A three-dimensional QP imaging of the shallowest subsurface of Campi Flegrei offshore caldera, southern Italy

To improve the knowledge of the shallowest subsurface of Campi Flegrei caldera, a 3-D P wave attenuation tomography of the area was performed. We analyzed about 18,000 active seismic traces, which provided a data set of 11,873 Δt* measurements, e.g., the differential travel times to quality factor ratios. These were inverted through an adapted tomographic inversion procedure. The 3-D tomographic images reveal an average QP about 70, interpreted as water-saturated volcanic and marine sediments. An arc-like, low-QP structure at 0.5–1 km depths was interpreted as a densely fractured, fluid-saturated rock volume, well matching the buried rim of Campi Flegrei caldera. The spatial distribution of high- and low-QP bodies in the inner caldera is correlated with low-Vp values and may reflect either the differences in the percentage of fluid saturation of sediments or the presence of vapor state fluids beneath fumarole manifestations.