首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   299篇
  免费   18篇
  国内免费   20篇
测绘学   7篇
大气科学   30篇
地球物理   84篇
地质学   123篇
海洋学   20篇
天文学   33篇
自然地理   40篇
  2024年   2篇
  2022年   4篇
  2021年   11篇
  2020年   16篇
  2019年   15篇
  2018年   13篇
  2017年   7篇
  2016年   11篇
  2015年   17篇
  2014年   10篇
  2013年   26篇
  2012年   19篇
  2011年   19篇
  2010年   15篇
  2009年   16篇
  2008年   21篇
  2007年   19篇
  2006年   12篇
  2005年   11篇
  2004年   11篇
  2003年   7篇
  2002年   12篇
  2001年   5篇
  2000年   6篇
  1999年   3篇
  1998年   5篇
  1997年   2篇
  1996年   5篇
  1995年   2篇
  1994年   2篇
  1992年   3篇
  1991年   1篇
  1990年   2篇
  1989年   1篇
  1981年   1篇
  1980年   1篇
  1979年   2篇
  1973年   1篇
  1972年   1篇
排序方式: 共有337条查询结果,搜索用时 890 毫秒
321.
322.
Cassini's first year examining Saturn and its moons has uncovered many surprises, not least on the enigmatic moon Titan. Nick Teanby reviews the progress made and discusses prospects for the future.  相似文献   
323.
324.
Nick Eyles   《Sedimentary Geology》2006,190(1-4):257-268
Water plays a dominant role in many glacial processes and the erosional, depositional and climatic significance of meltwaters and associated fluvioglacial processes cannot be overemphasized. At its maximum extent c. 20,000 years ago, the volume of the Laurentide ice sheet was 33 × 106 km3 (about the same as the volume of all ice present today on planet Earth). The bulk of this was released as water in little more than 10,000 years. Pulses of meltwater flowing to the Atlantic Ocean from large ice dammed lakes altered thermohaline circulation of the world's oceans and global climate. One such discharge event via Hudson Bay at 8200 years BP released 160,000 km3 of water in 12 months. Global sea levels recovered from glacial maximum low stands reached at about 20,000 years ago at an average rate of 15 m per thousand years but estimates of shorter term rates suggest as much as 20 m sea level rise in 1000 years and for short periods, rates as high as 4 m per hundred years. Meltwaters played a key role in lubricating ice sheet motion (and thus areal abrasion) across the inner portions of the ice sheet where it slid over rigid crystalline bedrock of the Canadian Shield. The recharge of meltwater into the ice sheets bed was instrumental in generating poorly sorted diamict sediments (till) by sliding-induced shearing and deformation of overpressured sediment and soft rock. The transformation of overpressured till into hyperconcentrated slurries in subglacial channels may have generated a highly effective erosional tool for selective overdeepening and sculpting of bedrock substrates. Some workers credit catastrophic subglacial ‘megafloods’ with the formation of drumlins and flutes on till surfaces. Subglacial melt river systems were instrumental in reworking large volumes of glaciclastic sediment to marine basins; it has been estimated that less than 6% of the total volume of glaciclastic sediment produced during the Pleistocene remains on land. Fluvioglacial and glaciolacustrine sediments and landforms dominate large tracts of the ‘glacial’ landscape in North America. The recharge of subglacial meltwater into underlying bedrock and sediment aquifers created transient reversals in the long-term equilibrium flow directions of basinal fluids. With regard to pre-Pleistocene glacial record, meltwaters moved enormous volumes of terrestrial ‘glaciclastic’ sediment to marine basins and thus played a key role in preserving a record of glaciation, a record otherwise almost entirely lost on land.  相似文献   
325.
Marram grass (Ammophila arenaria) and sea-wheat grass (Thinopyrum junceiforme) have been introduced to Australia and New Zealand. This study examines the morphology of incipient foredunes and established foredunes associated with these species at two sites, Mason Bay in southern New Zealand, and the Younghusband Peninsula in South Australia. Both species invaded the existing foredunes very rapidly. In both cases the antecedent topography comprised relatively sparsely vegetated, irregular foredunes. Invasion resulted in continuous, regular, evenly vegetated foredunes. At Mason Bay a massive foredune has formed since 1958, in conjunction with Ammophila. Thinopyrum has formed an incipient foredune, with a ramp or terrace morphology, along the Younghusband Peninsula, South Australia. In both cases gaps in the former foredune have been closed and the indigenous foredune vegetation has been displaced. Both species may decrease the frequency and severity of blowout development. They are likely to be resilient to aeolian processes of sedimentation compared with dunes formed by indigenous species. Ammophila survives burial, is tolerant of drought and is resistant to erosion associated with storm surge and high waves. Thinopyrum is very tolerant of salinity. These species may adversely affect the long-term development of coastal barriers by inhibiting transgressive dune development.  相似文献   
326.
327.
The Conway granite of New Hampshire is a highly radioactive intrusive into which a 1-km deep borehole was drilled and continuously cored. The formation is composed of two major granitic units, the Osceola and the Conway; and it is cut by three lamprophyre dikes. The suite of rocks used in this paper contained eight granitic and two lamprophyre samples on which acoustic and strain measurements were performed over the pressure range of 1 atm. to 0.4 GPa. The Vp data were subjected to cluster analyses to determine what, if any, similarities in pressure-dependence existed among the samples. Three distinct similarity groupings were found: the lamprophyre dike rocks and two groups each of which contains samples from both granitic units. These groupings by Vp behavior were also found to correlate exactly with sample similarity groupings formed by dynamic and static moduli data, crack spectra, and rock texture. Such sample similarities in the functional forms of the pressure-dependence were found not to correlate with mineral modes nor with original in situ depth, but rather with grain size and the degree to which the mineral phases were homogeneously mixed. A consideration of the results suggests that for these rocks, grain size and mineral phase homogeneity exerted a dominant influence on crack population formation. Further, it appears that the exhumation and coring of these samples may have intensified the amount of cracking over that in situ but it did not necessarily introduce any individual distinct crack populations.  相似文献   
328.
This paper tracks changes in relative centralisation and relative concentration of poverty for the 25 largest British cities, analysing change for poor and non-poor groups separately, and examining parallel changes in spatial segregation. The paper confirms that poverty is suburbanising, at least in the larger cities, although poverty remains over-represented in inner locations. Suburbanisation is occurring through both the reduction in low income populations in inner locations and the growth non-poor groups in these places, consistent with a process of displacement. Relative centralisation of poverty has fallen more stronglythan relative concentration of poverty, as the outward shift of poorer groups leaves them still living in denser neighbourhoods on average. The paper also shows that spatial segregation (unevenness) declined at the same time although it remains to be seen whether this indicates a long-term shift to less segregated urban forms or a transitional outcome before new forms of segregation emerge around suburban poverty concentrations.  相似文献   
329.
Effective national and regional policy guidance on climate change adaptation relies on robust scientific evidence. This two-part series of papers develops and implements a novel scenario-neutral framework enabling an assessment of the vulnerability of flood flows in British catchments to climatic change, to underpin the development of guidance for the flood management community. In this first part, the sensitivity of the 20-year return period flood peak (RP20) to changes in precipitation (P), temperature (T) and potential evapotranspiration (PE) is systematically assessed for 154 catchments. A sensitivity domain of 4,200 scenarios is applied combining 525 and 8 sets of P and T/PE mean monthly changes, respectively, with seasonality incorporated using a single-phase harmonic function. Using the change factor method, the percentage change in RP20 associated with each scenario of the sensitivity domain is calculated, giving flood response surfaces for each catchment. Using a clustering procedure on the response surfaces, the 154 catchments are divided into nine groups: flood sensitivity types. These sensitivity types show that some catchments are (very) sensitive to changes in P but others buffer the response, while the location of catchments of the same type does not show any strong geographical pattern. These results reflect the range of hydrological processes found in Britain, and demonstrate the potential importance of catchment properties (physical and climatic) in the propagation of change in climate to change in floods, and so in characterising the sensitivity types (covered in the companion paper).  相似文献   
330.
This article examines the effect of the Kyoto Protocol's Clean Development Mechanism (CDM) on the international transfer of wind power technologies. The analysis is conducted using patent data from over 100 countries during the period 1988–2008. It is found that transfers from Annex I countries to non-Annex I countries are significantly affected by the contemporaneous establishment of projects under the CDM. However, when taking into account the cumulative effect of CDM projects, the effect is negative. Finally, the effect of domestic absorptive capacity in the host country is positive and significant. Because involvement with the CDM may increase the latter, this is an important area for further research.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号