Application of SPAC method in dividing strata of the Quaternary strata

LI Baochun; SUN Shaohuai; LI Guoping

doi:10.16186/j.cnki.1673-9787.2023120033

Abstract：The SPAC method is one of the most effective methods of extracting phase velocity from environ-mental noise.The calculation of S-wave velocity structure by the SPAC method has important scientific sig-nificance for studying velocity structure.Objectives To verify the reliability of the SPAC method in dividing strata of the Quaternary strata,Methods this study used a multi-observation array to collect microtremor sig-nals within 2 to 40 Hz.It calculated the spatial autocorrelation coefficients among Stations A,B,and C groups and the central station.Furthermore,it used zero-order Bessel functions to fit the spatial autocorrela-tion coefficients,and extracted the phase velocity-frequency relationship from the Rayleigh surface wave to obtain the phase velocity dispersion curve.Compared with models with the Low-Velocity Soft Layer Model,the High-Velocity Hard Layer Model,and the Velocity-Increasing Model,the Forking Genetic Algorithm fGA has the advantage of not relying on the initial model for inverting dispersion curves.Results Taking the measured data from a certain location on the northern edge of the North China Craton as an example,it used the SPAC,transient Rayleigh surface wave,and wave velocity logging methods to invert the S-wave velocity structure.According to the drilling data,It found that at the depth of 6.7 to 9.0 m underground,the fully weathered gneiss exhibited S-wave velocities as low as 305 m/s,whereas the strongly weathered plagioclase gneiss showed higher S-wave velocities ranging from 430 to 540 m/s below 9.0 m.Conclusions Based on the comprehensive analysis of SPAC,transient Rayleigh surface wave,wave velocity logging methods and drilling data,it can be concluded that the accuracy of stratigraphic stratification and S-wave velocity value of the SPAC method are consistent with other results,which verifies the reliability of the SPAC method.