全文获取类型
收费全文 | 1319篇 |
免费 | 70篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 54篇 |
大气科学 | 108篇 |
地球物理 | 343篇 |
地质学 | 484篇 |
海洋学 | 123篇 |
天文学 | 211篇 |
综合类 | 7篇 |
自然地理 | 73篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 24篇 |
2020年 | 23篇 |
2019年 | 21篇 |
2018年 | 47篇 |
2017年 | 50篇 |
2016年 | 58篇 |
2015年 | 49篇 |
2014年 | 60篇 |
2013年 | 80篇 |
2012年 | 70篇 |
2011年 | 79篇 |
2010年 | 62篇 |
2009年 | 82篇 |
2008年 | 79篇 |
2007年 | 68篇 |
2006年 | 66篇 |
2005年 | 55篇 |
2004年 | 31篇 |
2003年 | 45篇 |
2002年 | 44篇 |
2001年 | 36篇 |
2000年 | 25篇 |
1999年 | 19篇 |
1998年 | 16篇 |
1997年 | 15篇 |
1996年 | 12篇 |
1995年 | 17篇 |
1994年 | 9篇 |
1993年 | 10篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 10篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 7篇 |
1973年 | 6篇 |
1972年 | 2篇 |
排序方式: 共有1403条查询结果,搜索用时 898 毫秒
751.
With the aid of perturbation analysis of vibration frequencies and mode shapes it is shown that any lower vibration mode of a torsionally coupled building may be approximated as a linear combination of three vibration modes of the corresponding torsionally uncoupled system (a system with coincident centres of mass and resistance but all other properties are identical to the actual system): one translational mode along each of the two principal axes of resistance and one mode in torsional vibration. This result provides the motivation for a simpler—relative to the standard—procedure for analysing the response of torsionally coupled multistorey buildings to earthquake ground motion. To illustrate the application and accuracy of this procedure two numerical examples are presented. 相似文献
752.
753.
This investigation deals with the rocking response of rigid blocks subjected to earthquake ground motion. A numerical procedure and computer program are developed to solve the non-linear equations of motion governing the rocking motion of rigid blocks on a rigid base subjected to horizontal and vertical ground motion. The response results presented show that the response of the block is very sensitive to small changes in its size and slenderness ratio and to the details of ground motion. Systematic trends are not apparent: The stability of a block subjected to a particular ground motion does not necessarily increase monotonically with increasing size or decreasing slenderness ratio. Overturning of a block by a ground motion of particular intensity does not imply that the block will necessarily overturn under the action of more intense ground motion. In contrast, systematic trends are observed when the problem is studied from a probabilistic point of view with the ground motion modelled as a random process. The probability of a block exceeding any response level, as well as the probability that a block overturns, increases with increase in ground motion intensity, increase in slenderness ratio of the block and decrease in its size. It is concluded that probabilistic estimates of the intensity of ground shaking may be obtained from its observed effects on monuments, minarets, tombstones and other similar objects provided suitable data in sufficient quantity is available, and the estimates are based on probabilistic analyses of the rocking response of rigid blocks, considering their non-linear dynamic behaviour. 相似文献
754.
Marc Leblanc Jean -Marie Dautria Michel Girod 《Contributions to Mineralogy and Petrology》1982,79(4):347-354
Large magnesian ilmenite nodules occur in a quaternary basanitic volcano near Silet in the Tahalra volcanic area (Central Ahaggar, Southern Algeria). Their texture, from foliated to polygonal, is ascribed to solid state flow and dynamic recrystallization in upper mantle conditions.Their composition is similar to that of kimberlitic Mg-ilmenite nodules, as regards Mg, Fe, Al and Cr contents; the geikielite content ranges from 25 to 37 mol %. No outwards Mg enrichment of the nodules has been observed. Peripheral secundary products (pseudobrookite, magnetite and hematite) result from reaction with the host magma.From textural relationships and microprobe analyses, the Mg-ilmenite nodules from Tahalra are thought to be early cumulates from a mafic melt fractionnated in the upper-mantle; later, after solid-state deformation, these ilmenitite rocks were sampled and carried up to the surface by a fast-rising basanitic liquid.Rocks almost exclusively made of ilmenite do therefore presently exist within the upper-mantle and form a new type of petrological heterogeneity. 相似文献
755.
In a soil developed on the Cretaceous chalk of the Eastern Paris basin, calcite dissolution begins at the surface. The soil water is rapidly saturated in calcite. Calcite dissolution follows two different pathways according to seasonal pedoclimatic conditions.During winter: the soil is only partly saturated in water and the CO2 partial pressure is low (Ca 10?3 atm.). As a consequence total inorganic dissolved carbon (TIDC) is a hundred times the carbon content of the gaseous phase. Equilibrium is usually observed between the two phases. It is a closed system. The measured carbon 14 activity (87,5%) and 13C content () of the drainage water are very close to theoretical values calculated for an ideal mixing system between gaseous and mineral phases (respectively characterized by the following isotopic values: ; ; ; ).During spring and summer: the soil moisture decreases, the input of biogenic CO2 induces an increase of the soil CO2 partial pressure (Ca from 3.10?3 atm to 7.10?3 atm). The carbon content of the gaseous phase is higher by an order of magnitude compared to winter conditions. Therefore the aqueous phase is undersaturated in CO2 with respect to the latter. This disequilibrium occurs as a result of unbalanced rates of CO2 dissolution and CO2 effusion toward atmosphère. It is an open system. The carbon isotopic ratio of the aqueous phase is regulated by that of the gaseous phase, as demonstrated by the agreement between measured and calculated isotopic compositions (respectively δL mes = from ?9,4%0 to ?11,5%0, δl calc = from ?9,8%0 to ?13,9%0 AL mes = 119%, AL calc = from 119% to 125%).The solutions originating from both systems (open and closed) move downwards without significant mixing together. It has also been observed that no significant variation of the TIDC isotopic composition occurs during precipitation of secondary calcite. 相似文献
756.
A.S. Mackenzie R.L. Patience J.R. Maxwell M. Vandenbroucke B. Durand 《Geochimica et cosmochimica acta》1980,44(11):1709-1721
The changes in configuration at a number of chiral centres in certain acyclic isoprenoid alkanes, steranes, rearranged steranes and triterpanes of the hopane type in a suite of fourteen Toarcian shales (Paris Basin) have been determined by gas chromatography and combined gas chromatography-mass spectrometry. A sequence of changes in the ratios of various stereoisomers occurs both with increasing maximum depth of burial and in a North-South direction for shallow samples (maximum burial depth < 1000m). These changes cover the full range of maturity shown by the samples. The presence of 5α(H), 14β(H), 17β(H)-steranes in immature samples indicates the presence of a reworked component. The extent of epimerisation at a number of the chiral centres suggests that it relates to the extent of steric hindrance at the centres. 相似文献
757.
758.
A general procedure for analysis of the response of concrete gravity dams, including the dynamic effects of impounded water and flexible foundation rock, to the transverse (horizontal) and vertical components of earthquake ground motion is presented. The problem is reduced to one in two dimensions, considering the transverse vibration of a monolith of the dam. The system is analysed under the assumption of linear behaviour for the concrete, foundation rock and water. The complete system is considered as composed of three substructures—the dam, represented as a finite element system, the fluid domain, as a continuum of infinite length in the upstream direction, and the foundation rock region as a viscoelastic half-plane. The structural displacements of the dam are expressed as a linear combination of Ritz vectors, chosen as normal modes of an associated undamped dam-rock system. The effectiveness of this analytical formulation lies in its being able to produce excellent results by considering only a few Ritz vectors. The generalized displacements due to earthquake motion are computed by synthesizing their complex frequency responses using Fast Fourier Transform procedures. The stress responses are calculated from the displacements. An example analysis is presented to illustrate results obtained from this analytical procedure. Computation times for several analyses are presented to illustrate the effectiveness of the procedure. 相似文献
759.
Optical examinations, elemental analysis, i.r. spectroscopy and thermogravimetric analysis have been used to characterize the kerogens from the Logbaba (Upper Cretaceous) cores from the Douala Basin (Cameroon). It is shown that those kerogens are derived from the same type of organic matter, having mainly a land-plant origin. This conclusion fits very well that obtained from the examination of the soluble organic matter (see Part I). Thus the evolution of the properties of kerogens during burial is not due to a change in the nature of organic matter and can be related mainly to catagenesis. Owing to the existence of a high geothermal gradient in the basin and a sampling along a 4000 m thick sedimentary column, this evolution can be followed over a large range and correlated to the formation of petroleum and gas. 相似文献
760.