基于站间天然电磁场单位脉冲响应的大地电磁时间序列去噪方法

针对天然大地电磁场信号在人文活动密集地区易受噪声干扰的问题,本文提出利用两个同步测点天然电磁场时间序列之间的单位脉冲响应,合成本地点受干扰时段的数据,从而去除大地电磁噪声.首先,选择高信噪比时段的数据,采用最小二乘法,估算本地点与参考点之间的单位脉冲响应,再根据卷积定律,结合参考磁场合成本地点的磁场和电场.最后用合成数据替换含噪声时段数据,实现时间域去噪.实测高信噪比数据和含噪数据的处理结果表明,该方法可以高精度合成本地点磁场与电场信号,有效去除本地点电场和磁场噪声,包括相关噪声,提高大地电磁数据质量.     

基于脉冲频响函数的MATLAB动力方程求解方法

提出一种基于脉冲频响函数并利用MATLAB求解的方法，由于脉冲频响函数利用卷积公式进行动力微分方法计算，因此，从理论上讲，它适合于任意外力下的动力方程求解，同时，MATAL又提供了卷积函数conv，所以，在MATLAB下求解动力方程将变得十分容易。     

基于MLS形函数处理边界的电阻率法有限单元正演模拟

电阻率法有限单元正演模拟中,采用第三类边界条件时为保证精度仍要求较大范围的计算域.无单元法为地球物理领域的新兴正演模拟方法,其计算效率低,但其中采用的移动最小二乘（MLS）形函数相比于有限单元法形函数具有良好的连续性,模拟精度高.本文将MLS形函数应用于电阻率法有限单元2.5维正演的第三类边界条件处理,提出电阻率法有限单元-移动最小二乘（FEM-MLS）耦合正演方法.通过不同正演方法的模型算例模拟结果对比,验证了本文算法的有效性,并讨论了各个参数选择对模拟结果的影响.本文数值模拟结果表明采用第三类边界条件时,在同等计算精度前提下,FEM-MLS耦合法相比于有限单元法可进一步缩小计算域并提高了计算效率,相比于采用较大计算域满足边界条件的有限单元法计算效率提高了约一倍,相比于采用相同小范围计算域的有限单元法平均精度提高了约一倍.

     

Derivation and relative performance of strings of line elements for modeling (un)confined and semi-confined flow

In the analytic element method, strings of line-sinks may be used to model streams and strings of line-doublets may be used to model impermeable walls or boundaries of inhomogeneities. The resulting solutions are analytic, but the boundary conditions are met approximately. Equations for line elements may be derived in two ways: through integration of point elements (the integral solution) and through application of separation of variables in elliptical coordinates (the elliptical solution). Using both approaches, two sets of line elements are presented for four flow problems: line-sinks and line-doublets in (un)confined flow, and line-sinks and line-doublets in semi-confined flow. Elliptical line elements have the advantage that they do not need a far-field expansion for accurate evaluation far away from the element. The derivation of elliptical line elements is new and applicable to both (un)confined flow and semi-confined flow; only the resulting expressions for elliptical line elements for semi-confined flow have not been found in the current groundwater literature. Existing solutions for elliptical line elements for (un)confined flow were intended for the modeling of isolated features. Four examples are presented, one for each flow problem, to demonstrate that strings of elliptical line elements may be used to obtain accurate solutions; elliptical line-doublets for semi-confined flow can only be strung together in combination with two integral line-doublets. For a zigzag canal in (un)confined flow, a string of elliptical line-sinks performed better than a string of integral line-sinks of the same order when the smallest angle between two adjacent segments is less than 130°. Elliptical line-doublets performed better than integral line-doublets for a square inhomogeneity in a uniform, confined flow field; the difference was smaller for an octagonal inhomogeneity. For semi-confined flow, the difference between the integral and elliptical line-sinks was small when modeling a zigzag canal.     

Earthquake damage detection in the Imperial County Services Building III: Analysis of wave travel times via impulse response functions

The majority of structural health monitoring methods are based on detecting changes in the modal properties, which are global characteristics of the structure, and are not sensitive to local damage. Wave travel times between selected sections of a structure, on the other hand, are local characteristics, and are potentially more sensitive to local damage. In this paper, a structural health monitoring method based on changes in wave travel times is explored using strong motion data from the Imperial Valley Earthquake of 1979 recorded in the former Imperial County Services (ICS) Building, severely damaged by this earthquake. Wave travel times are measured from impulse response functions computed from the recorded horizontal seismic response in three time windows—before, during, and after the largest amplitude response, as determined from previous studies of this building, based on analysis of novelties in the recorded response. The results suggest initial spatial distribution of stiffness consistent with the design characteristics, and reduction of stiffness following the major damage consistent with the spatial distribution of the observed damage. The travel times were also used to estimate the fundamental fixed-base frequency of the structure f₁ (assuming the building deformed as a shear beam), and its changes during this earthquake. These estimates are consistent with previous estimates of the soil–structure system frequency, f_sys, during the earthquakes (f₁<f_sys as expected from soil–structure interaction studies), and with other estimates of frequency (f₁ from ETABS models, and f_sys from ambient vibration tests, and “instantaneous” f₁ from high-frequency pulse propagation).     

A local model for analysis of pump and treat systems with vertical barrier walls

We present a mathematical model of local, steady groundwater flow near a vertical barrier wall. Flow features represented in the model include an impermeable arc-shaped barrier wall and multiple wells; distant boundary conditions are not included explicitly, but their effects on the local flow field are modelled by specifying a uniform flow at infinity and a constant areal recharge within a local domain. We develop an explicit closed-form solution to the boundary-value problem using the analytic element method. The solution is an extension of a harmonic solution presented by Anderson and Mesa [Anderson EI, Mesa E. The effects of vertical barrier walls on the hydraulic control of contaminated groundwater. Adv Water Resourc 2006;29(1):89–98] which does not include the effects of recharge. We demonstrate that the general solution with recharge consists of the harmonic solution superposed on a special case of the harmonic solution along with two elementary one-dimensional flow solutions. The results are used to investigate the effects of areal recharge on the capture zone envelopes of the pumping wells and on the reduction in discharge that can be achieved by including a barrier wall in a pump and treat design. We find that the benefits of including an open barrier wall in a design, measured as a reduction in the pumping rate required to contain a plume, increase for higher recharge rates. Dimensionless plots of capture zone envelopes are presented for a practical well and barrier wall configuration.     

航空瞬变电磁法关断电流斜坡响应的计算

深入分析了现行地面瞬变电磁法中存在关断电流斜坡响应的计算方法,指出其中存在的不合理之处,提出用冲激响应积分算法计算存在关断电流斜坡时的瞬变电磁响应,并应用于航空瞬变电磁法一维正演计算中.计算结果表明,当存在关断电流斜坡时,早期的感应电动势明显大于没有断电后沿的值,且受影响的时间范围约为十倍的电流后沿宽度;当后沿宽度较大时,甚至晚期的感应电动势也出现了明显的误差.由此得出,在航空瞬变电磁法仪器设计和实测资料处理过程中,关断电流斜坡的响应不容忽视.     

基于精确权函数的仪器响应失真校正方法与算例

当前,经仪器响应失真校正后,加速度计有效观测频带通常为0至1倍自振频率,这限制了强震仪应用领域的延拓。有鉴于此,本文提出一个基于精确权函数的仪器响应失真校正方法并给出一个验证算例。分析表明:较之现有方法,新方法兼有高精度和高效率的优点,并可以将力平衡式加速度计的有效观测频带至少延拓3倍,对于拓展强震仪的应用领域具有重要意义。     

In-plane seismic response of inhomogeneous alluvial valleys with vertical gradients of velocities and constant Poisson ratio

The propagation of seismic P and SV waves within inhomogeneous alluvial valleys has been investigated using the indirect boundary element method (IBEM). An improvement on the formulae published early, for the 2D Green's functions in an inhomogeneous medium is presented in this work. A modification has been done over these functions in view of its connection to the ray theory. An accuracy analysis validates these modified Green's functions computing the relative error in frequency domain for the SH case, and with a quantitative analysis obtaining envelope and phase misfits of the solution in time domain, for the P–SV case.     

幂函数剪切模量成层土非线性地震反应的半解析算法

运用文献[1]所建议的动态应力一应变关系及其推广的Masing加卸载准则,考虑土料在地震等产生的不规则加载条件下的非线性滞回特征,将增量法与相应场地地震线性反应解析解[2]相结合,提出了该动力非线性方程的半解析时域算法,基于改进的一维剪切梁模型,对剪切模量是其深度的某一幂函数的成层非均质土层,建立了求解土体地震反应的非线性分析技术。针对文献[2]中的土层剖面,做了计算、分析和讨论。     

耦合有限单元法扩边的直流电阻率勘探无单元Galerkin法正演

本文分析了目前直流电阻率正演模拟中的无单元Galerkin法（EFGM）和有限单元法（FEM）的优缺点,针对采用第一类边界条件需要足够大的计算域时EFGM计算成本高的问题,在计算域外围区域采用FEM扩边,提出了直流电阻率的无单元Galerkin-有限单元耦合法（EFG-FE）.采用具有Kronecker delta函数性质的径向基点插值法（RPIM）构造EFGM形函数,在外围区域将EFGM与FEM直接耦合,无需其他处理手段,消除了传统EFGM与FEM耦合中存在的界面耦合困难.EFG-FE将模型计算域分割为EFGM区域和FEM区域,模型核心区域采用EFGM计算,发挥EFGM灵活性、适应性强和高精度的优点,使得模型建立简单方便,对任意复杂地电模型适应性强,同时获得高精度模拟结果.在模型计算域外围采用快速扩展的FEM单元网格进行剖分,利用其数值稳定性和高效性,使用少量FEM节点和单元网格将计算域大范围扩大满足第一类边界条件,同时不大幅增加计算成本,进而提高计算效率.最后,通过不同正演方法的模型算例的模拟结果对比,验证了本文提出的EFG-FE有效可行,其模拟结果具有很高的模拟精度,且相比于采用第三类边界条件的EFGM提高了计算效率,具有更好的模拟性能.

     

伪随机编码源电磁响应的精细辨识

与传统阶跃源激励方式相比, 采用m序列伪随机编码对发射源波形进行编码, 提高了电磁探测的深度和分辨能力.然而受这种编码源发射波形自相关旁瓣效应的影响, 使得对大地冲激响应的精细辨识效果受到一定限制.为了解决这一问题, 在以往相关辨识方法研究的基础上, 进一步考虑发射自相关旁瓣的影响, 首先提出一种由收发互相关中高精度提取大地冲激响应的数学方法; 然后通过数值模拟给出了m序列编码源大地冲激响应的精细辨识结果; 同时对以m序列为发射波形的勘探系统相关参数选择进行了分析; 最后利用本文提出的方法对野外实测数据进行辨识处理, 通过与其他EM方法结果进行对比, 证明了本文提出方法的可靠性.     

高层框支剪力墙结构地震反应分析的超元法

西方嵩层框支剪力墙结构地震反应分析的超元法，即将每榀框支墙中的上部剪力墙和落地墙简化处理成等效柱，底部框架简化处理成等代柱，楼板视为深梁，将连续分布的质量离散化，建立二维空间结构的“串并联质点系”振动模型，采用超级单元进行结构的地震作用计算及内力分析，此法计算简便并适用于结构竖向变刚度和考虑楼板变形等情况。     

基于混合阶矢量基函数的海洋可控源电磁三维谱元法数值模拟

在前人工作的基础上,本文推导了电导率任意各向异性介质的海洋可控源电磁三维谱元法正演方程.采用一次场/二次场分离算法结合混合阶矢量基函数,可以有效避免源点的奇异性的影响,从而提高数值解的精度.采用任意六面体单元离散研究区域,有利于模拟复杂地形和地电结构.利用不完全LU分解的Induced Dimension Reduction（IDR（s））迭代算法求解线性方程组,有效地提高了求解的效率.设计典型的地电模型进行正演计算,并将计算结果与有限元解进行对比,对比结果表明本文提出的基于混合阶矢量基函数的海洋可控源电磁三维谱元数值模拟算法是正确的、有效的.本文算法具有良好的通用性,可推广用于电导率呈任意各向异性的陆地电磁、井中电磁等数值模拟研究.

     

液化场地地震反应的离散元数值模拟

采用一个描述饱和砂土中固相颗粒和液相流体耦合的离散元数值模型,固相颗粒采用离散元的颗粒流理论(PFC)模拟,液相流体通过求解平均Navier-Stokes方程的计算流体动力学(CFD)技术计算.采用上述模犁数值模拟了在地震荷载作用液化场地的加速度、剪应力、剪应变时程以及超孔隙水压力和土层表面位移的发展过程.数值模拟结果...     

A quadratic element method for evaluating groundwater flow by the inversion of surface tilt with application to the Tono Area, Japan

By modifying a previous method with constant elements, we developed a quadratic element method for more accurately estimating groundwater flow by the inversion of tilt data. In this method: (1) a region of groundwater flow is divided into quadratic elements in which the change in groundwater volume per unit volume of rock (Δv) and the Skempton coefficient (B) vary in a quadratic manner with the coordinates, (2) the values of Δv are set to zero at the boundaries of the region of groundwater flow and (3) the sum of the squared second derivatives of Δv is adopted as a constraining condition that is weighted and added to the sum of the squared errors in tilt. First, analyses were performed for a flow model to determine the accuracy of this method for estimating groundwater flow and also to clarify the effect of the assumed size of a region of groundwater flow. These analyses showed that the quadratic element method proposed in this study gives a much better estimation of Δv than the constant element method and that a large region of groundwater flow should be assumed, rather than a small region, since the values of Δv at points outside of the actual region of groundwater flow are estimated to be nearly zero when a large region is assumed while these values are greatly overestimated when an excessively small region is assumed. Finally, the quadratic element method was applied to the site of the Mizunami Underground Research Laboratory in the Tono area, Japan. Inverse analyses were performed for tilt data measured by four tiltmeters with a resolution of 10⁻⁹ radians during the excavation of two shafts under the assumption that the rock mass is an isotropic and homogeneous half- space. The results showed that the method proposed in this study reproduced the tilt data very accurately. Thus, the distribution of Δv was estimated without sacrificing the reproducibility of the tilt data. The contour maps of B(1 + ν)Δv (ν: Poisson’s ratio) showed that the heterogeneous flow of groundwater occurred at the site and that groundwater volume decreased mainly in the area surrounded by two faults. The latter result is consistent with the finding obtained by previous investigations that these faults have low permeability in the direction perpendicular to the strike and may act as a flow barrier.     

Uncertainty analysis of strain modal parameters by Bayesian method using frequency response function

Structural strain modes are able to detect changes in local structural performance, but errors are inevitably intermixed in the measured data. In this paper, strain modal parameters are considered as random variables, and their uncertainty is analyzed by a Bayesian method based on the structural frequency response function (FRF). The estimates of strain modal parameters with maximal posterior probability are determined. Several independent measurements of the FRF of a four-story reinforced concrete frame structural model were performed in the laboratory. The ability to identify the stiffness change in a concrete column using the strain mode was verified. It is shown that the uncertainty of the natural frequency is very small. Compared with the displacement mode shape, the variations of strain mode shapes at each point are quite different. The damping ratios are more affected by the types of test systems. Except for the case where a high order strain mode does not identify local damage, the first order strain mode can provide an exact indication of the damage location.     

Implementation of configuration dependent stiffness proportional damping for the dynamics of rigid multi-block systems

The distinct element method (DEM) has been used successfully for the dynamic analysis of rigid block systems. One of many difficulties associated with DEM is modeling of damping. In this paper, new procedures are proposed for the damping modeling and its numerical implementation in distinct element analysis of rigid multi-block systems. The stiffness proportional damping is constructed for the prescribed damping ratio, based on the non-zero fundamental frequency effective during the time interval while the boundary conditions remain essentially constant. At this time interval, the fundamental frequency can be estimated without complete eigenvalue analysis. The damping coefficients will vary while the damping ratio remains the same throughout the entire analysis. A new numerical procedure is developed to prevent unnecessary energy loss that can occur during the separation phases. These procedures were implemented in the development of the distinct element method for the dynamic analyses of piled multi-block systems. The analysis results for the single-block and two-block systems were in a good agreement with the analytic predictions. Applications to the seismic analyses of piled fourblock systems revealed that the new procedures can make a significant difference and may lead to much-improved results.