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11.
M. E. Bailey 《Celestial Mechanics and Dynamical Astronomy》1992,54(1-3):49-61
Reasons for interest in the origin of short-period comets and the difficulties of computing their long-term dynamcal evolution are reviewed. The relative advantages of a source region in an extended inner core of the Oort cloud or a compact comet belt just beyond the planetary system are finely balanced, and it is premature to consider the problem solved. A complication is that some comets belonging to the Jupiter family may be part of a time-dependent system, possibly the remains of a giant comet such as Chiron which could have been part of the system 104 yr ago. The origin of short-period comets plays a pivotal role in many areas of solar system science: planet formation, the source of water (possibly life) on the terrestrial planets, the cratering record on the terrestrial planets and satellites of the outer planets, and the environmental impact posed by massive bodies and their decay products in the Earth's near-space environment. 相似文献
12.
北京市地铁暗挖施工沉降控制探讨 总被引:1,自引:0,他引:1
仅以北京市地铁4#线张自忠车站主体暗挖施工的沉降监控项目为研究背景,对穿越城市主干道的车站结构暗挖施工进行连续性的沉降变形监测;从地下构筑物暗挖施工的工序变化与地表沉降的关系进行试验研究.在预测基础上对获取的数据进行多层次的统计与分析,以研究其变形规律与稳定性,实现了信息化施工. 相似文献
13.
14.
机载激光雷达平均树高提取研究 总被引:16,自引:3,他引:13
为了研究机载激光雷达(LiDAR)树高提取技术,以山东省泰安市徂徕山林场为实验区,于2005年5月进行了机载LiDAR数据获取和外业测量.通过对LiDAR点云数据的分类处理,分别得到了试验区的地面点云子集、植被点云子集和高程归一化的植被点云子集.基于高程归一化的植被点云子集计算了上四分位数处的高度,与实地测量的数据进行了比较,并结合中国森林调查规程进行了实用性分析.结果表明:对于较低密度的点云数据,使用分位数法可以较好地进行林分平均高的估计;机载激光雷达技术对树高估计是可行的,精度都高于87%,总体平均精度为90.59%,其中阔叶树的精度高于针叶树.该试验精度可以满足中国二类森林调查规程中平均树高因子的一般商品林和生态公益林的精度要求,对国有商品林小班的调查精度要求(5%)存在一点差距,需要在国有商品林区进一步开展验证工作.对本试验区而言,已经可以满足其作为森林公园生态公益林的调查要求. 相似文献
15.
16.
The dynamics and thermodynamics of large ash flows 总被引:6,自引:6,他引:0
Ash flow deposits, containing up to 1000 km3 of material, have been produced by some of the largest volcanic eruptions known. Ash flows propagate several tens of kilometres
from their source vents, produce extensive blankets of ash and are able to surmount topographic barriers hundreds of metres
high. We present and test a new model of the motion of such flows as they propagate over a near horizontal surface from a
collapsing fountain above a volcanic vent. The model predicts that for a given eruption rate, either a slow (10–100 m/s) and
deep (1000–3000 m) subcritical flow or a fast (100–200 m/s) and shallow (500–1000 m) supercritical flow may develop. Subcritical
ash flows propagate with a nearly constant volume flux, whereas supercritical flows entrain air and become progressively more
voluminous. The run-out distance of such ash flows is controlled largely by the mass of air mixed into the collapsing fountain,
the degree of fragmentation and the associated rate of loss of material into an underlying concentrated depositional system,
and the mass eruption rate. However, in supercritical flows, the continued entrainment of air exerts a further important control
on the flow evolution. Model predictions show that the run-out distance decreases with the mass of air entrained into the
flow. Also, the mass of ash which may ascend from the flow into a buoyant coignimbrite cloud increases as more air is entrained
into the flow. As a result, supercritical ash flows typically have shorter runout distances and more ash is elutriated into
the associated coignimbrite eruption columns. We also show that one-dimensional, channellized ash flows typically propagate
further than their radially spreading counterparts.
As a Plinian eruption proceeds, the erupted mass flux often increases, leading to column collapse and the formation of pumiceous
ash flows. Near the critical conditions for eruption column collapse, the flows are shed from high fountains which entrain
large quantities of air per unit mass. Our model suggests that this will lead to relatively short ash flows with much of the
erupted material being elutriated into the coignimbrite column. However, if the mass flux subseqently increases, then less
air per unit mass is entrained into the collapsing fountain, and progressively larger flows, which propagate further from
the vent, will develop.
Our model is consistent with observations of a number of pyroclastic flow deposits, including the 1912 eruption of Katmai
and the 1991 eruption of Pinatubo. The model suggests that many extensive flow sheets were emplaced from eruptions with mass
fluxes of 109–1010 kg/s over periods of 103–105 s, and that some indicators of flow "mobility" may need to be reinterpreted. Furthermore, in accordance with observations,
the model predicts that the coignimbrite eruption columns produced from such ash flows rose between 20 and 40 km.
Received: 25 August 1995 / Accepted: 3 April 1996 相似文献
17.
Cloud droplet chemistry is modelled for the first 150 m of rise in a wintertime, mid-latitude, marine stratus cloud using observations made at and near the Cape Grim Baseline Station as a source of input parameters. The emphasis in this work was to study the variation in droplet chemistry as a function of both droplet size and nucleus composition, with a particular focus on the way in which oxidation of dissolved sulfur dioxide varied.At 150 m above the condensation level, solute concentration as a function of droplet size was found to increase by as much as 2 to 3 orders of magnitude for only a factor of 2 increase in droplet radius, primarily as a consequence of the 1/r dependence in the droplet growth equation. This type of size dependence exists at all levels in the model cloud, and has a significant influence on oxidation rate of sulfur dioxide in droplets growing on sulfate nuclei, oxidation by ozone being favoured in the smallest droplets, but oxidation by hydrogen peroxide being favoured in the larger droplets. Oxidation by ozone is favoured at all sizes in droplets formed on sea-salt nuclei as a result of the initially high alkalinity of these droplets, and in the cloud overall is calculated to be the more important oxidation pathway. Although based on a simplified chemical scheme, these results suggest that both size-dependent and nucleus-dependent chemistry of cloud droplets may need to be considered explicitly in cloud modelling work.Volume-weighted mean pH values in the range 5 to 6 were predicted from sensitivity studies in which input variables were varied over reasonable ranges, in agreement with two sets of bulk cloud-water pH data obtained by aircraft near Cape Grim. 相似文献
18.
Application of X-ray computed tomography for analyzing cleat spacing and cleat aperture in coal samples 总被引:2,自引:2,他引:2
S. Mazumder K.-H.A.A. Wolf K. Elewaut R. Ephraim 《International Journal of Coal Geology》2006,68(3-4):205-222
New techniques to determine distributions of cleat aperture, cleat orientation and cleat spacing from CT scans have been developed. For cleat orientation and spacing distributions, two different coal blocks were scanned. The CT scans have been analyzed for the three orthogonal directions. Histograms of the cleat orientations are bimodal, expressing the typical cleat texture of face and butt cleats and bedding perpendicular relaxation fractures. Deviations up to 20° from the peak values in the cleat orientation distributions were used as input for automated image analysis of cleat spacing. Distributions of the cleat spacing measurements are related to the face and butt cleat directions. The term “relevant cleat length” is introduced as a measure to extract the amount of cleat length involved with the cleat spacing measurements. The ratio ranges from 0.03 to 0.38 and expresses the difference in cleat texture in both samples. Cleat spacing versus relevant cleat length shows sample specific patterns for face cleat, butt cleat and bedding. To describe cleat aperture quantitatively, peak height and missing attenuation have been used. The image of a cleat was seen as a convolution of a rectangular fracture profile with a Gaussian point spread function. 相似文献
19.
本文介绍了大范围卫星影像彩色镶嵌图的制作流程.对于大范围卫星影像彩色镶嵌图制作的技术难点(去云、拼接、调色),文章进行了重点介绍并提出技术解决办法.利用MODIS卫星数据制作了两幅全国彩色影像镶嵌图:一幅为红波段(B1)、绿波段(B4)和蓝波段(B3)构成的真彩色合成影像:一幅为红波段(B1).近红外波段(B2)和蓝波段(B3)构成的假彩色合成影像.两幅影像色彩精美,信息量丰富. 相似文献
20.
“国际地层表”与GSSP 总被引:11,自引:2,他引:11
近年来,“国际地层表”不断被国际地层委员会更新,用GSSP来定义年代地层单位而建立起来的“国际地层表”,使得大家在叙述地质历史时有一种共同的语言、有一个全球性的标准。在显生宇的96个阶(统)中已有4 6个阶(统)确立了GGSP,其余的有望在2 0 0 8年全部确立。我国已取得了4枚金钉子,即将取得3枚,还有可能再获得4—6枚。根据区域特点,我国已编制了“中国区域年代地层(地质年代)表”并正在开展区域性建阶工作 相似文献