全文获取类型
收费全文 | 539篇 |
免费 | 29篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 28篇 |
大气科学 | 94篇 |
地球物理 | 122篇 |
地质学 | 216篇 |
海洋学 | 27篇 |
天文学 | 45篇 |
综合类 | 3篇 |
自然地理 | 35篇 |
出版年
2023年 | 2篇 |
2022年 | 4篇 |
2021年 | 7篇 |
2020年 | 12篇 |
2019年 | 9篇 |
2018年 | 24篇 |
2017年 | 29篇 |
2016年 | 38篇 |
2015年 | 22篇 |
2014年 | 18篇 |
2013年 | 33篇 |
2012年 | 29篇 |
2011年 | 33篇 |
2010年 | 35篇 |
2009年 | 35篇 |
2008年 | 31篇 |
2007年 | 28篇 |
2006年 | 20篇 |
2005年 | 22篇 |
2004年 | 12篇 |
2003年 | 10篇 |
2002年 | 18篇 |
2001年 | 15篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 3篇 |
1997年 | 8篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1978年 | 2篇 |
1977年 | 6篇 |
1969年 | 1篇 |
1965年 | 1篇 |
1961年 | 1篇 |
1960年 | 1篇 |
1958年 | 1篇 |
1953年 | 1篇 |
1951年 | 1篇 |
1950年 | 1篇 |
1948年 | 1篇 |
1910年 | 2篇 |
排序方式: 共有570条查询结果,搜索用时 625 毫秒
541.
In this paper, vertical peak floor acceleration (PFAv) demands on elastic multistory buildings are statistically evaluated using recorded ground motions. These demands are applicable to the assessment of nonstructural components that are rigid in the vertical direction and located at column lines or next to columns. Hence, PFAv demands of the floor system away from column lines and their effects on nonstructural components are not addressed. This study is motivated by the questionable general assumption that typical buildings are considered to be relatively flexible in the horizontal (lateral) direction but relatively rigid in the vertical (longitudinal) direction. Consequently, only few papers address the evaluation of vertical component acceleration demands throughout a building, and there is no consensus on the relevance of vertical accelerations in buildings. The results presented in this study show that the vertical ground acceleration demands are amplified throughout the column line of a steel frame structure. This amplification is in many cases significant, depending on the vertical stiffness of the load‐bearing system, damping ratio, and the location of the nonstructural component in the building. From these outcomes it can be concluded that the perception of a rigid‐body response of the column lines in the vertical direction is highly questionable, and further research on this topic is required. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
542.
Konstantinos Mykoniou Christoph Butenweg Britta Holtschoppen Sven Klinkel 《地震工程与结构动力学》2016,45(11):1779-1796
A refined substructure technique in the frequency domain is developed, which permits consideration of the interaction effects among adjacent containers through the supporting deformable soil medium. The tank‐liquid systems are represented by means of mechanical models, whereas discrete springs and dashpots stand for the soil beneath the foundations. The proposed model is employed to assess the responses of adjacent circular, cylindrical tanks for harmonic and seismic excitations over wide range of tank proportions and soil conditions. The influence of the number, spatial arrangement of the containers and their distance on the overall system's behavior is addressed. The results indicate that the cross‐interaction effects can substantially alter the impulsive components of response of each individual element in a tank farm. The degree of this impact is primarily controlled by the tank proportions and the proximity of the predominant natural frequencies of the shell‐liquid‐soil systems and the input seismic motion. The group effects should be not a priori disregarded, unless the tanks are founded on shallow soil deposit overlying very stiff material or bedrock. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
543.
544.
Compositional data analysis was performed on chemical compositions of martian surface materials in order to unravel scenarios of past and present weathering and to evaluate the role of meteoritic accumulation. The observed chemical variability is analyzed by means of principal component analysis. Potential reservoirs that may have contributed primary material to soil formation are assessed. Chemical alteration in the course of in situ weathering is described in terms of alteration vectors that link the compositions of fresh rocks and their weathering crusts. The interplay of localized chemical alteration and global scale re-distribution and mixing of fines material is documented through the identification of different soil forming branches. These branches emanate from distinct compositional domains, which comprise basaltic and basalt-andesitic primary materials, and they converge to a global dust composition, which represents the product of chemical and physical disintegration and subsequent global mixing. Mass balance considerations applied to localized weathering phenomena are in line with findings from experimental acid-sulfate weathering on olivine-bearing basalts and the persistence of secondary silica in evaporitic rocks. In addition the composition and oxidation state of involved volcanic gases is deduced. Our findings corroborate the past activity of volcanic exhalation products in combination with liquid water. We conclude that average martian crust is dominated by basaltic materials at its topmost level and that the amount of meteoritic accumulation may contribute about 6 wt% to the martian fines. From the meteoritic contribution minimum soil formation rates of 60±20 cm/Gyr are derived. Sequestration of atmospheric oxygen during weathering of primary materials may account for the oxygen deficiency of the martian atmosphere. A 4-14-m-thick layer of oxidized martian fines may account for the estimated deficit of 1.7×1018 mol O2 in the martian atmosphere depending on the primary oxidation state of volatile volcanic emanations. 相似文献
545.
An elementary integral equation technique is used to construct strong and weak stationary shock solutions from the one-dimensional Vlasov equation. It is shown that the plasma is Penrose stable in all points in space under certain conditions.This work was supported in part by the Atomic Energy Commission Grant No. AT(11-1)-2059, and NSF Grant GA-31676.supported by a CSIR bursary. 相似文献
546.
Carbon,sulfur, oxygen and strontium isotope records,organic geochemistry and biostratigraphy across the Permian/Triassic boundary in Abadeh,Iran 总被引:6,自引:0,他引:6
Christoph?KorteEmail author Heinz?W.?Kozur Michael?M.?Joachimski Harald?Strauss Ján?Veizer Lorenz?Schwark 《International Journal of Earth Sciences》2004,93(4):565-581
Pelagic deposits at Abadeh represent a complete biostratigraphic record across the Permian/Triassic boundary (PTB). The presumed water depth during deposition of these sediments was between 60 and 90 m. Similar to other Permian/Triassic boundary sections, the succession at Abadeh is characterised by a negative carbon isotope shift of approximately 4. The values start to decrease in the lower C. changxingensis - C. deflecta s.l. Zone, reach –0.12 (V-PDB) in the uppermost Permian just below the PTB, remain low to the early I. isarcica Zone (–0.32) and increase subsequently in the upper I. isarcica Zone. For the time interval of the PTB negative carbon isotope excursion, between the C. iranica and the I. isarcica Zones, no correlation exists between the 13Ccarb and the 18Ocarb. The above observations argue against the conclusion of Heydari et al. (2001) that the carbon isotope event at the P/T transition is an alteration artefact and not a global signal. The decrease in 13Ccarb is accompanied by a ~5 (and potentially up to 10) increase in 34SSSS. Together, these features are thought to reflect a complex global event, notably the development of widespread anoxic oceans with anoxic bottom layers rising onto the shelves. For the carbon isotope drop, other factors, such as the collapse of ocean primary productivity may also have played a role. The 87Sr/86Sr ratios of Dzhulfian seawater show only a minor increase from 0.70705 to 0.70710, reaching 0.70720 in the Dorashamian. The increase becomes steeper in the Early Triassic reaching 0.70754 in the N. dieneri Zone. The rise of the strontium isotope values is thought to be related to enhanced continental weathering under humid climatic conditions in the uppermost Permian (C. meishanensis - H. praeparvus Zone) and the lack of a dense land vegetation in the Early Triassic, prior to the Spathian (Upper Olenekian). 相似文献
547.
Landslide risk management in Switzerland 总被引:1,自引:2,他引:1
548.
In the deep subsurface of the northeast German basin products of extensive volcanism are present that formed during the waning
phase of the Variscan orogeny (Permian–Carboniferous boundary). Large volumes (≥48,000 km3) of dominantly felsic magma were emplaced in the northern foreland of the Variscan mountains in a terrestrial semi-arid environment.
Most of these units were inferred by previous authors to represent the products of explosive volcanic eruptions such as ignimbrites.
However, a detailed analysis of the volcanic texture and lithofacies association shows that most units are best interpreted
as coherent lavas emplaced either as extrusive units or as shallow intrusions, with important implications for the reconstruction
of the paleo-environment. This study shows that detailed textural examination of drill cores provide important observations,
which can be applied as criteria for the interpretation of the mode of volcanic eruptions. Furthermore, phenocryst logging
and geochemical fingerprinting based on immobile element ratios can be employed for the classification and discrimination
of individual emplacement units. 相似文献
549.
Wen-chao Su Christoph A. Heinrich Thomas Pettke Xing-chun Zhang Rui-zhong Hu Bin Xia 《矿物岩石地球化学通报》2006,25(Z1):192
The Shuiyindong and Yata Carlin-type gold deposits, located in Southwestern Guizhou, China, are hosted by Permianbioclastic limestone in the form of stratabound mineralization and Middle Triassic calcareous clastic rocks as fault-controlled mineralization, respectively. In these deposits, quartz crystals in the veins or veinlets associated with mineralization have contained several populations of fluid inclusions with relatively quite big size and clear paragenetic relationship of entrapment. Petrography, microthermometry, Raman and LA-ICPMS of fluid inclusions analyses are used to characterize fluids chemistry and their evolution of Carlin-type system in Guizhou. 相似文献
550.