Hilbert-Huang 变换与大地电磁噪声压制

大地电磁信号具有非线性、非平稳、非最小相位特征,不符合以Fourier变换为基础的传统功率谱估计的基本要求. Hilbert-Huang变换是近年发展起来的处理非线性、非平稳信号的完全局部时频分析方法. 本文在简要介绍Hilbert-Huang变换基本原理与算法基础上,以实际数据分析为例,探讨了它在大地电磁信号处理及噪声压制中的应用. 提出利用Hilbert时-频能量谱对大地电磁信号进行时段筛选,以提高信号品质,增强数据处理的质量和资料的可解释性. 利用经验模态分解方法及其多尺度滤波特征,可以有效地分析MT信号中的噪声分布特征,并进行干扰压制.     

基于OC-CCI数据的南海高叶绿素a浓度水域面积的时空变化研究

浮游植物是海洋生态系统食物链的基础组成, 并通过光合作用影响着海表二氧化碳通量变化。文章基于高叶绿素a浓度水域面积指标构建南海浮游植物生物量的估算体系。利用遥感数据, 采用经验正交函数分解插值方法, 重构长时间序列的南海叶绿素a浓度场, 并研究了南海高叶绿素a浓度水域面积特征的时空分布。结果发现: 高叶绿素a浓度水域面积变化有着显著季节特征, 在冬季面积达到最大值, 在夏季达到最小值, 但是该水域对应的叶绿素a浓度却在冬季达到最小值, 在夏季达到最大值, 这一特征可能是由于风驱动的海表动力过程使得海表叶绿素重新分布; 空间分布上, 高叶绿素a浓度水域常年存在于海岸附近, 特别是在中国沿海、越南沿岸、泰国湾以及婆罗洲岛附近。在巽他陆架与湄公河口东部中央海盆, 高叶绿素a浓度区域面积呈年际变化。受厄尔尼诺调控的南海季风, 导致不同年份湄公河口东南沿海存在不同程度的北部冷水侵入, 北部冷水入侵可能是引起局地浮游植物生物量增减的原因。     

Shoreline variability via empirical orthogonal function analysis: Part II relationship to nearshore conditions

The method of empirical orthogonal function (EOF) or principal component analysis (PCA) was used to investigate the spatial and temporal variability of shoreline data sets from Duck, North Carolina, the Gold Coast, Australia, and the United States Pacific Northwest. In the present work, an attempt is made to relate the individual modes of shoreline variability identified by the EOF analyses to select parameterizations of the nearshore environment. The parameters considered include the wave energy (E), the cross-shore and longshore wave energy fluxes (F_x and F_y), the wave steepness (H_o/L_o), the non-dimensional fall velocity parameter (Ω), the profile parameter (P), the surf-similarity parameter (ζ), and a surfzone Froude number (F_r). Correlation analyses were used to evaluate the linear relationship between each of these parameters and the temporal eigenfunctions, c_k(t), associated with individual modes of shoreline change. Typically, strong correlations were observed between longshore uniform modes and the monthly means of several of the nearshore parameters.     

Predicting wave run-up on rubble-mound structures using M5 model tree

Prediction of run-up level is a key task in design of the coastal structures. For the design of the crest level of coastal structures, the wave run-up level with a 2% exceedance probability, R_u2%, is most commonly used. In this study, the performance of M5 model tree for prediction of the wave run-up on rubble-mound structures was investigated. The main advantage of model trees, unlike the other soft computing tools, is their easier use and more importantly their understandable mathematical rules. Experimental data set of Van der Meer and Stam was used for developing model trees. The conventional governing parameters were selected as the input variables and the obtained results were compared with Van der Meer and Stam’s formula, recommended by the Coastal Engineering Manual (CEM, 2006). The predictive accuracy of the model tree approach was found to be superior to that of Van der Meer and Stam’s empirical formula. Furthermore, to judge the generalization capability of the model tree method, the model developed based on laboratory data set was validated with the prototype run-up measurements on the Zeebrugge breakwater, Belgium. Results show that the model tree is more accurate than empirical formulas and TS Fuzzy approach in estimating the full-scale run-up.     

Estimation of tail dependence coefficient in rainfall accumulation fields

Extreme rainfall events are of particular importance due to their severe impacts on the economy, the environment and the society. Characterization and quantification of extremes and their spatial dependence structure may lead to a better understanding of extreme events. An important concept in statistical modeling is the tail dependence coefficient (TDC) that describes the degree of association between concurrent rainfall extremes at different locations. Accurate knowledge of the spatial characteristics of the TDC can help improve on the existing models of the occurrence probability of extreme storms. In this study, efficient estimation of the TDC in rainfall is investigated using a dense network of rain gauges located in south Louisiana, USA. The inter-gauge distances in this network range from about 1 km to 9 km. Four different nonparametric TDC estimators are implemented on samples of the rain gauge data and their advantages and disadvantages are discussed. Three averaging time-scales are considered: 1 h, 2 h and 3 h. The results indicate that a significant tail dependency may exist that cannot be ignored for realistic modeling of multivariate rainfall fields. Presence of a strong dependence among extremes contradicts with the assumption of joint normality, commonly used in hydrologic applications.     

用经验正交函数构造武汉地区电子浓度总含量的经验模式

本文利用经验正交函数（Empirical Orthogonal Function, 简称EOF）分析了武汉电离层台站一个太阳黑子周期（1980年4月～1990年12月）的实测电离层电子浓度总含量（TEC）资料,在此基础上构建了一个武汉地区的TEC月中值的经验模式.结果表明：（1）以EOF分析观测数据具有一定的物理意义,其基函数表征TEC的日变化,系数表征TEC的长期变化;（2）EOF技术展开收敛速度快,很少数低阶项即能反映TEC的主要变化,采用经验正交函数建模可以用较少的参数表征较复杂的物理过程.此外,本文还就TEC模式的外部驱动量的改进问题作了一些探讨,发现用F107A（10.7cm太阳辐射通量F107的81天滑动平均值）作TEC建模的外部驱动量能够更好地提高EOF模式的精度和稳定性.     

Near-source ground motion attenuation relationship for PGA and PSA of the vertical and horizontal components

In this paper, empirical ground-motion models for the vertical and average horizontal components of peak ground-motion and acceleration response spectra from shallow crustal earthquakes are derived using near-source database. These attenuation relationships were derived using a worldwide dataset consisted of corrected and processed accelerograms of 678 strong-motion records recorded with 60 km of the rupture plane of earthquakes between M_w 5.2 and 7.9. Ground motion models are functions of earthquake mechanism, distance from source to site, local average shear wave velocity, nonlinear soil response, sediment depth, depth-to-top of the rupture, hanging wall effects and faulting mechanism.     

Statistical EOF analysis of spatiotemporal glacier mass-balance variability: a case study of Mittivakkat Gletscher,SE Greenland

An Empirical Orthogonal Function (EOF) variance analysis was performed to map in detail the spatiotemporal variability in individual stake mass-balances (ba) on Mittivakkat Gletscher (MG) – in a region where at present five out of ~20.000 glaciers have mass-balance observations. The EOF analysis suggested that observed ba was summarized by two modes: EOF1 and EOF2 represented 80% (significant) and 6% (insignificant) of the explained variance, respectively. EOF1 captured a decline in ba that was uniformly distributed in space at all stakes. The decline was correlated with albedo observations and air temperature observations from nearby stations. EOF2, however, described variations in ba that were heterogeneously distributed among stakes and associated with local slope and aspect. Low elevation stakes (~<400 m a.s.l.) showed relatively negative (out of phase) correlation and higher elevated stakes relatively positive (in phase) eigenvector correlation values with EOF2. Such relatively negative and positive eigenvector correlation values were present where the constituted of exposed glacier ice or snow cover, respectively. The results from this study show how EOF analyses can provide information on spatiotemporal patterns of glacier mass-balance. Understanding such detailed variabilities in mass-balance on a Greenlandic glacier is of interest because a fifth of the Arctic contribution from glaciers and ice caps to sea-level rise originates from Greenland.     

海上消防系统风险分析探讨

平台消防系统可靠与否直接影响到平台及平台操作人员的安全。文章在采用故障树方法对平台消防系统发生失效的路径及可能性进行分析。针对有些事件的发生频率极小但危险性极大的问题,引入危险度的概念,对可靠性进行量化,即计算产生概率,同时提出了计算模糊事件概率期望值的方法。为有效评估油气生产系统的安全性,以及建立起稳定可靠的功能服务系统提供了理论依据。     

磁宁静期核心等离子体层电子密度分布的统计研究

等离子体层已有数十年研究历史,但对其核心等离子体区域却一直没有一个相对准确的界限和模型定义.基于范阿伦辐射带卫星RBSP-A在2012年9月18日至2014年10月13日约两年的观测,我们统计研究了磁宁静期间核心等离子体层电子密度随磁地方时（MLT）及磁壳指数（L-value）的分布特征.发现了核心等离子体层电子密度在不同MLT条件下随L值的变化趋势几乎一致,但与以前的等离子体层经验模式计算的电子密度存在较大的偏差.在不同L值下电子密度随MLT的变化趋势也相差不大,而且随MLT存在明显的逐日和半日变化.最后我们获得了等离子体层电子密度随L值和MLT变化的经验公式.研究结果对空间等离子体层建模及研究具有重要的意义.