利用K-SVD字典学习算法压制海洋大地电磁噪声

海洋大地电磁数据处理的结果会影响数据反演和解释的准确性.在浅水中, 由海浪引起的强电磁干扰和大地电磁场叠加在一起, 使得噪声所在频段内的视电阻率和相位曲线出现畸变.针对这种强干扰, 本文将K-SVD字典学习算法应用于海洋大地电磁信号处理中, 通过稀疏表示海浪感应磁噪声实现大地电磁磁场信号的重构, 并结合视电阻率信息进行相位校正.仿真数据和南黄海实测数据的处理结果表明, 该方法能够有效压制海浪感应磁噪声, 恢复后的视电阻率和相位曲线更加连续、光滑.

     

基于自适应多分辨率奇异值分解的大地电磁数据处理

针对强电磁干扰极易掩盖微弱的大地电磁有用信号,本文结合奇异值分解在去噪方面的优越性,提出基于自适应多分辨率奇异值分解（Adaptive Multi-Resolution Singular Value Decomposition,AMRSVD）的大地电磁数据处理方法.首先对大地电磁数据构建Hankel矩阵,利用MRSVD得到不同分辨率的近似信号和细节信号;然后选用近似信号和细节信号的标准差差值,对大地电磁数据进行信噪辨识;接着结合MRSVD和相邻细节信号的标准差差值,提出先验信息未知情况下的AMRSVD法;最后对辨识出的强干扰运用AMRSVD去除噪声,重构有用信号.实验结果表明,该方法的处理效率高,能有效分离出相关性较强的噪声,时间序列和视电阻率-相位曲线均得到有效改善.

     

地球曲率对长周期大地电磁测深法的影响

基于电磁理论,导出了层状球体大地条件下长周期大地电磁响应的计算公式,编制了计算机程序,并对相应理论模型进行正演计算。根据计算结果,分析了地球曲率对长周期大地电磁测深法视电阻率和阻抗相位的影响。理论模型的计算结果显示,随着探测周期的增大,地球曲率对长周期大地电磁响应的影响逐渐增大,视电阻率曲线会明显下降,阻抗相位曲线逐渐趋于90°;在一维情况下,对于目前的仪器测量范围可以忽略地球曲率带来的影响,但随着仪器技术的进一步发展,地球曲率对长周期大地电磁测深法的影响不可忽视。     

基于匹配追踪和遗传算法的大地电磁噪声压制

针对匹配追踪计算量大、大地电磁数据处理效率低的问题,提出基于匹配追踪和遗传算法的大地电磁噪声压制方法.首先,利用Gabor原子构建过完备原子库,并对过完备原子库集合进行划分.然后,借助遗传算法的自适应性,快速搜寻最优匹配原子及所在位置.最后,运用最优匹配原子对待处理信号进行稀疏分解,重构有用信号.通过对计算机模拟的典型强干扰和矿集区实测大地电磁数据进行分析处理,实验结果表明,相对于匹配追踪和正交匹配追踪,文中所提方法能从过完备原子库中快速、自适应地选取最优匹配原子与不同噪声干扰类型高精度的匹配,极大地提升了计算效率;大地电磁时间域序列中的大尺度强干扰被有效剔除,视电阻率曲线更为光滑、连续,低频段的数据质量得到明显改善.

     

广义S变换及其在大地电磁测深数据处理中的应用

广义S变换是一种优于短时窗傅立叶变换和小波变换的时频分析方法,利用广义S变换能够准确定位大地电磁资料中存在的噪声,通过定义时频窗对噪声进行滤除,从而明显提高阻抗视电阻率与相位的估算质量.本文基于S变换和大地电磁测深资料处理的基本原理,研究了基于广义S变换的大地电磁测深资料的处理流程和方法.对理论模拟信号及实测大地电磁场时间序列数据的处理,证实了方法的有效性.     

大地电磁测深和电偶源频率电磁测深资料反演

在研究大地电磁响应函数频散关系的基础上,构制了一套滤波系数算法,以用于由一套视电阻率资料估算相应的阻抗相位.理论模型和实际大地电磁观测资料的数字试验表明,该法是行之有效的.由频散关系估算的相位值与观测的相位资料的比较,可用于检验观测资料是否满足频散关系.利用经频散关系校正的阻抗相位值,进行大地电磁阻抗的联合反演则可望获得更为可靠的结果.研究了观测频带相互衔接的电偶源频率电磁测深和大地电磁测深视电阻率的一维联合反演问题.对两个实测点两种电磁法的观测资料进行了联合反演试验,与钻井资料对比表明,所获得的电性分层参数是较为可信的.在补充了由频散关系获取的电偶源频率电磁相位资料后,对于两种电磁法的视电阻率————阻抗相位、阻抗实部视电阻率-阻抗虚部视电阻率进行了拟大地电磁反演,获得了相近的反演结果.     

基于循环神经网络的大地电磁信号噪声压制研究

由于天然电磁场源信号微弱,观测数据极易受到噪声干扰,严重影响反演和解释结果.传统去噪方法依赖于人工对时间序列和功率谱的筛选,去噪效率低,主观性强.本文提出利用循环神经网络对大地电磁时域信号进行特征噪声的识别和提取,进而重构出去噪后的大地电磁信号.在对大地电磁时域信号进行大量分析的基础上,对噪声进行分类并搭建含噪信号数据库,利用该数据库训练了两个循环神经网络,并选取长短时记忆单元优化循环神经网络结构,分别实现含噪数据段筛选和噪声形态提取.对仿真和实测数据分别进行了测试,循环神经网络均能准确筛选出大地电磁信号中的噪声段,本方法在避免人为操作主观性的同时提高了工作效率,视电阻率和相位曲线质量得到明显改善.

     

基于机器学习的大地电磁时域数据处理的研究现状

大地电磁测深法基于天然电磁场,其存在信号弱频带宽的特点,因此很难获得无噪声的数据,而传统的大地电磁数据处理方法面临着噪声环境日益复杂、对于海量数据的处理效率不高和难以剔除持续性噪声等诸多挑战.机器学习方法是一种基于统计分析的数据驱动方法,能够发现数据内部的结构和规律,从而对未来时刻和未知状态进行预测和判断.本文从大地电磁噪声分析入手,基于目前在大地电磁数据处理中研究和应用较为广泛的神经网络、字典学习等机器学习方法,分析了其方法原理、应用领域及优缺点,并与传统方法做了对比.发现机器学习方法在海量数据处理、大地电磁噪声模式识别和特征提取、数据预测等方面具有巨大的优势.同时,提高泛化能力和将机器学习方法与大地电磁理论相结合等仍是目前需要进一步探索的问题.随着计算机技术的发展和三维探测的实用化,机器学习方法在大地电磁数据处理中的应用将更加广泛.     

矿区近场源噪声对大地电磁测深数据的影响及其压制方法

大地电磁测深采集的是天然电磁场,具有振幅微弱、频带宽的特点.在人文活动密集地区,电磁噪声干扰日趋严重,导致大地电磁测深曲线具有明显的近场源效应.本文利用电偶极子理论模型分析了电磁相关噪声对大地电磁测深数据的影响特征,并对湖南衡阳某矿区噪声实验数据进行了讨论,分析了矿区噪声的特点.分别利用远参考方法和基于大地电磁时间序列依赖关系的去噪方法对实验数据进行了处理.研究结果表明:矿区近场源噪声的频率一般大于0.1 Hz,远参考方法可以压制高频部分近场源噪声,但在0.1 Hz附近仍存在跳点;基于大地电磁时间序列依赖关系的去噪方法也可以压制矿区强相关噪声,抑制近场源效应,其效果要好于远参考的处理;两种方法结合可以更好的压制强噪声,基本消除了近场源效应,有效提高了大地电磁测深数据的信噪比.     

芦山M_S7.0地震余震期间大地电磁视电阻率变化

以2013年4月20日芦山MS7.0地震震后5月3—24日距主震震中约7km处所观测到的大地电磁数据为基础,分析地下电阻率变化与较大余震(MS3.0)活动的关系.为确保用于地震异常分析的观测数据的可靠性,基于大地电磁方法的特点,使用稳定估算(robust)技术和相干度因子约束等方法得到各频点一定数量的自功率谱和互功率谱,再根据视电阻率和相位曲线的分布形态,剔除引起视电阻率和相位曲线突变的谱值之后得到高质量的视电阻率和相位曲线.此外,根据曲线形态和误差分布来判断数据质量,剔除可能受到干扰影响的数据,以获得能够真实地反映地下电阻率变化的视电阻率分析结果.结果显示:观测期内的yx极化模式的视电阻率值总体呈非单调的增大趋势,最大增幅达27%;yx极化模式的视电阻率值几乎在每个相对集中的较大余震丛集后均会出现高值,而较大余震丛集期间对应的则是视电阻率低值.从物理机制上分析地震的电阻率效应源于微裂隙内孔隙流体的变化,故推测芦山地震余震期间电阻率的增加趋势是由于震后应变释放区应力恢复和孔隙度恢复所致.     

A Monte Carlo study of rainfall forecasting with a stochastic model

A procedure for short-term rainfall forecasting in real-time is developed and a study of the role of sampling on forecast ability is conducted. Ground level rainfall fields are forecasted using a stochastic space-time rainfall model in state-space form. Updating of the rainfall field in real-time is accomplished using a distributed parameter Kalman filter to optimally combine measurement information and forecast model estimates. The influence of sampling density on forecast accuracy is evaluated using a series of a simulated rainfall events generated with the same stochastic rainfall model. Sampling was conducted at five different network spatial densities. The results quantify the influence of sampling network density on real-time rainfall field forecasting. Statistical analyses of the rainfall field residuals illustrate improvement in one hour lead time forecasts at higher measurement densities.     

Earthquake Engineering and Engineering Vibration Aims And Scope

正This journal is established by the Institute of Engineering Mechanics(IEM),China Earthquake Administration,to promote scientific exchange between Chinese and foreign scientists and engineers so as to improve the theory and practice of earthquake hazards mitigation,preparedness,and recovery.To accomplish this purpose,the journal aims to attract a balanced number of papers between Chinese and     

Foreword

Destructive earthquakes have caused great damage in China and the United States and collapsing buildings havecaused many deaths and injuries. The field of earthquake engineering studies earthquake hazards, the occurrence ofearthquakes of various magnitudes, the nature of the ground shaking during an earthquake, the vibration of structuresduring earthquakes, the strengthening of existing structures and the design of new structures to be earthquake resistant,and finally, how to cope with earthquake damage and restore a city to normal functioning. Such efforts are in progressin both countries, but unfortunately, the language barrier interferes with the free flow of information between China andthe Untied States. It would be mutually beneficial if some means could be developed to promote the exchangeof information across the Pacific Ocean. This new journal has been established for this purpose and its success willbe an important step in promoting earthquake engineering in China and the United States.     

WORLD ASSOCIATION FOR SEDIMENTATION AND EROSION RESEARCH(WASER)

正President:Giampaolo Di Silvio,Italy Vice Presidents:Ulrich C.E.Zanke,Germany Zhao-yin Wang,China The World Association for Sedimentation and Erosion Research(WASER),inaugurated on Oct.19,2004,is an independent non-governmental,non-profit organization.The mission of WASER is to promote international co-operation on the study     

Enhancing remote sensing research on global change to improve our understanding on Earth system processes

正Global Change includes climate change and other environmental changes caused by the joint interaction among various layers of Earth. From the positive side, global change provides new opportunities to human and other living forms on Earth. In the meantime, it creates tremendous challenges and negative impact. At present, the negative impacts have reached all primary processes of the global ecosystem and every aspect of human society, especially causing degradation of the ecosystem. For instance, intensive deforestation causes decline of biodiversity; global warming causes sea level rise and increases     

Solute concentrations in the lower niger river and the source rock contribution

Water quality analyses for the Niger River for the 1980/81 hydrological year are presented. The samples were collected from the main river at Lokoja, and from two main tributaries, the Kaduna and the Benue Rivers. Different water types were distinguished by the concentrations of major ions. The type Ca > Na > Mg > K - HCO₃ > SO₄ > Cl was represented at all stations during at least part of the year. Chloride was found to dominate the sulphate ion in the Kaduna and Niger, while the Benue maintained a higher concentration of sulphate relative to chloride all year round. Distinct patterns of seasonal variation in the ion concentrations were observed, particularly for the samples collected at Lokoja. Low ion concentrations were prominent during periods of high discharge, while low flow periods coincided with high dissolved ion concentrations. The contribution of rainwater to the total dissolved solids in the river waters was assessed indirectly using rainwater chemistry data from the Gulf of Guinea. The estimated rainwater contribution to the Lower Niger amounts to 5.15 mg 1^?1. Geochemical weathering calculations involving reactions of the four major minerals of granitic rocks - anorthite, biotite, albite, and K-feldspar - with carbon dioxide and water, can account for the average water composition of the Lower Niger. The proportion of the ionic components was also related to the occurrence of the respective element in the minerals.