全文获取类型
收费全文 | 258678篇 |
免费 | 4323篇 |
国内免费 | 3346篇 |
专业分类
测绘学 | 6875篇 |
大气科学 | 19075篇 |
地球物理 | 54142篇 |
地质学 | 89605篇 |
海洋学 | 21453篇 |
天文学 | 56345篇 |
综合类 | 1004篇 |
自然地理 | 17848篇 |
出版年
2021年 | 2242篇 |
2020年 | 2586篇 |
2019年 | 2846篇 |
2018年 | 3502篇 |
2017年 | 3178篇 |
2016年 | 5713篇 |
2015年 | 4193篇 |
2014年 | 6932篇 |
2013年 | 14234篇 |
2012年 | 6480篇 |
2011年 | 7865篇 |
2010年 | 6999篇 |
2009年 | 9642篇 |
2008年 | 8356篇 |
2007年 | 7820篇 |
2006年 | 9606篇 |
2005年 | 7701篇 |
2004年 | 7601篇 |
2003年 | 7100篇 |
2002年 | 6695篇 |
2001年 | 5972篇 |
2000年 | 5928篇 |
1999年 | 5204篇 |
1998年 | 5229篇 |
1997年 | 5013篇 |
1996年 | 4675篇 |
1995年 | 4420篇 |
1994年 | 4106篇 |
1993年 | 3837篇 |
1992年 | 3625篇 |
1991年 | 3592篇 |
1990年 | 3761篇 |
1989年 | 3519篇 |
1988年 | 3301篇 |
1987年 | 3848篇 |
1986年 | 3408篇 |
1985年 | 4232篇 |
1984年 | 4745篇 |
1983年 | 4411篇 |
1982年 | 4311篇 |
1981年 | 3929篇 |
1980年 | 3648篇 |
1979年 | 3511篇 |
1978年 | 3495篇 |
1977年 | 3278篇 |
1976年 | 3042篇 |
1975年 | 2962篇 |
1974年 | 2915篇 |
1973年 | 3080篇 |
1972年 | 2028篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
541.
The impact of wave loads and pore-water pressure generation on initiation of sediment transport 总被引:3,自引:0,他引:3
Edward C. Clukey Fred H. Kulhawy Philip L. -F. Liu George B. Tate 《Geo-Marine Letters》1985,5(3):177-183
The build-up of pore-water pressure by waves can lead to sediment liquefaction and subsequent transport by traction currents. This process was investigated by measuring pore-water pressures both in a field experiment and laboratory wave tank tests. Liquefaction was observed in the wave tank tests. The results suggest that sand is less susceptible than silts to wave-induced liquefaction because of the tendency to partially dissipate pore-water pressures. However, previous studies have determined that pore-water pressures must approach liquefaction before current velocities necessary to initiate transport are reduced. Once liquefaction has occurred more sediment can be transported. 相似文献
542.
543.
544.
In this paper the peculiarities of compiled oceanographic data sets, which consist primarily in the presence of some characteristic information as well as in the planned incomplete filling of the files with observations, are discussed. A number of special ways are suggested which essentially improve the packing density of computerized data without applying special archiving algorithms. The effectiveness of these methods has been tested by treating thein situ hydrological/chemical data file compiled in the Black Sea in 1988.Translated by Vladimir A. Puchkin. 相似文献
545.
546.
M. V. Ramana 《Geo-Marine Letters》1986,6(1):1-5
Satellite imagery and offshore magnetic data were analysed to correlate regional tectonic elements on the inner continental
shelf off Konkan and the adjacent Deccan plateau. Three statistically important lineament trends—N-S, WNW-ESE and ENE-WSW—that
prevail on land are correlatable well with the offshore trends. This positive correlation suggests simultaneous deformation.
The major magnetic lineament observed off Jaigad Bay, west coast of India, may be the extension of onshore lineaments. The
correlation of both the offshore and onshore trends indicates that the fracture pattern of the crystalline basement has also
controlled the offshore structural pattern. 相似文献
547.
This paper presents a method to evaluate statistical properties of half-cycle excursions including extreme values. The probability density function for half-cycle excursions for an arbitrarily given wave spectrum is developed based on the Gaussian assumption. The results of numerical computations carried out using wave data obtained during hurricane Camille show that the half-cycle probability density function agrees well with the histogram constructed from the data. The extreme wave height for design consideration computed with risk parameter 0.01 is approximately 20% greater than the observed extreme height. 相似文献
548.
Iapetus (S8) is unique in our solar system in that the albedo of its leading hemisphere is only 0.05 while that of the trailing side is 0.5. Several existing hypotheses are examined and found inadequate. Photometric studies of the dark side are compared to comet nuclei and class D asteroids. It is hypothesized that in the last 106–108 yrs the leading side suffered a high-velocity collision with a cometary body of mass 1013–1015 kg and traveling at a speed of 20 km s–1. About 5–16% of the excavated material was ejected into space, where the vaporized ices dissipated while the dark carbonaceous/silicate material was reaccreted on the leading side. The collision, although not sufficient to break Iapetus' tidal lock, resulted in a period of oscillation of about 5 yr. Until tidal friction reasserted a lock, the oscillation gave rise to the longitude effect, viz., the observed fact that the dark material covers more than 220 of longitude but only 110 of latitude. 相似文献
549.
A special fine structure (slowly drifting chains of narrowband fiber bursts), firstly observed during the solar type-IV radio burst on April 24, 1985, is interpreted as the radio signature of whistler waves periodically excited by a switch-on/switch-off process of a loss-cone instability in a localized wave packet of the fast magnetoacoustic mode. 相似文献
550.
Bernard R. Durney 《Solar physics》1989,123(2):197-216
The following points are discussed:
Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation. 相似文献
(i) | The dependence of the angular velocity, , on the spatial coordinates near the lower boundary, R c, of the solar convection zone (SCZ) can be obtained from an integration with respect to r of a sound approximation to the azimuthal equation of motion. Here P 2 (cos ) is the second-order Legendre polynomial and is the polar angle. Estimates of 0, 2 (the primes denote derivatives with respect to r), based on the best available values for the Reynolds stresses and anisotropic viscosity coefficients, suggest that 0 < 0,=">2 0 for r = R c. Since a reliable theory of anisotropic turbulent coefficients does not exist at present, positive values of 0 are conceivable. |
(ii) | In the lower SCZ the latitudinal variations of the superadiabatic gradient vanish if is constant along cylinders. The uniformity of the superadiabatic gradient is, however, inconsequential: the physically meaningful rotation law is the one that insures the uniformity of the convective flux. |
(iii) | With the exception of the polar regions, the angular momentum transport in thin azimuthal convective rolls is towards the equator. |
(iv) | It is suggested that buoyancy uncorrelates horizontally separated regions in the lower SCZ preventing the generation of magnetic fields with small wave numbers: in consequence, the cycle magnetic field must be generated in a region of weak buoyancy whereas the lower SCZ generates a weak rather stochastic magnetic field. The dependence on rotation of these two types of magnetic field could differ. |
(v) | In the context of helioseismology it is customary to expand the perturbations (induced by rotation) of the eigenfrequencies in the following form: , where the notation is standard. The observations reveal that to a good approximation a 1 is independent of l. It is shown that this is the case if is constant with r. For a simple viscous, rotating fluid in the steady state (r) is constant with r if the angular momentum loss vanishes. Let J(ri dr) be the angular momentum of a thin shell of radius r and thickness dr. Since , the constancy of (r) implies that each shell of radius r has the same angular momentum as if the Sun were rotating uniformly with an angular velocity given by . It is discussed whether, alternatively, the observations simply indicate that 0(r) is a slowly varying function of r. |