Complex wave propagation in the Campi Flegrei Caldera, Italy, from source- and receiver-array analysis of sea-shot recordings

We investigate wave propagation in the complex shallow crust of Campi Flegrei Volcanic Complex, Italy, using array recordings of air-guns. We apply source-and receiver-array analysis to define the independent variation of horizontal slowness at both the source and receiver regions. This method allows the identification of asymmetric ray-paths associated with near-source and near-observer velocity heterogeneities. P-wave wave-vectors at both the source and receiver arrays depict discrepancies as large as 50 degrees with respect to the values expected for the 3D velocity structure of the Gulf. At the source region, these discrepancies may be associated with either un-modelled complexities in the geometry of the buried caldera rim, or with velocity variations beneath the source-array. At the receiver array, the inferred anomalies may be attributed to velocity variations marking the Solfatara crater rim, or to a near-receiver, low-velocity body whose position would coincide with negative gravimetric anomalies and a low Vp/Vs ratio region inferred by independent geophysical and seismological studies.