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.