全文获取类型
收费全文 | 33752篇 |
免费 | 1600篇 |
国内免费 | 3077篇 |
专业分类
测绘学 | 2471篇 |
大气科学 | 3289篇 |
地球物理 | 6588篇 |
地质学 | 16605篇 |
海洋学 | 2528篇 |
天文学 | 2080篇 |
综合类 | 2758篇 |
自然地理 | 2110篇 |
出版年
2024年 | 64篇 |
2023年 | 181篇 |
2022年 | 372篇 |
2021年 | 442篇 |
2020年 | 366篇 |
2019年 | 466篇 |
2018年 | 5177篇 |
2017年 | 4440篇 |
2016年 | 3093篇 |
2015年 | 714篇 |
2014年 | 670篇 |
2013年 | 754篇 |
2012年 | 1638篇 |
2011年 | 3434篇 |
2010年 | 2666篇 |
2009年 | 2947篇 |
2008年 | 2542篇 |
2007年 | 2953篇 |
2006年 | 655篇 |
2005年 | 665篇 |
2004年 | 765篇 |
2003年 | 754篇 |
2002年 | 662篇 |
2001年 | 430篇 |
2000年 | 333篇 |
1999年 | 214篇 |
1998年 | 135篇 |
1997年 | 108篇 |
1996年 | 81篇 |
1995年 | 92篇 |
1994年 | 51篇 |
1993年 | 56篇 |
1992年 | 48篇 |
1991年 | 44篇 |
1990年 | 33篇 |
1989年 | 24篇 |
1988年 | 20篇 |
1987年 | 30篇 |
1986年 | 19篇 |
1985年 | 18篇 |
1984年 | 29篇 |
1983年 | 31篇 |
1982年 | 22篇 |
1981年 | 38篇 |
1980年 | 46篇 |
1979年 | 15篇 |
1977年 | 16篇 |
1976年 | 11篇 |
1975年 | 15篇 |
1974年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 5 毫秒
41.
— Seismic responses of weathered and non-weathered ridge models were simulated to study the ridge effects on the ground motion characteristics. The range of ridge slope from 19.98° to 45° was considered to produce a possible set of generalized results. 2.5-D modeling based on parsimonious staggered grid approximation of elastodynamic wave equations was adopted in simulations. Computed results reveal an increase of amplitude of incoming waves with both elevation and the slope of the ridge. Further, the characteristics of surface waves are highly ridge slope dependent. The analysis of responses of weathered and non-weathered ridge models reveals that ridge has caused a strong generation of surface waves near its top. The surface waves are not dominating on the top of the ridge but at some lower elevation. The increase of weathering of ridge further intensified the ridge effect. Analysis in frequency domain, based on spectral ratio method, does not indicate any pattern in the spectral amplification factor and is very much sensitive to slope, source focal mechanism and location. However, on an average there is a continuous decrease of amplification with slope in the vertical component and increase in the transverse component, and it is increasing in the radial component up to slope =38.0° and thereafter decreasing. 相似文献
42.
Tidal mixing plays an important role in the modification of dense water masses around the Antarctic continent. In addition to the vertical (diapycnal) mixing in the near-bottom layers, lateral mixing can also be of relevance in some areas. A numerical tide simulation shows that lateral tidal mixing is not uniformly distributed along the shelf break. In particular, strong mixing occurs all along the Ross Sea and Southern Weddell Sea shelf breaks, while other regions (e.g., the western Weddell Sea) are relatively quiet. The latter regions correspond surprisingly well to areas where indications for cross-shelf exchange of dense water masses have been found. The results suggest that lateral tidal mixing may account for the relatively small contribution of Ross Sea dense water masses to Antarctic Bottom Water. 相似文献
43.
44.
45.
Amitabha Mukhopadhyay 《Hydrogeology Journal》2003,11(3):343-356
The ability of artificial neural network to differentiate water samples from the two aquifers of Kuwait on the basis of their
major ion chemistry has been demonstrated. The major ion concentration distribution in the groundwater of the Kuwait Group
and the Dammam Formation aquifers of Kuwait appears very similar. Cross-plots, supported by the discriminant function analysis
of the data, however, suggest that there are some subtle differences in the overall composition of the water from the two
aquifers that make it possible to differentiate the water from the two aquifers in almost 80% of the cases. An artificial
neural network improved the differentiation capability to 90% of the cases. It is also possible to estimate the fraction of
Kuwait Group water in the flow stream of dually completed wells with the help of an artificial neural network developed for
this purpose.
Electronic Publication 相似文献
46.
Ya. N. Pavlyuchenkov B. M. Shustov V. I. Shematovich D. S. Wiebe Zhi-Yun Li 《Astronomy Reports》2003,47(3):176-185
We consider radiative transfer in C18O, HCO+, and CS molecular lines in a spherically symmetrical, coupled, dynamical and chemical model of a prestellar core whose evolution is determined by ambipolar diffusion. Theoretical and observed line profiles are compared for the well-studied core L1544, which may be a collapsing protostellar cloud. We study the relationship between the line shapes and model parameters. The structure of the envelope and kinematic parameters of the cloud are the most important factors determining the shape of the lines. Varying the input model parameters for the radiative transfer—the kinetic temperature and microturbulent velocity—within the limits imposed by observations does not result in any substantial variations of the line profiles. The comparison between the model and observed spectra indicates that L1544 displays a flattened structure, and is viewed at an oblique angle. A two-dimensional model is needed to reproduce this structure. 相似文献
47.
48.
49.
50.
The compatibility of vanadium (V) during mantle melting is a function of oxygen fugacity (fO2): at high fO2’s, V becomes more incompatible. The prospects and limitations of using the V content of peridotites as a proxy for paleo-fO2 at the time of melt extraction were investigated here by assessing the uncertainties in V measurements and the sensitivity of V as a function of degree of melt extracted and fO2. V-MgO and V-Al2O3 systematics were found to be sensitive to fO2 variations, but consideration of the uncertainties in measurements and model parameters indicates that V is sensitive only to relative fO2 differences greater than ∼2 log units. Post-Archean oceanic mantle peridotites, as represented by abyssal peridotites and obducted massif peridotites, have V-MgO and -Al2O3 systematics that can be modeled by 1.5 GPa melting between FMQ − 3 and FMQ − 1. This is consistent with fO2’s of the mantle source for mid-ocean ridge basalts (MORBs) as determined by the Fe3+ activity of peridotitic minerals and basaltic glasses. Some arc-related peridotites have slightly lower V for a given degree of melting than oceanic mantle peridotites, and can be modeled by 1.5 GPa melting at fO2’s as high as FMQ. However, the majority of arc-related peridotites have V-MgO systematics overlapping that of oceanic mantle peridotites, suggesting that although some arc mantle may melt under slightly oxidizing conditions, most arc mantle does not. The fact that thermobarometrically determined fO2’s in arc peridotites and lavas can be significantly higher than that inferred from V systematics, suggests that V retains a record of the fO2 during partial melting, whereas the activity of Fe3+ in arc peridotitic minerals and lavas reflect subsequent metasomatic overprints and magmatic differentiation/emplacement processes, respectively.Peridotites associated with middle to late Archean cratonic mantle are characterized by highly variable V-MgO systematics. Tanzanian cratonic peridotites have V systematics indistinguishable from post-Archean oceanic mantle and can be modeled by 3 GPa partial melting at ∼FMQ − 3. In contrast, many South African and Siberian cratonic peridotites have much lower V contents for a given degree of melting, suggesting at first glance that partial melting occurred at high fO2’s. More likely, however, their unusually low V contents for a given degree of melting may be artifacts of excess orthopyroxene, a feature that pervades many South African and Siberian peridotites but not the Tanzanian peridotites. This is indicated by the fact that the V contents of South African and Siberian peridotites are correlated with increases in SiO2 content, generating data arrays that cannot be modeled by partial melting but can instead be generated by the addition of orthopyroxene through processes unrelated to primary melt depletion. Correction for orthopyroxene addition suggests that the South African and Siberian peridotites have V-MgO systematics similar to those of Tanzanian peridotites. Thus, if the Tanzanian peridotites represent the original partial melting residues, and if the South African and Siberian peridotites have been modified by orthopyroxene addition, then there is no indication that Archean cratonic mantle formed under fO2’s significantly greater than that of modern oceanic mantle. Instead, the fO2’s inferred from the V systematics in these three cratonic peridotite suites are within range of modern oceanic mantle. This also suggests that the transition from a highly reducing mantle in equilibrium with a metallic core to the present oxidized state must have occurred by late Archean times. 相似文献