Are transients carrying useful information for estimating H/V spectral ratios?

Abstract

Defining a procedure to calculate the H/V spectral ratio of seismic noise has recently been the focus of several studies. Although some issues have been addressed and a general procedure to follow in choosing the optimal seismological equipment, carrying out the measurements and analyzing the data has been agreed upon by the majority of specialists in this field, some points still remain open. One open question is whether transients should be avoided when H/V spectral ratios are calculated. Starting from contradictory results of previous papers, an ad-hoc experiment at the Nauen test site (Germany) was conducted to verify the influence of transients on the H/V spectral ratio of seismic noise. We show, consistent with some previous studies, that transients generated by high frequency content sources can amplify spectral ratio peaks at low frequencies. After careful polarization analysis of the data and estimation of the spectral amplitude decay with distance, we provide an explanation in terms of the composition of the wavefield over certain distance ranges. Our results highlight the importance of a better understanding of the wavefield composition for better exploiting the potential of H/V spectral ratio method.