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
binning; 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.
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