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排序方式: 共有432条查询结果,搜索用时 671 毫秒
81.
李剑颖 《CT理论与应用研究》2007,16(2):60-65
本文介绍了Light Speed VCT的技术特性,包括硬件、软件和图像重建的技术进步,以及这些技术进步带来的图像质量上的改进和临床应用.这些先进的临床应用包括5秒心脏成像、胸痛三联诊等.探讨了降低病人剂量的重要性以及实现图像质量和X射线剂量的和谐和同一. 相似文献
82.
The appearance of shear banding in granular materials has been investigated intensively during the last decades and is still of ongoing importance in terms of understanding the stress–strain behaviour of the material, the localization phenomena and the interaction between soil and structure. Only less attention has been paid to the occurrence of systems of shear bands although such systems can be identified in geotechnical structures as well as in geological formations. In this paper we present results of experiments on sand specimens under extensional load in natural gravity as well as in increased gravity in the centrifuge where the influence of the stress level on the geometry of a shear band pattern, specified by the spacing of the shear bands and the angle between failure surfaces and minor stress direction, has been investigated. X‐ray technique has been used to visualize the failure zones inside the specimen, an optical measurement system called Digital Image Correlation has been applied to identify and observe the appearing deformation mechanism on the sides of the specimens in natural gravity as well as during the flight in the centrifuge. It can be shown that the geometry of the shear band pattern is sparsely influenced by the change of the stress level. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
83.
垂直指向法标定是双偏振雷达的差分反射率因子ZDR标定有效方法的一种,可以订正ZDR测量误差,提高雷达数据的质量。本文基于双发双收模式的X波段双线偏振天气雷达系统,通过分析雷达硬件系统和垂直指向法标定的数据,诊断了观测数值的一些质量问题,初步判断是方位旋转关节损坏而导致ZDR的数值在某一固定区域上出现较大偏差。更换新的方位旋转关节后,对比处理问题前后的垂直指向标定数据证明能有效解决此问题。最后,经过多次试验分析表明,当垂直指向法标定在回波强度为25.5~30.5dB之间的中等范围稳定的层状云降水情况下使用时,其标定的数据能有效订正系统ZDR偏移量。 相似文献
84.
MICHAEL D. CHEETHAM ANNABELLE F. KEENE RICHARD T. BUSH LEIGH A. SULLIVAN WAYNE D. ERSKINE 《Sedimentology》2008,55(6):1905-1913
Grain‐size distribution is a fundamental tool for interpreting sedimentary units within depositional systems. The techniques assessed in this study are commonly used to determine grain‐size distributions for sand‐dominated sediments. However, the degree of consistency and differences in interpretation when using a combination of grain‐size methods have not yet been assessed systematically for sand‐dominated fluvial sediments. Results obtained from laser diffraction, X‐ray attenuation and scanning electron microscopy grain‐size analysis techniques were compared with those obtained from the traditional sieve/hydrometer method. Scanning electron microscopy was shown to provide an inaccurate quantitative analysis of grain‐size distributions because of difficulties in obtaining representative samples for examination. The X‐ray attenuation method is unsuitable for sand‐dominated sediments because of its upper size range of only 300 μm. The consistently strong correlation between the laser diffraction results and the sieve/hydrometer results shows that these methods are comparable for sand‐dominated fluvial sediments. Provided that sample preparation is consistent, the latter two methods can be used together within a study of such sediments while maintaining a high degree of accuracy. These results indicate that data for sand‐dominated fluvial sediments gained from the long‐established sieve/hydrometer method can be compared with confidence to those obtained by modern studies using laser diffraction techniques. 相似文献
85.
南堡凹陷晚新生代X型断层形成机制及其对油气运聚的控制 总被引:9,自引:6,他引:3
通过高分辨率三维地震资料解释、平衡地质剖面分析,并结合前人的构造物理模拟实验结果,指出南堡凹陷地震剖面显示的断层基本组合形式为复杂X型,继而对其发展模式及成因机制进行探讨,认为:(1)南堡凹陷复杂X型断层组合形成自晚新生代馆陶组沉积时期,明化镇组沉积以来显著发育,并呈现对称性与非对称性两种形态;(2)南堡凹陷新生代构造经历了古近纪地壳单剪变形与新近纪以来地壳纯剪变形机制的叠加,X型断层的广泛发育受控于地壳的纯剪变形机制,并主要由基底先存断层的重新活动触发。在此基础上分析了X型断层对油气运聚的影响,指出X型断层夹持的地垒构造是深层有利油气圈闭,X型断层下部大量微小断层分支能够取代主干油源断层,成为沟通烃源岩与浅层圈闭的有效运移通道。 相似文献
86.
以吉林省长白山地区的天然浮石为原料,成功地合成了13X型分子筛,对其合成路线,反应条件进行了研究,确定了合成13X型分子筛的最佳工艺条件,并通过X射线衍射法及差热分析等手段证明了产物的结构 相似文献
87.
从衍射几何学出发,推导出0-2θ型X射线衍射仪液体样品池窗口强度校正的相对透明因子,解释了由于超厚液体样品池上的窗口的吸收,反射法引起的低角强度损失和透射法引起的高角强度损失,获得了令人满意的的校正结果。 相似文献
88.
E. H. Geyer K. Jockers N. N. Kiselev G. P. Chernova 《Astrophysics and Space Science》1996,239(2):259-274
For observations of solar system objects an imaging polarimeter has been constructed as an auxiliary instrument for a f/15 focal reducer. With this instrument simultaneous measurement of theQ andU Stokes parameters is possible. It contains no moving parts such as a moving /2 retarder. The polarizer consists of two Wollaston prisms which are combined to form a single optical element. Their polarization angles differ by 45°. This compound polarizer is placed in the exit pupil of the afocal telescope-collimator system of the focal reducer and splits the exit pupil into halves. In this way four polarized beams with E-vector orientations 0°, 90°, 45°, and 135° emerge from the exit pupil. These are intercepted by the camera lens of the focal reducer system and imaged simultaneously on the CCD detector. With a properly designed field mask at the Cassegrain focus, the four beams are imaged without overlap.As a demonstration of the capabilities of this quadruple-beam Wollaston-prism polarimeter, we present observations of comet Tanaka-Machholz 1992 X in May 1992. The advantages, shortcomings, and possible improvements of the instrument are discussed.based on observations obtained at Hoher List Observatorypresently at Astronomical Observatory of Khar'kiv University, Sumskaya Street 35, 310022 Khar'kiv, Ukraine 相似文献
89.
90.
Chen Tieyun Liu Pu Wang Deyu Professor Shanghai Jiaotong University Shanghai . Postdoctor Shanghai Jiaotong University Shanghai 《中国海洋工程》1994,(3)
-A ring model is developed to investigate the ultimate strength of tubular X and XX joints in the case that the brace is compressed. In the present analysis, the tubular joint is made of elasto-perfectly-plastic material, and the arch element of the chord section intersecting with the brace is assumed to be rigid. It is found that when 6 plastic hinges for X joint and 8 plastic hinges Tor XX joint appear in the ring, the limit state is reached, and by means of the equivalent ring width Be formula proposed in a previous paper by the authors, the ultimate strength of tubular X and XX joints subjected to compression can be obtained. 相似文献