USE OF SHORT-PERIOD MICROTREMORS FOR VS PROFILING

A possible use of short-period microtremors is examined for the determination of shallow shear wave velocity (V(s)) profiles. Presented for this purpose is a hybrid version of Rayleigh wave methods, in which vertical ground-surface motions of either microtremors or forced vibrations are measured using an expanding circular array of six vertical sensors. Based on high-resolution frequency-wave number spectrum analysis of these data, Rayleigh wave dispersion curves can be determined. Microtremors cover the characteristics in a low-frequency range, while forced vibrations cover them in a high-frequency range. An inverse process that takes into account the effects of multiple Rayleigh modes on thc measured dispersion data enables one to estimate V(s) profiles from the ground surface to depths more than 50 m. The V(s) profiles inferred by the proposed method are compared with those obtained by the conventional down-hole method. The V(s) profiles from the two methods are in fairly good agreement, indicating that the use of short-period microtremors is promising as an economical means of V(s) profiling.