利用计算机断层成像方法由观测图像重建等离子体层全球密度分布——地球遮挡问题

本文是利用计算机断层成像(CT)方法中的滤波反投影法(FBP)和代数迭代法(ART),根据等离子体层的仿真模型,重建其全球密度分布.在重建过程中,地球遮挡是一个很重要的问题.计算结果表明两种方法都可以使用,但ART比FBP重建的效果好.ART重建图像的相关系数可达0.98,而FBP重建图像的相关系数仅为0.86.FBP重建的偏差是由地球遮挡引起,向阳面靠近地球区域的密度会减小.从定量分析中可以进一步看出地球遮挡所引起的偏差变化.     

山体遮挡对滇池风生流的影响初探

用二维风生流数值模型模拟滇池湖流运动。滇池在主导风向西南风作用下,假定湖面风场是均匀的,数值模拟的湖流流态与实测湖流结果相差很大。而考虑山体遮挡影响,根据实测湖流期间现有的风情资料,在湖面上构造一非均匀风场,数值模拟结果与实测值基本一致。山体遮挡对滇池风生流的影响是不容忽视的。建议进一步进行湖流和湖面风向、风速监测,并建立过山气流数学模型,深入研究山体遮挡对湖泊风生流的影响。     

悬挂网壳结构风压分布的环境影响因素研究

为了研究环境因素对机场大跨度悬挂网壳结构风荷载分布特性的影响,通过在SST k-ω湍流模型的基础上结合CFD技术对悬挂网格结构进行风洞试验数值模拟,验证了数值模拟方法的准确性并对比研究了周边建筑、围护结构、地势高低与支撑结构等不同环境因素下悬挂网壳风荷载分布规律,发现：周边建筑可以对风荷载起到遮挡效果,降低网壳表面风压,遮挡效果随着夹角的增大逐渐衰弱;有围护结构的封闭式悬挂网壳较开放式悬挂网壳存在更大的风压梯度,在一定程度上可以提高结构承风能力,但对于非规则网壳结构无规律可循,故在实际工程中,需根据情况具体分析;随着悬挂网壳结构所处地势高度的增加,其受到风压的影响程度有一定的增幅,但当地势变化差异不大时,结构受风扰动不明显;拱梁对结构表面风压分布趋势影响较小,对风荷载起到了遮挡作用,可以降低悬挂网壳结构风压系数值。     

射线透射成像中重物质遮挡下的轻物质检测技术研究

本文研究了不同厚度轻重物质在射线透射成像中的图像灰度关系，根据提取轻物质轮廓线的要求，找出了在不同厚度重物质遮挡下的可检测出轻物质的最小厚度曲线。     

基于视觉图像的震区人员搜救方法研究

传统基于信号传感技术的震区人员搜救方法,利用惯性导航传感技术融合移动距离和方向角对待搜救人员进行定位,容易受到天气状况不理想以及局部遮挡的影响,其搜救效率和精度低。提出基于视觉图像的震区人员搜救方法,利用图像采集设备采集震区初始视觉图像,采用小波降噪法降噪处理,提升可辨程度。通过色彩对比方法提取降噪后震区人员视觉图像的特征,并与震区原始图像特征对比,获取震区待搜救人员候选图像;根据候选图像分别采用Kalman滤波跟踪算法和Mean shift跟踪算法,在天气状况不理想以及局部遮挡的复杂震区环境中跟踪搜救人员目标。实验结果表明,所提方法查全率保持在98.5%以上,平均准确率约为98%,人员搜救平均时间约为23 s,说明所提方法能够进行高效、准确的震区人员搜救。     

地形对颗粒驱动重力流影响的模拟实验研究

颗粒驱动重力流由悬浮在流体中的颗粒提供驱动力并侵入到环境流体之中,交换流体实验研究是理解此类流体运动本质的主要手段之一。将颗粒驱动重力流与自然火山碎屑密度流的流体参数进行正确配比,能够理解火山碎屑密度流的流动和堆积过程的本质。本文介绍了火山碎屑密度流及其物理模拟实验研究,并对其物理本质(颗粒驱动重力流)的箱子模型进行推导。通过在实验水箱中模拟不同的地形变化,对颗粒驱动重力流在遇到地形变化时的运动特征和颗粒堆积特征进行研究。本论文设定了两种地形障碍对颗粒驱动重力流的影响模式:横亘的限定高度障碍物和侧向部分遮挡障碍物。横亘障碍物对流体底界形成整体阻挡;侧向部分遮挡障碍物在垂直方向上对流体侧界形成阻挡,对流体底界形成部分阻挡。通过记录流体遇到障碍物时的现象,获取流体前锋速度,测定颗粒堆积密度,评估横亘障碍物高度(小于或等于流体前锋高度),侧向遮挡程度对成分/颗粒驱动重力流流动特征和堆积特征的影响为:(1)成分重力流在通过有限高度横亘阻挡时,不同高度障碍物对流体的运动状态产生的影响体现为流体重新回到"惯性—浮力相"的距离长短和前锋减速特征不同。当障碍物高度分别相当于流体前锋高度的～1/3、～2/3和～1/1时,流体重回"惯性—浮力相"所需的距离为障碍物高度的35.1～38.6倍、12.0～18.1倍和2.1～4.3倍,三者不呈现线性关系;在通过上述高度横亘阻挡之后,流体前锋速度按照t~(5/6),t~(3/4)和t~(2/3)衰减。(2)颗粒驱动重力流体通过相当于其前锋高度的1/3、2/3和1/1横亘障碍物之后,无量纲化流体前锋运动特征的L/T斜率分别为～2/9、～1/9和～1/16。不同位置、相同高度的横亘阻挡对流体前锋减速模式产生相同的影响;流体前锋在通过较高障碍物之后的减速更加明显。h_b~(6.9)h_b~(7.2)h_b~(15)(3)与通过较高障碍物相比,当颗粒实验流体通过较低障碍物时,其堆积曲线下降较缓慢。当流体通过高度分别为流体前锋高度的～1/3、～2/3和～1/1的障碍物之后,堆积曲线呈现出、和的变化特征。与距离较近的障碍物相比,距离较远的障碍物对颗粒驱动重力流通过障碍物之后颗粒堆积的影响更大。(4)侧向遮挡对成分重力流体流动状态的影响具有滞后性。阻挡比例越小,这种滞后性越明显。当盐水流体通过占水箱宽度1/4挡板之后,其前锋位置的变化特征为t~(0.771),通过占水箱宽度1/2挡板之后为t~(0.700),通过占水箱宽度3/4挡板之后为t~(0.623),未呈现线性关系。(5)不同程度侧向遮挡对颗粒驱动流体前锋运动特征、沉积特征和堆积结果的影响表现出复杂性:在颗粒驱动重力流体通过侧向遮挡之后的最初一段距离内,流体前锋速度没有较大的改变。随着挡板宽度的增加,经过挡板之后的实验流体将会更快地进入减速阶段。与较宽挡板相比,颗粒驱动重力流在通过较窄挡板之后的堆积曲线下降较平缓。在经过相同宽度的挡板之后,初始约化重力大的流体,在经过挡板之后堆积曲线下降的越平缓。与参考实验流体相比,通过1/4遮挡的流体前锋速度与参考实验流体相当,并在后期超越参考实验流体,在遮挡位置之后的堆积物总量也大于参考实验流体;通过1/2遮挡的流体前锋速度在无量纲时间T100内明显下降,在T100之后加速超过参考实验流体,最终堆积总量小于参考实验流体;通过3/4遮挡的颗粒驱动重力流前锋速度和堆积结果都有显著下降。利用流体力学相似性原理,对实验流体的各项参数进行无量纲分析并与天然火山碎屑密度流进行对比。本论文的研究内容能够代表低密度火山碎屑流的物理特征,并适用于以下两种地质情况:(1)火山碎屑密度流翻越(破)火口缘的运动和堆积特征;(2)火山碎屑密度流通过曲折山谷的运动和堆积特征。根据本文实验的启示,对长白山天池火山八卦庙期火山碎屑流在火山口缘内外分布特征进行初步探讨;对1991年日本云仙岳火山碎屑流造成的人员伤亡原因进行初步探讨。本文的研究细化了火山碎屑密度流在遇到不同形态障碍物时的流态变化,为火山灾害评估提供更详细的解决方案;在实验过程中所摸索出来的一些经验和实验技术,为实验火山学的进一步发展提供经验。     

月球南极沙克尔顿撞击坑周围电势和尘埃静电输运的理论分析

近年来,月球南极地区因其特殊的光照条件以及撞击坑永久阴影区中可能存在的水冰而成为未来探测的焦点.沙克尔顿(Shackleton)撞击坑几乎位于月球南极点,已成为包括嫦娥七号任务在内的未来着陆任务最热门的目的地.然而,人们对该撞击坑周围的电势和尘埃环境仍然知之甚少.本文建立了一个理论模型来研究撞击坑周围的月表电势和尘埃的静电输运.研究发现,由于地形遮挡,该撞击坑的底部会充负电,背风侧坑壁表面电位最低可至-175V.相应地,有大量带电尘埃颗粒会从背风侧坑壁发射出来,迁移高度可达10km,水平迁移距离约为40km,这使得撞击坑附近可形成局部尘埃云.通过数值模拟对地形遮挡和局部尘埃云进行了验证,发现撞击坑附近确实产生了局部尘埃云,典型尘埃密度为10⁴～10⁵m^-3.研究结果对未来撞击坑附近月面探测的环境评估具有重要意义.此外,研究结果还有助于理解其他无大气天体的表面充电和尘埃静电输运过程.     

西山遮挡对滇池风生流影响的数值模拟

对于湖泊风生流的模拟,湖 面风场数据是决定精度高低的关键因素, 而湖泊周围的山地等复杂地势常常引起湖面风速风向的不均匀分布,致使湖流流态因地形因素而变化较大,本文建立了一个适合于湖泊区域的中小尺度三维过山气流数值模型,用以计算受山体遮挡的风场合沿水深平均的二维湖泊风生流数值模型模拟了实际地形条件下的滇池风生流,并与均匀风     

利用IMAGE卫星观测数据重建地球等离子体层的磁赤道面分布

美国利用IMAGE卫星的极紫外辐射（EUV）探测器对地球等离子体层进行了连续5年的遥感成像观测。由于IMAGE卫星数据是沿观测路径上的积分投影数据,并且存在地球“遮挡”、“阴影”、“数据缺失”等问题,无法直接利用传统的CT方法对等离子体层进行三维重建。本文利用地球磁场模型,基于地球等离子体层的物理性质,建立一个联系地球磁赤道面密度与投影数据的EUV成像模型,实现了从单个角度的EUV观测图像进行地球等离子层三维重构的方法。      

基于矩特征和局部特征的高原子序数液体容器识别

目前液体检查是地铁、机场安检工作中极为迫切的需求。本文提出了一种全新的基于全局和局部特征的高原子序数液体容器识别方法。该方法基于全局区域提取其组合矩特征,在识别初期首先迅速找到与模型库中相似的物品和差异较小的姿态,然后根据局部特征即容器壁投影特征和瓶底特征进行判断和提高识别精度。实验结果证明,该方法很好地结合了目标的整体和局部信息,能够解决容器在与高原子序数物品局部遮挡问题,同时具有较高的识别精度。     

A Monte Carlo study of rainfall forecasting with a stochastic model

A procedure for short-term rainfall forecasting in real-time is developed and a study of the role of sampling on forecast ability is conducted. Ground level rainfall fields are forecasted using a stochastic space-time rainfall model in state-space form. Updating of the rainfall field in real-time is accomplished using a distributed parameter Kalman filter to optimally combine measurement information and forecast model estimates. The influence of sampling density on forecast accuracy is evaluated using a series of a simulated rainfall events generated with the same stochastic rainfall model. Sampling was conducted at five different network spatial densities. The results quantify the influence of sampling network density on real-time rainfall field forecasting. Statistical analyses of the rainfall field residuals illustrate improvement in one hour lead time forecasts at higher measurement densities.     

Earthquake Engineering and Engineering Vibration Aims And Scope

正This journal is established by the Institute of Engineering Mechanics(IEM),China Earthquake Administration,to promote scientific exchange between Chinese and foreign scientists and engineers so as to improve the theory and practice of earthquake hazards mitigation,preparedness,and recovery.To accomplish this purpose,the journal aims to attract a balanced number of papers between Chinese and     

Solute concentrations in the lower niger river and the source rock contribution

Water quality analyses for the Niger River for the 1980/81 hydrological year are presented. The samples were collected from the main river at Lokoja, and from two main tributaries, the Kaduna and the Benue Rivers. Different water types were distinguished by the concentrations of major ions. The type Ca > Na > Mg > K - HCO₃ > SO₄ > Cl was represented at all stations during at least part of the year. Chloride was found to dominate the sulphate ion in the Kaduna and Niger, while the Benue maintained a higher concentration of sulphate relative to chloride all year round. Distinct patterns of seasonal variation in the ion concentrations were observed, particularly for the samples collected at Lokoja. Low ion concentrations were prominent during periods of high discharge, while low flow periods coincided with high dissolved ion concentrations. The contribution of rainwater to the total dissolved solids in the river waters was assessed indirectly using rainwater chemistry data from the Gulf of Guinea. The estimated rainwater contribution to the Lower Niger amounts to 5.15 mg 1^?1. Geochemical weathering calculations involving reactions of the four major minerals of granitic rocks - anorthite, biotite, albite, and K-feldspar - with carbon dioxide and water, can account for the average water composition of the Lower Niger. The proportion of the ionic components was also related to the occurrence of the respective element in the minerals.     

Scenario predictions for potential near-field and far-field earthquakes affecting Hong Kong

Historical earthquakes noted in the written records of the South China region, including Hong Kong, are not well delineated along identified prominent fault sources. Despite the lack of any definitive, localised trend in the spatial distribution of seismic activity in the region, there does appear to be some major disparity in the seismic activity rates (especially for large magnitude earthquakes) between the near-field and the far-field regions of Hong Kong. Despite this observation, previous studies of the regional seismic activity and seismic ground motion hazard (the latter using a probabilistic seismic hazard assessment, PSHA) have considered very broad source zone regions, in which uniform levels of seismic activity have been assumed. The present paper further scrutinises this broad source zone (BSZ) approach by adopting a novel expanding circular disc (ECD) method to determine the rates of earthquake recurrence. Such a method is intended to counter-check previously developed models by determining earthquake scenario events in terms of magnitude–distance (M–R) pairs or combinations, having defined values of average return period. Unlike the BSZ approach, the ECD method specifically accounts for the supposed variations in the seismic activity rates between events in the near-field and the far-field of Hong Kong.The form of the developed method is particularly suited to the determination of design-level earthquake ground motions for bedrock sites, since it assumes a directionally-independent attenuation model as described in the companion paper. It is found that, whilst the BSZ approach may indicate the overall average levels of hazard that are representative of the South China region as a whole, it does not capture the large disparity in seismic activity rates between near-field and far-field events. This important feature is expected to have a significant impact upon engineering assessments of the seismic safety of structures in Hong Kong and elsewhere in the South China region. For example, it is found that for events with M≥6, the seismic activity rate (normalised by time and area) in the very far-field is around 3.5 times larger than in the near-field and medium-field of Hong Kong. The resulting design M-R combinations, covering a range of return periods from 70 to 2500 years, are limited, for very long return periods and for distant events, by the maximum credible earthquake (MCE) magnitude. Intensive research to determine this seismic hazard parameter is recommended, in order to refine further the results of the ECD analysis, which presently conservatively assumes the MCE to range between M=6 in the near-field of Hong Kong to M=8 in the very far-field, at distances greater than 280 km from Hong Kong.     

Depositional ages and characteristics of Middle–Upper Jurassic and Lower Cretaceous lacustrine deposits in southeastern Mongolia

Lower Cretaceous lacustrine oil shales are widely distributed in southeastern Mongolia. Due to the high organic carbon content of oil shale, many geochemical studies and petroleum exploration have been conducted. Although most of the oil shales are considered to be Early Cretaceous in age, a recent study reveals that some were deposited in the Middle Jurassic. The present study aims at establishing depositional ages and characteristics of the Jurassic and Cretaceous lacustrine deposits in Mongolia. The Lower Cretaceous Shinekhudag Formation is about 250 m thick and composed of alternating beds of shale and dolomite. The Middle Jurassic Eedemt Formation is about 150 m thick and composed of alternating beds of shale, dolomitic marl, and siltstone. The alternations of shale and dolomite in both formations were formed by lake level changes, reflecting precipitation changes. Shales were deposited in the center of a deep lake during highstand, while dolomites were formed by primary precipitation during lowstand. Based on the radiometric age dating, the Shinekhudag Formation was deposited between 123.8 ±2.0 Ma and 118.5 ±0.9 Ma of the early Aptian. The Eedemt Formation was deposited at around 165–158 Ma of Callovian–Oxfordian. The calculated sedimentation rate of the Shinekhudag Formation is between 4.7 ±2.6 cm/ky and 10.0 ±7.6 cm/ky. Shales in the Shinekhudag Formation show micrometer‐scale lamination, consisting of algal organic matter and detrital clay mineral couplets. Given the average thickness of micro‐laminae and calculated sedimentation rate, the micro‐lamination is most likely of varve origin. Both Middle–Upper Jurassic and Lower Cretaceous lacustrine oil shales were deposited in intracontinental basins in the paleo‐Asian continent. Tectonic processes and basin evolution basically controlled the deposition of these oil shales. In addition, enhanced precipitation under humid climate during the early Aptian and the Callovian–Oxfordian was another key factor inducing the widespread oil shale deposition in Mongolia.     

THEORETICAL APPROACH TO THE SOLUTION OF THE INFILTRATION PROBLEM

ABSTRACT

The one-dimensional transient downward entry of water in unsaturated soils is investigated theoretically. The mathematical equation describing the infiltration process is derived by combining Darcy's dynamic equation of motion with the continuity and thermodynamic state equations adjusted for the unsaturated flow conditions. The resulting equation together with the corresponding initial and boundary conditions constitues a mathematical initial boundary value problem requiring the solution of a nonlinear partial differential equation of the parabolic type. The volumetric water content is taken as the dependent variable and the time and the position along the vertical direction are taken as the independent variables. The governing equation is of such nature that a solution exists for t > 0 and is uniquely determined if two relationships are defined, together with the specified state of the system, at the initial time t = 0 and at the two boundaries. The two required relations are those of pressure versus permeability and pressure versus volumetric water content.

Since the partial differential equation has strong non-linear terms, a discrete solution is obtained by approximating the derivatives with finite-differences at discrete mesh points in the solution domain and integrated for the corresponding initial and boundary conditions. The use of an implicit difference scheme is employed in order to generate a system of simultaneous non-linear equations that has to be solved for each time increment. For n mesh points the two boundary conditions provide two equations and the repetition of the recurrence formula provides n—2 equations, the total being n equations for each time increment. The solution of the system is obtained by matrix inversion and particularly with a back-substitution technique. The FORTRAN statements used for obtaining the solution with an electronic digital computer (IBM 704) are presented together with the input data.

Analysis of the errors involved in the numerical solution is made and the stability and convergence of the solution of the approximate difference equation to that of the differential equation is investigated. The method applied is that of making a Fourier series expansion of a whole line of errors and then following the progress of the general term of the series expansion and also the behavior of each constituent harmonic. The errors (forming a continuous function of points in an abstract Banach space) are represented by vectors with the Fourier coefficients constituting a second Banach space. The amplification factor of the difference equation is shown to be always less than unity which guarantees the stability of the employed implicit recurrence scheme.

Experiments conducted on a vertical column packed uniformly with very fine sand, show a satisfactory agreement between the theoretically and experimentally obtained values. Many experimental results are shown in an attempt to explain the infiltration phenomenon with emphasis on the shape and movement of the wet front, and the effects of the degree of compaction, initial water content and deaired water on the infiltration rate.     

Late Permian post‐ophiolitic trondhjemites from Central Iran: a mark of subduction role in growth of Paleozoic continental crust

Late Permian trondhjemites in the Anarak area occur as stocks and dykes, which cross cut the Anarak ophiolite and its overlying metasedimentary rocks, and are exposed along the northern Anarak east–west main faults. These leucocratic intrusive bodies have enclaves of all ophiolitic units and metamorphic rocks. They are composed of amphibole, plagioclase (oligoclase), quartz, zircon and muscovite. Secondary minerals are chlorite (pycnochlorite), epidote, albite, magnetite and calcite. Whole‐rock major‐ and trace‐element analyses reveal that they are characterized by high SiO ₂ (67.8–71.0 wt%), Al ₂ O ₃ (14.9–17.1 wt%) and Na ₂ O (5.3–8.6 wt%), low K ₂ O (0.1–1.5 wt%; average: 0.8 wt%), low Rb/Sr ratio (0.01–0.40; average: 0.09), low Y (3–6 ppm), negative Ti, Nb and Ta anomalies, slightly negative or positive Eu anomaly, LREE enrichment and fractionated HREE. These rocks present 2 to 40 times enrichment in inclined chondrite‐normalized REE patterns. Geochemical characteristics of the Anarak trondhjemites all reflect melting of a mafic protolith at more than 10 kbar. The field evidence and whole‐rock chemistry reveal that these rocks have been crystallized from magmas derived from melting of subducted Anarak oceanic crust. This study reveals that melting of garnet amphibolite was an important element of continent formation in the study area.