GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.2 GNGTS 2024 HVSR analysis to investgate a possible correlaton to a gas shallow reservoir in a mud volcanic feld: the case of Nirano (MO) A. Brindisi 1 , D. Albarello 1,3 , N. Carfagna 1 , E. Paolucci 2 1 Dipartmento di Scienze Fisiche, della Terra e dell’Ambiente, Università degli Studi di Siena, Siena, Italy 2 Dipartmento di Fisica ed Astronomia, Università degli Studi di Bologna, Bologna, Italy 3 Consiglio Nazionale delle Ricerche, Isttuto di Geologia Ambientale e Geoingegneria, Rome, Italy Multple studies highlight the evidence of a trough within the low-frequency range in HVSRs measurements performed over a gas feld and atribute it to the presence of a hydrocarbon reservoir (Lambert et al., 2007; Saenger et al., 2007; Panzera et al., 2016; Antunes et al., 2022). To explain the natural emission of low-frequency signals Saenger et al. (2007) and Lambert et al. (2007) consider hydrocarbon-reservoir related microtremor, assuming that the reservoir itself acts as a (secondary) source of low-frequency seismic waves by a resonant amplifcaton efect. Furthermore, Panzera et al. (2016) observe that the minimum is identfed by an “inverse eye- shaped” feature in the Fourier spectra, related to an amplitude increase in the vertcal component of moton due to a velocity inversion. This study focuses on the investgaton of the spectral anomaly described above at Nirano mud volcano feld, conducted through the analysis of the results obtained by seismic arrays and three directonal velocimetric statons (HVSR) deployed in the site. Afer a cluster analysis carried out on HVSRs have been identfed 3 groups of measurements, one of which include HVSRs located in the caldera-like basin area, marked by a minimum in the seismic spectrum at 0.53 Hz. The joint inversion procedure based on Genetc Algorithms of the HVSR curves and the Rayleigh waves dispersion curve shows that the minimum is well reproduced even without a velocity inversion. This proves that it is not uniquely correlated to the mechanisms proposed above and that, therefore, it may be linked to a stratgraphic efect that unites all the measurements concentrated in the group under examinaton or to the surface wave model used. References Antunes V., Planès T., Obermann A., Panzera F., D’Amico S., Mazzini A., Sciarra A., Ricci T., Lupi M.; 2022: Insight into the dynamics of the Nirano Mud Volcano through seismic characterizaton of drumbeat signals and V/H analysis. Journal of Volcanology and Geothermal Research, 431, 107619.