全文获取类型
收费全文 | 865篇 |
免费 | 27篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 75篇 |
地球物理 | 128篇 |
地质学 | 276篇 |
海洋学 | 79篇 |
天文学 | 223篇 |
综合类 | 6篇 |
自然地理 | 98篇 |
出版年
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 16篇 |
2018年 | 14篇 |
2017年 | 19篇 |
2016年 | 18篇 |
2015年 | 28篇 |
2014年 | 16篇 |
2013年 | 30篇 |
2012年 | 18篇 |
2011年 | 26篇 |
2010年 | 27篇 |
2009年 | 47篇 |
2008年 | 37篇 |
2007年 | 28篇 |
2006年 | 31篇 |
2005年 | 39篇 |
2004年 | 25篇 |
2003年 | 24篇 |
2002年 | 23篇 |
2001年 | 17篇 |
2000年 | 23篇 |
1999年 | 12篇 |
1998年 | 18篇 |
1997年 | 9篇 |
1996年 | 8篇 |
1995年 | 11篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 13篇 |
1990年 | 15篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 6篇 |
1985年 | 18篇 |
1984年 | 17篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 11篇 |
1980年 | 15篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1975年 | 10篇 |
1974年 | 7篇 |
1973年 | 14篇 |
1972年 | 6篇 |
1971年 | 6篇 |
排序方式: 共有899条查询结果,搜索用时 15 毫秒
161.
Brice Ménard Daniel Nestor David Turnshek Anna Quider Gordon Richards Doron Chelouche Sandhya Rao 《Monthly notices of the Royal Astronomical Society》2008,385(2):1053-1066
Using a sample of almost 7000 strong Mg ii absorbers with W 0 > 1 Å and 0.4 < z < 2.2 detected in the SDSS DR4 data set, we investigate the gravitational lensing and dust extinction effects they induce on background quasars. After carefully quantifying several selection biases, we isolate the reddening effects as a function of redshift and absorber rest equivalent width, W 0 . We find the amount of dust to increase with cosmic time as τ( z ) ∝ (1 + z )−1.1±0.4 , following the evolution of cosmic star density or integrated star formation rate. We measure the reddening effects over a factor of 30 in E ( B − V ) and we find that τ∝ ( W 0 )1.9±0.1 , providing us with an important scaling for theoretical modelling of metal absorbers. We also measure the dust-to-metal ratio and find it similar to that of the Milky Way. In contrast to previous studies, we do not detect any gravitational magnification by Mg ii systems. We measure the upper limit μ < 1.10 and discuss the origin of the discrepancy. Finally, we estimate the fraction of absorbers missed due to extinction effects and show that it rises from 1 to 50 per cent in the range 1 < W 0 < 6 Å . We parametrize this effect and provide a correction for recovering the intrinsic ∂ N /∂ W 0 distribution. 相似文献
162.
Nele MUTTIK Kalle KIRSIMÄE Peeter Somelar Gordon R. Osinski 《Meteoritics & planetary science》2008,43(11):1827-1840
Abstract— Alteration of surficial suevites at Ries crater, Germany was studied by means of X‐ray diffraction and scanning electron microscopy. Here, we discuss the origin of hydrous silicate (clay) phases in these suevites that have been previously interpreted as resulting from post‐impact hydrothermal processes. The results of this study indicate that the dominant alteration phases are dioctahedral Al‐Fe montmorillonite and halloysite, which are typical low temperature clay minerals. We suggest that the surficial suevites are not altered by hydrothermal processes and that alteration occurred by low temperature subsurface weathering processes. If the surficial suevites were indeed hydrothermally modified during the early stages of post‐impact cooling, then the alteration was of limited character and is completely masked by later weathering. 相似文献
163.
Athena Coustenis Donald E. Jennings Yves Bénilan Sandrine Vinatier Gordon L. Bjoraker Ronald C. Carlson 《Icarus》2008,197(2):539-548
We report here the first detection of mono-deuterated acetylene (acetylene-d1, C2HD) in Titan's atmosphere from the presence of two of its emission bands at 678 and 519 cm−1 as observed in CIRS spectral averages of nadir and limb observations taken between July 2004 and mid-2007. By using new laboratory spectra for this molecule, we were able to derive its abundance at different locations over Titan's disk. We find the C2HD value () to be roughly constant with latitude from the South to about 45° N and then to increase slightly in the North, as is the case for C2H2. Fitting the 678 cm−1ν5 band simultaneously with the nearby C2H2 729 cm−1ν5 band, allows us to infer a D/H ratio in acetylene on Titan with an average of the modal values of 2.09±0.45×10−4 from the nadir observations, the uncertainties being mainly due to the vertical profile used for the fit of the acetylene band. Although still subject to significant uncertainty, this D/H ratio appears to be significantly larger than the one derived in methane from the CH3D band (upper limit of 1.5×10−4; Bézard, B., Nixon, C.A., Kleiner, I., Jennings, D.E., 2007. Icarus, 191, 397-400; Coustenis, A., Achterberg, R., Conrath, B., Jennings, D., Marten, A., Gautier, D., Bjoraker, G., Nixon, C., Romani, P., Carlson, R., Flasar, M., Samuelson, R.E., Teanby, N., Irwin, P., Bézard, B., Orton, G., Kunde, V., Abbas, M., Courtin, R., Fouchet, Th., Hubert, A., Lellouch, E., Mondellini, J., Taylor, F.W., Vinatier, S., 2007. Icarus 189, 35-62). From the analysis of limb data we infer D/H values of (at 54° S), (at 15° S), (at 54° N) and (at 80° N), which average to a mean value of 1.63±0.27×10−4. 相似文献
164.
Changes in solid solution limits, which must occur as crystallization proceeds, are used to interpret crystallization trends of pyroxenes from the Bushveld Intrusion, from oxidised and reduced basaltic lavas, and from lunar lavas. 相似文献
165.
166.
167.
168.
169.
170.
Improving neural network performance on the classification of complex geographic datasets 总被引:4,自引:0,他引:4
Neural Networks are now established computational tools used for search minimisation and data classification. They offer
some highly desirable features for landuse classification problems since they are able to take in a variety of data types,
recorded on different statistical scales, and combine them. As such, neural networks should offer advantages of increased
accuracy. However, a barrier to their general acceptance and use by all but `experts' is the difficulty of configuring the
network initially.
This paper describes the architectural problems of applying neural networks to landcover classification exercises in geography
and details some of the latest developments from an ongoing research project aimed at overcoming these problems. A comprehensive
strategy for the configuration of neural networks is presented, whereby the network is automatically constructed by a process
involving initial analysis of the training data. By careful study of the functioning of each part of the network it is possible
to select the architecture and initial weights on the node connections so the constructed network is `right first time'. Further
adaptations are described to control network behaviour, to optimise functioning from the perspective of landcover classification.
The entire configuration process is encapsulated by a single application which may be treated by the user as a `black box',
allowing the network to the applied in much the same way as a maximum likelihood classifier, with no further effort being
required of the user. 相似文献