全文获取类型
收费全文 | 95820篇 |
免费 | 1524篇 |
国内免费 | 800篇 |
专业分类
测绘学 | 2623篇 |
大气科学 | 7339篇 |
地球物理 | 19805篇 |
地质学 | 31940篇 |
海洋学 | 8267篇 |
天文学 | 21709篇 |
综合类 | 236篇 |
自然地理 | 6225篇 |
出版年
2021年 | 621篇 |
2020年 | 768篇 |
2019年 | 881篇 |
2018年 | 1742篇 |
2017年 | 1673篇 |
2016年 | 2290篇 |
2015年 | 1493篇 |
2014年 | 2271篇 |
2013年 | 4676篇 |
2012年 | 2410篇 |
2011年 | 3440篇 |
2010年 | 3054篇 |
2009年 | 4298篇 |
2008年 | 3855篇 |
2007年 | 3628篇 |
2006年 | 3624篇 |
2005年 | 2927篇 |
2004年 | 3023篇 |
2003年 | 2827篇 |
2002年 | 2775篇 |
2001年 | 2487篇 |
2000年 | 2391篇 |
1999年 | 2086篇 |
1998年 | 2050篇 |
1997年 | 2058篇 |
1996年 | 1751篇 |
1995年 | 1666篇 |
1994年 | 1498篇 |
1993年 | 1382篇 |
1992年 | 1274篇 |
1991年 | 1159篇 |
1990年 | 1318篇 |
1989年 | 1207篇 |
1988年 | 1062篇 |
1987年 | 1277篇 |
1986年 | 1136篇 |
1985年 | 1381篇 |
1984年 | 1653篇 |
1983年 | 1499篇 |
1982年 | 1425篇 |
1981年 | 1340篇 |
1980年 | 1162篇 |
1979年 | 1132篇 |
1978年 | 1190篇 |
1977年 | 1065篇 |
1976年 | 998篇 |
1975年 | 946篇 |
1974年 | 937篇 |
1973年 | 979篇 |
1972年 | 636篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
431.
R. J. Pankhurst R. D. Beckinsale C. K. Brooks 《Contributions to Mineralogy and Petrology》1976,54(1):17-42
The Kangerdlugssuaq intrusion, East Greenland, consists of quartzsyenites, syenites, pulaskites and foyaites. The age and petrogenesis of the intrusion has been investigated by strontium and oxygen isotope analyses of the major rock types (and some separated minerals) and the surrounding country rocks. Crystallization and rapid cooling of the intrusion close to 50 m.y. ago is indicated by concordance of an Rb-Sr mineral isochron (49.9±1.0 m.y.) and an Rb-Sr whole-rock isochron (50.0±1.9 m.y.) with previously published mineral dates. The feldspathoid-bearing rocks of the intrusion, which were the last to crystallize, have uniformly depleted oxygen (18O = +3.9, SMOW) and homogeneous initial 87Sr/86Sr ratios (0.70450±7). This is ascribed to equilibration of the magma prior to the crystallization of these rocks with about 10% by weight of meteoric ground water. The concommittant increase of
to about 1 Kb (the lithostatic load pressure) would depress the liquidus surfaces in the system Ne-Ks-Qz by about 200 ° C, allowing the magma to evolve continuously down temperature from oversatuated to undersaturated compositions. The chemical mechanism responsible for this trend has not been uniquely identified, but probably involved reduction of SiO2 content in an open system. The outer, quartz-normative, rocks of the intrusion have 18O values ranging up to +5.5 and initial 87Sr/86Sr ratios ranging up to 0.7095. This is due to interaction of the solid rocks, down to temperatures approaching 500 ° C, with ground water which had been enriched in 18O and 87Sr by previous exchange with the Precambrian country rocks. Minimum water/rock ratios are lower than in certain other known cases of interaction in the North Atlantic Tertiary Igneous Province. 相似文献
432.
433.
A new methodology for seismic design is proposed based on structural optimization with performance‐based constraints. Performance‐based criteria are introduced for the seismic design of new buildings. These criteria are derived from the National Guidelines for Seismic Rehabilitation of Buildings (Reference [19], Federal Emergency Management Agency (FEMA), ‘NHERP Guidelines for seismic rehabilitation of buildings’, Report Nos 273 and 274, Washington, DC, 1997) for retrofitting existing structures. The proposed design methodology takes into account the non‐linear behaviour of the structure. The goal is to incorporate in the design the actual performance levels of the structure, i.e. how much reserve capacity the structure has in an earthquake of a given magnitude. The optimal design of the structure minimizes the structural cost subjected to performance constraints on plastic rotations of beams and columns, as well as behavioural constraints for reinforced concrete frames. Uncertainties in the structural period and in the earthquake excitation are taken into account using convex models. The optimization routine incorporates a non‐linear analysis program and the procedure is automated. The proposed methodology leads to a structural design for which the levels of reliability (performance levels) are assumed to be quantifiable. Furthermore, the entire behaviour of the structure well into the non‐linear range is investigated in the design process. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
434.
Although geographers are highly visible in research into contemporary climate changes accompanying greenhouse gas increases (“global warming”), a relatively little explored component by them is the impact of aviation. This contrasts with the atmospheric science community of cloud physicists, chemists, and numerical modelers, who use geographic tools of trade. Geographers are well positioned to investigate aviation-climate impacts, especially related to contrail clouds, given geographic traditions of environment, spatial variation, regionalization, generalization, and the integrative approach to studying physical and human systems (here, climate, transportation, and energy). We highlight the role played by geographers in contrail-climate studies and advocate for a broader involvement in aviation policy formulation. 相似文献
435.
Regularized derivatives of potential fields and their role in semi-automated interpretation methods 总被引:2,自引:0,他引:2
Evaluation of higher derivatives (gradients) of potential fields plays an important role in geophysical interpretation (qualitative and/or quantitative), as has been demonstrated in many approaches and methods. On the other hand, numerical evaluation of higher derivatives is an unstable process – it has the tendency to enlarge the noise content in the original data (to degrade the signal-to-noise ratio). One way to stabilize higher derivative evaluation is the utilization of the Tikhonov regularization. In the submitted contribution we present the derivation of the regularized derivative filter in the Fourier domain as a minimization task by means of using the classical calculus of variations. A very important part of the presented approach is the selection of the optimum regularization parameter – we are using the analysis of the C-norm function (constructed from the difference between two adjacent solutions, obtained for different values of regularization parameter). We show the influence of regularized derivatives on the properties of the classical 3D Euler deconvolution algorithm and apply it to high-sensitivity magnetometry data obtained from an unexploded ordnance detection survey. The solution obtained with regularized derivatives gives better focused depth-estimates, which are closer to the real position of sources (verified by excavation of unexploded projectiles). 相似文献
436.
Neutron activation analysis of the Taal volcanic ash revealed the presence of unusual amount of scandium in the volcanic ash as compared to the standard basalt BCR-1. The BCR-1 value for Fe/Sc is 2760 while that of Taal ash is about 2649. It is suggested that the eruption was probably characterized by the ejection of scandium-rich materials. Scandium may be used as supplementary evidence in evaluating an impending future Taal volcanic activity. 相似文献
437.
Large scale seismic anisotropy in the Earth's mantle is likely dynamically supported by the mantle's deformation; therefore, tomographic imaging of 3-D anisotropic mantle seismic velocity structure is an important tool to understand the dynamics of the mantle. While many previous studies have focused on special cases of symmetry of the elastic properties, it would be desirable for evaluation of dynamic models to allow more general axis orientation. In this study, we derive 3-D finite-frequency surface wave sensitivity kernels based on the Born approximation using a general expression for a hexagonal medium with an arbitrarily oriented symmetry axis. This results in kernels for two isotropic elastic coefficients, three coefficients that define the strength of anisotropy, and two angles that define the symmetry axis. The particular parametrization is chosen to allow for a physically meaningful method for reducing the number of parameters considered in an inversion, while allowing for straightforward integration with existing approaches for modelling body wave splitting intensity measurements. Example kernels calculated with this method reveal physical interpretations of how surface waveforms are affected by 3-D velocity perturbations, while also demonstrating the non-linearity of the problem as a function of symmetry axis orientation. The expressions are numerically validated using the spectral element method. While challenges remain in determining the best inversion scheme to appropriately handle the non-linearity, the approach derived here has great promise in allowing large scale models with resolution of both the strength and orientation of anisotropy. 相似文献
438.
David D. J. Antia 《Geological Journal》1981,16(2):137-147
Faunal lists for 17 sections in the Overton Formation are presented. Faunal diversity appears to increase towards the formations base while faunal dominance appears to increase towards its top. 相似文献
439.
440.
A collection of brachiopods by the Institute of Oceanology, Academia Sinica (Qingdao), contains eight species from seven genera.
Six of the species have been recorded previously from China seas—Lingula adamsi, L. anatina, Discinisca stella, Pelagodiscus atlanticus, Campages mariae, Terebratalia coreanica. Two species (Terebratulina hataiana andFrenulina sanguinolenta) have been described from other parts of the Pacific area. The apparent absence of any endemic species is a noteworthy feature
of Chinese in comparison with Japanese faunas. 相似文献