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   202篇
  免费   19篇
  国内免费   25篇
测绘学   19篇
大气科学   8篇
地球物理   50篇
地质学   96篇
海洋学   13篇
天文学   2篇
综合类   4篇
自然地理   54篇
  2024年   2篇
  2023年   2篇
  2022年   1篇
  2021年   6篇
  2020年   3篇
  2019年   10篇
  2018年   8篇
  2017年   7篇
  2016年   9篇
  2015年   11篇
  2014年   10篇
  2013年   12篇
  2012年   7篇
  2011年   5篇
  2010年   10篇
  2009年   13篇
  2008年   15篇
  2007年   15篇
  2006年   7篇
  2005年   9篇
  2004年   7篇
  2003年   8篇
  2002年   10篇
  2001年   4篇
  2000年   10篇
  1999年   7篇
  1998年   5篇
  1997年   6篇
  1996年   3篇
  1995年   2篇
  1994年   3篇
  1993年   2篇
  1992年   1篇
  1990年   1篇
  1989年   2篇
  1988年   2篇
  1987年   3篇
  1986年   2篇
  1985年   2篇
  1984年   2篇
  1983年   1篇
  1977年   1篇
排序方式: 共有246条查询结果,搜索用时 15 毫秒
101.

Geological mapping of fault systems on the Gazelle Peninsula, eastern New Britain arc, combined with a reinterpretation of existing sea floor data indicate that faults previously thought to be a possible location of the boundary between the North and South Bismarck Plates, do not appear to be directly related to the plate boundary spreading centres and transform faults in the 3.5 Ma Manus Basin. Structure on the Gazelle Peninsula is dominated by the Mediva Fault (new name) and the Wide Bay Fault System, both north‐northwest trending, deep‐seated features. The Mediva Fault, an element of the Baining Mountain Horst and Graben Zone, is an extensional structure which has focused Middle Miocene intrusive activity, controlled Mio‐Pliocene sedimentation in the central Gazelle Peninsula, and displaced Quaternary volcanic deposits. The Wide Bay Fault System has been active since at least the Late Oligocene. One hundred kilometres of sinistral strike‐slip motion is likely on this fault since at least the late Middle Miocene, moving the Gazelle Peninsula in a north‐northwest direction with respect to the remainder of New Britain. The nature and timing of movements along these two major structures indicate that some other major tectonic process has operated (and presently continues) in this region of the New Britain arc to create these structures.  相似文献   
102.
循环荷载下半透水边界土层一维固结解析解   总被引:2,自引:0,他引:2  
李西斌  谢康和 《岩土力学》2005,26(1):155-159
研究了在循环荷载作用下具有双面半透水边界的饱和粘土层的一维固结问题。根据瞬时加载条件下的解答,采用积分方法得到了循环荷载条件下的解析解。根据解析解的结果,通过计算分析了边界条件和加载条件对土体固结过程的影响,可知:砂垫层和下卧层的渗透性减小会明显减缓固结过程;在循环荷载的一个周期内,孔压和固结度都会达到最大值和最小值各一次。  相似文献   
103.
The dynamics of social vulnerability are of key interest to many government agencies and departments. Identifying the geographic distribution of vulnerability within regions, and analysing how localised areas of social need change over time, is a key information requirement for decision-making, and the resultant allocation of resources. Typically, the delineation of areas for the determination of social vulnerability occurs using a combination of political and census boundaries. In many instances, the boundaries of these areas align to natural geographic features such as rivers or lakes. In other cases, a boundary is aligned to a man-made structure such as a road. The boundary may also be arbitrarily positioned based on some measure of distance and not align to any physical feature. In this research, we identify the various boundary types present in a political region. Using two measures of social vulnerability, we assess these boundaries as barriers to the continuity of social vulnerability. From our results, we identify motorways/highways and watercourses as potential barriers. We find no significant effects with lesser road structures suggesting there is no “wrong side of the street”. These results have implications for decision-makers and emphasise the need to recognise the “softness” of boundaries, and consider the relationships between areas, when allocating resources.  相似文献   
104.
This study analyzes metropolitan dynamics in a small country with an “island state” context of closed boundaries, using commuting data and mobile phone tracking data. We examine whether the Israeli context encourages the formation of a monocentric “metropolitan state,” characterized by increasing links between localities throughout the country and its principal metropolitan node (Tel Aviv)—rather than with secondary metropolitan areas—and by fuzzy, overlapping metropolitan boundaries. Commuting data from the 1995 and 2008 censuses show that metropolitan expansion processes in Israel are gradual. Mobile phone tracking data for 2013 reveal similar patterns, confirming the urban structure’s stability and the reliability of tracking data as a means of assessing metropolitan processes. The “island state” context supports growing monocentricity, but, when it comes to commuting and travel for other purposes, Israel is not yet a metropolitan state; metropolitan boundaries are not as fuzzy and rapidly changing as expected.  相似文献   
105.
A new analytic solution approach is presented for the modeling of steady flow to pumping wells near rivers in strip aquifers; all boundaries of the river and strip aquifer may be curved. The river penetrates the aquifer only partially and has a leaky stream bed. The water level in the river may vary spatially. Flow in the aquifer below the river is semi-confined while flow in the aquifer adjacent to the river is confined or unconfined and may be subject to areal recharge. Analytic solutions are obtained through superposition of analytic elements and Fourier series. Boundary conditions are specified at collocation points along the boundaries. The number of collocation points is larger than the number of coefficients in the Fourier series and a solution is obtained in the least squares sense. The solution is analytic while boundary conditions are met approximately. Very accurate solutions are obtained when enough terms are used in the series. Several examples are presented for domains with straight and curved boundaries, including a well pumping near a meandering river with a varying water level. The area of the river bottom where water infiltrates into the aquifer is delineated and the fraction of river water in the well water is computed for several cases.  相似文献   
106.
Spatial boundaries have become an indispensable part of regimes and tools for regulating fisheries, with examples including marine protected areas, regional fisheries management organizations and Exclusive Economic Zones. Yet, it is also widely acknowledged that boundaries are a social construct, which may be resisted by both fishers and fish ecology. The ensuing spatial and institutional mismatches have been shown to frustrate management efforts, exacerbating issues of non-compliance and ultimately leading to conflicts and overfishing. Interestingly, the often static and rigid nature of these boundaries has also led to a concomitant research interest in ‘transboundary’. This paradoxical situation of more boundary-setting entailing more transboundary thinking warrants a deeper understanding about boundaries and the role of transboundary research in fisheries. The aims of this review article are twofold: (1) a theoretical clarification on the meanings and uses of spatial boundaries drawing on geographical “boundary studies” literature; and (2) a construction of a typology that outlines how transboundary research is being articulated and envisioned. Together, the study reveals that transboundary scholarship in fisheries are mostly related to resources, fleets, trade and governance aspects and that dealing with the “boundary paradox” encompasses re-incorporating, re-scaling and re-imagining of boundaries. This article provides a conceptual basis for reflecting upon boundaries in world's fisheries and opens up discussions for a more nuanced boundary application that can better cope with multi-level interactions and dynamicity.  相似文献   
107.
Treaties, establishing boundaries to exclusive economic or exclusive fisheries zones, are generally based on environmental and economic circumstances. They delimit maritime entitlements, which are dependent upon coastal geography and they must, as a matter of law, represent equitable solutions. All perpetual treaties are based on the assumption that the circumstances leading to the conclusion of a treaty remain current as long as the treaty is enforceable. However, climate-related changes - such as sea level rise, coastal erosion, extreme weather events, ocean warming and ocean acidification - are altering the environmental circumstances that are essential to many maritime boundary agreements.An unforeseen fundamental change of circumstances can be invoked as grounds for terminating a treaty when the change relates to an essential basis of the treaty and radically transforms the obligations still to be performed. However, a fundamental change cannot justify termination of a treaty if it is caused by the party invoking it or if the treaty establishes a boundary. This rule is codified in Article 62 of the Vienna Convention on the Law of Treaties.Treaties delimiting sovereign rights to fisheries are not actual boundary treaties because the travaux préparatoires suggest that the boundary exclusion covers only boundaries delimiting territorial sovereignty. Furthermore, the reason for excluding boundary treaties - the need for permanence and stability - is less pertinent for maritime frontiers that fluctuate until settled through treaties or adjudication. Therefore, maritime boundaries can be subject to termination when their essential basis is affected by fundamental environmental changes.  相似文献   
108.
A quasi-3D method for the analysis of single piles and pile groups is presented. The method includes an equivalent linear constitutive model for nonlinear analysis, an 8-node pile element that simulates the effects of pile volume and energy transmitting boundaries which are especially important for the analysis of high frequency loading of machine foundations. The quasi-3D formulation and equivalent linear model result in orders of magnitude decreases in computational time. The accuracy and reliability of the approximate approach was validated by comparing results with 3D analytical results from MIT and by data from field tests on single piles and pile groups from Taiwan. The computed results compared very favorably with the analytical and field test data.  相似文献   
109.
Forest vegetation in the southeastern United States extends westward beyond the Ozark and Ouachita plateaus in Arkansas and Missouri into the Central Plains. Along this transect, luxuriant forests give way to mixed forests and grasslands that include smaller trees and progressively fewer tree species and eventually to grassland‐dominated landscapes in central Kansas, Oklahoma, and Texas. This transition is directly related to decreasing precipitation with distance to the west of the Mississippi River valley. Many species, however, have abrupt western range boundaries related to physiography and hydrogeomorphic processes. The western range limits for many trees correspond to Coastal Plain boundaries that at a regional scale impose sharper range boundaries than would be expected on the basis of decreasing precipitation. Also, riparian habitats within stream valleys extending westward from the Coastal Plain provide suitable habitats for trees in the dry regions of the Great Plains. The presence of riparian trees in this region is determined largely by the presence or absence of groundwater conditions necessary for survival. For floodplain trees, then, it is primarily habitat—not climate—that determines the location of range boundaries.  相似文献   
110.
Abstract

Geography is a part of scientific knowledge devoted to the study of two fundamental relations of human life: relations with the natural world and relations across space. These relations do not exist in isolation but are merely aspects of life as a whole. They must be understood and taught as parts of a total theory of human existence. Marxism is such an attempt to holistically understand the world. It is also, however, an attempt to change the world. Although Marxists recognize that capitalism has yielded enormous technical and material benefits for a minority of the world's people, we now find other competing purposes rising to primary positions in the list of urgent societal tasks. These purposes include equalizing living standards, finding jobs for the present doomed generation of children, removing the economic and social tensions underlying dangerous military antagonisms, and achieving a stable, managed relation with the natural environment. For purposes like these, Marxists consider capitalism outdated as a form of social and economic organization. Most Western Marxists are also critical of the centralized socialism of the Soviet Union. Socialism in the Soviet Union has achieved a modest standard of living for its working class. But the Soviet Union has yet to begin an approach towards a true model of socialism in the sense of decentralizing economic and political power directly to the mass of the people. Because we live in capitalist countries however, the focus of this critique is capitalist, rather than Soviet-style socialist society.  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