全文获取类型
收费全文 | 333篇 |
免费 | 18篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 16篇 |
地球物理 | 83篇 |
地质学 | 121篇 |
海洋学 | 30篇 |
天文学 | 82篇 |
自然地理 | 13篇 |
出版年
2022年 | 4篇 |
2021年 | 7篇 |
2020年 | 7篇 |
2019年 | 10篇 |
2018年 | 14篇 |
2017年 | 10篇 |
2016年 | 16篇 |
2015年 | 16篇 |
2014年 | 18篇 |
2013年 | 12篇 |
2012年 | 10篇 |
2011年 | 15篇 |
2010年 | 13篇 |
2009年 | 23篇 |
2008年 | 25篇 |
2007年 | 24篇 |
2006年 | 18篇 |
2005年 | 16篇 |
2004年 | 12篇 |
2003年 | 9篇 |
2002年 | 13篇 |
2001年 | 7篇 |
2000年 | 6篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1963年 | 1篇 |
1958年 | 1篇 |
1953年 | 2篇 |
1952年 | 2篇 |
1950年 | 1篇 |
1948年 | 1篇 |
1946年 | 1篇 |
排序方式: 共有353条查询结果,搜索用时 15 毫秒
121.
The Sardinia Radio Telescope (SRT) is a challeging scientific project managed by the National Institute for Astrophysics (INAF),
it is being developed at 30 km North of the city of Cagliari, Italy. The goal of the SRT project is to build a general purpose,
fully steerable, 64 m diameter radio telescope, capable of operating with high efficiency in the centimeter and millimeter
frequency range (0.3–100 GHz). In portions of this frequency range, especially towards the high end, astronomical observations
can be heavily deteriorated by non-optimal atmospheric conditions, especially by water vapor content. The water molecule permanent
electric dipole in fact, leads to pressure broadened rotational transitions around the 22.23 GHz spectral line. Furthermore,
water vapor’s continuum absorption and emission may influence higher frequency observations too. To a lower degree, cloud
liquid black body radiation can also affect centimeter and millimeter observations. In addition to this, inhomogeneities in
water vapor distributions can cause signal phase errors which introduce a great amount of uncertainty to VLBI mode observations.
The Astronomical Observatory of Cagliari (OA-CA) has obtained historical timeseries of radiosonde profiles conducted at the
airport of Cagliari. Through the radiosonde measurements and an appropriate radiative transfer model, we have performed a
statistical analysis of the SRT site’s atmosphere which accounts for atmospheric opacity at different frequencies, integrated
water vapor (IWV), integrated liquid water (ILW) and cloud cover distributions during the year. This will help to investigate
in which period of the year astronomical observations at different frequencies should be performed preferably. The results
show that, at the SRT site, K-band astronomical observations are possible all year round, the median opacity at 22.23 GHz
is 0.10 Np in the winter (Dec-Jan-Feb) and 0.16 Np in the summer (Jun-Jul-Aug). Integrated water vapor during winter months
ranges, on average, between 7 and 15 mm. Cloud cover is usually not present for more than 36% of the time during the year.
The atmospheric opacity study indicates that observations at higher frequencies (50–100 GHz) may be performed usefully: the
median opacity at 100 GHz is usually below or equal to 0.2 Np in the period that ranges from January to April. 相似文献
122.
The near-surface rock structure that covers an abandoned marl mine nearby the village of Montevecchia (Italy) was investigated through a combination of seismic surveys. The methods selected for these investigations were refraction seismics and multichannel analysis of surface waves (MASW). A sort of transillumination experiment was also attempted. All the methods were successful and gave complementary information. Refraction seismics was used to characterize the upper low velocity layer and the second layer of the near-surface structure. The MASW method was necessary to assess the existence of a velocity inversion revealing the presence of a low velocity layer between the 2nd layer and a 4th high velocity layer covering the upper mine gallery. The transillumination experiment validated the presence of the 4th layer and gave an estimate of the average velocity that represents a lower boundary for the P-wave velocity within this layer. Both the refraction and transillumination data were analysed to derive average estimates of attenuation level and rock quality factor. 相似文献
123.
124.
The Pitzer’s interaction parameters, λN–M, involving the Mth cationic Al species Al3+ or AlOH2+ or AlO+ and the Nth neutral species SiO2(aq) (at temperatures of 25–300 °C) or CO2(aq) (at temperatures of 25–150 °C), have been evaluated through empirical linear relationships between λN–M and the surface electrostatic field of the ionic species of interest. These relationships have been obtained starting from the known λN–M for both SiO2(aq) and CO2(aq) with the main dissolved cations. The Pitzer’s interaction parameter thus estimated for the pair CO2(aq)–Al3+ at 25 °C, 0.327, is 20–40% higher than the corresponding values obtained from CO2 solubilities in concentrated solutions of AlCl3, 0.272 ± 0.010 (2σ), and Al2(SO4)3, 0.232 ± 0.002 (2σ), partly corroborating the empirical approach adopted in this study. To test the Pitzer’s interaction coefficients for cationic Al species with aqueous SiO2, the log K values of the kaolinite dissolution reaction have been computed starting from available experimental data at 23–25 °C and ionic strengths of 0.0001–0.12 mol/kg adopting, alternatively, the Pitzer’s equations and the Debye–Hückel equation. A satisfactory agreement has been found between the log K values obtained through these two approaches, with maximum deviations of 0.11–0.12 log units. This good convergence of results is encouraging as it represents a necessary condition to prove the reliability of the Pitzer’s interaction coefficients estimated in this work. These results are a first step to take into account specific interactions among solutes in concentrated electrolyte solutions, such as those hosted in sedimentary basins or geothermal waters, for instance through the Pitzer’s equations. However, experimental or field data at higher ionic strengths are absolutely necessary to validate the reliability of the Pitzer’s interaction coefficients determined in this study. 相似文献
125.
Carmine Apollaro Marina Accornero Luigi Marini Donatella Barca Rosanna De Rosa 《Applied Geochemistry》2009
This work is aimed at investigating the weathering processes of the granodiorites cropping out in a small catchment of the Sila Massif. The mineral constituents in this granodiorite are plagioclase, often zoned with a Ca-rich core and a Na-rich rim, quartz, chlorite, K-feldspar, white mica and epidote. During this study, dolomite was discovered in local stream sediments, as separate monomineralic grains, probably resulting from erosion of veins cutting the crystalline rocks. Prevailing dissolution of foreign dolomite and a Ca-rich plagioclase is suggested by the Ca–Mg–HCO3 chemical composition of local groundwaters and stream waters, which is rather unexpected for waters interacting with granitoid rocks. These qualitative observations are quantitatively confirmed by reaction path modelling of the weathering processes occurring in the study area, which was carried out using the EQ3/6 software package, version 8.0, and the Double Solid Reactant Method. Indeed, it was possible to ascertain that the release of both major dissolved constituents and several trace elements (Ba, Co, Cr, Fe, Mn, Ni, Pb, Sr, V and Zn), from rocks to waters, is chiefly controlled by the dissolution of foreign dolomite and the Ca-rich core of zoned plagioclases. 相似文献
126.
Biochemical and microbial features of shallow marine sediments along the Terra Nova Bay (Ross Sea,Antarctica) 总被引:1,自引:0,他引:1
Franco Baldi Davide Marchetto Francesco Pini Renato Fani Luigi Michaud Angelina Lo Giudice Daniela Berto Michele Giani 《Continental Shelf Research》2010
Shallow marine sediments were collected from seven stations (three of which located at Gerlache Inlet, two at Tethys Bay, one at Adelie Cove and one just beneath the Italian Research Base) along the Terra Nova Bay coast (Ross Sea, Antarctica). Their chemical, biochemical and microbiological properties were studied in order to provide further insights in the knowledge of this Antarctic benthic ecosystem. Overall, the organic carbon (OC) represented the major fraction of total carbon (TC) and displayed concentrations similar to or slightly lower than those previously measured in Antarctic bottom sediments. The biopolymeric carbon within OC ranged from 4.1% to 19.9% and showed a wide trophic range (65–834 μg g−1 d.w.). Proteins (PRT) represented on average the main biochemical class contributing to labile organic carbon, followed by lipids (LIP) and carbohydrates (CHO). The activity of aminopeptidase, β-d-glucosidase, alkaline phosphatase and esterase was checked, giving the highest values at Tethys Bay and at the deepest water sediments. The principal component analysis, which was computed considering physical, chemical (elemental and biochemical sedimentary composition) and microbiological parameters (including bacterial abundance, ectoenzymatic activities, T-RFs richness and diversity indices), allowed to obtain two main clusters (“Tethys Bay” and “other stations”). Based on data obtained, two representative 16S rRNA clone libraries using samples from Tethys Bay and Gerlache Inlet were constructed. The sequences of 171 clones were compared to those available in public databases to determine their approximate phylogenetic affiliations. Both aerobic and anaerobic bacteria were disclosed, with the majority of them affiliated with the Gamma- and Deltaproteobacteria, Bacteroidetes and Acidobacteria. The occurrence of strictly anaerobic bacteria suggests that sediments might also undergo anoxic conditions that, in turn, could favor the accumulation of PRT in respect to CHO, assuming that fermentation of amino acids is slower than that of sugars from decomposing polysaccharides. 相似文献
127.
128.
129.
Increasing amount of crystals tends to reduce the mobility of magmas and modifies its elastic characteristics (e.g. [Caricchi, L. et al., 2007. Non-Newtonian rheology of crystal-bearing magmas and implications for magma ascent dynamics. Earth and Planetary Science Letters, 264: 402–419.; Bagdassarov, N., Dingwell, D.B. and Webb, S.L., 1994. Viscoelasticity of crystal- and bubble-bearing rhyolite melts. Physics of the Earth and Planetary Interior, 83: 83–99.]). To quantify the effect of crystals on the elastic properties of magmas the propagation speed of shear and compressional waves have been measured at pressure and temperatures relevant for natural magmatic reservoirs. The measurements have been performed in aggregates at variable particle fractions (? = 0–0.7). The measurements were carried out at 200 MPa confining pressure and temperatures between 300 K and 1273 K (i.e. across the glass transition temperature (Tg) from glass to melt). The specimens were mixtures of a haplogranitic melt containing 5.25 wt.% H2O and variable amounts of sub-spherical alumina particles. Additional experiments were carried out on a sample containing both, crystals and air bubbles. The temperature derivatives of the shear (dVs/dT) and compressional wave (dVp/dT) velocities for pure glass and samples with a crystal fraction of 0.5 are different below and above the glass transition temperature. For a crystal fraction 0.7, only dVp/dT changed above the Tg. In the presence of gas bubbles, Vp and Vs decrease constantly with increasing temperature. The bubble-bearing material yields a lower bulk modulus relative to its shear modulus. The propagation velocities of compressional and shear waves increase non-linearly with increasing crystal fraction with a prominent raise in the range 0.5 < ? < 0.7. The speed variations are only marginally related to the density increase due to the presence of crystals, but are dominantly related to the achievement of a continuous crystal framework. The experimental data set presented here can be utilized to estimate the relative proportions of crystals and melt present in a magmatic reservoir, which, in turn, is one of the fundamental parameters determining the mobility of magma and, consequently, exerting a prime control on the likelihood of an eruption from a sub-surficial magma reservoir. 相似文献
130.
We point out one problem of the grid advection schemes when used in wave models in coastal areas. The deficiency of the schemes is investigated by means of the ‘third' generation WAM wave model, in which the wave energy is advected by a first order upwind scheme. Two similar, alternative modifications of this scheme are analyzed, the second of which is shown to solve most of the problems encountered with advection along the co-ordinate axes. 相似文献