1-D P-velocity models of Mt. Vesuvius volcano from the inversion of TomoVes96 first arrival time data

We applied a revised version of the 1-D tau -p inversion method to first P-arrival times from the active seismic experiment performed at Mt. Vesuvius (southern Italy) in 1996 (TomoVes96 Project). The main objective of this work is to obtain 1-D velocity models of Mt. Somma-Vesuvius volcano complex and surrounding area. Moreover we show that combining the I-D information we provide a reliable 2-D initial model for perturbative tomographic inversions. Seismic and geological surveys suggest the presence of a refractor associated with the contrast between carbonate basement and volcanic/alluvial sediments; synthetic simulations, using a realistic topography and carbonate top morphology, allowed us to study the effect of topography on the retrieved velocity models and to check that the 1-D tau -p method can also approximately retrieve the refractor depth and velocity contrast. We analysed data from 14 on-land shots recorded at stations deployed along the in-profile direction. We grouped the obtained models in three subsets according to the geology of the sampling area: Models for carbonate outcrop area, models for the Campanian Plain surrounding the volcano edifice and models for Mt. Somma-Vesuvius volcano complex. The found I-D P-velocity models show important vertical and lateral variations. Very low velocities (1.5-2.5 km/s) are observed in the upper 200-500 m thick shallow layer. At greater depths (3 km is the maximum investigated depth) P velocities increase to values in the range of 4-6 km/s which are related to the presence of the carbonatic basement. Finally we interpolated the I-D models to demonstrate an example of misfit for a 2-D interpolated model whose residuals are confined in a narrow band around zero.