全文获取类型
收费全文 | 37457篇 |
免费 | 612篇 |
国内免费 | 592篇 |
专业分类
测绘学 | 945篇 |
大气科学 | 3134篇 |
地球物理 | 7516篇 |
地质学 | 12381篇 |
海洋学 | 3384篇 |
天文学 | 8749篇 |
综合类 | 123篇 |
自然地理 | 2429篇 |
出版年
2021年 | 214篇 |
2020年 | 273篇 |
2019年 | 272篇 |
2018年 | 628篇 |
2017年 | 580篇 |
2016年 | 904篇 |
2015年 | 641篇 |
2014年 | 871篇 |
2013年 | 1912篇 |
2012年 | 1059篇 |
2011年 | 1486篇 |
2010年 | 1215篇 |
2009年 | 1793篇 |
2008年 | 1595篇 |
2007年 | 1513篇 |
2006年 | 1418篇 |
2005年 | 1291篇 |
2004年 | 1215篇 |
2003年 | 1174篇 |
2002年 | 1095篇 |
2001年 | 983篇 |
2000年 | 985篇 |
1999年 | 924篇 |
1998年 | 840篇 |
1997年 | 847篇 |
1996年 | 728篇 |
1995年 | 646篇 |
1994年 | 563篇 |
1993年 | 516篇 |
1992年 | 512篇 |
1991年 | 482篇 |
1990年 | 478篇 |
1989年 | 419篇 |
1988年 | 403篇 |
1987年 | 454篇 |
1986年 | 434篇 |
1985年 | 526篇 |
1984年 | 586篇 |
1983年 | 565篇 |
1982年 | 528篇 |
1981年 | 467篇 |
1980年 | 435篇 |
1979年 | 402篇 |
1978年 | 418篇 |
1977年 | 364篇 |
1976年 | 329篇 |
1975年 | 338篇 |
1974年 | 336篇 |
1973年 | 341篇 |
1972年 | 203篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
242.
243.
244.
There continues to be reluctance in some jurisdictions to use chemical dispersants as a viable countermeasure for accidental oil spills. One argument used by some opponents to dispersant use is that “chemical dispersants do not work effectively in cold water”. To address this issue, the U.S. Minerals Management Service (MMS) funded and conducted two series of large-scale dispersant experiments in very cold water at Ohmsett - The National Oil Spill Response Test Facility, located in Leonardo, New Jersey in February-March 2006 and January-March 2007. Alaska North Slope, Endicott, Northstar and Pt. McIntyre crude oils and Corexit 9500 and Corexit 9527 dispersants were used in the two test series. The crude oils were tested both when fresh and after weathering. Results demonstrated that both Corexit 9500 and Corexit 9527 dispersants were 85-99% effective in dispersing the fresh and weathered crude oils tested at cold temperatures. The MMS expects that results from these test series will assist government regulators and responders in making science based decisions on the use of dispersants as a response tool for oil spills in the Arctic. 相似文献
245.
Summary. Four box cores collected from the Ontong—Java plateau during the Eurydice expedition have been used to make relative geomagnetic palaeo-intensity measurements. Rock magnetic measurements on the sediments show that they are characterized by a uniform magnetic mineralogy, and that they are suitable for relative intensity estimates. These are obtained by normalizing the NRM by an ARM imparted in a low DC bias field. the palaeoceanographic event known as the preservation spike is used to establish a crude time-scale for the record so that it may be compared with other data from the same region, and also with global palaeointensity estimates. the marine sediment data are quite similar to Australian intensity data from lake sediments and archaeomagnetic sources, but as might be expected exhibit some obvious differences from the global record. 相似文献
246.
Jerry C. Ritchie Vernon L. Finney Kenneth J. Oster Carole A. Ritchie 《Geomorphology》2004,61(3-4):347-360
Over the past 150 years, major land use changes have occurred in the Stemple Creek Watershed in northern California that have caused erosion to move soils from the upland to the flood plain, stream channels, and the bay. The purpose of this study is to document the recent (1954 to present) sediment deposition patterns in the flood plain area adjacent to Stemple Creek using the 137Cesium technique. Sediment deposition ranged from 0.26 to 1.84 cm year−1 for the period from 1964 to 2002 with an average of 0.85±0.41 cm year−1. Sediment deposition rates were higher for the 1954 to 1964 period with a range of 0.31–3.50 cm year−1 and an average of 1.29±1.04 cm year−1. These data indicate that sediment deposition in the flood plain has decreased since the middle 1950s, probably related to reduction in row crop agriculture and an increase in pasturelands. This study shows that the flood plains in the Stemple Creek Watershed are a significant sink for the soils being eroded from the upland area. Given the significance of the flood plain for trapping eroded materials before they reach the stream channels or the bay, efforts need to be made to manage these flood plain areas to insure that they do not change and become a source rather than a sink for eroded materials as improved management practices on the upland areas reduce sediment input to the flood plain. 相似文献
247.
248.
Recent laboratory studies show that the quantum yield, , from O2+ dissociative recombination varies considerably with the degree r of vibrational excitation. However, the suggestion that the high values for deduced from airglow and auroral observations can be explained by invoking vibrational excitation, creates a number of problems. Firstly, the rapid vibrational deactivation of O2+ ions by collisions with O atoms will keep r too low to account for the magnitude of ; secondly, r varies considerably from one atmospheric source to another but its relative values (which should be reliable) do not co-vary with those of ; thirdly, because r increases markedly above the peak of the dissociative recombination layer, the fits which theorists have obtained to the observed volume emission rate profiles would have to be regarded as fortuitious. It is tentatively suggested that is higher in the airglow and aurora than in the laboratory plasma studied by Zipf (1980) because of the electron temperature dependence of the specific recombination coefficient for O2+(v' ? 3) ions.The repulsive state of O2 does not provide a suitable channel for the dissociative recombination. A possible alternative is the bound state with predissociation to the repulsive state. 相似文献
249.
The oldest open clusters in our Galaxy set the lower limit to the age of the Galactic Disk (9–10 Gyr). Although they appear
to be very rich now, it is clear that their primordial populations were much larger. Often considered as transitional objects,
these populous open clusters show structural differences with respect to globular clusters so their dynamics and characteristic
evolutionary time scales can also be different. On the other hand, their large membership lead to different dynamical evolution
as compared with average open clusters. In this paper, the differential features of the evolution of rich open clusters are
studied using N-body simulations, including several of the largest (104 stars) published direct collisional N-body calculations so far, which were performed on a CRAY YMP. The disruption rate of rich open clusters is analysed in detail
and the effect of the initial spatial distribution of the stars in the cluster on its dynamics is studied. The results show
that cluster life-time depends on this initial distribution, decreasing when it is more concentrated. The effect of stellar
evolution on the dynamical evolution of rich clusters is an important subject that also has been considered here. We demonstrate
that the cluster's life-expectancy against evaporation increases because of mass loss by evolving high-mass stars.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
250.
The circulation and salinity distribution in the Hooghly Estuary have been studied by developing a two‐dimensional depth‐averaged numerical model for the lower estuary, where the flow is vertically well mixed. This has been coupled with a one‐dimensional model for the upper estuary, where the flow is assumed to be unidirectional and well mixed over the depth and breadth. The Hooghly River receives high freshwater discharge during the monsoon season (June to September), which has significant effect on the salinity distribution in the estuary. The model‐simulated currents, elevations, and salinities are in good agreement with observations during the dry season. However, during the wet season the computed salinities seem to deviate slightly from the observed values. 相似文献