全文获取类型
收费全文 | 240篇 |
免费 | 48篇 |
国内免费 | 27篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 26篇 |
地球物理 | 64篇 |
地质学 | 89篇 |
海洋学 | 27篇 |
天文学 | 15篇 |
综合类 | 8篇 |
自然地理 | 57篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 5篇 |
2021年 | 5篇 |
2020年 | 6篇 |
2019年 | 12篇 |
2018年 | 3篇 |
2017年 | 13篇 |
2016年 | 8篇 |
2015年 | 7篇 |
2014年 | 11篇 |
2013年 | 6篇 |
2012年 | 6篇 |
2011年 | 21篇 |
2010年 | 14篇 |
2009年 | 15篇 |
2008年 | 22篇 |
2007年 | 21篇 |
2006年 | 28篇 |
2005年 | 11篇 |
2004年 | 16篇 |
2003年 | 14篇 |
2002年 | 12篇 |
2001年 | 9篇 |
2000年 | 9篇 |
1999年 | 6篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有315条查询结果,搜索用时 78 毫秒
271.
The contribution of bound-free and free-free processes to the outward acceleration of Pup is studied and is found to be negligible. 相似文献
272.
Véronique Bommier 《Solar physics》1996,164(1-2):29-47
A synthesis work about the interaction of matter with polarized radiation, applied to solar magnetic field diagnostics, has recently been done by Stenflo (1994). This synthesis uses the classical theory of matter-radiation interaction - supplemented by the theory of partial redistribution of Omont, Smith, and Cooper (1972), on the one hand, and full quantum matter-radiation interaction theory, unable to take into account the partial frequency redistribution effects, on the other hand. The need of a full quantum approach taking into account the partial frequency redistribution effects appears as a unifying purpose; the present work, using the density matrix formalism, is a first attempt in this direction. 相似文献
273.
Non-LTE polarized radiative transfer in intermediate magnetic fields: Numerical problems and results
This paper presents some numerical results relative to a solution, based on the density matrix formalism, of the non-LTE, polarized radiative transfer problem for a two-level atom. The results concern the atomic upper level population and alignment, and the emergent radiation Stokes profiles, for a plane-parallel, static, isothermal atmosphere embedded in a magnetic field of intermediate strength, such that the Zeeman splitting has to be taken into account in the line profile. Zeeman coherences are neglected, whereas magneto-optical effects are taken into account, resulting in a full 4×4 absorption matrix. Induced emission is neglected and complete frequency redistribution, in the rest and laboratory frames, is assumed. Pure Doppler absorption profile (gaussian shape) has also been assumed. The presentation of the results is preceded by a brief discussion of their accuracy and of the numerical difficulties that were met in the solution of the problem.On leave from the Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 5, I-50125 Firenze, Italia 相似文献
274.
The concentrations of7Be,210Pb and major ions have been measured in acrosol and snow samples collected near Summit, Greenland (72°20N, 38°45W) in the summers of 1989 and 1990. Comparison to previous results from free tropospheric sampling of the North American Arctic indicates that some acrosol-associated species are as much as 50% depleted in near surface air over the Greenland Ice Sheet. It is shown that local atmospheric processes, particularly isolation of air masses beneath a near surface inversion, can exert dominant influence on the chemistry of surface-level air. These findings illustrate the extreme caution that must be taken if the results of surface-based atmospheric sampling are to be used to examine the relationship between the chemistry of the atmosphere and snow falling from it.Depth profiles of7Be in the surface layers of the snowpack near Summit suggest that up to half of the annual accumulation of snow may occur in the two to three month late spring-early summer period. If this is generally true for the Summit region, previous regional studies of snow chemistry that assumed linear dependence of age on depth to convert depth profiles to time series will have to be reassesed. However, spatial heterogeneity of near surface snow chemistry, that is currently not well understood, makes interpretation of the7Be profiles tentative at present. 相似文献
275.
Kerosene, a grade mineral oil, is commonly used to extract beetles from sediment. Here, the use of kerosene to extract chironomid
head capsules was tested on 10 samples from sediment of different lakes, with different organic matter content as measured
by loss on ignition, and estimated ages. Our results revealed that this flotation tool is very effective in extracting either
full or half chironomid head capsules. The mean extraction efficiency was 89.3 ± 8.0% with an estimated relative abundance
error ranging from −1% to 1% for 46 of the 57 identified taxa. Larger chironomids (400–500 μm width), which are often full
of sediment particles, have the highest relative abundance error, with a maximum of 4.3% for Corynocera oliveri-type. A canonical correspondence analysis showed that, despite this small bias, samples retrieved with the kerosene flotation
do not differ from the whole sample assemblages. These results give us confidence in the use of this flotation technique for
chironomid sample preparation. 相似文献
276.
Enlou Zhang Richard Jones Alan Bedford Peter Langdon Hongqu Tang 《Journal of Paleolimnology》2007,38(4):477-491
Previous studies have shown chironomids to be excellent indicators of environmental change and training sets have been developed
in order to allow these changes to be reconstructed quantitatively from subfossil sequences. Here we present the results of
an investigation into the relationships between surface sediment subfossil chironomid distribution and lake environmental
variables from 42 lakes on the Tibetan Plateau. Canonical correspondence analysis (CCA) revealed that of the 11 measured environmental
variables, salinity (measured as total dissolved solids TDS) was most important, accounting for 10.5% of the variance in the
chironomid data. This variable was significant enough to allow the development of quantitative inference models. A range of
TDS inference models were developed using Weighted Averaging (WA), Partial Least Squares (PLS), Weighted Averaging–Partial
Least Squares (WA–PLS), Maximum Likelihood (ML), Modern Analogues Technique (MAT) and Modern Analogues Techniques weighted
by similarity (WMAT). Evaluation of the site data indicated that four lakes were major outliers, and after omitting these
from the training set the models produced jack-knifed coefficients of determination (r
2) between 0.60 and 0.80, and root-mean-squared errors of prediction (RMSEP) between 0.29 and 0.44 log10 TDS. The best performing model was the two-component WA–PLS model with r
2
jack = 0.80 and RMSEPjack = 0.29 log10 TDS. The model results were similar to other chironomid-salinity models developed in different regions, and they also showed
similar ecological groupings along the salinity gradient with respect to freshwater/salinity thresholds and community diversity.
These results therefore indicate that similar processes may be controlling chironomid distribution across salinity gradients
irrespective of biogeographical constraints. The performance of the transfer functions illustrates that chironomid assemblages
from the Tibetan Plateau lakes are clearly sensitive indicators of salinity. The models will therefore allow the quantification
of long-term records of past water salinity for lacustrine sites across the Tibetan Plateau, which has important implications
for future hydrological research in the region. 相似文献
277.
The post-Variscan development of the British Isles within a regional transfer zone influenced by orogenesis 总被引:2,自引:0,他引:2
D.C.P. Peacock 《Journal of Structural Geology》2004,26(12):2225-2231
The break-up of Pangaea after the Variscan Orogeny included rifting extending southwards from the Barents Sea via the Norwegian–Greenland Rift and into the North Sea, and northwards from the Central Atlantic. These two major rift systems interacted to form an approximately 1200-km-wide transfer zone across the British Isles, where a complex network of basins developed during the Mesozoic. Fault patterns were commonly controlled by reactivation of Precambrian, Caledonian and Variscan structures. The two main rift systems were unable to breach this regional transfer zone, where the crust had been thickened by the Caledonian and Variscan orogenies, until the Eocene. Breaching did not occur down the North Sea and through the English Channel because of Alpine contraction in NW Europe. Instead, breaching occurred around the west of Ireland and NW Scotland, so the British Isles remained connected to Europe rather than to the North American Plate. 相似文献
278.
Diatom-based conductivity and water-level inference models from eastern Tibetan (Qinghai-Xizang) Plateau lakes 总被引:7,自引:0,他引:7
Xiangdong Yang Christian Kamenik Roland Schmidt Sumin Wang 《Journal of Paleolimnology》2003,30(1):1-19
Climate in central Asia is dominated by the Asian monsoon. The varying impact of the summer monsoon across the Tibetan (Qinghai-Xizang) Plateau provides a strong gradient in precipitation, resulting in lakes of different salinity. Diatoms have been shown to indicate changes in salinity. Thus, transfer functions for diatoms and salinity or related environmental variables represent an excellent tool for paleoclimatic reconstructions in the Tibetan Plateau. Forty freshwater to hypersaline lakes (salinity: 0.1 to 91.7 g l–1) were investigated in the eastern Tibetan Plateau. The relationship between 120 diatom taxa and conductivity, maximum water depth and major ions were analyzed using an indicator value approach, ordination and taxon response models. Canonical correspondence analysis indicated that conductivity was the most important variable, accounting for 10.8% of the variance in the diatom assemblages. In addition water depth and weathering were influential. Weighted Averaging (WA) and Weighted Averaging Partial Least Square (WA-PLS) regression and calibration models were used to establish diatom-conductivity and water depth transfer functions. An optimal two-component WA-PLS model provided a high jack-knifed coefficient of prediction for conductivity (r2
jack = 0.92), with a moderate root mean squared error of prediction (RMSEPjack = 0.22), a very low mean bias (0.0003), and a moderate maximum bias (0.26). A WA model with tolerance downweighting resulted in a slightly lower r2
jack (0.89) for water depth, with RMSEPjack= 0.26, mean bias = –0.0103 and maximum bias = 0.26. 相似文献
279.
280.
阐述了水价确定与调水工程良性运行的关系,通过对不同水价制定方法的经济学分析,论述了长期边际成本定价对调水资源优化配置的有效性,提出了南水北调工程水价制定原则:以长期边际成本定价思想为指导,实现容量水价与计量水价相结合,以动态调整和价格歧视为辅助措施的水价机制;适当提升南水北调水价,确保受水区水价和水源结构的平稳过渡。 相似文献