Insight Into the Wave Scattering Properties of the Solfatara Volcano, Campi Flegrei, Italy

Imaging methods able to discriminate type and content of fluids in volcanic areas and to track their migration with time are fundamental to get a picture of the volcanic structure and to constrain its shallow dynamics. In this study we provide an image of the Solfatara crater, located in the Campi Flegrei caldera, a volcanic area of Southern Italy, through a statistical description of the scatterers. We analyze active seismic data recorded in a 3D geometry during the first Repeated Induced Earthquake and Noise (RICEN) experiment, held in September 2013. After extraction of seismic sections, we evaluate the coherent and incoherent intensities, averaging over sources, and receivers sharing the same source-station distance. We thus compute the ratio between the two intensities for the direct surface wave, which is sensitive to the scattering mean free path in the area. We find that the intensity ratio in all the analyzed frequency bands exponentially decreases with distance, allowing for the computation of the scattering mean free path as a function of the frequency. We report that the scattering mean free path exhibits a small increase between 7.5 and 10 Hz from 50 to 60 m, and then it decreases down to about 10 m, at a frequency of 21.5 Hz. We thus model the scattering mean free path computing the backscattered field from a single scatterer of cylindrical shape in a homogeneous medium and we determine the model parameters through a fit with the mean free path inferred from data. We find that the best fit model is obtained for a size of the scatterer of about 10 m, with a small increase of the Rayleigh wave velocity inside the scatterer. The scatterers are here interpreted as regions richer in water with respect to the background and eventually due to the condensed steam running below the investigated area. The connection between the scattering mean free path and the type, and content of fluids retrieved here is of fundamental importance to image the volcanic structure. In addition, monitoring of this quantity with time can track fluid migration eventually related to magma injection and the unrest state of a volcano toward an eruption.