GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 1.3 247 Fig. 2 - Vp tomographic result of A (a) and B (b) profiles. Interpretation of the two Vp velocity models A (c) and B (d). Modifiet after Gammaldi et al. , 2017. the second was about 480 m long and oriented along a WNW-ESE direction (Fig. 1, a). We collected the records in Common Shot Gather (CSG) and displayed the signal relative to a shot acquired by the receivers. Then, we manually picked the first arrival time by visual inspection of each section obtaining a total amount of 9053 and 8841 picks for each array. The 3D elastic and anelastic images of the shallow (30-35 m) central part of Solfatara crater are obtained through an iterative, linearized, tomographic inversion of picked P-wave arrival times and of the measured t* values using a multiscale strategy. 2D velocity sections (60-70 m) are obtained using a non-linear travel-time tomography method based on the evaluation of a posteriori probability density with a Bayesian approach. Results. The 2D tomographic profiles provide evidence for a low velocity (500–1500 m/s) water saturated deeper layer at West near the outcropping evidence of the Fangaia, contrasted by a high velocity (2000-3200 m/s) layer correlated with a consolidated tephra deposit. The transition velocity range (1500-2000 m/s) layer suggests a possible presence of a gas-rich, accumulation volume (Fig. 2, a-b). The central fault represents the high-permeability pathway for hydrothermal fluids. The gasses at depths of 40–60 m are blocked by the tephra saturated in meteoric water, which is the main shallower cap rock. Starting from the middle of the crater going westward, the saturated tephra became thinner being replaced by the gas accumulation zone. The gas is channelled between the consolidated and unconsolidated tephra, and finally released by the ring faults bordering the Solfatara (Fig. 2, c-d). Taking into account that the presence of fluids and their circulation may greatly affect the rock volume, and therefore the average compressional wave velocity, we expect that the tomographic images can constrain the possible location and phase of permeating fluids. For this purpose, we compared our seismic tomography with 2D cross sections of resistivity (ρ) and with temperature and CO2 flux measurements (Fig. 3).