HVSR analysis to investigate a possible correlation to a gas shallow reservoir in a mud volcanic field: the case of Nirano (MO).

Multiple studies highlight the evidence of a trough within the low-frequency range in HVSRs measurements performed over a gas field and attribute it to the presence of a hydrocarbon reservoir (Lambert et al., 2007; Saenger et al., 2007; Panzera et al., 2016). 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 amplification effect. Furthermore, Panzera et al. (2016) observe that the minimum is identified by an “inverse eye-shaped” feature in the Fourier spectra, related to an amplitude increase in the vertical component of motion due to a velocity inversion. This study focuses on the investigation of the spectral anomaly described above at Nirano mud volcano field, conducted through the analysis of the results obtained by seismic arrays and three directional velocimetric stations (HVSR) deployed in the site. After a cluster analysis carried out on HVSRs have been identified 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 Genetic 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 stratigraphic effect that unites all the measurements concentrated in the group under examination or to the surface wave model used. References Lambert M., Schmalholz S. M., Saenger E. H. and Podladchikov Y. Y.; 2007: Low-frequency anomalies in spectral ratios of single-station microtremor measurements: Observations across an oil and gas field in Austria. In SEG Technical Program Expanded Abstracts 2007 (pp. 1352-1356). Society of Exploration Geophysicists. Panzera F., Sicali S., Lombardo G., Imposa S., Gresta S. and D’Amico S.; 2016: A microtremor survey to define the subsoil structure in a mud volcanoes area: the case study of Salinelle (Mt. Etna, Italy). Environmental Earth Sciences, 75, 1-13. Saenger E.H., Torres A., Rentsch S., Lambert M., Schmalholz S.M. and Mendez-Hernandez E.; 2007: A hydrocarbon microtremor survey over a gas field: Identification of seismic attributes. 77th SEG meeting, San Antonio, Texas, USA, Expanded Abstracts, 1277–1281.