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1.
— In this paper, the site characteristics of the Dahan downhole array are studied by analyzing the September 21, 1999 M 7.3 Chi-Chi earthquake sequence including the main shock and some aftershocks. The four-level array (0 m, 50 m, 100 m and 200 m) is located to the north of Hualien City in eastern Taiwan. Polarization analysis is used to check the orientation errors of the seismometers at different levels of depth. If the surface instrument is chosen as reference, the angle between the major polarization axes of the surface and any downhole records is the orientation error that must be corrected for the downhole accelerographs. The orientation errors at depths of 50 m, 100 m and 200 m are 32°, 120° and –84°. After the corrections, the coherency between the surface and downhole records is substantially improved. Spectral ratio analysis shows that the predominant frequency of the Chi-Chi main shock shifts to a lower frequency. We also simulate ground motions at different depths by using the Haskell method with a linear velocity structure model. The record at surface is chosen as the input motion. Compared with the observed data, ground acceleration can be well reproduced for the aftershocks (weak-motion events) of the September 21, 1999 M 7.3 Chi-Chi earthquake. However, for the Chi-Chi main shock, the synthetic waveform cannot match well with the observation neither in amplitude nor in phase. This indicates that large ground shaking probably induced the nonlinear site effect at that time, and the model used cannot support it.Acknowledgement. The authors would like to express their thanks to Dr. L.F. Bonilla and one anonymous reviewer for their valuable suggestions. This research was supported by the National Science Council under grant number NSC 89-2921-M-194-007. The Institute of Earth Sciences, Academia Sinica supplied the strong-motion data. The support of these organizations is gratefully acknowledged. 相似文献
2.
This paper shows that the polarization analysis of later arrivals could be used to estimate the orientation error and the tilt of the downhole accelerometers in the Dahan downhole strong-motion array. This four-level array is located to the north of Hualien city in eastern Taiwan. Polarization analysis of a windowed signal which includes significant phases (e.g. primary S wave) can give a stable major polarization axis. This axis can then be used as the reference direction for correcting the orientation error and the tilt of the downhole seismometers. If the surface instrument is chosen as the reference, the angle between the major polarization axes of the surface and any downhole recordings will be the orientation error needed to be corrected for the downhole accelerograph. This angle is a combination of the tilt and the orientation errors which correspond to the dip and strike of maximum polarization, respectively. At the Dahan downhole strong-motion array, the orientation errors of the seismometers at the depth of 50, 100 and 200 m were 1°, 90° and 84°. These orientation errors were consistent with the horizontal particle-motion analysis. The errors of horizontal plane (tilt) were almost zero. After correction, the coherency of the waveforms between the surface and the downhole recordings was significantly improved. 相似文献
3.
Four-level Dahan downhole recordings are used to investigate site amplification due to the near-surface structure. The downhole array is installed at the depths of 0 m (surface), 50, 100 and 200 m. Eight sets of data presenting well-separated incident and reflected waves (from the free surface) are selected to analyse the site amplification. Five techniques of spectral ratios have been tested to determine the most reliable way to estimate site response. The superior one is the ratio of Root-Mean-Square (RMS) Fourier amplitude spectra, because it has the minimum number of pseudopeaks. However, the pseudopeaks can be further reduced by omitting those ratios with small denominators. Results show that the amplification factors are nearly uniform at the depths of 50 and 100 m, where their values fall between one and two. This factor is frequency-dependent at the surface and in the range from two to five under 6 Hz. These results agree well with those from time-domain analyses and the transmission-coefficient estimation except that the latter overestimates the amplification factor of the surface station. This suggests that the shallow velocity structure provided by the geotechnical survey might be underestimated. Numerical modelling also supports the underestimation of velocity at near surface since the synthetic SH waves based on a modified velocity model can match these observations well. 相似文献
4.
H. C. HARDEE 《Geophysical Prospecting》1983,31(1):57-71
Downhole periodic seismic sources have been proposed as a means for studying fluid zones in hydrothermal/magma systems encountered in deep scientific holes drilled as part of the Continental Scientific Drilling Program. Scientific measurements in these holes will require the use of downhole, nondestructive, high resolution, seismic tools. Downhole periodic seismic sources tested in a shallow geothermal zone showed that a pneumatic oscillator could effectively generate downhole low-frequency (10–100 Hz) seismic waves and that these waves could effectively penetrate a highly fractured formation. An improved version of these downhole sources, capable of swept-frequency operation, is currently under development. This type of downhole seismic source has applications in crustal studies and in the evaluation of petroleum, groundwater and geothermal reservoirs. 相似文献
5.
We present a novel denoising scheme via Radon transform-based adaptive vector directional median filters named adaptive directional vector median filter (AD-VMF) to suppress noise for microseismic downhole dataset. AD-VMF contains three major steps for microseismic downhole data processing: (i) applying Radon transform on the microseismic data to obtain the parameters of the waves, (ii) performing S-transform to determine the parameters for filters, and (iii) applying the parameters for vector median filter (VMF) to denoise the data. The steps (i) and (ii) can realize the automatic direction detection. The proposed algorithm is tested with synthetic and field datasets that were recorded with a vertical array of receivers. The P-wave and S-wave direct arrivals are properly denoised for poor signal-to-noise ratio (SNR) records. In the simulation case, we also evaluate the performance with mean square error (MSE) in terms of signal-to-noise ratio (SNR). The result shows that the distortion of the proposed method is very low; the SNR is even less than 0 dB. 相似文献
6.
考虑场地土层不同物理参数的综合反演,改进了水平与竖向谱比(Horizontal-to-Vertical Spectral Ratio,简称HVSR)混合全局优化反演方法,进一步通过土层的S、P波波速、厚度、泊松比、密度和S、P波品质因子等土层参数反演的敏感性分析,形成了可同时反演场地土层厚度及剪切波速的混合全局优化反演方法.以美国GVDA和日本IWTH27竖向强震动观测台阵为例,分别以理论HVSR及加速度观测记录获得的HVSR曲线为目标,反演获得了场地浅层速度结构,并与观测台阵场地钻孔揭示的土层模型进行比较,验证了发展的反演方法的合理性和适用性.本文研究表明,基于地震加速度记录的HVSR全局优化反演方法是获取场地土层速度结构的一种有效的途径. 相似文献
7.
Downhole arrays are deployed to measure motions at the ground surface and within the soil profile, with some arrays instrumented to also record the pore pressure response within soft soil profiles during excitation. The measurements from these arrays have typically been used in conjunction with parametric and nonparametric inverse analysis approaches to identify soil constitutive model parameters for use in site response analysis or to identify averaged soil behavior between locations of measurement. The self-learning simulations (SelfSim) inverse analysis framework, previously developed and applied under total stress conditions, is extended to effective stress considerations and is employed to reproduce the measured motions and pore pressures from downhole arrays while extracting the underlying soil behavior and pore pressure response of individual soil layers. SelfSim is applied to the 1987 recordings from the Imperial Valley Wildlife Liquefaction Array. The extracted soil behavior suggests a new functional form for modeling the degradation of the shear modulus with respect to excess pore pressures. The extracted pore pressure response is dependent on the number and amplitude of shear strain cycles and has a functional form similar to current strain-based pore pressure generation models. 相似文献
8.
The propagation characteristics of blast-induced shock waves in a jointed rock mass have been monitored and studied. Accelerometers were set up on a rock surface along three lines, at 0°, 45° and 90° with respect to the orientation of the predominant joint strikes. Cylindrical charges were detonated in a charge hole, and ground accelerations in both vertical and radial directions at various points on the rock surface were recorded. Results show that rock joints have significant effects on the propagation characteristics of blast-induced shock waves. The amplitude and principal frequency of shock waves attenuate with the increase of distance from the charge centre, and the increase of incident angle between the joint strike and the wave propagation path. The measured data were compared with the empirical equations of shock wave attenuation proposed by other authors. The mechanism of rock joint effect, the attenuation of shock waves in relation to the propagation distance, the charge weight and the incident angle, are discussed in this paper. 相似文献
9.
The principal stress axes and eigenvalue ratios of the stress tensors from two active seismic regions in Alaska (eastern Aleutian Arc and eastern Gulf of Alaska) are computed. These results are obtained using focal mechanisms of earthquakes with magnitudes ranging from 5.5 to 8.2 and show for both regions slightly different compressional states with an orientation of the pressure axis of N26°W for the eastern Gulf of Alaska and N27°W for the east Aleutian Arc. The eigenvalue ratios for the eastern Gulf of Alaska and east Aleutian Arc are 0.84 and 0.99, respectively. The algorithm used in computations allows us to detect some focal mechanisms which are incoherent with respect to the obtained stress tensors. These incoherences are explained in terms of some tectonic features of the region. The orientations of possible pre-existing fractures which would need an unrealistically high maximum shear stress to start a slip on their fault planes are also investigated. These critical orientations depend on a constitutive frictional law as well as on eigenvalue ratios and eigenvectors of the stress tensors. The orientation, which is given in our case by the angle between the normal vector to a fracture and the regional tensional axis, is approximately 59° for the eastern Aleutian Arc. This angle ranges from 49° to 59° for the eastern Gulf of Alaska. It must be pointed out that fractures which need unrealistically high shear stress to start slip on their fault planes are defined by a very narrow band of possible angles between tensional direction and the normal vector to fault plane. 相似文献
10.
Records of the 1999 Chi–Chi earthquake, provided by the Dahan downhole array were analyzed, and nonlinear hysteretic stress–strain relations in the soil layers were estimated by the method previously used for studying the response of soils during the 1995 Kobe and the 2000 Tottori earthquakes. The obtained models of the soil behavior were applied for evaluating changes of the shear moduli in the soil layers and for the nonlinear identification of the soil behavior at Dahan site during the Chi–Chi earthquake. We found that reduction of the shear moduli in the soil layers did not exceed 5%, and the soil response was virtually linear. The content of the nonlinear components in the soil response was about 5% of the intensity of the response, and it was mostly due to the odd-order nonlinearities. A similarity in the stress–strain relations describing the behavior of soils during the 1995 Kobe earthquake, the 2000 Tottori earthquake, and the 1999 Chi–Chi earthquake was found, which indicates the possibility to describe the behavior of similar types of soils at different sites by similar stress–strain relations and predict soil behavior in future earthquakes. 相似文献
11.
J. A. Hunter B. Benjumea J. B. Harris R. D. Miller S. E. Pullan R. A. Burns R. L. Good 《Soil Dynamics and Earthquake Engineering》2002,22(9-12):931-941
Shear wave velocity–depth information is required for predicting the ground motion response to earthquakes in areas where significant soil cover exists over firm bedrock. Rather than estimating this critical parameter, it can be reliably measured using a suite of surface (non-invasive) and downhole (invasive) seismic methods. Shear wave velocities from surface measurements can be obtained using SH refraction techniques. Array lengths as large as 1000 m and depth of penetration to 250 m have been achieved in some areas. High resolution shear wave reflection techniques utilizing the common midpoint method can delineate the overburden-bedrock surface as well as reflecting boundaries within the overburden. Reflection data can also be used to obtain direct estimates of fundamental site periods from shear wave reflections without the requirement of measuring average shear wave velocity and total thickness of unconsolidated overburden above the bedrock surface. Accurate measurements of vertical shear wave velocities can be obtained using a seismic cone penetrometer in soft sediments, or with a well-locked geophone array in a borehole. Examples from thick soil sites in Canada demonstrate the type of shear wave velocity information that can be obtained with these geophysical techniques, and show how these data can be used to provide a first look at predicted ground motion response for thick soil sites. 相似文献
12.
利用SH地震波（偏振化方向垂直入射面的横波）在地层界面反射系数的附加相角导出了SH波Goos-Hänchen效应所引起的横向偏移和横向偏移渡越时间,给出了Goos-Hänchen效应正常时差公式,讨论了Goos-Hänchen效应对反射SH波正常时差的影响,绘出了横向偏移、横向偏移渡越时间、Goos-Hänchen效应正常时差及Goos-Hänchen效应正常时差校正量曲线.数值算例表明：对掠入射波或入射角在临界角附近的入射波,SH反射波的横向偏移、横向偏移渡越时间非常大,Goos-Hänchen效应对正常时差会产生较大的测量误差,在其他角度的入射波,横向偏移（横向偏移渡越时间）与波长（周期）为同一个数量级.横向偏移效应对SH反射波的传播走时影响是不可忽略的,因此在实际的地震资料处理中应进行横向偏移效应误差校正. 相似文献
13.
本文利用远震P和SH波反演得到2008年5月12日汶川大地震(MW=7.9)的一系列有限破裂模型。使用一种基于小波变换的模拟退火非线性反演方法, 我们将主断层划分成若干个子断层, 在反演时同时确定每个子断层上的滑移量、 滑动角、 上升时间(rise time)以及平均破裂速度。我们首先根据一个假定的破裂模型生成理论地震图, 将该理论地震数据作为输入进行反演, 对该有限破裂反演方法进行了一系列测试, 以验证反演对断层倾角、 平均破裂速度、 最大破裂深度等参数的敏感性。然后我们采用4个不同倾角的断层面来对汶川地震远震体波记录进行反演。结果表明, 若对只在一个断层面上模拟该地震, 30°倾角是个较为合适的值。反演的结果还表明, 此次地震有两个主要的能量释放区域, 并且主断层面存在倾角变化的可能性。在将来的研究中, 可以结合GPS, InSAR测地学以及强震等数据, 来对强震的破裂过程做更细致的研究。 相似文献
14.
In this paper, a global inversion method is developed for seismic moment tensor inversion by using the body wave forms. The
algorithm depends on neither the selection of starting model nor the forms of objective function and constraints. When the
error function, measure of the difference between the observed and synthetic waveforms, is chosen as the objective function,
the best fitting source model is found; when a certain combination in seismic moment tensor elements is selected as the objective
function and the values of error function are constrained in a suitable bound, the extreme source models can be produced by
minimizing or maximizing this combination. By changing the form of the combination of moment tensor elements, a variety of
different source characteristics can be considered. Therefore the extreme solution provides an estimation of the uncertainty
in the best fitting source model. The seismic waveform data was used to evaluate the effectiveness of this algorithm.
This research was supported by the National Natural Science Foundation of China. 相似文献
15.
Downhole monitoring with fibre-optic Distributed Acoustic Sensing (DAS) systems offers unprecedented spatial resolution. At the same time, costs are reduced since repeated wireline surveys can be replaced by the permanent installation of comparatively cheap fibre cables. However, the single component nature of fibre data requires novel approaches when designing a monitoring project such as cross-well seismics. At the example of the shallow CO2 injection test site in Svelvik, Norway, we model the evolution of velocity changes during CO2 injection based on rock physics theory. Different cross-well seismic design scenarios are then considered to evaluate the best design and the limits of this method to detect containment breach. We present a series of evaluation tools to compare the effect of different well spacings for cross-well seismic tomography. In addition to travel-times, we also consider characteristic amplitude changes along the fibre unique to DAS strain measurements, which might add a constraint to the inversion. We also compare the effect of using helical fibres instead of classical straight fibres. We thus present a toolbox to evaluate and compare different monitoring design options for fibre optic downhole installations for cross-well monitoring. 相似文献
16.
Todd H. Wiedemeier Barbara H. Wilson Mark L. Ferrey John T. Wilson 《Ground Water Monitoring & Remediation》2017,37(2):25-34
Magnetite is a natural component of many aquifers. Abiotic degradation of chlorinated solvents by magnetite can be an important mechanism for natural attenuation of these contaminants. The quantity of magnetite in aquifer materials can be estimated by measuring the magnetic susceptibility of the materials. This is most commonly done by determining the magnetic susceptibility of core samples in an analytical laboratory using a magnetic susceptibility meter. Unfortunately, the cost of acquiring core samples often makes an evaluation of abiotic degradation by magnetite economically unrealistic. Downhole sondes (probes) are available for the determination of magnetic susceptibility. In this study, a downhole sonde was evaluated as an affordable alternative to acquiring and analyzing core samples. The sonde was introduced into 10 monitoring wells. The data from the sonde were then compared to data from core samples that were collected from the same elevation as the sonde data. The core samples analyzed in the laboratory were used as the standard against which the sonde data were compared. The downhole sonde reported values that were similar to values reported on core samples. At most wells, the means of the two measurements could not be distinguished at the 95% confidence interval. When the means could be distinguished, they still agreed within a factor of two. 相似文献
17.
An exact formulation for borehole coupling, which is valid for all frequencies and all azimuthally symmetric and non-symmetric components, is presented. The borehole effects on downhole seismic measurements are studied in detail as functions of frequency, angle of incidence and polarization of an incident wave as well as geophone orientation. We found that correction for the borehole effect on downhole measurements should be made for frequencies above 500 Hz in a hard formation. In a soft formation, if the angle of incidence is well away from the resonance angle for SV incidence, no borehole correction is needed for frequencies below 300 Hz, while for frequencies above 300 Hz, the borehole can cause severe problems in downhole measurements. The borehole can also significantly alter the particle motion direction which implies that horizontal component rotation from data itself is unreliable for experiments with frequencies above 1 kHz in the hard formation and around 500 Hz in the soft formation. 相似文献
18.
The EISCAT UHF radar system was used to study the characteristics of E-region coherent backscatter at very large magnetic aspect angles (5–11°). Data taken using 60 s pulses during elevation scans through horizontally uniform backscatter permitted the use of inversion techniques to determine height profiles of the scattering layer. The layer was always singly peaked, with a mean height of 104 km, and mean thickness (full width at half maximum) of 10 km, both independent of aspect angle. Aspect sensitivities were also estimated, with the Sodankylä-Tromsø link observing 5 dB/degree at aspect angles near 5°, decreasing to 3 dB/degree at 10° aspect angle. Observed coherent phase velocities from all three stations were found to be roughly consistent with LOS measurements of a common E-region phase velocity vector. The E-region phase velocity had the same orientation as the F-region ion drift velocity, but was approximately 50% smaller in magnitude. Spectra were narrow with skewness of about –1 (for negative velocities), increasing slightly with aspect angle. 相似文献
19.
XIAOHONG XU 《Geophysical Prospecting》1990,38(2):139-168
A multichannel lattice filter structure is utilized to represent seismic waves propagating in adjacent layers in an elastic medium. Using this model, an explicit time-domain solution for arbitrary source and receiver locations is obtained as an ARMA (AutoRegressive and Moving-Average) process. The lattice and ARMA structures have given rise to an effective algorithm for the calculation of offset/downhole synthetic seismograms. A large range of recently developed offset/downhole seismic survey geometries, such as the ‘Yo-Yo’ arrangement, can thus be simulated. In addition, the explicit solutions for upgoing and downgoing waves provide new insight into the properties of general downhole seismic signals, including wave-mode conversion effects and multiple reflections. Furthermore, offset/downhole seismograms generated by a line source (i.e. 2D point source) can also be constructed by superposition of plane waves with different incidence angles. Synthetic seismograms generated using a different source-receiver arrangement indicate that the properties especially associated with offset/downhole seismic signals can be predicted by this modelling method. These properties include arrival times, amplitude attenuation and wave-mode conversion effects. Finally, utilizing this numerical modelling method to a real downhole survey with Yo-Yo geometry may lead to a proper data acquisition and processing procedure, and improves the interpretation confidence of the field section. 相似文献
20.
Mourad Zeghal Ahmed-W. Elgamal Ender Parra 《Soil Dynamics and Earthquake Engineering》1996,15(8):523-547
Downhole records of seismically-induced soil liquefaction are a valuable source of information on the associated mechanisms of stiffness degradation and lateral spreading. In this paper, free-field downhole array seismic records are employed to identify and model the recorded response at Wildlife Refuge (California, USA) and Port Island (Kobe, Japan) sites. The Wildlife Refuge site was instrumented in 1982 with a two-accelerometer array and six piezometers that recorded a case of seismically induced site liquefaction. At Port Island, a four-accelerometer down-hole array recorded strong motion during the recent 1995 Hyogoken-Nanbu earthquake. This earthquake resulted in widespread liquefaction and major ground deformations at Port Island. Using the recorded downhole accelerations at these two sites, the actual seismic shear stress-strain histories are directly evaluated. These histories provide valuable insight into the mechanisms of site liquefaction and associated loss of stiffness and strength. Based on the identified dynamic soil behaviors, computational simulations of the observed seismic response are performed. Optimization techniques are employed to estimate the necessary computational modeling parameters. This document constitutes the second part of a set of two companion papers about site amplification and liquefaction. 相似文献