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   91篇
  免费   15篇
测绘学   7篇
大气科学   4篇
地球物理   51篇
地质学   18篇
海洋学   8篇
天文学   13篇
自然地理   5篇
  2022年   1篇
  2020年   4篇
  2019年   4篇
  2018年   2篇
  2017年   8篇
  2016年   4篇
  2015年   5篇
  2014年   1篇
  2013年   4篇
  2012年   8篇
  2011年   7篇
  2010年   5篇
  2009年   2篇
  2008年   9篇
  2007年   4篇
  2006年   3篇
  2005年   1篇
  2004年   3篇
  2003年   4篇
  2002年   1篇
  2001年   1篇
  2000年   2篇
  1999年   4篇
  1998年   4篇
  1997年   2篇
  1996年   1篇
  1993年   2篇
  1986年   1篇
  1984年   1篇
  1979年   1篇
  1977年   2篇
  1976年   1篇
  1975年   1篇
  1974年   2篇
  1972年   1篇
排序方式: 共有106条查询结果,搜索用时 718 毫秒
61.
Regional administrations present themselves in many different ways on the internet. A content analysis of regional websites in the Netherlands and Northwest Germany revealed this diversity. Geographical factors can partly explain these differences. Mapping the five principal aspects of the geographical representations of regional administrations revealed some spatial clusters. These regional representations are also linked to the geographical characteristics of regions. Competitive mid-sized regions focus more on economic items. Different types of peripheries vary much more in how they present themselves on the internet. The way regions present themselves is linked to the polarising influence of regional competition, but there is still considerable freedom for regions to present a distinct region image.  相似文献   
62.
The Vicência meteorite, a stone of 1.547 kg, fell on September 21, 2013, at the village Borracha, near the city of Vicência, Pernambuco, Brazil. It was recovered immediately after the fall, and our consortium study showed it to be an unshocked (S1) LL3.2 ordinary chondrite. The LL group classification is based on the bulk density (3.13 g cm?3); the chondrule mean apparent diameter (0.9 mm); the bulk oxygen isotopic composition (δ17O = 3.768 ± 0.042‰, δ18O = 5.359 ± 0.042‰, Δ17O = 0.981 ± 0.020‰); the content of metallic Fe,Ni (1.8 vol%); the Co content of kamacite (1.73 wt%); the bulk contents of the siderophile elements Ir and Co versus Au; and the ratios of metallic Fe0/total iron (0.105) versus total Fe/Mg (1.164), and of Ni/Mg (0.057) versus total Fe/Mg. The petrologic type 3.2 classification is indicated by the beautifully developed chondritic texture, the standard deviation (~0.09) versus mean Cr2O3 content (~0.14 wt%) of ferroan olivine, the TL sensitivity and the peak temperature and peak width at half maximum, the cathodoluminescence properties of chondrules, the content of trapped 132Xetr (0.317 × 10?8cm3STP g?1), and the Raman spectra for organic material in the matrix. The cosmic ray exposure age is ~72 Ma, which is at the upper end of the age distribution of LL group chondrites. The meteorite is unusual in that it contains relatively large, up to nearly 100 μm in size, secondary fayalite grains, defined as olivine with Fa>75, large enough to allow in situ measurement of oxygen and Mn‐Cr isotope systematics with SIMS. Its oxygen isotopes plot along a mass‐dependent fractionation line with a slope of ~0.5 and Δ17O of 4.0 ± 0.3‰, and are similar to those of secondary fayalite and magnetite in the unequilibrated chondrites EET 90161, MET 96503, and Ngawi. These data suggest that secondary fayalite in Vicência was in equilibrium with a fluid with a Δ17O of ~4‰, consistent with the composition of the fluid in equilibrium with secondary magnetite and fayalite in other unequilibrated ordinary chondrites. Secondary fayalite and the chondrule olivine phenocrysts in Vicência are not in isotopic equilibrium, consistent with low‐temperature formation of fayalite during aqueous alteration on the LL parent body. That alteration, as dated by the 53Mn‐53Cr chronology age of secondary fayalite, took place 4.0 ? 1.1 + 1.4 Ma after formation of CV CAIs when anchored to the quenched angrite D'Orbigny.  相似文献   
63.
High-resolution depth imaging with sparseness-constrained inversion   总被引:2,自引:0,他引:2  
An imaging technique is developed which exceeds the resolution limitation prescribed by conventional seismic imaging methods. The high‐resolution imaging is obtained by introducing a sparseness‐constrained least‐squares inversion into the imaging process of prestack depth migration. This is implemented by a proposed interference technique. In contrast to conventional depth migration, a decomposed signal or combined event, instead of the source wavelet, is needed in the proposed scheme. The proposed method aims to image a small local region with a higher resolution using the prestack data set. It should be applied following conventional depth imaging if a higher resolution is needed in a target zone rather than replacing the conventional depth imaging for the entire medium. Synthetic examples demonstrate the significant improvements in the resolution using the proposed scheme.  相似文献   
64.
In order to effectively mitigate climate change, people need to adopt environmentally-friendly actions. We argue that some people act in an environmentally-friendly manner without external incentives to do so, but rather because they are intrinsically motivated to do so. There is some initial evidence to suggest that this is particularly likely for people with a strong environmental self-identity. However, not much is known about how environmental self-identity influences pro-environmental actions. In this research, we aimed to test whether, and if so, via which process environmental self-identity is related to environmentally-friendly behaviour. We conducted three studies to test our hypotheses. In the first study, our hypotheses were confirmed in a sample of the general population using a correlational design. In the second study, we replicated our findings with a different indicator of moral obligation and with a different dependent variable. In the third study, we tested our model in an experimental design. Again, we found support for our hypotheses that environmental self-identity is related to one's obligation-based intrinsic motivation (that is, feelings of moral obligation) to act pro-environmentally, which in turn affects pro-environmental actions. As expected, the obligation-based intrinsic motivation mediates the relationship between environmental self-identity and environmentally-friendly behaviour. Our findings suggest that strengthening environmental self-identity may be a cost-effective way to promote pro-environmental actions, as people with a strong environmental self-identity are likely to act in an environmentally-friendly manner without an external incentive to do so.  相似文献   
65.
Abstract– We measured cosmogenic radionuclides and noble gases in the L3–6 chondrite breccia Northwest Africa (NWA) 869, one of the largest meteorite finds from the Sahara. Concentrations of 10Be, 26Al, and 36Cl in stone and metal fractions of six fragments of NWA 869 indicate a preatmospheric radius of 2.0–2.5 m. The 14C and 10Be concentrations in three fragments yield a terrestrial age of 4.4 ± 0.7 kyr, whereas two fragments show evidence for a recent change in shielding, most likely due to a recent impact on the NWA meteoroid, approximately 105 yr ago, that excavated material up to approximately 80 cm deep and exposed previously shielded material to higher cosmic‐ray fluxes. This scenario is supported by the low cosmogenic 3He/21Ne ratios in these two samples, indicating recent loss of cosmogenic 3He. Most NWA samples, except for clasts of petrologic type 4–6, contain significant amounts of solar Ne and Ar, but are virtually free of solar helium, judging from the trapped 4He/20Ne ratio of approximately 7. Trapped planetary‐type Kr and Xe are most clearly present in the bulk and matrix samples, where abundances of 129Xe from decay of now extinct 129I are highest. Cosmogenic 21Ne varies between 0.55 and 1.92 × 10?8 cm3 STP g?1, with no apparent relationship between cosmogenic and solar Ne contents. Low cosmogenic (22Ne/21Ne)c ratios in solar gas free specimens are consistent with irradiation in a large body. Combined 10Be and 21Ne concentrations indicate that NWA 869 had a 4π cosmic‐ray exposure (CRE) age of 5 ± 1 Myr, whereas elevated 21Ne concentrations in several clasts and bulk samples indicate a previous CRE of 10–30 Myr on the parent body, most probably as individual components in a regolith. Unlike many other large chondrites, NWA 869 does not show clear evidence of CRE as a large boulder near the surface of its parent body. Radiogenic 4He concentrations in most NWA 869 samples indicate a major outgassing event approximately 2.8 Gyr ago that may have also resulted in loss of solar helium.  相似文献   
66.
Kees Terlouw 《GeoJournal》2012,77(5):707-721
Regions and regional identity have become more important over the last decades. At the same time regions have become less discernable as distinct historically rooted spatial entities. Globalisation and the decline of collective identities through individualisation transform both this regional reality and how regions are conceptualised. This article analyses the shifts in types of regional identities used by regional administrations in an increasingly competitive environment. It uses the contrast between ‘thick’ traditional and historical rooted well-established regional identities, and ‘thin’ regional identities which are more transitory and focus more on economic competitiveness. These concepts are used to analyse the regional identity of regional administrations in Northwest Germany and the Netherlands. Hybrid regional identities combining a locally specific mix of thick and thin elements of regional identity, and which link up with regional identities at other relevant scales, appear to be the most effective regional identities for regional administrations facing the challenges of both globalisation and the decline in collective identities.  相似文献   
67.
68.
A Weichselian Late Pleniglacial fluvio-aeolian deposit has been investigated in the southern Netherlands. Three main structural lineaments have been distinguished: (1) very small, vertical platy structures (microjoints), in a parallel and a columnar configuration; (2) large joints and normal faults with minor displacement (‘Grubbenvorst type’), arranged in a conjugate fault-system; (3) large joints and normal faults (‘wedge-type’), located adjacent to ice-wedge casts. Since clay is absent, the occurrence of the vertical platy structures cannot be attributed to desiccation cracking. The vertical platy structures are interpreted as the result of thermal contraction cracking of a relatively thin layer, due to a sudden temperature drop. The large joints and normal faults of the Grubbenvorst type are the result of failure of the sediment due to the melting of the permafrost in the Late Pleniglacial, just before the formation of the Beuningen Gravel Bed. In other areas large periglacial convolutions have been formed during the same period. The normal faults and joints of the wedge type are more generally known. They are the result of failure of the sediment adjacent to a melting ice wedge.  相似文献   
69.
Tick populations and tick‐borne diseases like Lyme borreliosis have been steadily increasing since the mid‐1990s. Realizing the threat that ticks pose to public health, two Dutch citizen science projects have collected tick bite reports since 2006. This unique volunteered geographical dataset, which currently has nearly 35,000 reports, was used to identify environmental and other circumstantial factors associated with tick bites. For this, we first enriched the tick bite reports with temperature, precipitation, vegetation and volunteered data associated with the location of the tick bite. Using this enriched dataset, we then derived a series of features to characterize the environmental and volunteer‐related conditions in which each tick bite occurred. Next, we discretized these features using the Jenks Natural Breaks algorithm and, after that, we mined frequent environmental patterns associated with tick bites using the AprioriClose algorithm. Finally, we checked that these patterns are specifically associated with the tick bites by comparing them with the frequent patterns mined from pseudo‐random locations. The frequent patterns were visualized using heat maps and ring maps and two representative patterns associated with tick bites were projected into geographic space to study their spatio‐temporal distribution. Our results show that factors linked to human activity are more relevant to model tick bites than seasonal accumulations of temperature, vegetation or precipitation. In particular, the number of warm and dry days per season are present in a significant number of patterns and the majority of tick bites are produced within a distance of half a kilometer of a forest, recreational or built‐up area. The study of patterns in the time‐series revealed that there are several persistent patterns consistently occurring each year and the validation process showed that the volunteer tick bites collection is capturing environmental conditions associated with tick bites, suggesting that these reports have a high scientific value. These results support the creation of a Dutch tick bite risk map that, in turn, will open the door to the design of public health interventions to reduce the incidence of Lyme disease.  相似文献   
70.
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