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1.
利用峰值频率移动法估算零偏VSP资料的品质因子Q．该方法用Ricker子波和匹配地震子波分别逼近零相位和混合相位的震源子波，得到了峰值频率移动法估计Q值的公式．进而针对常规方法估计的地震子波峰值频率精度不高的问题，提出了估计地震子波峰值频率的特征结构法．通过合成零偏VSP资料的仿真试验，验证了峰值频率移动法估计Q值的正确性．仿真结果表明，与快速Fourier变换和Burg最大熵方法相比较，特征结构法得到的峰值频率和Q值精度高一些．仿真结果也表明，用峰值频率移动法估计Q值时需要选取恰当的子波参数,否则影响Q值估计的精度． 相似文献
2.
近年来,随着我国对矿产资源需求量的逐年增加,人工源大地电磁法作为地球电性探测的感应类电法得到了迅猛发展,自主研制的地球物理仪器陆续推出.电磁法仪器的出现逐步摆脱了对国外相关仪器的依赖,可控源音频大地电磁法(CSAMT)发射机作为该方法的电偶极子源具有举足轻重的作用,然而,当前野外施工所用的CSAMT发射机普遍存在高频电流发射受阻,发射功率较小,收发距的大小受到很大的限制;到低频区,较小的收发距,使得接收机进入近场区,待后续数据处理时需要做近场校正,不仅增加了一个校正工序,也带来了校正数据误差,同时,较小的收发距,限制了野外施工的效率.另外,随着CSAMT的研究逐步深入,CSAMT三维阵列式探测也逐渐引起学者的重视,实现高频大电流的发射,为CSAMT三维阵列式探测提供了有利支持.本文详细分析了实现高频大功率发射技术的方法,通过模型建立和方程求解,获得了影响发射电流的主要因素,进而,有望达到高频发射电流大于低频发射电流的目的. 相似文献
3.
预应力组合网架结构的自振特性分析及现场测试 总被引：6，自引：2，他引：4
本文采用子空间迭代法计算了预应力组合网架的自振频率与振型,分析总结了结构的自振特性及其影响因素,诸如索预应力、跨高比、边界条件、板和梁肋偏心、附加质量以及节点刚度的影响,并回归得出预应力组合网架基本周期简化计算公式。同时使用“脉动法”对天津一中新建活动中心楼层预应力组合网架进行了现场动力测试,结果表明预应力组合网架动力计算所采用的计算模型合理、计算分析结果可信,最后使用自由振动法及半功率点法得出了预应力组合网架结构的阻尼比,此值可作为钢与混凝土预应力组合网架结构的参考阻尼比。 相似文献
4.
The continuous connection method of analysis is extended to deal with the free vibrations of a coupled shear wall structure. The natural modes and frequencies are determined from the Galerkin technique, and the dynamic response following an imposed lateral displacement is evaluated. A comparison is made between theoretical predictions of natural frequencies and the results from tests on model structures. 相似文献
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A method for the dynamic analysis of staggered wall-beam frames is developed using consistent mass terms which are derived and given in simple terms. The method uses effective stiffnesses for wall-beam elements developed in an earlier paper. Experiments using a nine storey 1 : 15 scale perspex model are described. The first three natural frequencies of the model were obtained using two methods: sinusoidal external excitation of the structure with the base fixed and white noise excitation employing a single degree-of-freedom shake table, in the latter method with and without the addition of mass throughout the model. Agreement between analytical predictions of the first three natural frequencies and mode shapes and experimentally determined values is considered satisfactory, particularly for the first two modes. The lumped mass assumption gave reasonable results for these two cases, whereas the consistent mass theory gave reasonable results for the first three natural frequencies. 相似文献
8.
Broadband strong motion simulation in layered half-space using stochastic Green’s function technique
Y. Hisada 《Journal of Seismology》2008,12(2):265-279
The stochastic Green’s function method, which simulates one component of the far-field S-waves from an extended fault plane
at high frequencies (Kamae et al., J Struct Constr Eng Trans AIJ, 430:1–9, 1991), is extended to simulate the three components of the full waveform in layered half-spaces for broadband frequency
range. The method firstly computes ground motions from small earthquakes, which correspond to the ruptures of sub-faults on
a fault plane of a large earthquake, and secondly constructs the strong motions of the large earthquake by superposing the
small ground motions using the empirical Green’s function technique (e.g., Irikura, Proc 7th Japan Earthq Eng Symp, 151–156, 1986). The broadband stochastic omega-square model is proposed as the moment rate functions of the small earthquakes,
in which random and zero phases are used at higher and lower frequencies, respectively. The zero phases are introduced to
simulate a smooth ramp function of the moment function with the duration of 1/fc s (fc: the corner frequency) and to reproduce
coherent strong motions at low frequencies (i.e., the directivity pulse). As for the radiation coefficients, the theoretical
values of double couple sources for lower frequencies and the theoretical isotropic values for the P-, SV-, and SH-waves (Onishi
and Horike, J Struct Constr Eng Trans AIJ, 586:37–44, 2004) for high frequencies are used. The proposed method uses the theoretical Green’s functions of layered half-spaces
instead of the far-field S-waves, which reproduce the complete waves including the direct and reflected P- and S-waves and
surface waves at broadband frequencies. Finally, the proposed method is applied to the 1994 Northridge earthquake, and results
show excellent agreement with the observation records at broadband frequencies. At the same time, the method still needs improvements
especially because it underestimates the high-frequency vertical components in the near fault range. Nonetheless, the method
will be useful for modeling high frequency contributions in the hybrid methods, which use stochastic and deterministic methods
for high and low frequencies, respectively (e.g., the stochastic Green’s function method + finite difference methods; Kamae
et al., Bull Seism Soc Am, 88:357–367, 1998; Pitarka et al., Bull Seism Soc Am 90:566–586, 2000), because it reproduces the full waveforms in layered media including not only random characteristics at
higher frequencies but also theoretical and deterministic coherencies at lower frequencies. 相似文献
9.
Both approximate and exact formulations for the interaction of an incident elastic wave with a cased borehole are presented. In the approximate method, simple and explicit formulae are derived for the pressure in fluid at low frequencies. In the exact method, elastic potentials in each annulus are represented as a superposition of fundamental solutions to the Helmholtz equations. Continuity of displacements and stresses across layer boundaries are used to determine unknown coefficients. A global matrix algorithm is employed to compute simultaneously these coefficients in individual layers. Calculations show that, in cased boreholes, the borehole effects on downhole seismic measurements are more significant than in open boreholes. A strong resonance occurs in the fluid for SV-wave incidence from a soft formation. This resonance is prominent even at very high frequencies because the tube-wave velocity is raised well above the formation shear velocity by the steel pipe. At a particular angle of incidence of a plane P-wave, the pressure in the fluid is near zero at low frequencies (the cased borehole screening phenomenon). For hard formations and frequencies above 1 kHz, the cased borehole influence on a downhole geophone measurement is significant, especially at grazing incidence. For soft formations, both the pressure in the fluid and the solid displacement on the borehole wall show strong dependence on frequency and angle of incidence, even at low frequencies. 相似文献
10.
<正>This paper focuses on developing an online structural condition assessment technique using long-term monitoring data measured by a structural health monitoring system.The seasonal correlations of frequency-temperature and beam-end displacement-temperature for the Runyang Suspension Bridge are performed,first.Then,a statistical modeling technique using a six-order polynomial is further applied to formulate the correlations of frequency-temperature and displacement-temperature,from which abnormal changes of measured frequencies and displacements are detected using the mean value control chart.Analysis results show that modal frequencies of higher vibration modes and displacements have remarkable seasonal correlations with the environmental temperature and the proposed method exhibits a good capability for detecting the micro damage-induced changes of modal frequencies and displacements.The results demonstrate that the proposed method can effectively eliminate temperature complications from frequency and displacement time series and is well suited for online condition monitoring of long-span suspension bridges. 相似文献
11.
New approximate formulas are proposed to determine the natural frequencies of structures considering the effects of panel zone flexibility and soil-structure interaction. Several structures with various earthquake resisting systems are idealized as prismatic cantilever flexural-shear beams. Floor masses are considered as lumped masses at each story level and masses of columns are evenly distributed along the cantilever beam. Soil-structure interaction is considered as axial and rotational springs, whose potential energy are formulated and incorporated into overall potential energy of the structure. Subsequently, natural frequency equations are derived on the basis of energy conservation principle. The effect of axial forces on natural frequency is also considered in the proposed formulas. Using the method presented in this study, natural frequencies are computed using a simplified method with no complex numerical modeling. The proposed formulas are verified via experimental and numerical methods. Close agreement between the results from these three approaches are observed. Furthermore, the effects of panel zone flexibility, continuity plates and doubler plates on the natural frequencies of buildings are investigated. 相似文献
12.
A method of adaptation of the ionospheric channel parameters to the current conditions is proposed. The observational data on the temporal behavior of signals of radio stations with known location and set of working frequencies (the so-called reference stations) are used as information describing the current state of the channel. A facility that makes possible to determine by simple means the moments of radio rises and declines on the reference paths has been created. A technique that relates for these instants working frequencies of the stations with maximum usable frequencies of the paths has been developed. By means of an approximate solution of the inverse problem using the method of characteristics for the calculation of radiowave propagation and the monthly average semiempirical model of the ionosphere, the height profile of electron density in a certain spatial region is refined. The efficiency of the proposed method of adaptation of the ionospheric channel parameters using the data of vertical sounding is assessed and the expediency of practical application of such adaptation is shown. 相似文献
13.
Dong Yinfeng Li Yingmin Xiao Mingkui Lai Ming 《Soil Dynamics and Earthquake Engineering》2008,28(1):7-19
Some limitations of the Hilbert–Huang transform (HHT) for nonlinear and nonstationary signal processing are remarked. As an enhancement to the HHT, a time varying vector autoregressive moving average (VARMA) model based method is proposed to calculate the instantaneous frequencies of the intrinsic mode functions (IMFs) obtained from the empirical mode decomposition (EMD) of a signal. By representing the IMFs as time varying VARMA model and using the Kalman filter to estimate the time varying model parameters, the instantaneous frequencies are calculated according to the time varying parameters, then the instantaneous frequencies and the envelopes derived from the cubic spline interpolation of the maxima of IMFs are used to yield the Hilbert spectrum. The analysis of the length of day dataset and the ground motion record El Centro (1940, N–S) shows that the proposed method offers advantages in frequency resolution, and produces more physically meaningful and readable Hilbert spectrum than the original HHT method, short-time Fourier transform (STFT) and wavelet transform (WT). The analysis of the seismic response of a building during the 1994 Northridge earthquake shows that the proposed method is a powerful tool for structural damage detection, which is expected as the promising area for future research. 相似文献
14.
传统CSEM一般只提取主频信号,或以谐波与主频的振幅比为依据提取部分低阶谐波信号,但缺乏判断标准,实际操作中存在很大的不确定性.本文基于小波变换和希尔伯特解析包络提出一种新的CSEM信号噪声评价方法,首先在时间域中基于混合基快速傅里叶变换获得原始信号准确功率谱;其次在频率域中根据CSEM频率位置相邻频率幅值进行频谱预处理,基于离散小波变换将预处理后的频谱分成低频部分和高频部分,基于希尔伯特变换识别高频部分的上包络线,并与低频部分重构得到频谱的整体上包络线;最后根据包络线与对应CSEM频率振幅的比值估计噪声的影响幅度,根据阈值筛选出高信噪比的主频和谐波信号.本方法不需增加野外工作量即可提取大量的频率信号,特别是高阶谐波信号,实现频率加密,提高CSEM的纵向分辨能力和能源利用率. 相似文献
15.
A filter method is presented which allows a qualitative and quantitative identification of wave modes observed with plasma experiments on satellites. Hitherto existing mode filters are based on the MHD theory and thus they are restricted to low frequencies well below the ion cyclotron frequency. The present method is generalized to cover wave modes up to the characteristic ion frequencies. The spectral density matrix determined by the observations is decomposed using the eigenvectors of the linearized Hall-MHD equations. As the wave modes are dispersive in this formalism, a precise determination of the κ-vectors requires the use of multi-point measurements. Therefore the method is particularly relevant to multi-satellite missions. The method is tested using simulated plasma data. The Hall-MHD filter is able to identify the modes excited in the model plasma and to assign the correct energetic contributions. By comparison with the former method it is shown that the simple MHD filter leads to large errors if the frequency is not well below the ion cyclotron frequency. Further the range of validity of the linear theory is examined rising the simulated wave amplitudes. 相似文献
16.
A general method is outlined for the determination of natural frequencies of cylindrical shells with any boundary conditions when the effects of rotatory inertia and transverse shear deformation are included in the analysis. This is applied to cylindrical shells with both ends clamped. It is shown that the inclusion of these effects tends to have a greater effect upon frequencies of cylindrical shells with clamped ends than it does for corresponding shells with simply supported ends, for which numerical results are available. The authors suggest an empirical relation, which together with the latter results enables rapid estimates to be made of the effects of rotatory inertia and shear deformation on the frequencies of a wide range of cylindrical shells with clamped ends. An assessment of the accuracy of the theory with these effects included is made by comparing frequencies with values from a three-dimensional elasticity theory, but this comparison has to be restricted to cylindrical shells with simply supported ends. 相似文献
17.
In this paper, the role that the site dynamic characteristics play in soil–structure interaction is studied on a simple model in which the site is represented as a soil layer over bedrock (half-space), and using the indirect boundary-element method (IBEM). For the purpose of comparison with published analytical solutions, the structure is represented as a shear wall supported by a semi-circular rigid foundation, subjected to incident plane SH waves. The accuracy of the method is verified, numerical results are analyzed, and the model response is compared with earthquake observations at the Hollywood Storage Building. It is shown that the effects of dynamic soil–structure interaction may become more significant near the characteristic frequencies of the site, and that the resonance of the system shifts to lower frequencies. The thickness of the soil layer, the stiffness of the bedrock, and the mass and the stiffness of the superstructure all influence the values of the system frequencies and system amplitudes. 相似文献
18.
This study developed a finite element method with the effect of soil–fluid–structure interaction to calculate bridge natural frequencies. The finite element model includes bridge girders, piers, foundations, soil, and water. The effective mass above the soil surface was then used to find the first natural frequency in each direction. A field experiment was performed to validate that the natural frequencies calculated using the proposed finite element method had acceptable accuracy. The calculated natural frequencies with the fluid–structure interaction effect are always smaller than those without this effect. However, the frequency change due to the fluid effect is not obvious, so using the soil–structure interaction model is accurate enough in the bridge natural frequency analysis. The trend of the frequency decreases with the increase of the scour depth, but the curve is not smooth because of non-uniform foundation sections and layered soils. However, when the scour depth is such that pile cap is exposed, the changes in natural frequency with the scour depth are more obvious, and this is useful for measurement of the depth using bridge natural frequencies. 相似文献
19.
Comparison of Frequency-Selection Strategies for 2D Frequency-Domain Acoustic Waveform Inversion 总被引：1，自引：0，他引：1
Youngseo Kim Hobum Cho Dong-Joo Min Changsoo Shin 《Pure and Applied Geophysics》2011,168(10):1715-1727
Several frequency-selection strategies have been used to obtain global minimum solutions in waveform inversion. One strategy,
called the discretization method, is to discretize frequencies with a large sampling interval to minimize redundancy in wavenumber
information. Another method, the grouping method, groups frequencies with redundancy in wavenumber information. The grouping
method can be carried out in two ways. With the first method, the minimum frequency is fixed and the maximum frequency is
gradually extended upward (i.e., the overlap-grouping method). Under the second method, frequencies are not overlapped across
the groups and waveform inversion proceeds from lower to higher frequency groups (i.e., the individual-grouping method). In
this study, we compare these three frequency-selection strategies using both synthetic and real data examples based on logarithmic
waveform inversion. Numerical examples for synthetic and real field data demonstrate that the three frequency-selection methods
provide solutions closer to the global minimum compared to solutions resulting from simultaneously performed waveform inversion,
and that the individual-grouping method yields slightly better resolution for the velocity models than the other methods,
particularly for the deeper part. These results may imply that using either too small or too large data sets at every stage
slightly deteriorates inversion results, and that grouping data in appropriately sized aggregations improves inversion results. 相似文献
20.
The response of rectangular rigid footings resting on an elastic soil of shear modulus decreasing monotonically with depth is studied. Such profiles are typically encountered after ground improvement. The propagation characteristics of SV/P surface waves are investigated, showing the appearance of cut-off frequencies above which surface waves do not exist. The semi-analytical method of the subdivision of the footing/soil contact area is then used for solving the boundary value problem, whereby the influence functions for the sub-regions are determined by integration of the corresponding surface-to-surface Green׳s functions. Impedance functions are presented over a wide range of frequencies for typical values of the non-homogeneity parameters, the Poisson׳s ratio and the foundation geometry. The salient features that are associated with the non-homogeneity and the appearance of cut-off frequencies are elucidated. 相似文献