共查询到20条相似文献，搜索用时 15 毫秒
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Summary . Synthetic seismograms represented by integrals generally display signals associated with the limits of integration. Sometimes these 'end-point' contributions are spurious (e.g. in the WKBJ seismogram) and sometimes they are the main physical interest (e.g. the Kirchhoff integral for an edge). The end-point contributions may be asymptotically approximated using integration by parts or Laplace's method and it may then be possible to reduce them if desired. We describe examples in the WKBJ seism ogram for reflected or transmitted waves in homogeneous layers and for turning waves. We also study signals due to discontinuities in reflection coefficients, by partitioning the real slowness integral so that the discontinuities lie at end points. Examples are the head wave, which is a physically correct signal, and spurious diffractions caused by using plane-wave coefficients for grazing rays in the WKBJ seismogram. 相似文献
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流域土地覆被变化水文效应的模拟——以长江上游源头区梭磨河为例 总被引：43，自引：3，他引：40
为分析地表覆被变化的水文效应,以半分布式的地形指数模型 (TOPOMODEL) 为基础,对梭磨河流域1960~1999年逐日流量过程进行了模拟。结果表明,对于流域面积2536km2 的梭磨河流域,该模型也能取得较好的模拟结果。模拟了流域40年来气候波动和地表覆被变化对流域水文的影响。最后在4种不同的流域土地覆被情景下模拟了1960~1999年逐日径流过程。对于实际蒸散发与潜在蒸发有300多mm差距的梭磨河流域,在其它条件不变的情况下,随着流域土地覆被和冠层最大截流量的增加,冠层截流蒸发和流域总蒸发增加,植被蒸腾和土壤表面蒸发减少,土壤水分增加而流域水分含量和饱和层含水量减少。地表径流、地表以下径流、总径流减小。重现期小于20年的洪峰流量减小,但对40年一遇的洪峰流量影响很小,甚至有增加洪峰流量的作用。 相似文献
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Brian S. McKenzie 《Urban geography》2013,34(1):133-151
This research assesses neighborhood differences in access to supermarkets for the Portland, Oregon region. Five-year American Community Survey data for the 2006–2010 period are used to compare a travel time and distance measure of supermarket access for neighborhoods of concentrated poverty relative to other neighborhoods, taking into account their distance to the city center. Poor and nonpoor outer neighborhoods have longer average travel times and distances to nearby supermarkets than their more centralized counterparts. Poor outer neighborhoods fare better on travel distance than nonpoor outer neighborhoods do, but endure penalties in longer travel times. This research demonstrates the potential for variation in results across methods measuring access to neighborhood amenities and suggests that more nuanced methodology will be required for us to understand sociospatial disparities in access. 相似文献
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Social science literature suggests that neighborhood-based visual cues have substantive effects on individuals' littering behaviors. Experimental research on this topic typically alters the appearance of a selected public location, and then monitors changes in littering due to these tightly controlled esthetic changes. The general finding is that littering occurs more frequently in relatively disorderly settings. The current paper extends this work by testing whether or not the same finding holds in more “real life” situations, wherein the operative visual cues come directly from the environment rather than through experimental manipulation. Specifically, the paper empirically identifies two study sites that serve similar functions in their urban system, but that differ markedly in contextual features. At the first site (#1), visual cues include liquor stores, vacant structures, and a noisy interstate highway. Notable visual cues at site #2 are scenic vistas, well-maintained housing, and greenspaces. When the same quasi-natural littering experiment was conducted at both locations, the littering rate was significantly higher at site #1 compared to site #2 (after controlling for individual-level demographic attributes). This result adds value to the applied geography community, as it confirms that spatially-based attributes do influence individual behaviors in a manner consistent with controlled, comparatively acontextual experimental findings. Additionally, the results implicate policy strategies that might be useful for counteracting antisocial urban behavior. 相似文献
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1978-2012年武陵山区公路通达性演化及其对经济发展的影响 总被引：1，自引：0，他引：1
利用通达性系数和加权平均出行时间,结合GIS网络分析和空间分析方法,探讨了1978-2012年武陵山区64个县（市、区）与武陵山区及其外围38个地级市以上城市之间通达性演化的格局和特征,并分析了2000-2012年武陵山区经济发展格局,采用柯布—道格拉斯生产函数模型,定量模拟了通达性对经济发展的影响。研究发现：1978-2000年,武陵山区通达性系数的稳定性较好,到2012年,通达性系数较小值具有向武陵山区东南部集聚的趋势。1978-2000年武陵山区的经济加权平均出行时间和人口加权出行时间的优劣程度均具有由东向西逐渐下降的“圈层式”布局的特点,到2012年,经济加权平均出行时间和人口加权平均出行时间较小值均具有向武陵山区东南部迁移的趋势,原因在于武陵山区及其外围的东部和南部城市较多且路网发展更为迅速和完善。2000-2012年,武陵山区经济发展格局相对稳定,形成了“东部和西北部强、西南部和中北部弱”的经济发展格局,但经济发展的绝对差距逐步扩大,两极分化的趋势越来越明显。2000-2012年武陵山区通达性的重要程度发生变化,通达性的优劣程度对于GDP的影响明显上升,通达性的改善在整体上较好地促进了武陵山区GDP的增长,对武陵山区经济发展的作用变得越来越重要。同时,通达性改善削弱了第一产业的发展,但对第二产业发展具有逐渐上升的正向影响,对第三产业发展的正向影响显著上升。研究充实了贫困地区交通与经济发展关系的实证和理论,对武陵山区通达性改善与经济发展具有重要的指导意义,可以为相关政策的制定提供参考依据。 相似文献
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外资企业对内资企业出口能力提升和出口市场拓展具有重要影响。本文基于2000—2016年中国地级市出口数据,分析中国内资企业出口市场拓展格局,研究外资效应对内资企业进入新出口市场和出口额增长的影响,并依据经济和技术复杂度检验外资效应的区域和产业异质性。描述性分析发现：① 内资企业出口在国内呈多级连片格局,逐渐向非洲和拉丁美洲等新兴市场拓展。② 处于低复杂度城市/产业的内资企业更倾向于拓展新出口市场,处于高复杂度城市/产业的内资企业则倾向于提升出口额。计量模型结果表明：① 外资企业推动了内资企业出口额增长,但抑制了其进入新出口市场,分别表现出溢出效应和竞争效应。② 从区域和产业异质性来看,外资企业仅在高复杂度城市/产业阻碍了内资企业出口市场拓展,表现出竞争效应;而在低复杂度城市/产业则表现为溢出效应。③ 相较于低经济复杂度城市,高经济复杂度城市的内资企业受到的竞争和溢出效应均更强。 相似文献
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Pip Wallace 《New Zealand geographer》2005,61(2):124-130
Abstract: The focus of this article is on examining how resource management plans prepared under the Resource Management Act 1991 can be used to control the effects of activities for the purpose of preserving the integrity of an organic farm. The article examines effects that can potentially threaten an organic farm and identifies methods that could be employed in a District or Regional Plan to avoid or mitigate those effects. It concludes that the use of resource management plans represents a substantial opportunity to secure the position of the organic farm in New Zealand. 相似文献
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Seasonal variation of the soil seed bank of grasses in central Argentina as related to grazing and shrub cover 总被引：1，自引：0，他引：1
The objectives of this research were to (a) study seasonal variation of the seed bank of grasses in two areas with different grazing history, (b) analyse the effect of the presence of shrubs on the seed bank and (c) study seed viability and dormancy in buried seeds of Piptochaetium napostaense and Stipa tenuis. The seed density of grasses was in general low and showed seasonal variation. Most of the grasses showed maximum seed density in December when seed dispersal occurs. Shrub cover did not show a marked influence in seed accumulation. The relatively high number of damaged seeds suggests that predation may be an important factor in seed bank dynamics in the Caldenal. Seeds of dominant perennial forage grasses showed dormancy that could explain, at least in part, persistence of these species in spite of periodical disturbance. 相似文献
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S. O. Ogunade 《Geophysical Journal International》1995,121(1):162-172
Geomagnetic variations, observed at 11 sites in south-western Nigeria, have been analysed to derive interstation transfer functions with the site at Ile-Ife as reference station. The study involves frequencies from 1 to 6 c.p.h. The reference station Ile-Ife is 160 km northward of the continental slope off the Nigerian coastline and 400 km southward of the dip equator. The analysis has been carried out separately with selected data sections of a few hours length during daytime and during the night. Thus expressed linear relations between field components are in the case of day events of dual implication: (1) for the source field structure of the equatorial electrojet, (2) for internal conductivity conditions, including the coast effect from the Bight of Benin. Conductivity anomalies are the sole cause for an observed spatial variability of night events. A 2-D thin-sheet conductivity model has been derived taking both the source and the coast effect into consideration. This model provides a reasonably good fit between observed and computed transfer functions during day and night. 相似文献
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T. L. Gipprich R. K. Snieder R. W. Jibson W. Kimman 《Geophysical Journal International》2008,172(2):770-778
Dynamic stresses generated by earthquakes can trigger landslides. Current methods of landslide analysis such as pseudo-static analysis and Newmark's method focus on the effects of earthquake accelerations on the landslide mass to characterize dynamic landslide behaviour. One limitation of these methods is their use Mohr–Coulomb failure criteria, which only accounts for shear failure, but the role of tensile failure is not accounted for. We develop a limit-equilibrium model to investigate the dynamic stresses generated by a given ground motion due to a plane wave and use this model to assess the role of shear and tensile failure in the initiation of slope instability. We do so by incorporating a modified Griffith failure envelope, which combines shear and tensile failure into a single criterion. Tests of dynamic stresses in both homogeneous and layered slopes demonstrate that two modes of failure exist, tensile failure in the uppermost meters of a slope and shear failure at greater depth. Further, we derive equations that express the dynamic stress in the near-surface in the acceleration measured at the surface. These equations are used to approximately define the depth range for each mechanism of failure. The depths at which these failure mechanisms occur suggest that shear and tensile failure might collaborate in generating slope failure. 相似文献
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以经济发达的宁波市区为例,通过水环境敏感性与水质现状两方面综合评价城市内部空间水环境效应,从中心城区至外围地区,因人口集聚水平、产业发展类型和水污染处理能力的差异,水环境效应指数不断降低;探究其空间异质性,各乡镇评价单元水环境效应指数存在正向相关;运用地理加权回归（GWR）模型,定量分析水环境效应影响机理,其中经济发展水平是水环境演变的基础,产业发展与人口集聚则是造成水环境压力的直接原因。人口空间集聚带来的水污染排放,产生的水环境效应远高于其它因素,是城市内部水环境下一步治理的重点。 相似文献
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近40年西藏怒江河谷盆地的气候特征及变化趋势 总被引：15，自引：1，他引：14
利用1971-2008年西藏怒江流域9个气象站逐月平均气温、平均最高气温、平均最低气温、降水量、日照时数等资料,分析了近40a怒江流域年、季气象要素的变化趋势.并计算讨论了变化趋势与经纬度、海拔高度的相关.结果表明:近40a流域年平均气温以0.26℃/10a的速率显著升高,增暖趋势比中国东北和西北弱.比淮河和华南强,与青藏高原相近.最高气温和最低气温都出现了增高的趋势,年平均气温日较差表现为一致的减小趋势,为-0.13-0.57℃/10a:年降水量以21.O mm/10a的速宰显著增加,各季降水均呈现增加趋势;年日照时数平均每10年减少31.7 h,以夏季减幅最突出.减幅比黄河流域、青藏高原东侧大;地温升高明显.流域上游季节性冻土最大冻结深度呈显著变浅趋势,以安多最明显.日照时数与降水量负相关显著.与夏、秋季大气水汽压呈显著的负相关;大部分站点日照时数的减少,与大气水汽压的显著增加和降水量的增多关系密切.总云量与日较差存在显著负相关.与夏季降水量呈显著正相关,相对湿度的变化不是总云量减少的主要原因.随着纬度的增加和海拔高度的上升,年平均气温升温幅度在加大,年降水量增幅也增大.日照时数的变化趋势仅与经度有着很好的正相关,随着经度的增加,冬季和夏季日照时数的减幅都在减小. 相似文献
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Young-Long Kim 《International journal of geographical information science》2020,34(6):1235-1256
ABSTRACTThis study takes a data-driven approach to define urban nighttime by examining the spatiotemporal dynamics of urban vitality. Using micro-scale spatiotemporal analysis, this paper empirically provides a comprehensive, yet granular, picture of collective human behaviors in cities. Using Seoul, South Korea as a case study site, it prioritizes the spatiotemporal context in order to mitigate uncertain contextual effects inherent in such forms of data-driven analysis. Instead of leaving the data re-grouping up to researcher’s arbitrary decision, this paper employs a functional principal component analysis (FPCA) to systematically transform a set of discrete data to a continuous functional form. This paper applies FPCA on 24-hour-based dataset of pedestrian traffic in Seoul in order to make a data-driven extraction of principal components that characterize the city’s unique patterns of urban vitality. Extracting principal components allows for less statistically obvious phenomena to be measured that would have otherwise been hidden within the data. This approach proved successful in capturing nighttime vitality patterns that are eclipsed by the overwhelming trend of daytime patterns. Additionally, this paper compares differences between regions and seasons to examine what the differences can tell about the definition of nighttime. 相似文献
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