全文获取类型
收费全文 | 253891篇 |
免费 | 5157篇 |
国内免费 | 3345篇 |
专业分类
测绘学 | 6752篇 |
大气科学 | 18817篇 |
地球物理 | 53268篇 |
地质学 | 87944篇 |
海洋学 | 21108篇 |
天文学 | 55766篇 |
综合类 | 986篇 |
自然地理 | 17752篇 |
出版年
2021年 | 2205篇 |
2020年 | 2574篇 |
2019年 | 2831篇 |
2018年 | 3140篇 |
2017年 | 2838篇 |
2016年 | 5462篇 |
2015年 | 4164篇 |
2014年 | 6868篇 |
2013年 | 14158篇 |
2012年 | 6305篇 |
2011年 | 7483篇 |
2010年 | 6585篇 |
2009年 | 9216篇 |
2008年 | 8114篇 |
2007年 | 7460篇 |
2006年 | 9578篇 |
2005年 | 7586篇 |
2004年 | 7541篇 |
2003年 | 7035篇 |
2002年 | 6648篇 |
2001年 | 5936篇 |
2000年 | 5905篇 |
1999年 | 5183篇 |
1998年 | 5214篇 |
1997年 | 5013篇 |
1996年 | 4659篇 |
1995年 | 4407篇 |
1994年 | 4081篇 |
1993年 | 3827篇 |
1992年 | 3617篇 |
1991年 | 3576篇 |
1990年 | 3749篇 |
1989年 | 3506篇 |
1988年 | 3288篇 |
1987年 | 3840篇 |
1986年 | 3399篇 |
1985年 | 4208篇 |
1984年 | 4726篇 |
1983年 | 4398篇 |
1982年 | 4303篇 |
1981年 | 3913篇 |
1980年 | 3631篇 |
1979年 | 3503篇 |
1978年 | 3475篇 |
1977年 | 3279篇 |
1976年 | 3036篇 |
1975年 | 2957篇 |
1974年 | 2916篇 |
1973年 | 3074篇 |
1972年 | 2024篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
391.
Roberto Mínguez Fernando Delgado Ignacio Escuder Manuel G. de Membrillera 《国际地质力学数值与分析法杂志》2006,30(10):1019-1037
Empirical criteria have been used successfully to design filters of most embankment large dam projects throughout the world. However, these empirical rules are only applicable to a particular range of soils tested in laboratory and do not take into account the variability of the base material and filter particle sizes. In addition, it is widely accepted that the safety of fill dams is mainly dependent on the reliability of their filter performance. The work herein presented consists in a new general method for assessing the probability of fulfilling any empirical filter design criteria accounting for base and filter heterogeneity by means of first‐order reliability methods (FORM), so that reliability indexes and probabilities of fulfilling any particular criteria are obtained. This method will allow engineers to estimate the safety of existing filters in terms of probability of fulfilling their design criteria and might also be used as a decision tool on sampling needs and material size tolerances during construction. In addition, sensitivity analysis makes possible to analyse how reliabilities are influenced by different sources of input data. Finally, in case of a portfolio risk assessment, this method will allow engineers to compare the safety of several existing dams in order to prioritize safety investments and it is expected to be a very useful tool to evaluate probabilities of failure due to internal erosion. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
392.
393.
A. S. Samodurov 《Physical Oceanography》2006,16(3):129-140
We study the dynamic features of propagation of subsurface gravitational lenses and the conditions of changes in the modes
of control over their motion by various internal and external forces. As these forces, we study the action of drag, molecular
and turbulent viscosity due to the presence of internal and external forces, and deceleration caused by the effects of double
diffusion. We also analyze the modes of propagation of small-scale gravitational lenses. The information about these lenses
taken from the literature is supplemented and generalized.
__________
Translated from Morskoi Gidrofizicheskii Zhurnal, No. 6, pp. 3–14, May–June, 2006. 相似文献
394.
Miller B.S. Zosuls A.L. Ketten D.R. Mountain D.C. 《Oceanic Engineering, IEEE Journal of》2006,31(1):87-94
Previous research on the cetacean auditory system has consisted mostly of behavioral studies on a limited number of species. Little quantitative physiologic data exists on cetacean hearing. The frequency range of hearing varies greatly across different mammalian species. Differences among species correlate with differences in the middle-ear transfer function. Middle-ear transfer functions depend on the mechanical stiffness of the middle ear and the cochlear input impedance. The purpose of this study was to measure the middle-ear stiffness for the bottlenose dolphin (Tursiops truncatus), a species specialized for underwater high-frequency hearing and echolocation. Middle-ear stiffness was measured with a force probe that applied a known displacement to the stapes and measured the restoring force. The average middle-ear stiffness in ten dolphin ears was 1.37 N//spl mu/m, which is considerably higher than that reported for most terrestrial mammals. The relationship between middle-ear stiffness and low-frequency hearing cutoff in Tursiops was shown to be comparable to that of terrestrial mammals. 相似文献
395.
396.
The Electron Energy Spectrum from Large Solar Flares 总被引:2,自引:0,他引:2
G. M. Simnett 《Solar physics》2006,237(2):383-395
We report on the differential electron spectrum for intense transient events seen at one AU by the EPAM instrument on the
Advanced Composition Explorer (ACE) spacecraft. Over an observing period from September 1997 to September 2005, there were 45 major events that could be
reliably identified with a source flare on the Sun. In the ∼40 – 300 keV energy range, the electron spectral index was between
one and three for all but two of the events. Twenty-five of the events were associated with Geostationary Operational Environmental
Satellites (GOES) X-ray class X flares. We compare this result with the spectral index measured from electron pulse events,
lasting approx. one hour or less, where the spectral index is typically much softer than three. This suggests that the measured
spectral index of near-relativistic electrons at one AU may be a reliable indicator of the source. We also examine the likelihood
that fast coronal mass ejections (CMEs) are responsible in themselves for accelerating near-relativistic electrons and conclude
that they do not. 相似文献
397.
Hector Socas-Navarro David Elmore Anna Pietarila Anthony Darnell Bruce W. Lites Steven Tomczyk Steven Hegwer 《Solar physics》2006,235(1-2):55-73
The Spectro-Polarimeter for Infrared and Optical Regions (SPINOR) is a new spectro-polarimeter that will serve as a facility
instrument for the Dunn Solar Telescope at the National Solar Observatory. This instrument is capable of achromatic polarimetry
over a very broad range of wavelengths, from 430 to 1600 nm, allowing for the simultaneous observation of several visible
and infrared spectral regions with full Stokes polarimetry. Another key feature of the design is its flexibility to observe
virtually any combination of spectral lines, limited only by practical considerations (e.g., the number of detectors available, space on the optical bench, etc.).
Visiting Astronomers, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc.
(AURA), under cooperative agreement with the National Science Foundation. 相似文献
398.
399.
A new approach to constraining seawater δ34S and sulphate concentration using francolite‐bound sulphate reveals an abrupt increase in δ34S to +50‰ around the Early–Middle Cambrian boundary. Such high δ34S values are best explained by increased rates of pyrite burial due to ocean anoxia coupled with an increased sensitivity of the ocean sulphate reservoir to perturbations due to low sulphate concentrations of 500–700 μgL?1. We argue that the spread of anoxic waters at this time was partly the result of greenhouse warming related to the eruption of the Kalkarindji Large Igneous Province of northern Australia and that it triggered the collapse of early metazoan reef ecosystems during the latest Early Cambrian. Mass extinctions of the last 260 Myr have all coincided with enhanced volcanic activity, while several are also associated with positive shifts in seawater δ34S. Extending this correlation back in time further implicates volcanically induced climate change as a major determining factor in biosphere evolution. Terra Nova, 18, 257–263, 2006 相似文献
400.
The probabilities of discovering detached close binary (type DM) stars as eclipsing variables are calculated as a function
of the mass of the main component, mass ratio, major semiaxis, and angle of inclination of the orbit. The case of total limb
darkening (hypothesis “D”) is examined. This is compared with earlier results for uniformly bright stellar disks (hypothesis
“U”). Based on data from Svechnikov and Kuznetsova’s Catalog of Approximate Photometric and Absolute Elements of Eclipsing
Variables, the spatial density of stars of this type in the neighborhood of the sun is estimated to be ≈ 460 · 10
−6
pc−3.
__________
Translated from Astrofizika, Vol. 49, No. 1, pp. 151–169 (February 2006). 相似文献