Reliability Evaluation on Weak Signal Extraction for Airgun Source Surveys

Based on the data synthesis simulation and the actual processing of the airgun seismic source signal, three quantitative indicators of signal-to-noise ratio, waveform correlation coefficient and phase offset, are superimposed. We systematically evaluate the functions of the following three stack methods including linearity, phase weighting and S-transform in the extraction of weak signals under strong background noise and quantitatively estimate the reliability of the stack results. Through the comprehensive discussion of the above three methods of stack results, the preliminary comparative analysis believes that the linear stack signal-to-noise ratio is low, but the waveform distortion is minimal; the phase-weighted superimposed signal-to-noise ratio is high and the phase offset is small, but the results of the waveform quality and linear stack are larger than the deviation; the S-transform stack has a relatively higher signal-to-noise ratio and a small loss of waveform amplitude, but there is a certain phase shift phenomenon. It is therefore suggested that linear stack technology should be used when the requirements of both waveform quality and time precision are high. However, the selection of the stack method when the airgun source excitation is limited should be emphasized. If high fidelity is required, the S-transform stack method should be selected; if the required time is high, accuracy can be selected by phase-weighted stack method to achieve reasonable extraction of weak signals.     

基于EWT和Meyer自适应阈值的微震降噪

露天矿采场边坡的微震信号具有振幅弱、噪声多、信噪比低的特征,基于经验小波变换(EWT)及Meyer自适应阈值提出分频降噪方法.首先,针对微震信号的频谱特征,设计一种频谱分割策略,将信号和噪声分解为不同的模态;其次,分析模态的频谱和能量,采用Meyer阈值函数自适应降噪,提高有效信号振幅;最后,借助阈值函数处理包含较少有用信号的模态,重构微震信号以保持其连续性,结果表明,降噪后微震信号信噪比(SNR)高、有效信息多,噪声少;通过多种方法对比,并用信噪比与均方根误差(RMSE)对本次降噪效果进行评价,本文方法较好的保留了微震信号有效信息,降噪效果显著.     

Numerical analysis of seismic wave amplification in Nice (France) and comparisons with experiments

The analysis of site effects is very important since the amplification of seismic motion in some specific areas can be very strong. In this paper, the site considered is located in the centre of Nice on the French Riviera. Site effects are investigated considering a numerical approach (Boundary Element Method) and are compared with experimental results. The experimental results are obtained thanks to real earthquakes (weak motion) and microtremor measurements. The investigation of seismic site effects through numerical approaches is interesting because it shows the dependency of the amplification level on such parameters as wave velocity in surface soil layers, velocity contrast with deep layers, seismic wave type, incidence and damping.In this specific area of Nice, experimental measurements obtained for weak motion lead to strong site effects. A one-dimensional (1D) analytical analysis of amplification does not give a satisfactory estimation of the maximum reached levels. A boundary element model is then proposed considering different wave types (SH, P, SV) as the seismic loading. The alluvial basin is successively assumed as an isotropic linear elastic medium and an isotropic linear viscoelastic solid with Zener type behaviour (standard solid). The influence of frequency and incidence is analysed. The thickness of the surface layer, its mechanical properties, its general shape as well as the seismic wave type involved have a great influence on the maximum amplification and the frequency for which it occurs. For real earthquakes, the numerical results are in very good agreement with experimental measurements for each motion component. The boundary element method leads to amplification values very close to the actual ones and much larger than those obtained in the 1D case. Two-dimensional basin effects are then very strong and are well reproduced numerically.     

少震区前兆异常的特征分析

以陕西前兆台网24年的资料为基础，分析了少震区前兆异常的一些特性。在这个建网以来未发生过5级以上地震，每年平均发生2～3次3级多地震，年最大震级的平均值仅3.6级的陕西前兆台网中，每年年度会商所提出的各类测项异常比在11%～20%之间，综合异常比为15%。这些异常比的变化曲线与陕西省内地震频次的强度变化曲线不同步，它们是与地震无关的测值变化，可能是未被人们认识的外界干扰，也可能是地壳内部应力积聚又释放但又无地震发生的一种地壳蠕动结果。     

海安县优质弱筋小麦气候区划研究

本文利用海安县及周边6个基本台站(27个自动站)1990—2009年气象资料,通过对海安县农业气候资源和小麦生产气候条件的调查分析,在地理信息系统平台的支持下,采用样条函数法对气候因子和区划因子进行网格细化,运用综合评价法对海安县优质弱筋小麦种植实现基于小网格的气候适宜性区划,分为最佳种植区和适宜种植区,结果显示:海安中西部地区为优质弱筋小麦最佳种植区,东部沿海为适宜种植区。这为开发和利用海安县农业气候资源、高质高效生产种植弱筋小麦提供科学依据。     

A survey of numerical methods for stochastic differential equations

The development of numerical methods for stochastic differential equations has intensified over the past decade. The earliest methods were usually heuristic adaptations of deterministic methods, but were found to have limited accuracy regardless of the order of the original scheme. A stochastic counterpart of the Taylor formula now provides a framework for the systematic investigation of numerical methods for stochastic differential equations. It suggests numerical schemes, which involve multiple stochastic integrals, of higher order of convergence. We shall survey the literature on these and on the earlier schemes in this paper. Our discussion will focus on diffusion processes, but we shall also indicate the extensions needed to handle processes with jump components. In particular, we shall classify the schemes according to strong or weak convergence criteria, depending on whether the approximation of the sample paths or of the probability distribution is of main interest.     

一种提取弱异常的方法

本文提出一种基于互相关原理的提取弱异常的方法,用于从复杂异常形态中提取出有用的弱异常,给出了应用实例.     

The effects of quartz recrystallization and reaction on weak phase interconnection,strain localization and evolution of microstructure

We conducted axial compression and general shear experiments, at T = 900 °C and P = 1.5 GPa, on samples of banded iron formation (BIF) and synthetic aggregates of quartz, hematite and magnetite to investigate how dynamic recrystallization of quartz promotes strain localization, and the role of weak second phases (oxides) on the rheology and microstructural evolution of the aggregates. Experiments showed strain localization into oxide rich layers, and that the oxide content and oxide distribution are key factors for the strength of the aggregate. Only 2–10 wt.% hematite leads to pronounced weakening and increasing hematite content above ∼10% has only a minor additional effect. Where oxide grains are dispersed, the initial strength contrast with quartz induces stress concentrations at their tips, promoting high stress recrystallization-accommodated dislocation creep of quartz. Fine recrystallized quartz reacts with oxide, forming trails of fine reaction product (ferrosilite/fayalite) leading to the interconnection/percolation of a weaker matrix. The strength contrast between the quartz framework and these fine-grained trails promotes strain localization into micro-shear zones, inducing drastic strain weakening. Thus dynamic recrystallization of quartz promotes syn-deformational reactions leading to a microstructurally-controlled evolution of phase strength contrast. It results in a rheologic transition from load-bearing framework to a matrix-controlled rheology, with transition from S–C′ to S–C fabric with increasing strain.     

锦屏一级水电站坝基软弱岩带水泥-化学复合灌浆与检测评价

锦屏一级水电站坝基地质缺陷发育,采用水泥-化学复合灌浆法对断层、挤压带和岩脉等软弱岩带进行灌浆处理。灌浆后检测成果显示,水泥-化学复核灌浆对坝区岩块岩屑型、岩屑夹泥型及煌斑岩脉等结构面在抗渗性能、变形性能等方面均有较大改善。岩块岩屑型软弱岩带的声波波速较灌前提高约12%～18%,灌后钻孔变模值约为6.0～6.5GPa;岩屑夹泥型软弱岩带声波波速灌前提高6.8%,钻孔变模改善作用较为有限。灌后声波波速、变形模量以及灌后各性能指标提高幅度等能为其他类似工程提供借鉴和参考。     

软弱层带夹泥粒度成分的分形研究

应用分形几何理论，对软弱层带夹泥粒度成分的研究表明，粒度分布具有分形结构特征，其分维值在２－３之间。分维值可作为定量描述夹泥粒度成分的参数。分维值的大小，不仅能表征夹泥所处的演化阶段及环境条件，而且还与夹泥的粘粒含量，粘土矿物成分及物理力学特征等密切相关。