全文获取类型
收费全文 | 294篇 |
免费 | 17篇 |
国内免费 | 2篇 |
专业分类
大气科学 | 33篇 |
地球物理 | 67篇 |
地质学 | 88篇 |
海洋学 | 34篇 |
天文学 | 72篇 |
自然地理 | 19篇 |
出版年
2022年 | 2篇 |
2021年 | 3篇 |
2020年 | 3篇 |
2019年 | 6篇 |
2018年 | 6篇 |
2017年 | 11篇 |
2016年 | 10篇 |
2015年 | 15篇 |
2014年 | 7篇 |
2013年 | 7篇 |
2012年 | 15篇 |
2011年 | 11篇 |
2010年 | 11篇 |
2009年 | 22篇 |
2008年 | 23篇 |
2007年 | 19篇 |
2006年 | 29篇 |
2005年 | 16篇 |
2004年 | 8篇 |
2003年 | 10篇 |
2002年 | 9篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1999年 | 10篇 |
1998年 | 14篇 |
1997年 | 1篇 |
1996年 | 4篇 |
1995年 | 1篇 |
1994年 | 8篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 5篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1973年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有313条查询结果,搜索用时 15 毫秒
281.
282.
James MacDonald Margarita Hernanz & Jordi José 《Monthly notices of the Royal Astronomical Society》1998,296(3):523-530
We investigate the evolution of cooling helium atmosphere white dwarfs using a full evolutionary code, specifically developed to follow the effects of element diffusion and gravitational settling on white dwarf cooling. The major difference between this work and previous work is that we use more recent opacity data from the OPAL project. Since, in general, these opacities are higher than those available 10 years ago, at a given effective temperature, convection zones go deeper than in models with older opacity data. Thus convective dredge-up of observationally detectable carbon in helium atmosphere white dwarfs can occur for thicker helium layers than found by Pelletier et al. We find that the range of observed C to He ratios in different DQ white dwarfs of similar effective temperature is well explained by a range of initial helium layer mass between 10−3 and 10−2 M⊙, in good agreement with stellar evolution theory, assuming a typical white dwarf mass of 0.6 M⊙. We also predict that oxygen will be present in DQ white dwarf atmospheres in detectable amounts if the helium layer mass is near the lower limit compatible with stellar evolution theory. Determination of the oxygen abundance has the potential of providing information on the profile of oxygen in the core and hence on the important 12 C(α,γ)16 O reaction rate. 相似文献
283.
Jesús Carrera Xavier Sánchez-Vila Inmaculada Benet Agustín Medina Germán Galarza Jordi Guimerà 《Hydrogeology Journal》1998,6(1):178-190
Matrix diffusion has become widely recognized as an important transport mechanism. Unfortunately, accounting for matrix diffusion
complicates solute-transport simulations. This problem has led to simplified formulations, partly motivated by the solution
method. As a result, some confusion has been generated about how to properly pose the problem. One of the objectives of this
work is to find some unity among existing formulations and solution methods. In doing so, some asymptotic properties of matrix
diffusion are derived. Specifically, early-time behavior (short tests) depends only on φ
m
2
R
m
D
m
/ L
m
2, whereas late-time behavior (long tracer tests) depends only on φ
m
R
m
, and not on matrix diffusion coefficient or block size and shape. The latter is always true for mean arrival time. These
properties help in: (a) analyzing the qualitative behavior of matrix diffusion; (b) explaining one paradox of solute transport
through fractured rocks (the apparent dependence of porosity on travel time); (c) discriminating between matrix diffusion
and other problems (such as kinetic sorption or heterogeneity); and (d) describing identifiability problems and ways to overcome
them.
Received, October 1997 · Revised, November 1997 · Accepted, December 1997 相似文献
284.
Jordi Vilà-Guerau de Arellano Peter G. Duynkerke 《Journal of Atmospheric Chemistry》1993,16(2):145-155
A second-order modelling technique is used to investigate the influence of turbulence on chemical reactions. The covariance and variance equations for the NO-O3-NO2 system are developed as a function of the ratio of the timescale of turbulence (
t
) and the timescale of chemistry (Ch): the first Damköhler number (
t
/Ch). Special attention is given to the calculation of the covariance between NO and O3 normalized by the product of their means, the so-called intensity of segregation (I
S
). This parameter quantifies the state of mixing of two chemical species.The intensity of segregation is calculated as a function of the flux of NO and the first Damköhler number. The model results presented illustrate the importance of taking the effect of turbulence on chemical reactions into account for higher values of the NO flux, for values of the ratio O3/NO larger than 12.5 and for values of the ratio
t
/CH larger than 0.1. For such cases, the effective reaction rates are slower than if the chemical species are assumed to be uniformly mixed. 相似文献
285.
Colombo Celso Gaeta Tassinari Fabio Diaz Pinzon Juaquin Buena Ventura 《Ore Geology Reviews》2008,33(3-4):505-518
Epigenetic gold mineralization occurs in the Marmato mining district, within the Calima Terrain of the Setentrional Andes, Colombia. Regional rocks associated with this mineralization include: graphite- and chlorite-schists of the Arquia Complex; metamorphosed during the Cretaceous, Miocene sandstones, shales and conglomerates of the Amagá Formation; as well as pyroclastic rocks (clasts of basalt, andesites and mafic lavas) and subvolcanic andesitic/dacitic bodies of the Combia Formation (9 to 6 Ma). The subvolcanic Marmato stock hosts mesothermal and epithermal low-sulfidation Au–Ag ores in the form of distensional veins, stockwork, and quartz veinlets within brecciated zones. Ore minerals are pyrite, sphalerite and galena with subordinate chalcopyrite, arsenopyrite, pyrrhotite, argentite and native gold/electrum.Sericitized plagioclase from a porphyry dacite yielded a K–Ar age of 5.6 ± 0.6 Ma, interpreted as the age of ore deposition. This is in close agreement with the age of reactivation of the Cauca–Romeral Fault System (5.6 ± 0.4 Ma), which bounds the Calima Terrain. A porphyry andesite–dacite (6.7 ± 0.1 Ma), hosting the Au–Ag veins, shows a measured 87Sr/86Sr between 0.70440 and 0.70460, εNd between + 2.2 and + 3.2 and 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of 18.964 to 19.028; 15.561 to 15.570; and 38.640 to 38.745, respectively. The 87Sr/86Sr and εNd values of rocks from the Arquia Group range from 0.70431 to 0.73511 and − 12.91 to + 10.0, respectively, whereas the corresponding Pb isotopic ratios (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) range from 18.948 to 19.652; 15.564 to 15.702; and 38.640 to 38.885, respectively. 87Sr/86Sr and εNd values obtained on sulfides from the gold quartz veins, which occur at shallow and intermediate levels, range from 0.70500 to 0.71210 and from − 1.11 to + 2.40. In the deepest veins, εNd values lie between + 1.25 and + 3.28 and the 87Sr/86Sr of calcite and pyrite fall between 0.70444 and 0.70930. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of all mineralization are in the ranges 18.970 to 19.258; 15.605 to 15.726 and 38.813 to 39.208, respectively. Carbonates have an average 87Sr/86Sr ratio of 0.70445, which is within the range of values measured in the host dacite. The Sr isotopic data indicate that carbonic fluids have a restricted hydrothermal circulation within the host igneous body, while the Sr, Pb and Nd isotopic compositions of the sulfides suggest that the fluids not only circulated within the Marmato stock, but also throughout the Arquia Complex, inferring that these rocks offer a potential target for mineral exploration. Based on geological and geochronological evidence, the epizonal Marmato gold ores formed during the Miocene to Pliocene, as a result of cooling of the Marmato stock and reactivation along a crustal-scale fault zone related to thermal processes in an accretionary oceanic–continental plate orogen. 相似文献
286.
The objective of this study was to investigate the removal mechanism of U(VI) from groundwater by magnetite as the main product of anoxic steel corrosion. For this purpose, a systematic sequence of batch experiments was conducted to focus the active role of magnetite in the reduction of U under different conditions. Results indicated that under anoxic conditions U(VI) was sorbed at the magnetite surface, whereas under reducing conditions at different H2(g) pressures, U was present in tetravalent form as amorphous UO2. 相似文献
287.
Assessing aquifer vulnerability to pollutants by electrical resistivity tomography (ERT) at a nitrate vulnerable zone in NE Spain 总被引:1,自引:0,他引:1
There is an increasing demand for groundwater vulnerability maps which illustrate the exposure of aquifers against pollution.
These maps show areas of greatest potential for groundwater contamination on the basis of local subsurface conditions. Parameters
affecting vulnerability are mainly permeability and thickness of each protective layer. For unconsolidated sediments, the
permeability is strongly related to the clay content, which can be deduced from indirect resistivity methods, like electrical-imaging.
Such geophysical methods can be of great help in groundwater vulnerability studies because they disturb neither the structure
nor the dynamics of the soil. Sensibility analysis was performed of the electrical resistivity tomography method for accurately
mapping soil media. Managers and public administrators may effectively use this method for assessing the potential risk of
groundwater contamination. In the studied zone, electrical resistivity exhibits a wide range of variability that can be easily
correlated to soil parameters, such as clay content and hydraulic conductivity. A numerical index of protection has been assessed
from the geophysical information derived from 2D electrical resistivity tomography. This work represents a preliminary approach
on the natural vulnerability evaluation of shallow aquifers at the Empordà basin (NE Spain) that is highly affected by diffuse
pollution by nitrates. 相似文献
288.
Robert J. Diaz Donald C. Rhoads James A. Blake Roy K. Kropp Kenneth E. Keay 《Estuaries and Coasts》2008,31(6):1184-1197
A combination of methods (infaunal grabs and sediment profile cameras) were used to monitor the response of Boston Harbor
benthic habitats to reductions in wastewater associated with movement of the outfalls to the mouth of the harbor and then
offshore. From 1992 to 2006, there was strong evidence that benthic habitats within Boston Harbor have shifted from a more
anaerobic state to a more aerobic state and that these changes are directly related to changes in carbon loading associated
with outfall placement and improvements in wastewater treatment. Over the period of 1992 to 2000, when the ocean outfall started
to operate, there was >90% reduction in organic loadings to Boston Harbor from 11,400 to 1,200 t C per year. There were also
corresponding decreases in primary production due to reduced nutrient loadings. The most apparent change in harbor benthos
was the widespread increase in 1992 and subsequent decline by 2005 in Ampelisca spp. tube mats. The long-term increase in thickness of the apparent color redox potential discontinuity layer was consistent
with reductions in organic loading and increases in bioturbation. The optimal organic loading for maintaining large areas
of amphipod tube mats and high bioturbation rates was around 500 g C per square meter per year. Above and below this level,
the area of tube mats in Boston Harbor declined. 相似文献
289.
Antonio Pulido‐Bosch José Delgado Fernando Sola Ángela Vallejos Fernando Vicente Juan M. López‐Sánchez Jordi J. Mallorquí 《水文研究》2012,26(5):731-740
Ground subsidence of detrital deposits in the Almería basin (SE Spain) was studied using the remote sensing technique of Differential Interferometry SAR (DInSAR). This basin is one of the most arid in Europe, receiving an average rainfall of 250 mm per year. Over the last 60 years the region has experienced an enormous agricultural and urban expansion, whose water demand has been largely supplied from groundwater, leading to the current situation of overexploitation of water resources. This paper outlines the likely relationship between groundwater abstraction and subsidence. To this end, 34 ERS and Envisat images, taken between 2003 and 2009, were analysed to estimate ground surface deformations, and hence, compared with water table variations measured in a number of piezometers in the basin. The analysis shows a clear parallelism between the variations in piezometric level and deformation of the ground surface. In addition, the zones of greatest subsidence coincide with those areas where groundwater abstractions are concentrated. Subsidence over the examined period varies from 10 to 30 mm, with extreme values as high as 50 mm, which translates to a rate of between 1·7 and 5 mm/year, reaching maximum rates of 8 mm/year at some points. Given such subsidence rates, damage to urban infrastructures are, for the moment, incipient. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
290.
Josep M. Trigo-Rodríguez Esko Lyytinen Daniel C. Jones José M. Madiedo Alberto J. Castro-Tirado Iwan Williams Jordi Llorca Stanislav Vítek Martin Jelínek Blanca Troughton Francisco Gálvez 《Monthly notices of the Royal Astronomical Society》2007,382(4):1933-1939
The existence of asteroidal meteoroid streams capable of producing meteorite-dropping bolides has long being invoked, but evidence is scarce. Recent modelling of previously reported associations suggests that the time-scales to keep the orbital coherence of these streams producing meteorites are too short. We present an unequivocal association between near earth object (NEO) 2002NY40 and at least one bright fireball detected over Finland in 2006 August. Another two additional fireballs recorded from Spain and Finland seem to be related, together producing a fireball-producing stream (β Aquarids). On the basis of historical data, the 2006 finding suggests the existence of a meteoroid complex capable of producing meteorites. Taking into account present time-scales for orbital decoherence, if 2002NY40 has large meteoroids associated with it, such behaviour would be the consequence of a relatively recent asteroidal fragmentation. Supporting our claim, the heliocentric orbits of two recently discovered NEOs, 2004NL8 and 2002NY40, were found to exhibit a good similarity to each other and also to the orbits of the three bolides. The fireball spectra of the two Finish bolides showed that the chemical abundances of these objects are consistent with the main elements found in chondrites. This result is consistent with the probable Low iron, Low metal (LL) chondritic mineralogy of asteroid 2002NY40. Consequently, this asteroid may be delivering LL chondrites to the Earth. Additional fireball reports found in the literature suggest that the associated β Aquarid complex may have been delivering meteorites to the Earth during, at least, the last millennium. 相似文献