医用CT图像常见伪影成因及矫正分析

鉴于目前市场上的医用CT基本上都采用第三代CT原理.本文在比较理想C T系统与实际系统差异的基础上,分析了第三代CT图像常见伪影、产生因素和矫正技术.以达到有效帮助CT理论研究工作者、医务工作者和维修工程师加深对CT图像伪影的理解和正确快速采取措施矫正或减少图像伪影的目的.     

International students’ post-graduation migration plans and the search for home

Research on international student’s post migration plans treats migration as a binary stay-return category and focuses on push-pull factors as the cause of this migration. In this paper we expand the definition of migration and consider the role of life experiences and aspirations, particularly the concept of home. We ask, what are the different conceptualizations of home and how are these tied to differential migratory plans? We analyze data from 232 interviews with international students from more than 50 countries who attended a flagship public university in Canada from 2006–2013.We find that students have four ways of thinking about home: as host, as ancestral, as cosmopolitan, and as nebulous. These understandings of home correspond to particular post-migration plans. While students who view home as a host plan to stay, and those who view home as ancestral plan to return, those with cosmopolitan and nebulous conceptions of home have more open migration plans.     

东亚大陆大三角地震区的强震成组活动迁移图像分析

本文分析了1900—2008年东亚大陆大三角地震区7.5级以上大地震的地震活动时空图像, 认为: ① 1902—1974年东亚大陆大三角地震区的7.5级以上浅源地震成组活动的主体地区, 沿着大三角地震区的边界顺时针迁移, 到了1997—2001年开始转移到大三角地震区的内部; ② 对于整个大三角地震区而言, 近代7.5级以上浅源地震的震级(M)-时间(t)过程表明, 地震活动是活跃和平静交替出现的幕式过程; ③ 大三角地震区近代7.5级以上浅源地震的时空迁移图像的力学机制, 可能是地震区内地震断层应力转移和耦合的相互作用。     

数据自相关多次波偏移成像

在常规偏移方法中一般都需要压制地震数据中的多次波,仅利用一次波信息成像,把自由表面反射的多次波视为噪声,但是在多次波中也包含着地下结构信息,应该将其充分利用到成像中来.事实上,已经有不少成像方法试图利用多次波信息,但是大部分方法都需要对多次波进行预测.本文提出了基于傅里叶有限差分偏移算子的数据自相关偏移方法.在这种偏移方法中,对含有一次波和多次波的地震数据,分别进行下行和上行延拓,然后直接利用常规的互相关成像条件成像.由于波场延拓采用了傅里叶有限差分算子,其计算效率高,能够很好地对复杂介质中的地震数据进行延拓.在数值试验中,使用了一个含散射点的三层模型和Marmousi模型.合成数据测试结果表明,这种方法可以对更大范围的地下构造成像,比常规的只利用一次波的傅里叶有限差分法照明度更好,并且在浅层可以提供更高的分辨率.我们提出的数据自相关策略易于实现且避免了繁杂的多次波预测,这对于复杂地下构造成像可能有着重大意义.     

海水速度差异对中深层偏移成像的影响分析

本文针对深水环境下中深层偏移成像质量差的问题,考虑海水速度变化对中深层偏移成像质量的影响,从在大水深中加入深海声道模型入手,分析在偏移成像当中海水速度的不同选取对水平层状介质、倾斜层介质以及较复杂介质模型偏移成像质量的影响,通过在同一模型上改变海水速度进行成像,分析中深层成像效果可以得出:在大水深反射资料数据处理当中应该考虑真实的海水速度进行成像处理,否则会由于海水速度的选取不当而造成偏移成像层位的空间位置和中深层层位几何形态的变化.     

Prestack exploding reflector modelling and migration for anisotropic media

The double‐square‐root equation is commonly used to image data by downward continuation using one‐way depth extrapolation methods. A two‐way time extrapolation of the double‐square‐root‐derived phase operator allows for up and downgoing wavefields but suffers from an essential singularity for horizontally travelling waves. This singularity is also associated with an anisotropic version of the double‐square‐root extrapolator. Perturbation theory allows us to separate the isotropic contribution, as well as the singularity, from the anisotropic contribution to the operator. As a result, the anisotropic residual operator is free from such singularities and can be applied as a stand alone operator to correct for anisotropy. We can apply the residual anisotropy operator even if the original prestack wavefield was obtained using, for example, reverse‐time migration. The residual correction is also useful for anisotropic parameter estimation. Applications to synthetic data demonstrate the accuracy of the new prestack modelling and migration approach. It also proves useful in approximately imaging the Vertical Transverse Isotropic Marmousi model.     

Measurement of iodine-129 in seawater samples collected from the Japan Sea area using accelerator mass spectrometry: Contribution of nuclear fuel reprocessing plants

The concentrations of ¹²⁹I in seawater samples from two sites (off Sekine and the Toyama Bay) in the Japan Sea were determined by accelerator mass spectrometry. The observed concentrations exceed amounts expected from natural origin and globally distributed fallout due to nuclear weapons testing. Because the fraction of natural origin and global fallout is 2% and 8.9–13.8%, respectively, the residual more than 80% of the concentration must come primarily from nuclear fuel reprocessing plants. This result indicates a rapid distribution of ¹²⁹I through atmospheric transport on a global scale. The depth distribution of ¹²⁹I at the Toyama Bay in the Japan Sea shows that the ¹²⁹I maximum is in the mixed layer and that concentrations decrease with depth. The inventory of ¹²⁹I in water column is four times higher than that measured in the Gulf of Mexico which has almost the same depth at the Toyama Bay. This higher inventory probably reflects: (1) the rapid water sinking in the Japan Sea, (2) the difference of distance in sampling locations with respect to major ¹²⁹I releasing plants and (3) the strong increase in emissions from nuclear fuel reprocessing plants after the profile of the Gulf of Mexico was taken.     

镇江谏壁钼矿床钼的迁移成矿机理初探

通过对有关钼配合物的形成条件、物理化学性质的考察、碳酸盐、CO2与CO、碳氢化物的相关性以及有关矿床的矿物流体包裹体液相、气相化学成分等的研究，结合镇江谏壁钼矿床成矿的地质背景、矿床特征、钼矿物的共生、伴生矿物组合等的实际情况，认识到镇江谏壁钼矿中钼是呈易挥发的羰基配合物从地球深部迁移至地壳浅部，在有利部位被氧化、硫化沉淀富集成矿。     

板块俯冲中水分运移过程及其地球物理含义

流体/水分在地球演化过程中具有重要作用,它的存在可以提高地质体的扩散及蠕变,降低岩石/地幔的固液相限,对地质体的地球物理性质、地质灾害诱发、地质构造演化等都有着重要的作用.在板块俯冲区,流体/水分的影响贯穿了整个动力学过程,但板块深部脱水还存在着争议,且目前系统地研究水分在整个俯冲中的迁移过程及其地球物理意义的工作还较...     

傅里叶有限差分法保幅叠前深度偏移方法

地震数据中饱含有丰富的走时信息和振幅信息. 为解决传统偏移方法中几何扩散和入射角变化引起的振幅误差问题,本文提出了一种实用的波动方程保幅地震偏移方法. 该方法从全声波方程出发进行单程波保幅分解,得到直观、高效率的直接面对地震波传播波场的压力分量进行延拓的保幅偏移单程波方程,进而推导出一个含有6项的傅里叶有限差分法保幅偏移的算子方程;修改边界条件和成像条件,使修改后的边界条件和成像方程中考虑振幅补偿,从而从三方面补偿几何扩散损失和入射角变化对振幅的影响. 脉冲响应测试、单炮记录的数值试验以及SEG/EAGE盐丘模型的叠前偏移结果表明,该方法不但可以使散射能量聚焦、归位,提高成像精度;而且可以输出正确反映地下反射系数的振幅信息,为后续的地震属性分析(如AVO/AVA)提供更真实的地震信息.