全文获取类型
收费全文 | 85篇 |
免费 | 3篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 4篇 |
大气科学 | 16篇 |
地球物理 | 11篇 |
地质学 | 16篇 |
海洋学 | 19篇 |
天文学 | 12篇 |
自然地理 | 11篇 |
出版年
2019年 | 1篇 |
2018年 | 2篇 |
2016年 | 3篇 |
2015年 | 1篇 |
2014年 | 3篇 |
2013年 | 9篇 |
2012年 | 5篇 |
2011年 | 6篇 |
2010年 | 3篇 |
2009年 | 3篇 |
2008年 | 4篇 |
2007年 | 2篇 |
2006年 | 4篇 |
2005年 | 4篇 |
2004年 | 4篇 |
2003年 | 4篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 3篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1983年 | 1篇 |
排序方式: 共有89条查询结果,搜索用时 671 毫秒
61.
Michael Mitchell Rod Griffith Paul Ryan Greg Walkerden Brian Walker Valerie A. Brown 《社会与自然资源》2014,27(3):299-314
Natural resource management (NRM) organizations are increasingly looking to resilience thinking to provide insights into how social and environmental systems interact and to identify points of intervention. Drawing on complex systems analysis, resilience thinking emphasizes that landscapes constantly change from social and ecological interactions, and focuses NRM planners’ attention on identifying key variables, feedbacks, and thresholds that can help improve intervention strategies. More deliberative approaches are being developed to use resilience thinking in ways that engage and build human capacity for action. This article documents experiences shared with NRM agencies in rural Australia as we developed new approaches to link resilience thinking with collective learning principles. We present an emerging framework through which heuristics associated with resilience thinking is being used as part of a planning-by-doing process. The framework is being tested to assess whether and how it can enable change agents to advance their capacities for adaptation and transformation. 相似文献
62.
The concentration of hydrogen peroxide was measured in waters off the coast of Peru during June and July 1983. The study period coincided with the end of the 1982/83 El Niño and the onset of coastal upwelling. Depth profiles of hydrogen peroxide concentration exhibit surface maxima and decrease with depth to the base of the mixed layer. Surface peroxide concentrations ranged from 0.8 to 5 × 10?8 M. Below the mixed layer hydrogen peroxide was below the detection limit (5 × 10?9 M). Diel variations were observed, with surface peroxide levels increasing during the day and decreasing at night. The nearshore station exhibited lower hydrogen peroxide concentrations than offshore stations, a reversal of the trend found in other coastal regions. This is attributed to the lack of coastal vegetation and runoff, and to active coastal upwelling of deeper water with low hydrogen peroxide concentrations. 相似文献
63.
There are fundamental links between population health, environment and economic development. The ultimate goal of economic
development is to improve the quality of human life and increase people's longevity. This goal cannot be achieved without
a clean and prosperous Earth. There has been increasing evidence showing that the expansion of economic activities can have
significant impacts on the environment and can cause large-scale and sometimes irreversible environmental changes. It is beyond
reasonable doubt that these changes will have serious public health consequences. How to tackle these issues is a unprecedented
challenge to scientists and policy-makers.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
64.
Rod Bryant 《GPS Solutions》2002,6(3):138-148
A key requirement for emergency call location (e.g. E911), for robust operation of location-based m-commerce systems and for
telematics systems is that the location technology be able to operate in urban canyons and inside buildings. We start from
a definition of the target environments, which includes multi-level parking garages, office buildings and homes, but not underground
parking garages or tunnels. Based on experience in these target environments and understanding of typical applications we
derive specific requirements for sensitivity and acquisition speed. The primary problems associated with weak signal operation
are as follows. (1) In conventional GPS receivers sampling at the correlator output typically occurs at a sampling interval
of the order of 1 ms. With weak signals, however, the signal-to-noise ratio of these samples is too low to support lock-in
of a phase-locked or frequency-locked loop. (2) With weak signals, the signal-to-noise ratio is too low to support the extraction
of the 50BPS navigation message from the signal. Therefore, aiding data is required from an external source. (3) Because the
data cannot be extracted, it is not possible for the receiver to synchronize to the incoming bits, words or subframes. Therefore,
it is not possible to construct pseudoranges without prior information. (4) The paper describes Sigtec Navigation's subATTO technology. This technology provides sensitivity down to –185 dBW (19 dBHz assuming NF of 1.5 dB and no other implementation
loss). This is 5 dB below an attoWatt (10–18 W) and has been shown to provide reliable positioning inside buildings, multi-level parking garages and in urban canyons
without any aiding at all. The paper describes the patented signal processing scheme, how ambiguity resolution and time synchronization
are achieved, the wireless assistance technique, the acquisition strategy and the use of scanning channels. Results are presented
from trials in a multi-level parking garage. The results obtained in most parking garages are similar to these in terms of
availability of fixes, signal strengths received and location accuracy achieved. The performance achieved in multi-level parking
garages is rarely worse than this. One of the major impediments to practical application of weak signal-processing schemes
is the limited dynamic range imposed by the GPS C/A code signal structure. This problem is discussed along with the problems
of multipath distortion in the context of telematics operation in urban canyons. A realistic urban accuracy goal of 20 m for
95% of fixes is proposed based on experience with GPS and dead reckoning. Enhancements under development will provide sensitivity
of –188 dBW, which will provide continuous availability within a broader range of indoor environments. For practical applications,
this will require the use of modern 'search engine' hardware for acceptable acquisition speed. As the paper shows, this sensitivity
is near the practical limit of sensitivity with acceptable acquisition times and dynamic capability.
Electronic Publication 相似文献
65.
A rule-based model for managing uncertainty in spatial databases is presented. The overall goal of the model is to allow
a user to assign to a single map class each polygon whose class is not entirely certain using more information than only the
map class attributes of such polygons (that are herein termed abjects). This situation might arise when multiple map realizations of an area are available and interpreters/cartographers are not
in agreement as to what class is present at a given location or when a digital image is classified by algorithmic/probabilistic
means. The scale-based model developed relies on attribute, geometric, and neighborhood measures of abjects arranged in a hierarchical rule-based structure. Structural knowledge of these measures leads to the procedural knowledge
that determines what action – e.g., merge, reclassify, retain – is to be taken for a given abject. The wider applicability of the model and associated methodology is also discussed.
Received: 5 July 2001 / Accepted: 11 April 2002 相似文献
66.
67.
Climatic hazards are a key feature of life. It is vital that teachers are knowledgeable about these phenomena in order to develop their students’ understanding of them. This study used a mixed methods approach to examine the accuracy and depth of preservice primary teachers’ (n = 430) knowledge of tropical cyclones. The findings suggest that prospective primary teachers hold alternative conceptions about the causes, spatial distribution, and impacts of these phenomena and that their explanations of processes lack structural complexity and integration. Issues raised by these findings and implications for teachers’ geographical knowledge and their professional standards and preservice teacher education are discussed. 相似文献
68.
Jones MA Stauber J Apte S Simpson S Vicente-Beckett V Johnson R Duivenvoorden L 《Marine pollution bulletin》2005,51(1-4):448-458
Port Curtis is one of Australia's leading ports for which substantial industrial expansion is proposed over the next decade. However, there has been little attempt to date to assess the extent of contamination in waters, sediments and biota or to characterize the potential impacts of contaminants on aquatic biota. Contaminants of potential concern to biota and human health were investigated in the Port Curtis estuary using a screening-level risk assessment approach. Dissolved metal concentrations in waters were below [ANZECC/ARMCANZ, 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Vol. 1. The Guidelines, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand] trigger values, suggesting low risk of these contaminants. In sediments, arsenic, nickel and chromium concentrations exceeded interim sediment quality guidelines-low (ISQG-low), but were also high in the reference zone suggesting a natural origin. Historical data on naphthalene in Harbour sediments showed that it was also of potential concern. Bioaccumulation of contaminants in a range of biota was also used as an indicator of contaminant exposure. Biota were generally enriched in metals and tributyltin, which was also elevated in water and sediments. Although not unique to Port Curtis, mercury in barramundi was identified as a potential risk to human health. 相似文献
69.
Fluoride in Nebraska's Ground Water 总被引:1,自引:0,他引:1
David C. Gosselin Jacqueline Headrick F. Edwin Harvey Rod Tremblay Kimberly McFarland 《Ground Water Monitoring & Remediation》1999,19(2):87-95
Fluoride concentrations in ground water are generally low but play an important role in dental health. This study evaluates the vertical and spatial distribution of fluoride in Nebraska's ground water and examines the geological and geochemical processes that control its concentration. Data from 1794 domestic wells sampled by the Nebraska Department of Health and Human Services. Regulation, and Licensure (NDOH) had a range of fluoride concentrations from <0.1 to 2.6 mg/L. and a median concentration of 0.3 mg/L. The median fluoride concentrations for Nebraska's 13 ground water regions varied from 0.2 to 0.7 mg/L. In each of these regions, individual wells may have either insufficient or overabundant F concentrations; we recommend that individual private water systems be tested for fluoride. Based on these data, system-specific recommendations can be made regarding the necessity for fluoridation.
Geochemical data indicated that the majority of fluoride occurs as F. Dissolution of F-bearing minerals controls fluoride occurrence. Apatite plus minor amounts of fluorite along with significant ground water residence times are the primary factors controlling F in the water from the Dakota Formation in Knox County, as well as in other parts of northeastern Nebraska. In western and southwestern Nebraska, dissolution of volcanic glass is the most probable source of F Long residence times plus fluorite also may contribute to the F concentrations in the Chadron Formation. 相似文献
Geochemical data indicated that the majority of fluoride occurs as F. Dissolution of F-bearing minerals controls fluoride occurrence. Apatite plus minor amounts of fluorite along with significant ground water residence times are the primary factors controlling F in the water from the Dakota Formation in Knox County, as well as in other parts of northeastern Nebraska. In western and southwestern Nebraska, dissolution of volcanic glass is the most probable source of F Long residence times plus fluorite also may contribute to the F concentrations in the Chadron Formation. 相似文献
70.
Heather Tallis Sarah E. LesterMary Ruckelshaus Mark PlummerKaren McLeod Anne GuerrySandy Andelman Margaret R. CaldwellMarc Conte Stephen CoppsDavid Fox Rod FujitaSteven D. Gaines Guy GelfenbaumBarry Gold Peter KareivaChoong-ki Kim Kai LeeMichael Papenfus Scott RedmanBrian Silliman Lisa WaingerCrow White 《Marine Policy》2012,36(1):303-306
Policies are arising around the world, most recently in the United States, that mandate the implementation of marine spatial planning as a practical pathway towards ecosystem-based management. In the new United States ocean policy, and several other cases around the globe, ecosystem services are at the core of marine spatial planning, but there is little guidance on how ecosystem services should be measured, making it hard to implement this new approach. A new framework is shown here for practical, rigorous ecosystem service measurement that highlights contributions from both natural and social systems. The novel three-step framework addresses traditional shortcomings of an ecosystem services approach by giving managers and scientists the tools to assess and track: (1) the condition of the ecosystem (supply metrics), (2) the amount of ocean resources actually used or enjoyed by people (service metrics), and (3) people's preference for that level of service (value metrics). This framework will allow real world progress on marine spatial planning to happen quickly, and with a greater chance for success. 相似文献