S-Wave Velocity Profiling by Inversion of Microtremor H/V Spectrum

A method for estimating the S -wave velocity ( V S ) profile of subsurface soils is proposed, based on inversion of the horizontal-to-vertical (H/V) spectrum of microtremors observed with a three-component sensor. For this purpose, theoretical formulas are derived for computing the H/V spectrum of surface waves propagating on a layered half-space in which the effects of the fundamental and higher modes are taken into account. An inverse analysis using microtremor H/V spectrum is then presented for estimating S -wave velocity profiles of subsurface soils. Assuming that either the V S values or the thicknesses of the shallow soil layers are known, the remaining unknowns are sought. The inverse analyses are performed using the H/V spectra observed at six sites, and their shallow V S profiles are estimated. The inverted S -wave velocity profiles are consistent with available downhole velocity logs at the sites. The standard error ratios of the inverted values are less than about 0.1, with a maximum of 0.2.