Detailed 3D bedrock geometry in the Firenze area from HVSR seismic noise measurements, seismic noise inversion and dispersion curves of surface waves

The 3D bedrock geometry in the Firenze area was reconstructed from seismic noise measurements and borehole data. A total of ~ 300 measurements of seismic noise, collected since 2002 with various instruments (Le3D-Lite, Le3D-5sec, Tromino) were used to derive the fundamental frequency using the HVSR methodology. The fundamental frequencies obtained range from 0.4 to 12 Hz and provide robust constraints for site effect characterization. Using borehole data, the relationship between frequency and sediment thickness was quantified through nonlinear regression, yielding h= 137 f ^-1.147.  Among the investigated locations, the Mantignano area in western Florence was selected for detailed study with an array of 13 seismic stations equipped with Le3D-Lite seismometers, where inversion of HVSR spectra was performed and dispersion curve of surface wave was measured. For the HVSR inversion we employed the MATLAB- based OpenHVSR program. The inversion workflow incorporates an integrated misfit- minimization algorithm, allowing detailed reconstruction of the 3D subsurface structure at the Mantignano site. The results show that the bedrock position in Mantignano governs the stable low-frequency peak of all the HVSR curve, whereas the higher- frequency peaks reflect the near-surface horizons. 

Additionally, phase-velocity information from surface waves, obtained using both CPSD measurements and the theoretical Bessel J0 model, provides consistent constraints on frequency–velocity pairs, improving the reliability of the dispersion characteristics obtained from Cross Spectral phase data.