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141.
Basílio X. Santiago Rebecca A. W. Elson Steinn Sigurdsson & Gerard F. Gilmore 《Monthly notices of the Royal Astronomical Society》1998,295(4):860-868
We present V and I photometry of two open clusters in the LMC down to V ∼26. The clusters were imaged with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope ( HST ), as part of the Medium Deep Survey Key Project. Both are low-luminosity ( MV ∼−3.5), low-mass ( M ∼103 M⊙) systems. The chance discovery of these two clusters in two parallel WFPC2 fields suggests a significant incompleteness in the LMC cluster census near the bar. One of the clusters is roughly elliptical and compact, with a steep light profile, a central surface brightness μ V (0)∼20.2 mag arcsec−2 , a half-light radius r hl ∼0.9 pc (total visual major diameter D ∼3 pc) and an estimated mass M ∼1500 M⊙. From the colour–magnitude diagram and isochrone fits we estimate its age as τ∼(2–5)×108 yr. Its mass function has a fitted slope of Γ=Δlogφ( M )/Δlog M =−1.8±0.7 in the range probed (0.9≲ M /M⊙≲4.5). The other cluster is more irregular and sparse, having shallower density and surface brightness profiles. We obtain Γ=−1.2±0.4, and estimate its mass as M ∼400 M⊙. A derived upper limit for its age is τ≲5×108 yr. Both clusters have mass functions with slopes similar to that of R136, a massive LMC cluster, for which HST results indicate Γ∼−1.2. They also seem to be relaxed in their cores and well contained in their tidal radii. 相似文献
142.
Beat Schmid John M. Livingston Philip B. Russell Philip A. Durkee Haflidi H. Jonsson Donald R. Collins Richard C. Flagan John H. Seinfeld Santiago Gassó Dean A. Hegg Elisabeth Öström Kevin J. Noone Ellsworth J. Welton Kenneth J. Voss Howard R. Gordon Paola Formenti Meinrat O. Andreae 《Tellus. Series B, Chemical and physical meteorology》2000,52(2):568-593
We report on clear‐sky column closure experiments (CLEARCOLUMN) performed in the Canary Islands during the second Aerosol Characterization Experiment (ACE‐2) in June/July 1997. We present CLEARCOLUMN results obtained by combining airborne sunphotometer and in‐situ (optical particle counter, nephelometer, and absorption photometer) measurements taken aboard the Pelican aircraft, space‐borne NOAA/AVHRR data and ground‐based lidar and sunphotometer measurements. During both days discussed here, vertical profiles flown in cloud‐free air masses revealed 3 distinctly different layers: a marine boundary layer (MBL) with varying pollution levels, an elevated dust layer, and a very clean layer between the MBL and the dust layer. A key result of this study is the achievement of closure between extinction or layer aerosol optical depth (AOD) computed from continuous in‐situ aerosol size‐distributions and composition and those measured with the airborne sunphotometer. In the dust, the agreement in layer AOD (λ=380–1060 nm) is 3–8%. In the MBL there is a tendency for the in‐situ results to be slightly lower than the sunphotometer measurements (10–17% at λ=525 nm), but these differences are within the combined error bars of the measurements and computations. 相似文献
143.
Juan Diego Jaramillo Juan David Gómez Doriam Restrepo Santiago Rivera 《地震工程与工程振动(英文版)》2014,13(3):535-543
A method is proposed to estimate the seismic soil-structure-interaction (SSI) effects for use in engineering practice. It is applicable to 2D structures subjected to vertically incident shear waves supported by homogenous half-spaces. The method is attractive since it keeps the simplicity of the spectral approach, overcomes some of the difficulties and inaccuracies of existing classical techniques and yet it considers a physically consistent excitation. This level of simplicity is achieved through a response spectra modification factor that can be applied to the free-field 5%-damped response spectra to yield design spectral ordinates that take into account the scattered motions introduced by the interaction effects. The modification factor is representative of the Transfer Function (TF) between the structural relative displacements and the free-field motion, which is described in terms of its maximum amplitude and associated frequency. Expressions to compute the modification factor by practicing engineers are proposed based upon a parametric study using 576 cases representative of actual structures. The method is tested in 10 cases spanning a wide range of common fundamental vibration periods. 相似文献
144.
Every year the Earth crosses or passes near one of the dust trails left by Comet 55P/Tempel-Tuttle in its pass through the Solar System every 33.2 years. This produces a meteor shower Commonly called the Leonid. The 2001 Leonid meteor shower is one of the strongest in recent years. We present observations made by the 50 MHz all-sky meteor radar located at the Platteville Atmospheric Observatory in Colorado (40° N, 105° W). The spatial and temporal distributions of the meteor activity detected by the radar during the 2001 Leonid shower differs from the observed sporadic activity detected by VHF radars. Estimation of the radiant flux of the meteor shower of the shower by a well-known methodology is presented, and the intensity of the phenomena is discussed. 相似文献
145.
Dmitri Rouwet Salvatore Inguaggiato Yuri Taran Nicholas Varley José A. Santiago S. 《Bulletin of Volcanology》2009,71(3):319-335
This study presents baseline data for future geochemical monitoring of the active Tacaná volcano–hydrothermal system (Mexico–Guatemala).
Seven groups of thermal springs, related to a NW/SE-oriented fault scarp cutting the summit area (4,100m a.s.l.), discharge
at the northwest foot of the volcano (1,500–2,000m a.s.l.); another one on the southern ends of Tacaná (La Calera). The near-neutral
(pH from 5.8 to 6.9) thermal (T from 25.7°C to 63.0°C) HCO3–SO4 waters are thought to have formed by the absorption of a H2S/SO2–CO2-enriched steam into a Cl-rich geothermal aquifer, afterwards mixed by Na/HCO3-enriched meteoric waters originating from the higher elevations of the volcano as stated by the isotopic composition (δD
and δ18O) of meteoric and spring waters. Boiling temperature fumaroles (89°C at ~3,600m a.s.l. NW of the summit), formed after the
May 1986 phreatic explosion, emit isotopically light vapour (δD and δ18O as low as −128 and −19.9‰, respectively) resulting from steam separation from the summit aquifer. Fumarolic as well as bubbling
gases at five springs are CO2-dominated. The δ13CCO2 for all gases show typical magmatic values of −3.6 ± 1.3‰ vs V-PDB. The large range in 3He/4He ratios for bubbling, dissolved and fumarolic gases [from 1.3 to 6.9 atmospheric 3He/4He ratio (R
A)] is ascribed to a different degree of near-surface boiling processes inside a heterogeneous aquifer at the contact between
the volcanic edifice and the crystalline basement (4He source). Tacaná volcano offers a unique opportunity to give insight into shallow hydrothermal and deep magmatic processes
affecting the CO2/3He ratio of gases: bubbling springs with lower gas/water ratios show higher 3He/4He ratios and consequently lower CO2/3He ratios (e.g. Zarco spring). Typical Central American CO2/3He and 3He/4He ratios are found for the fumarolic Agua Caliente and Zarco gases (3.1 ± 1.6 × 1010 and 6.0 ± 0.9 R
A, respectively). The L/S (5.9 ± 0.5) and (L + S)/M ratios (9.2 ± 0.7) for the same gases are almost identical to the ones calculated for gases in El Salvador, suggesting an
enhanced slab contribution as far as the northern extreme of the Central American Volcanic Arc, Tacaná. 相似文献
146.
S. Santiago R. L. Thomas L. McCarthy J. L. Loizeau G. Larbaigt C. Corvi D. Rossel J. Tarradellas J. P. Vernet 《水文研究》1992,6(2):227-240
Ten large volume water samples were taken from the Rhône River (Switzerland-France) in November, 1989 for recovery of total suspended sediment by continuous flow centrifugation. the samples were freeze-dried and analysed for particle size, organic carbon, total nitrogen, and carbonate. for comparative purposes, four bed sediments collected in July, 1989 are also described. the Rhône can be subdivided into three sections on the basis of the origins of the water. the first section is the Upper Rhône River draining into Lake Geneva. Waters are derived from glaciers, with low temperature and conductivity and high turbidity. Suspended sediment is coarse, has a bimodal distribution, and is low in both organic matter and carbonate. the second reach is from Lake Geneva to the confluence with the Saône at Lyon and has warmer water with higher conductivity and very low turbidity. Suspended sediment is higher in organic matter, with high carbonate originating from the lake. the final section is from Lyon to Arles, with warmer water and higher conductivity and turbidity due to modification by the Saône. Sediment is rich in organic matter, which May, account for an observed decline in oxygen in the river waters downstream from Lyon. Carbonate in these sediments also decreases due to increased turbidity from the Saône. Suspended sediments other than from the Upper Rhône show a remarkable consistency in grain size, predominantly in the fine silts (mode 9-11 μm). This consistency indicates a high degree of suitability for geochemical analysis. Bed sediments were bimodal throughout, with a dominant coarse population in two out of the four samples. Grain size statistical parameters could be easily explained by application of the theory of mixing of two major populations in the sand size (bed traction load) and the fine silt/clay size (suspended sediment load). 相似文献
147.
The justification for the presence of graphite in the interstellar medium is examined. The conditions necessary for growth of graphite (single crystals) and other structurally disordered carbons are discussed. The effect of structural imperfections on the mechanical, thermal, electrical, magnetic, optic and chemical properties is illustrated. From this information it is concluded that the existence of graphite (single crystals) in the interstellar medium is unlikely. While various forms of highly disordered carbon grains may exist, none would satisfy the physical properties necessary to explain both polarization and extinction simultaneously. 相似文献
148.
Gustavo E. Romero Jorge A. Combi Santiago E. Perez Bergliaffa Luis A. Anchordoqui 《Astroparticle Physics》1996,5(3-4)
The ultra-high energy cosmic rays recently detected by several air shower experiments could have an extragalactic origin. In this case, the nearest active galaxy Centaurus A might be the source of the most energetic particles ever detected on Earth. We have used recent radio observations in order to estimate the arrival energy of the protons accelerated by strong shock fronts in the outer parts of this southern radio source. We expect detections coresponding to particles with energies up to 2.2 × 1021 eV and an arrival direction of (l ≈ 310°, b ≈ 20°) in galactic coordinates. The future Southern Hemisphere Pierre Auger Observatory might provide a decisive test for extragalactic models of the origin of the ultra-high energy cosmic rays. 相似文献
149.
Validation and Evaluation of Predictive Models in Hazard Assessment and Risk Management 总被引:12,自引:4,他引:12
Santiago Beguería 《Natural Hazards》2006,37(3):315-329
The paper deals with the validation and evaluation of mathematical models in natural hazard analysis, with a special focus
on establishing their predictive power. Although most of the tools and statistics available are common to general classification
models, some peculiarities arise in the case of hazard assessment. This is due to the fact that the target for validation,
the propensity to develop a dangerous characteristic, is not really known and must be estimated from a (usually) very small
sample. This implies that the two types of errors (false positives and false negatives) should be given different meanings.
Related to this, a very frequent situation is the presence of prevalence (different proportion of positive and negative cases)
in the sample. It is shown that sample prevalence can have a dramatic effect in some very common validation statistics, like
the confusion matrix and model efficiency. Here some statistics based on the confusion matrix are presented and discussed,
and the use of threshold-independent approaches (especially the ROC plot) is shown. The ROC plot is also proposed as a convenient
tool for decision-taking in a risk management context. A general scheme for hazard predictive modeling is finally proposed. 相似文献
150.
Palynological analyses (pollen and dinocysts) of a sediment core taken in the Kara-Bogaz Gol have been used to reconstruct rapid and catastrophic environmental changes over the last two centuries (chronology based on 210Pb). A natural cyclicity (65 years) of water level changes in the Caspian Sea (CS) and in the Kara-Bogaz Gol (KBG) and anthropogenic factors (building of a dam separating the CS and the KBG waters) combine to induce rapid changes in water levels of the KBG, in the salinity of its waters and in vegetation cover of its surroundings. The impact of low water levels on the dinocysts is marked by a lower diversity and the survival of two species that are typical of the KBG, the Caspian Sea species present in the KBG having disappeared. During periods of higher water levels (AD 1871–1878), the lake is surrounded by a steppe-like vegetation dominated by Artemisia; whereas during periods of low water levels (AD 1878–1913 and AD 1955–1998), the emerged shore are colonised by Chenopodiaceae. The period of AD 1913–1955 corresponding to decreasing water levels has an extremely low pollen concentration and a maximum of reworking of arboreal taxa. During the last low-level period, humans responded by abandoning the shores of the bay. What happened to the KBG can be used as an example of what may happen in the future for the Aral Sea.A problem of reworking of Tertiary dinocysts into modern deposits has been detected owing to the knowledge of the modern dinoflagellate assemblages recently made available through a water survey. A comparison to modern surface pollen samples from Central Asia (Anzali, Caspian Sea south and central basins, Aral Sea, Lake Balkhash, Lake Issyk-Kul and the Chinese Tien-Shan range) allows us to establish the potential reworking of at least five arboreal pollen taxa possibly by run-off and dust storms. 相似文献