全文获取类型
收费全文 | 260671篇 |
免费 | 5237篇 |
国内免费 | 3406篇 |
专业分类
测绘学 | 7020篇 |
大气科学 | 19248篇 |
地球物理 | 54892篇 |
地质学 | 90822篇 |
海洋学 | 21721篇 |
天文学 | 56609篇 |
综合类 | 1028篇 |
自然地理 | 17974篇 |
出版年
2021年 | 2253篇 |
2020年 | 2604篇 |
2019年 | 2856篇 |
2018年 | 3776篇 |
2017年 | 3382篇 |
2016年 | 5893篇 |
2015年 | 4233篇 |
2014年 | 6958篇 |
2013年 | 14279篇 |
2012年 | 6645篇 |
2011年 | 8162篇 |
2010年 | 7164篇 |
2009年 | 9842篇 |
2008年 | 8573篇 |
2007年 | 8076篇 |
2006年 | 9687篇 |
2005年 | 7769篇 |
2004年 | 7697篇 |
2003年 | 7201篇 |
2002年 | 6791篇 |
2001年 | 6055篇 |
2000年 | 5972篇 |
1999年 | 5217篇 |
1998年 | 5242篇 |
1997年 | 5044篇 |
1996年 | 4695篇 |
1995年 | 4449篇 |
1994年 | 4118篇 |
1993年 | 3858篇 |
1992年 | 3648篇 |
1991年 | 3602篇 |
1990年 | 3769篇 |
1989年 | 3528篇 |
1988年 | 3311篇 |
1987年 | 3857篇 |
1986年 | 3419篇 |
1985年 | 4238篇 |
1984年 | 4755篇 |
1983年 | 4418篇 |
1982年 | 4316篇 |
1981年 | 3942篇 |
1980年 | 3648篇 |
1979年 | 3511篇 |
1978年 | 3499篇 |
1977年 | 3281篇 |
1976年 | 3048篇 |
1975年 | 2965篇 |
1974年 | 2916篇 |
1973年 | 3082篇 |
1972年 | 2029篇 |
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
991.
Coincident seismic reflection/refraction studies of the continental lithosphere: a global review 总被引:5,自引:0,他引:5
Summary. Vertical-incidence reflection profiling has identified several characteristic features of the continental Iithosphere including a generally transparent upper crust, a reflective lower crust, reflections from the crust-mantle boundary, and a commonly transparent upper mantle. The underlying physical causes of these characteristic features remain poorly understood. This review summarizes additional information brought to bear on the physical properties of these characteristic crustal structures through the use of coincident wide-angle refraction profiling. 相似文献
992.
993.
994.
In this paper, the currently accepted correlation of the Early Pleistocene Ludhamian stage of England with the Tiglian‐A sub‐stage of the Netherlands is challenged. Recent investigations of Early Pleistocene marine North Sea deposits from a borehole near Noordwijk (the Netherlands) yielded evidence from molluscs, dinoflagellate cysts and sporomorphs for an alternation of warm‐temperate and arctic intervals within the Praetiglian and Tiglian stages. Marine equivalents of the terrestrial‐based pollen sub‐stages Tiglian A and B have been recognised in the upper part of the sequence. A Praetiglian age can be assigned to the lower part of the sequence on the basis of mollusc analysis. Within the Praetiglian, an alternation of warm and cold phases has been recognised from both the dinoflagellate cyst and molluscan records. Three cold phases within the Praetiglian are tentatively correlated with marine isotope stages (MIS) 96–100. The molluscan assemblages provide evidence for climate forcing of the sea level: highest sea levels are reached in the warm‐temperate intervals. Within the Praetiglian, an interval with an acme zone of the dinoflagellate cyst Impagidinium multiplexum, is correlated with the Ludhamian and tentatively linked to MIS 97 and/or MIS 96. The cold molluscan assemblages from the Noordwijk borehole include an acme zone of Megayoldia thraciaeformis, the first and only occurrence of this North Pacific bivalve in the North Sea Basin. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
995.
The paper employs statistical hypothesis tests to explore the question of whether natural hazards (hail and tornadoes being considered here) are or are not intertemporally random. The answer to this question, at least for these two hazards, is surprising and has important policy implications: hazards appear to be more likely in year t if an event was experienced in year t?1: hence, apparent ‘Gambler's Fallacy’ behaviour on the part of farmers and others may indeed be rational. Should this result hold for a full range of climate-related hazard types. as we suspect, hundreds of millions of dollars of crop loss and other damages may be preventable (e.g., rotating to frost-resistant crops if past events suggest greater than usual likelihood of early or late frost). The degree of positive serial correlation is also seen to vary a great deal regionally, with individuals in some states being unable to benefit from knowledge of prior hazard occurrence. 相似文献
996.
997.
Erosion of man-made, forestry drainage channels occurs when the plough cuts through the surface peat layer into the underlying erodible mineral soil. A procedure is developed, based on hydraulic considerations, which will allow the drainage engineer to design stable drainage networks in upland forestry plantations. An example design chart is given for an erodible sandy loam type soil. 相似文献
998.
F. Begemann 《Meteoritics & planetary science》1996,31(2):171-176
Abstract— Noble gases repeatedly have served to widen the scope of meteorite research. During the first half century of such measurements, the emphasis was on the determination of U, Th/He-gas retention ages of iron meteorites, which is the most unsuitable class of meteorites for such studies. With the realization that the He in these meteorites results from the interaction of cosmic rays with meteoritic matter, meteorites became to be used as “the poor man's space probe” that yielded information on the constancy in time and space of the cosmic radiation. Another widening of scope came with the discovery of extremely high noble gas contents in the outermost layers of the individual grains that make up stony meteorites. These gases are of solar origin; they have been implanted as low-energy solar wind (SW) or as solar energetic particles (SEP) into the grains before their compaction. Presently they offer the only opportunity to precisely measure the isotopic composition of solar matter and to learn about potential changes of the Sun in time. Stony meteorites of the “carbonaceous” variety contain “stardust” that carries the undiluted nucleosynthesis products of individual stars that yield incredibly detailed information concerning the parameters that prevailed during the synthesis. 相似文献
999.
1000.
M. Guainazzi F. Fiore G. Matt G.C. Perola 《Monthly notices of the Royal Astronomical Society》2001,327(1):323-328
We have studied the correlation among X-ray absorption, optical reddening and nuclear dust morphology in Seyfert 2 galaxies. Two main conclusions emerge: (i) the Balmer decrement and the amount of X-ray absorption are anticorrelated over a wide range of column density, – the correlation no longer applies to Compton-thick objects , although they span a comparable range in Balmer decrement; (ii) Compton-thin Seyfert 2s seem to prefer nuclear environments, which are rich in dust on scales of hundreds of parsecs. On the other hand, Compton-thick Seyferts indifferently exhibit 'dust-poor' and 'dust-rich' environments. These results support an extension of the Seyfert unification scenario (as recently proposed by Matt ), where Compton-thick Seyfert 2s are observed through compact 'torii', whereas Compton-thin ones are obscured by dust on much larger scales. 相似文献