Array tecniques and spectral ratios applied to seismic noise to investigate subsoil structures in Campi Flegrei (Italy)

Abstract

The purpose of this work is to study the subsoil structure (from shallow to deep layers) of the Campi Flegrei area using both spectral ratios and array techniques applied to seismic noise. The dataset consists of seismic noise recorded in the underground by the seismic Array “ARF” and the broadband stations of the mobile network. We estimated the dispersion curves of Rayleigh waves propagating through the array by applying the Frequency–Wavenumber (f-k) technique and the MSPAC method. The phase velocity dispersion curve of the fundamental mode of Rayleigh waves was obtained by plotting the inverse of slowness, as a function of frequency, and selecting the part of the curve bounded by the resolution limits defined through kmin and kmax. For the MSPAC method we divided the array in semi-circular sub arrays called Rings, which radius are defined by the sensor’s spacing, and we calculated the spatial autocorrelation coefficients for all the possible pairs of sensors. Further constrains on the dispersion curves are obtained from the estimate of the phase velocity of transient coherent signals recorded by the array. We used the DINVER computer code to perform a joint inversion of the autocorrelation coefficients, the dispersion curves, the single coherent noise values and the mean HVSR of the mobile network station to obtain a shallow shear wave velocity model for the first 200 m, which is in a good agreement with the stratigraphic information of the wells available in the area. The study of the velocity models is crucial to recognize anomalies due to the presence of fluids connected to the volcanic activity and a high quality characterization of the medium allows us to correctly identify, in the geophysical observables, the contribution of the volcanic source.