical drought occurrence is at ∼60–64 years and is between the historical
drought series and the solar proxy Be10. For modern droughts we notice that the coherence is similar among AMO, SOI and the solar indices. We can conclude that the
sea surface temperatures (AMO) and solar activity leave their signal in terms of severe droughts in the Maya lands, however
in the long term, the influence of the SOI on this type of phenomenon is less clear. 相似文献