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在多孔隙含水层中地面磁共振（surface nuclear magnetic resonance，SNMR）信号呈现多弛豫衰减特性，常规盲源分离方法和单指数拟合方法引起信号严重失真和信息缺失等问题.本文提出了基于稀疏表示的随机噪声背景下多弛豫SNMR信号的提取方法.根据SNMR信号的衰减特征，设计了精确刻画SNMR信号且与随机噪声不相关的离散衰减余弦冗余字典.其次，针对多弛豫SNMR信号稀疏度未知的问题，通过设置合理的残差比阈值控制迭代次数，改进了广义正交匹配追踪（generalized orthogonal matching pursuit，gOMP）算法，使得该方法应用于SNMR信号的提取时，具有更好的自适应性和普适性.再次，鉴于SNMR测量数据为多次独立重复采集的结果，提出了基于数据流的SNMR信号提取策略，在提高算法鲁棒性的同时，保证了信号提取结果的唯一性.最后，通过仿真和实测数据证明了基于gOMP算法的稀疏表示方法可以显著地提升多弛豫SNMR信号的提取质量，降低随机噪声对含水层反演结果的影响，提高SNMR探测能力. 相似文献
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For autonomous manipulation in water, an underwater vehicle-manipulator system (UVMS) should be able to generate trajectori9es for the vehicle and manipulators and track the planned trajectories accurately. In this paper, for trajectory generation, we suggest a performance index for redundancy resolution. This index is designed to minimize the restoring moments of the UVMS during manipulation, and it is optimized without impeding the performance of a given task. As a result, the restoring moments of the UVMS are decreased, and control efforts are also reduced. For tracking control of the UVMS, a nonlinear H∞ optimal control with disturbance observer is proposed. This control is robust against parameter uncertainties, external disturbances, and actuator nonlinearities. Numerical simulations are presented to demonstrate the performance of the proposed coordinated motion control of the UVMS. The results show that control inputs for tracking are reduced, and the UVMS can successfully track generated trajectories. 相似文献
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There is a pressing need to determine the relationships between driving variables and landscape transformations. Human activities shape landscapes and turn them into complex assemblages of highly diverse structures. Other factors, including climate and topography, also play significant roles in landscape transitions, and identifying the interactions among the variables is critical to environmental management. This study analyzed the configurations and spatial-temporal processes of landscape changes from 1998 to 2011 under different anthropogenic disturbances, identified the main variables that determine the landscape patterns and transitions, and quantified the relationships between pairs of driver sets. Landsat images of Baicheng and Tekes from 1998, 2006 and 2011 were used to classify landscapes by supervised classification. Redundancy analysis (RDA) and variation partitioning were performed to identify the main driving forces and to quantify the unique, shared, and total explained variation of the sets of variables. The results indicate that the proportions of otherwise identical landscapes in Baicheng and Tekes were very different. The area of the grassland in Tekes was much larger than that of the cropland; however, the differences between the grassland and cropland in Baicheng were not as pronounced. Much of the grassland in Tekes was located in an area that was near residents, whereas most of the grassland in Baicheng was far from residents. The slope, elevation, annual precipitation, annual temperature, and distance to the nearest resident were strong driving forces influencing the patterns and transitions of the landscapes. The results of the variation partitioning indicated complex interrelationships among all of the pairs of driver sets. All of the variable sets had significant explanatory roles, most of which had both unique and shared variations with the others. The results of this study can assist policy makers and planners in implementing sustainable landscape management and effective protection strategies. 相似文献
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The influence of topography and land use on water quality of Xiangxi River in Three Gorges Reservoir region 总被引：5，自引：0，他引：5
A self-organizing map (SOM) was used to cluster the water quality data of Xiangxi River in the Three Gorges Reservoir region.
The results showed that 81 sampling sites could be divided into several groups representing different land use types. The
forest dominated region had low concentrations of most nutrient variables except COD, whereas the agricultural region had
high concentrations of NO3N, TN, Alkalinity, and Hardness. The sites downstream of an urban area were high in NH3N, NO2N, PO4P and TP. Redundancy analysis was used to identify the individual effects of topography and land use on river water quality.
The results revealed that the watershed factors accounted for 61.7% variations of water quality in the Xiangxi River. Specifically,
topographical characteristics explained 26.0% variations of water quality, land use explained 10.2%, and topography and land
use together explained 25.5%. More than 50% of the variation in most water quality variables was explained by watershed characteristics.
However, water quality variables which are strongly influenced by urban and industrial point source pollution (NH3N, NO2N, PO4P and TP) were not as well correlated with watershed characteristics. 相似文献
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H.K. F&#x;r&#x;nc&#x;olu S.S. Seefeldt B. ahin M. Vural 《Journal of Arid Environments》2009,73(12):1149-1157
In the semi-arid steppe rangelands of Central Turkey, Festuca valesiaca and Thymus sipyleus ssp rosulans have become the dominant species on degraded pastures. We hypothesized that decreases in species richness and abundance are correlated with increasing prevalence of these two species. Therefore, our objectives were to determine whether there are patterns in examined vegetation; how dominant species contribute to these patterns; and how patterns differ between grazed and ungrazed vegetation. We determined that protection from grazing increased species richness. Grazing significantly changed composition through decreasing total plant, forb, grass and F. valesiaca covers, while substantially increasing T. sipyleus cover. Topography, soil and grazing appear to impact the dominance of plant communities where F. valesiaca and T. sipyleus prevail. These two dominant species had a significant effect in shaping vegetation patterns. Based on regression analysis, alterations in species richness with changes in cover of forbs and shrubs were evident, and spatial heterogeneity of F. valesiaca and T. sipyleus indicated unstable vegetative patterns in heavily grazed pastures and successional changes in protected pastures. Our study results identify F. valesiaca and T. sipyleus as indicator species of vegetation suppression in condition assessments of degraded steppe rangelands. 相似文献
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本文研究了北京翠湖湿地公园夏、秋季水华优势藻血红裸藻(Euglena sanguinea)从水华发生到衰亡过程的细胞密度、形态特征及其与环境因子的关系.结果发现,2016年8月下旬至9月中旬血红裸藻密度稳定增加,9月20日藻密度达到峰值5.13×106cells/L;9月下旬至11月底藻密度显著下降;血红裸藻在衰亡过程中呈现细胞形态由大变小、由细长变为圆形,细胞颜色由深变浅,运动速率由快变慢的趋势,衰亡后形成具透明胶被孢子的休眠体;血红裸藻群落生长的主控因子为水温和p H,其适宜的环境阈值范围为水温23~27℃,p H 7.3~8.0;其受光照强度影响存在分层现象,弱光强区域藻密度相对较低;血红裸藻的衰亡过程与铵态氮浓度呈显著正相关,低浓度时加速衰亡;与总氮浓度呈负相关,衰亡后总氮浓度显著升高;受总磷浓度影响相对较小,与溶解氧浓度无显著相关关系. 相似文献
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分析了等效源曲面延拓方法积分方程核函数的自相似性、冗余性，根据Fredholm积分方程核函数特征，提出了一种小波余弦非线性阈值压缩算法，实现了大型Fredholm积分方程的降阶，使得Bhattacharyya等（1977年）提出的等效源曲面延拓方法能够处理大面积、大数据量的资料. 理论模型结果表明，当压缩比为405%时硬阈值压缩方法的曲面延拓可以达到很高的精度，当压缩比为811%时, 硬阈值压缩方法精度降低，而采用我们提出的余弦非线性阈值可以明显提高曲面延拓的精度. 将硬阈值、软阈值和非线性阈值压缩三种不同方法用于川东北气田MT－1线资料的处理，当压缩比达79%时，三种不同方法曲化平结果都可以达到很好的效果，但非线性阈值压缩的曲化平结果失真最小，它能够客观地反映杨家河局部隆起，为在该区寻找与油气有关的局部构造提供重要依据. 相似文献
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The contribution to each component of the deformation vector due to a marginally undetected blunder in an observation is split into two parts, and so are the deformation measures at a point. The first part, called the local component (or self component), signifies the contribution due to the redundancy-related multiplier belonging to the observation itself, while the second part, called the complementary component, shows the contribution due to the non-redundancy-related multipliers belonging to the observation. A larger redundancy results in a smaller local component for each component of the deformation vector. However, robustness due to an observation at a point is correlated with redundancy to a significant extent. The first-order global positioning system (GPS)-geodetic network of Taiwan was examined for its robustness. The experiments seem to indicate that: (1) large deformations tend to be found at points where the group redundancies are small; (2) the local components monopolize deformation measures at the perimeter stations of the network where very small redundancy numbers are found; (3) the largest deformation at a point may be due to an observation not directly tied to the point of interest; and (4) except for local twisting, deformation measures and mean positional precisions at individual points are highly correlated.Acknowledgement The authors thank Drs. A.A. Simkooei, M. Schmidt, and M.R. Craymer greatly for their constructive comments and suggestions, which improved the quality of the paper. This research was supported by National Science Council of Taiwan (contract number NSC-91-2211- E-002-085). 相似文献