井间电磁波数值模拟及成像技术

系统地介绍了井间电磁波数值模拟与成像方法的研究现状。正演方面主要讨论了有限差分、有限元和积分方程法,分析了各种方法的优缺点、适用范围,并指出了未来发展趋势;反演方面主要讨论了线性反演和非线性反演,根据井间电磁波成像的性质,对时间域和频率域线性反演方法分别做了详细论述,并给出了未来发展趋势。     

磁敏感加权成像临床应用和在血管瘤评估中的研究进展

磁敏感加权成像是一种近年发展的MRI新技术,在中枢神经系统疾病中的诊断及鉴别诊断中的重要作用已经得到了广大临床大夫的认可,具有很高的应用价值和拓展前景。SWI作为常规MRI检查的重要补充序列,采用3D梯度回波扫描,完全速度补偿,射频脉冲扰相等技术,具有三维、高分辨率、高信噪比的特点,可以更好地分析软组织血管瘤的结构成分,鉴别活动性出血、机化血栓、钙化及铁沉积等。而且SWI相位图能够清楚显示瘤体内部新生血管、瘤体周围静脉血管位置和形态改变,更有利于准确评估病变范围及程度,为血管瘤定位及定性诊断可以提供更多的影像信息,对于临床诊断及治疗方案的制定、预后评估都具有重要的应用价值。     

Range imaging: a new method for high‐resolution topographic measurements in small‐ and medium‐scale field sites

Topographic measurements are essential for the study of earth surface processes. Three‐dimensional data have been conventionally obtained through terrestrial laser scanning or photogrammetric methods. However, particularly in steep and rough terrain, high‐resolution field measurements remain challenging and often require new creative approaches. In this paper, range imaging is evaluated as an alternative method for obtaining surface data in such complex environments. Range imaging is an emerging time‐of‐flight technology, using phase shift measurements on a multi‐pixel sensor to generate a distance image of a surface. Its suitability for field measurements has yet not been tested. We found ambient light and surface reflectivity to be the main factors affecting error in distance measurements. Low‐reflectivity surfaces and strong illumination contrasts under direct exposure to sunlight lead to noisy distance measurements. However, regardless of lighting conditions, the accuracy of range imaging was markedly improved by averaging multiple images of the same scene. For medium ambient lighting (shade) and a light‐coloured surface the measurement uncertainty was approximately 9 mm. To further test the suitability of range imaging for field applications we measured a reach of a steep mountain stream with a horizontal resolution of approximately 1 cm (in the focal plane of the camera), allowing for the interpolation of a digital elevation model on a 2 cm grid. Comparison with an elevation model obtained from terrestrial laser scanning for the same site revealed that both models show similar degrees of topographic detail. Despite limitations in measurement range and accuracy, particularly at bright ambient lighting, range imaging offers three‐dimensional data in real time and video mode without the need of post‐processing. Therefore, range imaging is a useful complement or alternative to existing methods for high‐resolution measurements in small‐ to medium‐scale field sites. Copyright © 2012 John Wiley & Sons, Ltd.     

Evaluation of the saltation process of bed materials by video imaging under altered bed roughness

The purpose of this paper is to examine the nature of particle saltation and movement over the beds of fixed roughness from flume experiments. A series of experiments are carried out to study the saltation of individual sand particles of different sizes over rough beds under different flow conditions. A 3‐D acoustic Doppler velocimeter is used to record the fluid velocity components; subsequently, under different flow conditions, the images of released sand particles are recorded using high‐speed video imaging technique. Systematic analysis is made with regard to the forces acting on the grains and the variation of their magnitudes along the saltation trajectories of the grains. Relations between the saltation parameters, flow intensity and bed roughness are developed. The distributions of the angle of orientations during a single saltation follows almost a Gaussian distribution. The shape of the Gaussian distribution depends on the particle size and bed roughness. Particle collisions with rough beds and the resulting coefficients of restitution are also discussed. A theoretical framework is developed to compute the mean particle velocity considering the spin in the energy balance equation. Results of the detailed analysis using the imaging technique are much better than in previously reported studies. Copyright © 2013 John Wiley & Sons, Ltd.     

Backscattering of individual LiDAR pulses from forest canopies explained by photogrammetrically derived vegetation structure

In recent years, airborne LiDAR sensors have shown remarkable performance in the mapping of forest vegetation. This experimental study looks at LiDAR data at the scale of individual pulses to elucidate the sources behind interpulse variation in backscattering. Close-range photogrammetry was used for obtaining the canopy reference measurements at the ratio scale. The experiments illustrated different orientation techniques in the field, LiDAR acquisitions and photogrammetry in both leaf-on and leaf-off conditions, and two-waveform recording LiDAR sensors. The intrafootprint branch silhouettes in zenith-looking images, in which the camera, footprint, and LiDAR sensor were collinear, were extracted and contrasted with LiDAR backscattering. An enhanced planimetric match (refinement of strip matching) was achieved by shifting the pulses in a strip and searching for the maximal correlation between the silhouette and LiDAR intensity. The relative silhouette explained up to 80–90% of the interpulse variation. We tested whether accounting for the Gaussian spread of intrafootprint irradiance would improve the correlations, but the effect was blurred by small-scale geometric noise. Accounting for receiver gain variations in the Leica ALS60 sensor data strengthened the dependences. The size of the vegetation objects required for triggering a LiDAR observation was analyzed. We demonstrated the use of LiDAR pulses adjacent to canopy vegetation, which did not trigger a canopy echo, for canopy mapping. Pulses not triggering an echo constitute the complement to the actual canopy. We conclude that field photogrammetry is a useful tool for mapping forest canopies from below and that quantitative analysis is feasible even at the scale of single pulses for enhanced understanding of LiDAR observations from vegetation.     

基于高分辨率SAR图像成像机理的震害信息分析

与中低分辨率SAR图像相比,高分辨率SAR图像受目标复杂性和成像噪声等因素的干扰更为严重,致使应用高分辨率SAR图像检测震害目标变得更加困难.传统的适用于中低分辨率SAR图像的震害信息提取方法对于高分辨率SAR图像有一定的局限性.为了从高分辨率SAR图像中更加准确地提取震害信息,从SAR图像成像机理和目标的后向散射特征出发,对建筑物和道路、桥梁的震害特征进行了细致分析,为寻求合适的目标检测和提取方法提供了思路.     

基于暗原色的航空遥感影像薄雾去除与改进

本文对航空遥感影像的薄雾进行了研究,根据雾图像成像模型和基于暗原色先验知识,结合图像对比度的自适应调整,提出了一种对航空遥感影像薄雾去除的改进算法。采用数字航空相机DSS439获取的不同地物特征类型的应急遥感影像进行去雾实验,并对实验结果进行分析、比较与评价,验证了该方法的正确性和有效性。     

Spectral material mapping using hyperspectral imagery: a review of spectral matching and library search methods

Imaging spectroscopy is an emerging and versatile technique that finds applications in diverse fields concerned with remote identification, discrimination and mapping of materials. The large amount of spectral data produced by hyperspectral imaging necessitates the development of automated techniques that convert imagery directly into thematic maps. Spectral library search method, a method of choice for organic compound identification by the mass spectroscopy, has caught the attention of researchers as one of the appropriate methods for an efficient exploitation of high quality spectral data available from the hyperspectral imaging systems. Given the apparent increase in the number of papers appearing on the subject as well as the variety of methods proposed, it is reasonable to say that the field of automated interpretation of reflectance spectral data has passed its infancy now gaining important space in the scientific community. We present an overall view of the literature relevant to the development of library search method, the various search algorithms and systems available in the purview for developing an automated hyperspectral data analysis system for material identification.     

基于源波场重建与波场分解的逆时偏移

逆时偏移以其高精度、适于强变速地质体和不受反射层倾角限制的特点,已经成为处理盐丘、高陡倾角反射层和推覆构造等复杂地质体的标准成像手段。然而,其成像条件需要具有同一时间上的源波场与检波点波场,这在大规模数据上会导致存储的不足与I /O 时间过长。这种成像条件也会在波场传播路径上产生假象。鉴于此,提出了一套可以有效解决这些问题的逆时偏移方案。对于存储与I / O 问题,用源波场重建技术来使源波场与检波点波场在时间上同步,即在源波场正演过程中,仅需存储成像区域边界的波场和最后两层时间的波场作为源波场重建的边界条件和初始条件,从而大幅度节约存储与I /O。对于假象问题,利用上下左右行波分解成像条件与Laplace 滤波技术结合,以达到去除假象的目的。经测试,该方案取得较好的成像效果。其意义在于改进了逆时偏移去假象的方案,使其可以有效地对复杂地质情况进行高质量的成像,并通过源波场重建技术来减少实现逆时偏移所需要的存储与I /O 时间。     

Oxidation state of lithospheric mantle along the northeastern margin of the North China Craton: implications for geodynamic processes

Quaternary volcanic rocks in the Kuandian (KD), Longgang (LG), Changbaishan (CBS), Wangqing (WQ), and Jilin (JL) volcanic centres in eastern Liaoning and southern Jilin provinces contain mantle xenoliths of spinel-facies lherzolites and minor harzburgites. Among the study sites, the KD, LG, and CBS volcanic fields are located on the northeastern margin of the North China Craton (NCC), whereas the WQ and JL fields lie on the southern margin of the Xing'an–Mongolia Orogenic Belt (XMOB). The (Fo) components of olivine (Ol) and Cr# (=Cr/(Cr + Al)) of spinel, together with trace element abundance of clinopyroxene, suggest that the subcontinental lithospheric mantle (SCLM) in the study area has undergone a low degree (4–6%) of partial melting. The rocks do not show modal metasomatism, but clinopyroxene grains in selected samples show elevated large ion lithophile element compositions, suggesting that the mantle xenoliths underwent minor cryptic metasomatism by exchange with a silicate melt. Two-pyroxene thermometry yielded equilibration temperatures ranging from 740°C to 1210°C. The corresponding oxygen fugacity (fO₂) was calculated to range from FMQ –2.64 to +0.39 with an average of –0.59 (n?=?53). The oxidation state is comparable to that of abyssal peridotites and the asthenospheric mantle. We failed to discover differences in equilibration temperatures and oxidation state between lherzolites and harzburgites, suggesting that partial melting did not affect fO₂ values. In addition, similar fO₂ of non-metasomatized and metasomatized samples suggest that metasomatism in the region did not affect fO₂. Our data suggest that the present SCLM beneath the northeastern margin of the NCC and the southern margin of the XMOB are very similar and likely formed from a fertile asthenosphere after delamination of an old lithospheric keel below the NCC in response to the west-dipping subduction of the Pacific oceanic plate since early to middle Mesozoic time.