Geoelectric investigation of the Hellenic subduction zone using long period magnetotelluric data

The strongest evidence up to date for a subduction zone in the Hellenic region is a clearly identified Wadati-Benioff zone below the central Aegean Sea, to a maximum depth of 180 km. Alternative seismic tomography models suggest that subduction process continues deeper than the Wadati-Benioff zone to a maximum depth of at least 600 km. So far the lack of deep electrical studies in the region impeded scientists from imposing other control factors than seismic to the proposed models for the Hellenic Subduction Zone (HSZ). A Long Period Magnetotelluric (LMT) study was carried out in the southern part of the Greek mainland to study the deep electrical characteristics of the HSZ and examine whether prominent modelled features correlate with structures identified by the seismic methods. The study comprised collection, processing and modelling of magnetotelluric (MT) data in the period range 100–10000 s from ten sites located along a 250 km NE–SW trending profile. The dimensionality of the data was examined at a pre-modelling stage and it was found that they do not exhibit three-dimensional (3-D) features. The latter enabled to construct both one-dimensional (1-D) and two-dimensional (2-D) models. The proposed geoelectric model for HSZ was based on 2-D modelling, since it had better maximum depth resolution of about 400 km, and revealed structures not detected by 1-D modelling attempts. The model structure which was related to the African and Euro-Asian lithosphere is relatively resistive (> 800 Ω-m) and has an average thickness of 150–170 km. Although the bottom of the lithosphere is adequately resolved, the Wadati-Benioff zone that delineates the top of the subducting lithospheric slab is not identified by any electrical feature. The modelled structure associated with the subducting part of the African lithosphere penetrates a relatively conductive (< 200 Ω-m) asthenosphere with a dip angle of 42°. Intermediate electrical resistivities (200–800 Ω-m) are attributed to the ascending melting part of the lithosphere below the region of the Hellenic Volcanic Arc (HVA) and to a dipping zone below the south-western part of the profile, at 170–220 km depths.     

基于高光谱遥感技术的土地质量信息挖掘研究

土地资源的高效管理对土地资源与生态环境监测提出更高的要求,高光谱遥感图像精细的光谱线可提高土地动态监测的精度、广度和深度,在土地资源与土地质量监测中具有广阔的应用前景。本文基于高光谱遥感技术,详细探讨了土地质量指标,主要是土壤有机质土壤水分信息提取的技术流程和关键技术。通过地物光谱分析认为,有机质含量越高的土壤,其光谱发射率越低;重度沙化土地实验室光谱曲线光谱反射率最高,依次排序为中度沙化、轻度沙化和无退化土地;重度水蚀土地光谱反射率较高,但中度水蚀土地与轻度水蚀和无退化土地实验室光谱曲线无明显差异。在光谱分析基础上,利用多元统计分析技术建立了土壤光谱在677、1202、2074和1509nm四个波段的有机质反演模型和在1423、1524、1746nm三个波段的预测土壤含水量的回归模型;并将有机质反演模型应用于宜兴成像光谱遥感数据中,进行宜兴试验区土壤有机质参数成图。最后探讨了反演模型应用推广所存在的问题。     

快速成像：磁共振成像技术的进展

本文复习了大部分快速成像磁共振扫描技术，包括：半傅里叶技术，矩形视野，多层激励，快速自旋回波，快速梯度回波和回波平面技术。磁共振快速成像技术的使用可减少检查时间和运动伪影，提高病人检查耐受性以及获取动态影像等。使磁共振扫描有一更广的临床适应症。     

磁共振扫描对鼻咽癌放疗后局部复发与纤维化的鉴别诊断

本文对３２例鼻咽癌病人进行了Ｇｄ－ＤＴｈＡ增强前后磁共振成像（ＭＲＩ）检查。放疗后局部肿瘤复发（ＲＮＰＣ），纤维化（ＲＦ）的病例。结果表明；Ｇｄ－ＤＴＰＡＭＲ增强扫描下加权像能够提高病块与周围结构的区别。副鼻窦及鼻咽部粘膜看明显的强化作用。ＮＰＣ及ＲＮＰＣ皆为轻度到中等度均匀或四块状强化，ＲＦ组一般无强化，但有５例３８．５％呈轻度索条状强化，且多发生于放疗后间隔时间小于１年内者，ＲＮＰＣ与ＲＦ两组的强化率平均值有显著差别（Ｐ＜０．０５），虽其Ｘ±ＳＤ有部分重叠，但如能结合病灶强化程度和强化形式，对ＲＮＰＣ与ＲＦ的鉴别可提供重要信息．     

A new approach to pre-stack seismic processing

Pre-stack processing of seismic reflection data is significantly simplified if the data organization is the same as that in which the data were acquired in the field; that is, in time slices through common-source gathers. For an impulsive source, the entire processing stream reduces to two elements: velocity analysis and reverse-time pre-stack migration. Many of the steps that are applied as independent operations in standard processing (including demultiplexing, sorting into common-midpoint gathers, elevation corrections, near-surface velocity static corrections, first break muting, ground roll removal, and both normal and dip move-out corrections) either are eliminated, or are applied implicitly during migration. This approach is ideally suited to parallel processing, and can be implemented in machines with small processor memories.     

一种倾角时差校正和叠前成象方法

本文从测量射线参数出发进行反向射线追踪,导出倾角时差校正（DMO）的公式。经过DMO后,可以从一组等炮检距剖面得出共分角线点道集。用于对这些道集进行叠加的速度值与界面倾角无关。对经过DMO的资料的等时切片进行叠前成象（PSI）,就可以把分布在圆上的绕射能量沿圆弧加起来,并放在圆弧上对应于最大炮检距的位置。经过这两种处理,再应用标准的速度分析和叠加方法,就可得出偏移后的剖面。这两种处理均与速度无关。最后用物理模型试验说明了DMO和PSI的效果是好的。