Use of peaks and troughs in the horizontal-to-vertical spectral ratio of ambient noise for Rayleigh-wave dispersion curve picking

Abstract

To assist in the identification of fundamental-mode dispersion curves of Rayleigh waves in dispersion diagrams, we explore the relation between the shape of the horizontal-to-vertical spectral-ratio (HVSR) of ambient seismic noise and the shape of the dispersion curves for phase and group velocities in a stratified medium. We propose to use the information coming from the HVSR to identify the osculation zones and multi-mode effects and to locate inflection points and critical points in the observed phase and group dispersion diagrams of Rayleigh waves. The relationship between these curves has been numerically investigated for some models consisting of one and two homogeneous layers overlying a half-space, with velocities increasing downwards. It is primarily found that the first minimum in the HVSR appears close to the frequency of the inflection point of the fundamental mode of phase velocity. In addition, the osculation and multimode effects occur between frequencies of the fundamental peak and the first minimum of the HVSR. On the other hand, the frequencies of the minima in HVSR closely approximate the critical points of the fundamental-mode group-velocity dispersion curve, even better than the inflection points of the fundamental-mode phase-velocity curve. Finally, we show an example of experimental identification of fundamental-mode phase and group dispersion curves supported by the shape of the HVSR, obtaining a reliable velocity profile through the simultaneous inversion of these three curves.