Converted phase identification and retrieval of Vp/Vs ratios from move-out reflection analysis: application to the Campi Flegrei caldera

Abstract

Here, we propose a method for the determination of Vp/Vs ratios in a horizontally layered propagation media using maximization of a coherency function along theoretical travel-times of PS reflected phases. The theoretical travel-times are computed using the information about the propagation media that is extracted by velocity analysis or by topographic analysis performed on the first arrivals. The method is also a valid tool for the identification of the PS phases associated with a fixed seismic reflector, and it is particularly suitable for data that is stored in common mid-point and common conversion point bin- ning; for this kind of data the hypothesis of horizontally and layered media can usually be verified. We applied the method to both simulated and real datasets. The use of the real data that was acquired in the Campi Flegrei caldera (southern Italy) allowed us to estimate a relatively high Vp/Vs ratio (3.5 ± 0.6) for a very shallow layer (maximum depth, 600 m). This hypothesis has been tested by theoretical rock physical modeling of the Vp/Vs ratios as a function of porosity, suggesting that the shallow layer appears to be formed of unconsolidated, water-saturated, volcanic and marine sediments that filled Pozzuoli Bay during the post-caldera activity.