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61.
Multiple geographies of the Arab Internet 总被引:1,自引:0,他引:1
The Arab world plays a relatively minor role in the rapidly changing geographies of global cyberspace. This paper explores the multiple geographies of the Arab Internet. First, it addresses Internet penetration rates, which averaged 7.8 per cent in 2006, although these varied widely among and within the region's countries. Between 2000 and 2006, the number of users jumped by 830 per cent, indicating these geographies are in rapid flux. It then examines the telecommunications infrastructure of the Middle East and North Africa, including fixed and mobile telephone networks and Internet cafes. Third, it turns to the reasons why the Internet has experienced relatively late adoption among Arab countries, including the dominance of the Latin alphabet, high access costs reflecting state-owned telecommunications monopolies, low Arab literacy rates, and restrictive gender relations that keep the proportion of female users low. The paper pays special attention both to government censorship of the Arab Internet as well as resistance to such controls and attempts to utilize the Internet counter-hegemonically. Finally, it explores the impacts of the Internet on some Arab societies, including the opening of discursive communities of politics, the Palestinian–Israeli conflict, Iraq and electronic commerce. 相似文献
62.
63.
A new mesh refinement strategy for generating high quality unstructured meshes of the Northwestern European continental shelf, the continental slope and the neighbouring ocean is presented. Our objective is to demonstrate the ability of anisotropic unstructured meshes to adequately address the challenge of simulating the hydrodynamics occurring in these three regions within a unique mesh. The refinement criteria blend several hydrodynamic considerations as the tidal wave propagation on the continental shelf and the hydrostatic consistency condition in steep areas. Several meshes illustrate both the validity and the efficiency of the refinement strategy. The selection of the refinement parameters is discussed. Finally, an attempt is made to take into account tidal ellipses, providing another cause for anisotropy in the mesh. 相似文献
64.
利用Fourier Bessel级数展开法,给出了表面具有覆盖层的圆弧形凹陷地形对入射平面SV波散射问题的一个解析解,并利用该解分析了不同深宽比凹陷地形表面覆盖层刚度和厚度对入射SV波的影响.结果表明,凹陷地形表面覆盖层的存在,即使厚度很薄,也会显著增加对入射SV波的放大作用,该放大作用可达到单一凹陷地形的2.5倍以上.覆盖层刚度和厚度的变化对入射平面SV波也具有很大影响. 相似文献
65.
Vincent Jomelli & Pascal Bertran 《Geografiska Annaler: Series A, Physical Geography》2001,83(1-2):15-28
We analyse the morphology and sedimentology of 25 dirty snow avalanche deposits in the French Alps. The deposits typically have either a snow-ball structure or a massive structure with sliding planes. The snow balls show a longitudinal and a vertical sorting that reflects a sieve effect, similar to that observed in other rapid inertial granular flows. The massive type results from snow compaction when the avalanche is channelled by a gully or when it reaches the distal part of the scree. Velocity decrease and compaction limit the deformation to a zone at the base of the snow mass and cause the formation of distinctive sliding planes. These appear as smooth recrystallised surfaces due to local melt from frictional heating. The flow can be assimilated to a frictional granular flow. No systematic variation of size and shape of the rock debris has been observed along the profiles in both types of deposit. The distribution of rock debris and its fabric suggest that the clasts are transported passively and do not undergo any sorting during displacement. Snow melt after avalanching causes a redistribution of rock debris particularly when the snow thickness is important. This redistribution does not generate new sedimentological characteristics such as enhanced sorting or fabric. 相似文献
66.
Kerrie M. Swadling Herbert J. G. Dartnall John A. E. Gibson milie Saulnier-Talbot Warwick F. Vincent 《Quaternary Research》2001,55(3):380
Early Holocene sediments from a continental Antarctic lake (Ace Lake, Vestfold Hills, East Antarctica) contained abundant fossil rotifers of the genus Notholca. The fossil is similar to specimens of Notholca sp. present in modern-day Ace Lake and other fresh and brackish lakes of the Vestfold Hills. Cyanobacteria and protists (chrysophyte cysts, dinoflagellate cysts, and rhizopod tests) were also recovered from the core samples. These sediments were deposited early in the freshwater phase of Ace Lake, soon after deglaciation of the area. The occurrence of this trophically diverse assemblage of organisms at an early stage in the evolution of the lake suggests either that they were part of an endemic Antarctic flora and fauna which pre-dated the last glacial maximum and survived in glacial refugia or that efficient intercontinental dispersal had occurred. 相似文献
67.
Vincent J Realmuto Ken Hon Anne B Kahle Elsa A Abbott David C Pieri 《Bulletin of Volcanology》1992,55(1-2):33-44
Multispectral thermal infrared radiance measurements of the Kupaianaha flow field were acquired with the NASA airborne Thermal Infrared Multispectral Scanner (TIMS) on the morning of 1 October 1988. The TIMS data were used to map both the temperature and emissivity of the surface of the flow field. The temperature map depicted the underground storage and transport of lava. The presence of molten lava in a tube or tumulus resulted in surface temperatures that were at least 10° C above ambient. The temperature map also clearly defined the boundaries of hydrothermal plumes which resulted from the entry of lava into the ocean. The emissivity map revealed the boundaries between individual flow units within the Kupaianaha field. In general, the emissivity of the flows varied systematically with age but the relationship between age and emissivity was not unique. Distinct spectral anomalies, indicative of silica-rich surface materials, were mapped near fumaroles and ocean entry sites. This apparent enrichment in silica may have resulted from an acid-induced leaching of cations from the surfaces of glassy flows. Such incipient alteration may have been the cause for virtually all of the emissivity variations observed on the flow field, the spectral anomalies representing areas where the acid attack was most intense. 相似文献
68.
John A. E. Gibson Warwick F. Vincent Patrick Van Hove Claude Belzile Xioawa Wang Derek Muir 《Aquatic Geochemistry》2002,8(2):97-119
The geochemical processes occurring within meromictic Lake A (maxdepth 120 m) on northern Ellesmere Island, Canada, were investigated to determinethe history of the lake and to provide a baseline for future studies. The lake contained seawaterdiluted by freshwater input that had been mixed prior to the lake's isolation from tidalaction. Input of freshwater after isolation of the lake created vertical stratification resultingin the creation of distinct oxic, suboxic and anoxic zones. Dissolved oxygen was present to 13 m,and sulphide beneath 32 m. Manganese and iron cycling dominated the redox chemistrybetween these depths. Total manganese concentrations reached 176 M, higher thanin most other natural stratified lake or marine environments. 相似文献
69.
Estimates of spatial and temporal variations in suspended sand concentrations (SSC) made with a multi-transducer Acoustic Backscatter Sensor (ABS) under a repeated wave group over a mobile rippled bed in the wave research flume at the National Hydraulics Laboratory in Ottawa, Canada, reveal an number of complex and intriguing patterns. Ensemble averages of 8 nearly identical wave groups provided much more robust estimates of SSC and allowed a detailed examination of the wave group effects. The largest SSC near the bed (< 0.10 m) occurs in phase with the largest waves in the group. Above approximately 0.10 m elevation, SSC lags behind the near bed SSC by as much as 2–3 waves; introducing significant curvature (on a semi-log plot) to the SSC profile. The log linear segments of the SSC profile grow and decay systematically on the scale of the wave group. The range in lengths of log-linear profile segments ( 0.03–0.355 m) suggest that the boundary layer thickness also fluctuates throughout the passage of the wave group. Furthermore, there are significant variations in the patterns of SSC, which occur under the largest and smallest waves in the group. Under the largest waves vertical bands of alternating high and low SSC produce an intra-wave modulation in the upper water column ( 0.075–0.30 m). The equivalent horizontal excursion of these bands scales to the ripple length. Under the smaller waves the intra-wave modulation of the SSC disappears and is replaced by temporally homogenous suspension that expands vertically through several individual wave cycles. The former pattern of homogenous suspension appears to be associated with growth of a boundary layer due to the persistent uni-directional horizontal flow during this part of the group together with the persistence of antecedent bed generated turbulence and vorticity which maintains the suspension. The latter pattern of bands of high and low SSC indicates a strong temporal and spatial constraint on the SSC (phase coupling) induced by the presence of the bedforms which may be enhanced by strong reversals in both flow and vorticity under the large waves in the group. 相似文献
70.
Introduction Scattering of incident waves by local topographies is one of the most attractive topics in the field of engineering seismology. They can be resolved by either a numerical method or an analyti-cal method. The numerical methods include finite difference method, finite element method, boundary element method, etc. The analytical method is the wave function expansion method. Al-though numerical methods can be used for arbitrary-shaped topographies, the analytical solutions are still … 相似文献