Microtremor Survey For Shear Wave Velocity In Site With Hard Interlayer

The Expend Spatial Auto-Correlation(ESPAC)method can extract accurately and steadily the Fundamental Mode Rayleigh Dispersion Curve(FMRDC) from observation of microtremors at an irregular array. Focus on shear wave inversion at hard interlayer site, an efficient and reliable scheme for shear wave velocity exploration has been developed on basis of ESPAC and GA(Ggenetic Algorithm). We apply this scheme for site characterization in one domestic nuclear power plant site, of which the result is consistent with the shear wave velocity structure given by bore-hole investigation.For construction of such a scheme, following research work has been carried out in this paper:(1) The efficiency and accuracy of various methods for comupation of the FMRDC have been compared. Results show that fast scalar transfer algorithm method is superior in computation efficiency and is as same accuate as other methods, thus we use this method for later compuation of FMRDC. Based on that method, we validate the fact that multi-solution can exist for shear wave velocity inversion based on FMRDC, that is, the FMRDC of soil layers with significant different structure can be very close. The characteristic of FMRDC of hard interlayer site has been analysised and discussed, such as the cutoff frequency, which provides a solid foundation for the subsequent inversion work.(2) We illustrate the full process for the applicationof GA in shear wave velocity inversion. We show that the initial model will significantly affect the GA’s convergence speed and precision. Based on this work, we introudce an initial model building strategy and a new objective function for GA. The effect of the cutoff frequency on shear wave velocity inversion has also been analyized.(3) We discuss the factors that influence the extraction of the FMRDC based on ESPAC, such as the choice of the instrument, array layout. Based on various examples, we validate the reliability of ESPAC(4) We combins the ESPAC, GA and the fast scalar transfer algorithm to create an shear wave exploration scheme and elaborates the application this scheme in the shear wave velocity of the hard interlayer site in one domestic nuclear power plant site. The inversion results is consistent with the shear wave velocity structures given by bore-hole, which proves that the scheme is effective.