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   548篇
  免费   18篇
  国内免费   2篇
测绘学   11篇
大气科学   42篇
地球物理   140篇
地质学   191篇
海洋学   94篇
天文学   38篇
自然地理   52篇
  2021年   6篇
  2020年   5篇
  2019年   14篇
  2018年   9篇
  2017年   10篇
  2016年   15篇
  2015年   11篇
  2014年   10篇
  2013年   36篇
  2012年   19篇
  2011年   27篇
  2010年   20篇
  2009年   29篇
  2008年   31篇
  2007年   23篇
  2006年   21篇
  2005年   12篇
  2004年   16篇
  2003年   16篇
  2002年   18篇
  2001年   6篇
  2000年   12篇
  1999年   11篇
  1998年   7篇
  1997年   4篇
  1996年   10篇
  1995年   6篇
  1994年   4篇
  1993年   12篇
  1992年   15篇
  1990年   6篇
  1989年   3篇
  1988年   7篇
  1987年   3篇
  1986年   9篇
  1985年   14篇
  1984年   15篇
  1983年   9篇
  1982年   5篇
  1981年   8篇
  1980年   10篇
  1979年   5篇
  1977年   8篇
  1975年   2篇
  1974年   2篇
  1972年   3篇
  1969年   2篇
  1968年   2篇
  1966年   2篇
  1959年   2篇
排序方式: 共有568条查询结果,搜索用时 62 毫秒
231.
Landslides are a significant hazard in many parts of the world and exhibit a high, and often underestimated, damage potential. Deploying landslide early warning systems is one risk management strategy that, amongst others, can be used to protect local communities. In geotechnical applications, slope stability models play an important role in predicting slope behaviour as a result of external influences; however, they are only rarely incorporated into landslide early warning systems. In this study, the physically based slope stability model CHASM (Combined Hydrology and Stability Model) was initially applied to a reactivated landslide in the Swabian Alb to assess stability conditions and was subsequently integrated into a prototype of a semi-automated landslide early warning system. The results of the CHASM application demonstrate that for several potential shear surfaces the Factor of Safety is relatively low, and subsequent rainfall events could cause instability. To integrate and automate CHASM within an early warning system, international geospatial standards were employed to ensure the interoperability of system components and the transferability of the implemented system as a whole. The CHASM algorithm is automatically run as a web processing service, utilising fixed, predetermined input data, and variable input data including hydrological monitoring data and quantitative rainfall forecasts. Once pre-defined modelling or monitoring thresholds are exceeded, a web notification service distributes SMS and email messages to relevant experts, who then determine whether to issue an early warning to local and regional stakeholders, as well as providing appropriate action advice. This study successfully demonstrated the potential of this new approach to landslide early warning. To move from demonstration to active issuance of early warnings demands the future acquisition of high-quality data on mechanical properties and distributed pore water pressure regimes.  相似文献   
232.
Marine sedimentary section across the Paleocene/Eocene (P/E) boundary interval is preserved in the Dungan Formation (Lower Indus Basin), Pakistan. Four dinoflagellate zones in the P/E interval of the Rakhi Nala section (Lower Indus Basin) are identified and correlated. The quantitative analysis of the dinoflagellate cyst assemblages together with geochemical data are used to reconstruct the palaeoenvironment across the P/E interval. The dinocyst assemblages allow the local correlation of the Dungan Formation (part) of the Sulaiman Range with the Patala Formation (part) of the Upper Indus Basin and global correlation of the Zone Pak-DV with the Apectodinium acme Zone of the Northern and Southern hemispheres. The onset of the carbon isotopic excursion (CIE) associated with Paleocene Eocene Thermal Maximum (PETM) is used globally to identify the P/E boundary. The CIE for the total organic carbon (fine fraction) δ13CFF is of a magnitude of ?1.7‰ is recorded for the first time in the Indus Basin. The Apectodinium acme precedes and straddles the onset of the CIE in the Indus Basin. This Apectodinium acme is also accompanied by a planktonic and benthonic foraminifera “barren zone.” The CIE in the Indus Basin, coupled with the changes in the dinocyst distribution and the benthonic and planktonic foraminifera assemblages, provides evidence of the changes associated with the PETM in this little-known part of the world. The benthonic foraminiferal assemblage indicates bathyal environment of deposition at the time of P/E boundary interval; the presence of dominantly open marine dinoflagellates and high planktonic foraminiferal ratio suggest that the water column at this site was well connected with the rest of the Tethys.  相似文献   
233.
Abstract

This article investigates future greenhouse gas emission scenarios for Russia's electricity sector, a topic of importance since Russia's ratification of the Kyoto Protocol in November 2004. Eleven scenarios are constructed to the year 2020 considering economic and technological details in both the demand and supply sides of the sector. The scenarios are based upon a thorough review of the different factors controlling carbon dioxide emissions, including potential economic growth, changes in energy efficiency and technological development, and that Russia may export large amounts of natural gas to European and Asian markets. The most likely scenario is that Russia will double industrial output over the next 10 years, increase energy efficiency in the demand sector, will remain consistent to the goals of the Energy Strategy 2020 and will implement more efficient technology in the electricity supply sector. Consequently, carbon dioxide emissions will still be 102 million tonnes below 1990 levels in 2010, representing a significant source for emission reduction credits available to be sold on international markets or transferred to the next crediting period.  相似文献   
234.
235.
The government of Ontario, Canada, has passed legislation to protect large parts of the southern portions of the province from most development. An extensive plan for a Greenbelt that surrounds the Greater Toronto Area and other regional growth centers was introduced. This article looks at the new policy as a spatial strategy that shifts the scales of environmental and growth management policy in Ontario. The legislation also sets the framework for a state spatial project; that is, a set of changes in how the regional state internally operates. The current Greenbelt legislation is a new step in a longer term development by which governments in Ontario have attempted to regulate the relationships between cities and regions, town and hinterland. Overlapping strongly with what is usually called the Toronto bioregion between the Niagara Escarpment, Oak Ridges Moraine, and Lake Ontario, the Greenbelt reorganizes space in southern Ontario in ways that would further ecosystem policies and practices in the area. Theoretically guided by newer debates on rescaling and regionalism, and based on close reading of the planning and policy documents on the Greenbelt as well as a series of expert interviews, we argue that the current Greenbelt legislation is an act of up-scaling traditional urban-regional regulation in southern Ontario, which we shall call extended metropolitanization. Such rescaling recasts traditional political conflicts in new terms. We conclude that extended metropolitanization in southern Ontario has been a process that has brought nature, the state, and governance together into a new regional sustainability fix.  相似文献   
236.
The Amelia Creek impact structure is located in Australia's Northern Territory in folded Palaeoproterozoic strata of the Davenport Ranges (20°51'S, 134°53′E). An impact origin is confirmed by presence of unequivocal shatter cones with apices that point upwards, and by planar microstructures in quartz grains from target sandstones of the Hatches Creek Group. Aeromagnetic, advanced spaceborne thermal emission and reflection radiometer (ASTER), and X-band synthetic aperture radar (X-SAR) images show an area of anomalous deformation in which smooth regional trends are disrupted by arcuate features at a 10 km radius to the north and south of the shock-metamorphosed rocks. However, no arcuate forms are apparent to the east and west of these shocked rocks, and instead, large south-southwest-trending faults are present about 6 km away on both sides. Despite pervasive shatter coning, typical of the central region of complex impact structures, no structural uplift is apparent, but instead the shocked rocks lie at the southern toe of a north-northeast-trending syncline. These shatter cones overprint and post-date the Palaeoproterozoic regional deformation, and thus, the impact structure has not been refolded and its abnormal form is likely due to pre-existing structure in the target rocks and/or an oblique impact. Small pockets of undeformed Late Neoproterozoic and Middle Cambrian strata are exposed in palaeovalleys in the central region of the structure, constraining the time of the impact to the Proterozoic.  相似文献   
237.
Sm‐Nd and Rb‐Sr isotopic data for Archaean gneisses from three localities within the eastern Yilgarn Block of Western Australia indicate that the gneisses define a precise Rb‐Sr whole rock isochron age of 2780 ± 60 Ma and an initial 87Sr/86Sr of 0.7007 ± 5. The Sm‐Nd isotopic data do not correspond to a single linear array, but form two coherent groups that are consistent with a c. 2800 Ma age of crust formation, with variable initial Nd. These results indicate that the gneiss protoliths existed as continental crust for a maximum period of only c. 100 Ma, and probably for a much shorter time, prior to the formation of the 2790 ±30 Ma greenstones.  相似文献   
238.
239.
Abstract

Rough and smooth skates (Dipturus nasutus (Banks 1841) and/), innominatus (Garrick &; Paul 1974)) were aged by counting growth bands on X‐rays of thick sections of vertebral centra. Band counts were imprecise, but there was no between‐reader bias. Age estimates were not validated. The oldest rough skate was 9 years old, but few were more than 6 years old. Females may live longer than males. The combined sexes von Bertalanffy growth curve was Lr = 91.3 (1 ‐ e?0.16[t + 1.20]). Half the males matured by c. 52 cm pelvic length (PL) and 4 years, and females by 59 cm PL and 6 years. The oldest smooth skate in the sample was 24 years, but longevity probably exceeds that. Females appear to live longer than males. The combined sexes von Bertalanffy growth curve was: Lt = 150.5 (1 ‐ e?0.095[t + 1.06]). Half the males matured by c. 93 cm PL and 8 years, and females by 112 cm PL and 13 years. Smooth skate are late maturing and long‐lived relative to other skates, whereas rough skate are early maturing with a moderate life span.  相似文献   
240.
Sub-fossil insect remains have the potential to characterise changing environmental conditions in both lentic and lotic water systems, however, relatively few studies have been undertaken in riverine environments. This paper uses sub-fossil caddisfly larvae (Trichoptera) and aquatic beetles (Coleoptera) to reconstruct river flow conditions for a large paleochannel (from multiple monoliths) using the Lotic invertebrate Index for Flow Evaluation (LIFE). Examination of the larval Trichoptera and Coleoptera remains indicated a marked change in the community and flow environment, as suggested by paleoLIFE scores within the profile of three of the monoliths examined. At the base of the channel the community was characterised by taxa indicative of high energy lotic habitats with predominantly mineral substrates (e.g. Trichoptera: Hydropsyche contubernalis and Brachycentrus subnubilis, Coleoptera: Elmis aenea and Esolus parallelepipedus). Within three of the monoliths there was a change in community composition to one indicative of a low energy backwater/lentic environment with abundant submerged and emergent vegetation (e.g. Trichoptera: Phryganea bipunctata and Limnephilus flavicornis, Coleoptera: Colymbetes fuscus and Hydrobius fuscipes). Detrended Correspondence Analysis (DCA) and loss of mass on ignition (LOI) indicated the presence of a strong environmental gradient within the data, associated with river flow. The utilisation of two aquatic insect orders provides clear evidence of temporal changes within the instream community and when combined with knowledge of ecological and habitat associations, allows differences between the two groups to be interpreted more clearly.  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