Estimation of the Thickness of Multi-layer Soils from the Higher Peak of H/V Spectral Ratio

Based on inversion of the horizontal-to-vertical spectral ratio of microtremors (H/V), the empirical H/V spectrum of surface waves propagating on a layered half-space was studied and the effects of soil layers on fundamental and higher peak frequencies were considered. Using the Nakamura H/V spectral method and comparing it with the analytical site-to-reference (RF) method, inverse analysis is performed. These procedures were carried out at several stations around Iran. The results indicate that the transfer function of upward propagation is equal to the downward transfer functions, and is independent of the direction of wave propagation. Furthermore, the resonant peaks depend on the sedimentary cover depth; the next peaks in the H/V spectrum (the peaks after the resonant peak) were generated either in the higher mode of resonant amplification in thick layers and/or from wave interference, subsurface topography and different impedance between two adjacent thin layers as well as resonance amplification. In addition, the analysis indicates that fewer deep sedimentary layers instead of multi-layers do not lead to variation in transfer functions. In deep-deposit layers, the soil layer difference can amplify the amplitude, and does not create a new peak in H/V spectral ratio.