JOINT INVERSION OF SITE VELOCITY STRUCTURE BY MICRO-TREMOR ARRAY RECORD: A CASE STUDY OF THE OBSER-VATION SITE 3# OF XIANGTANG IN TANGSHAN

doi:10.3969/j.issn.0253-4967.2020.06.005

Abstract

Abstract: The research on the exploration method of velocity structure of the site soil layer that is efficient, economic and easy for promotion and application is of great significance considering the importance of shear wave velocity structure in shallow underground for prediction and prevention of geological hazards. With no dependence on special hypocenter, no need for destructive drilling and a wide range of detectable depths, microtremor array applies to densely populated cities and plain areas, and has become one of the new research focuses in the field of geophysical exploration at home and abroad in recent years.
In the study of inversion record of wave velocity profile on shallow soil layer by using array observation records, surface wave dispersion(DC)or microtremor horizontal-to-vertical spectral ratio(MHVSR)inversion is generally carried out separately at present, but the velocity structure of inversion is often of obvious multi-solution. The dispersion curve mainly constrains the shear wave velocity of the loose sedimentary layer while the predominant frequency estimated from the peak value of MHVSR mainly constrains the thickness of the overburden. In addition, various results of the Rayleigh-wave dispersion curve indicate that the calculated frequency range of the phase velocity is higher than the predominant frequency. In view of this, a joint inversion method of DC and MHVSR is developed, and a new inversion strategy is proposed in this paper. Different from the existing inversion methods, in this paper, firstly the Rayleigh-wave dispersion curve is obtained from the data of microtremor array by Modified Space Autocorrelation Method(MSPAC)and Frequency-wavenumber analysis method(F-K), the results are compared with the theoretical fundamental order and one high order Rayleigh wave dispersion curve calculated from the borehole data, and the measured dispersion curve is fitted. Secondly, the dominant frequency and its corresponding amplification coefficient of the site are analyzed based on the Microtremor Horizontal-to-Vertical Spectral Ratio(MHVSR)recorded from a single station. As a result of the correlation between the dominant frequency and the thickness of the overburden, the depth of the site bedrock is determined, and then the initial velocity structure of the site is obtained by the improved half-wavelength method. Finally, the best velocity structure of the site is determined by the joint inversion of DC and MHVSR, and the S-wave transfer functions caused by the vertical incidence of the inversion model and the measured borehole model obtained by different inversion methods are compared.
The advantages of the inversion method in this paper lie in two aspects. On the one hand, in the extraction of surface wave dispersion curve, the comprehensive application of Modified Space Autocorrelation Method(MSPAC)and Frequency-wavenumber analysis method (F-K) widens the frequency range of extracting dispersion curve by a single method. On the other hand, in the determination of initial velocity structure, the problem of relying on certain prior information of the current other inversion methods is better solved with the improved half-wavelength method.
In this paper, the effectiveness and stability of the new inversion strategy are verified by a theoretical example and an array observation example. It is observed that the MHVSR of the single DC inversion model is different from that of the theoretical example model after the peak frequency(especially in the high frequency segment)under the initial model, and the DC of the single MHVSR inversion model is different from that of the theoretical example model in the lower frequency segment. However, the joint inversion of the two makes up for the high frequency difference of the MHVSR of the DC inversion model and the low frequency of the DC of the MHVSR inversion model, thus greatly reducing the multi-solution of the inversion model and reflecting well the site characteristics(amplification effect and predominant frequency)when the inversion model approaches the real site. Compared with the traditional seismic and electromagnetic exploration methods, the joint inversion method based on the microtremor array records in this paper is of more practical value in acquiring the velocity structure of shallow site.

Key words: microtremor survey method, dispersion curves, microtremor horizontal-to-vertical spectral ratio(MHVSR), velocity structure, joint inversion

摘要： 目前, 利用台阵观测记录反演浅地表土层波速剖面的研究一般单独进行面波频散曲线(Dispersion Curve, 简称DC)或微动水平与竖向谱比(Microtremor Horizontal-to-Vertical Spectral Ratio, 简称MHVSR)反演, 但其反演的速度结构往往存在明显的多解性。鉴于此, 文中发展了DC与MHVSR联合反演方法, 提出了一种新的反演策略。区别于已有的反演方法, 文中方法首先利用微动台阵记录获得面波频散曲线, 进而采用改进的半波长法获得场地初始速度结构, 最后通过DC和MHVSR的联合反演得到场地最佳速度结构。该反演方法的优势体现在2个方面: 1)在面波频散曲线提取环节, 综合应用修正空间自相关(MSPAC)法和频率-波数(F-K)法, 拓宽了采用单一方法提取频散曲线的频率范围; 2)在初始速度结构确定环节采用了改进半波长法, 可较好解决当前其他反演方法依赖一定先验信息的难题。文中分别以一理论算例及台阵观测算例验证了新反演策略的有效性及稳定性。结果表明, 单独的DC反演模型的理论MHVSR在峰值频率之后(特别在高频段)与理论算例模型的MHVSR有差异, 单独的MHVSR反演所得模型的DC在较低频段内与理论算例模型的DC有差异, 而二者联合反演可弥补DC反演模型的MHVSR高频差异和MHVSR反演模型的DC低频差异, 极大地降低了反演模型的多解性。相比已有的浅层速度结构反演方法, 文中提出的基于微动台阵记录的联合反演方法更具有实用价值。

关键词: 微动探测, 频散曲线(DC), 微动水平与竖向谱比(MHVSR), 速度结构, 联合反演

CLC Number:

P315.2

WANG Ji-xin, RONG Mian-shui, FU Li-yun, FU Lei. JOINT INVERSION OF SITE VELOCITY STRUCTURE BY MICRO-TREMOR ARRAY RECORD: A CASE STUDY OF THE OBSER-VATION SITE 3^# OF XIANGTANG IN TANGSHAN[J]. SEISMOLOGY AND GEOLOGY, 2020, 42(6): 1335-1353.

王继鑫, 荣棉水, 符力耘, 傅磊. 用微动台阵记录联合反演场地浅层速度结构——以唐山响嘡台3^#场地为例[J]. 地震地质, 2020, 42(6): 1335-1353.

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