全文获取类型
收费全文 | 256388篇 |
免费 | 4973篇 |
国内免费 | 3356篇 |
专业分类
测绘学 | 7034篇 |
大气科学 | 18926篇 |
地球物理 | 53519篇 |
地质学 | 89233篇 |
海洋学 | 21152篇 |
天文学 | 56044篇 |
综合类 | 1013篇 |
自然地理 | 17796篇 |
出版年
2021年 | 2229篇 |
2020年 | 2574篇 |
2019年 | 2841篇 |
2018年 | 3383篇 |
2017年 | 3115篇 |
2016年 | 5643篇 |
2015年 | 4191篇 |
2014年 | 6916篇 |
2013年 | 14230篇 |
2012年 | 6429篇 |
2011年 | 7742篇 |
2010年 | 6764篇 |
2009年 | 9392篇 |
2008年 | 8197篇 |
2007年 | 7589篇 |
2006年 | 9604篇 |
2005年 | 7672篇 |
2004年 | 7557篇 |
2003年 | 7054篇 |
2002年 | 6666篇 |
2001年 | 5951篇 |
2000年 | 5924篇 |
1999年 | 5198篇 |
1998年 | 5229篇 |
1997年 | 5029篇 |
1996年 | 4674篇 |
1995年 | 4413篇 |
1994年 | 4100篇 |
1993年 | 3845篇 |
1992年 | 3631篇 |
1991年 | 3595篇 |
1990年 | 3753篇 |
1989年 | 3509篇 |
1988年 | 3299篇 |
1987年 | 3838篇 |
1986年 | 3407篇 |
1985年 | 4224篇 |
1984年 | 4732篇 |
1983年 | 4405篇 |
1982年 | 4310篇 |
1981年 | 3928篇 |
1980年 | 3643篇 |
1979年 | 3508篇 |
1978年 | 3491篇 |
1977年 | 3274篇 |
1976年 | 3040篇 |
1975年 | 2956篇 |
1974年 | 2914篇 |
1973年 | 3073篇 |
1972年 | 2023篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
561.
A. P. M. Vaughan L. A. Dowling F. J. G. Mitchell S.‐E. Lauritzen A. M. McCabe P. Coxon 《第四纪科学杂志》2004,19(6):577-590
Organic‐rich deposits, uncovered during overburden removal from mantled gypsum karst at Knocknacran opencast gypsum mine, Co. Monaghan, are the best candidate to date for a last interglacial record in Ireland. The two till and organic‐rich deposits (preserved at different quarry elevations) were emplaced on to a Tertiary dolerite surface during high‐energy flood events and subsequently folded and faulted by movement towards sinkholes in underlying gypsum. Uranium–thorium disequilibrium dating suggests that the organic‐rich deposits in the upper section were hydrologically isolated at ca. 41 ka and those in the lower section at ca. 86 ka. Interpretation of the pollen content, although tentative because of the depositional and post‐depositional history of the material, suggests that the organic material originated in a warm stage possibly warmer than the post‐Eemian interstadials. The unusual setting of preservation may indicate that in situ, last interglacial deposits have generally been removed by erosion in Ireland. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
562.
We discuss the difficulties encountered when the Heisenberg-Kolmogoroff model for turbulence is applied to the large-scale turbulence in: (A) molecular clouds (specifically the velocity vs size relationship) and (B) stars (specifically, the estimate of convective fluxes).A new model for large-scale turbulence is, therefore, needed. 相似文献
563.
Seismic reflection profiles and long- and medium-range sidescan sonar were used to investigate a salt diapir complex and area of slope instability near the base of the Continental Slope off North Carolina. Within the area of investigation three diapirs are bounded on their upslope side by a scarp 60 m high and 50 km long. The slope above the scarp is characterized by a series of shallow rotational normal faults. The bottom below the scarp is furrowed by slide tracks, which were probably carved by large blocks that broke off the scarp face and slid downslope leaving rubble and scree lobes.Extensive slumping in this area appears to be a result of uplift and faulting associated with salt intrusion, which has fractured and oversteepened the slope leading to instability and failure. Sharply defined slide tracks suggest that slope failure above the breached diapir complex is a continuing process, in contrast to much of the surrounding slope area where few instability features were observed. 相似文献
564.
We report on a year's study of spatial and seasonal patterns of zooplankton abundance in Port Phillip and Westernport Bays, July 1982 to August 1983. These two bays, closely adjacent on the southern coast of Victoria, Australia, differ in several respects: Port Phillip is a nearly landlocked bay with a broad basin, while Westernport is an open tidal embayment with extensive mud and seagrass banks. Both bays have a resident zooplankton fauna distinct from that of Bass Strait. Although these resident communities have many species in common, patterns of abundance and dominance are quite different. We found that the holoplankton of Port Phillip was about half copepods, mostly Paracalanus indicus, with 23% Caldocera and 21% larvaceans. Westernport Bay zooplankton was dominated by Acartia tranteri, with no resident cladoceran fauna. Bass Strait species were more often found in Westernport than in Port Phillip Bay, but the resident community of Port Phillip Bay was more similar to that of Bass Strait than to that of Westernport.Although this study was undertaken in an exceptionally dry year, the available historical data show that the overall patterns found in 1982–1983 are typical for these bays. The differences in community composition probably relate to differences in depth profile, predator abundance, and suspended matter between the bays. 相似文献
565.
The problem of collapse of a dust ball has been studied in detail from the point of view of an external observer (O). It is seen that although there is no material pressure from the point of view of the comoving observer, there is a non-vanishing material pressure in the ball from the point of view of O. In the early stages of collapse (w.r.t. O), this pressure is positive everywhere inside the ball, while during the later stages, if one goes outwards from the centre towards the surface of the ball, this pressure is negative up to a certain value of the radial coordinate, and then changes its sign thereafter. On the basis of this pressure, one can understand the physics of the whole scenario of collapse w.r.t. O; in particular, the important feature is that in the ultimate stages an event horizon is formed asymptotically and there is no collapse to a point (as takes place from the point of view of the comoving observer). 相似文献
566.
An analysis previously developed for rough surface scattering with narrow-beam reception is extended to wide-beam or omnidirectional reception. The source is considered to be a pulsed electric dipole. The analysis includes multipathing effects on the received signal. The results are used to develop a model for the backscattered radar cross section of the ocean surface, up to a second-order approximation. 相似文献
567.
The Zürich sunspot relative number R
z
series has been analysed by the cyclogram method. The amplitude and the frequency variations of the Fourier 11 yr component between 1700–1983 A.D., were determined in a continuous way.Four distinct time intervals with significantly different characteristics of the periodicities are observed and discussed.Their second harmonics are also considered. The periodicity changes are contemporary to those of the 11 yr cycles.Around the year 1903 it seems that an important event has happened in the Sun. In fact the 11.4 yr cycle periodicity, that was very stable since at least 1825 started to change gradually to smaller values and similarly it happened to the second harmonic which also stopped and abruptly changed of phase of 90°. 相似文献
568.
569.
570.
Yanjun ZHANG Fenglong ZHANG Full QI Fuquan NI 《东北亚地学研究》2006,9(2):199-203
In view of the situation of excavation of open coal mine for the underground water disaster,we should carry out simulation studies for the numerical value of the water lowering project and improve the accuracy and the level of the water lowering project.On the basis of the hydrological geological conditions of certain open mine digging,a more reasonable seepage numerical model was built according to MODFLOW.It was simulated in advance that the process of the confined water level descending with the time,and combining with the actual observations to test the correctness of the model.The calculation showed that the results coincided well with the results of actual measurement.Based on this,different water lowering numerical simulations were built for the open coal mine digging.It could be simulated and forecast that the changes of the groundwater level in drainage process within and outside the mine pit,and it was quantitatively assessed that the possible water lowering result of the opencast water drainage process,which provide an important basis for the actual water lowering project and the possible project disposal. 相似文献