Application of SPAC method in dividing strata of the Quaternary strata

The SPAC method is one of the most effective methods of extracting phase velocity from environmental noise. The calculation of S-wave velocity structure by the SPAC method has important scientific significance for studying velocity structure.ObjectivesTo verify the reliability of the SPAC method in dividing strata of the Quaternary strata，Methodsthis study used a multi-observation array to collect microtremor signals 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 autocorrelation 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.ResultsTaking 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.ConclusionsBased 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.