Improved half-wavelength method for determining the velocity structure of site soil layer

The initial velocity structure model is generally necessary in the study of geophysical inversion for shallow surface soil wave velocity profile. In order to avoid excessive reliance on the surveying data instead, and to make full use of observation records of passive sources such as earthquakes and ambient noise to quickly and easily construct the initial model of soil inversion, this paper proposes an improved half-wavelength method based on the dispersion curve and the horizontal-to-vertical spectral ratio （HVSR） of the soil layer. Firstly, the method deter-mines the predominant frequency of the soil layer by HVSR, and then obtains the depth of overlying soil, and determines the frequency band range required for the inversion. Next, invert initial velocity structure of the site by using the variation of the partial derivative of the Rayleigh wave velocity to the shear wave velocity with the depth of the soil layer. Combined with the half-wavelength method in engineering site exploration, three types of soil layer models frequently encountered and the measured model of Kushiro site in Japan were analyzed, and the error analysis of the velocity structure was carried out and compared with the theoretical or measured velocity structures. The research shows that the initial velocity structure obtained by the improved half-wavelength method has a maximum error not more than 35%, which can be used to reconstruct a small searching model space in inversion for the wave velocity profile of engineering site by using the geophysical method, so as to improve the speed of inversion calculations and the reliability of result.