We propose a strategy for merging both active and passive data sets in linearized
tomographic inversion. We illustrate this in the reconstruction of 3D images of a
complex volcanic structure, the Campi Flegrei caldera, located in the vicinity of the
city of Naples, southern Italy. The caldera is occasionally the site of significant unrests
characterized by large ground uplifts and seismicity. The P and S velocity models
of the caldera structure are obtained by a tomographic inversion based on travel
times recorded during two distinct experiments. The first data set is composed of 606
earthquakes recorded in 1984 and the second set is composed of recordings for 1528
shots produced during the SERAPIS experiment in 2001. The tomographic inversion is
performed using an improved method based on an accurate finite-difference traveltime
computation and a simultaneous inversion of both velocity models and earthquake
locations. In order to determine the adequate inversion parameters and relative data
weighting factors, we perform massive synthetic simulations allowing one to merge
the two types of data optimally. The proper merging provides high resolution velocity
models, which allow one to reliably retrieve velocity anomalies over a large part of the
tomography area. The obtained images confirm the presence of a high P velocity ring
in the southern part of the bay of Pozzuoli and extends its trace inland as compared
to previous results. This annular anomaly represents the buried trace of the rim of
the Campi Flegrei caldera. Its shape at 1.5 km depth is in good agreement with the
location of hydrothermalized lava inferred by gravimetric data modelling. The Vp/Vs
model confirms the presence of two characteristic features. At about 1 km depth a
very high Vp/Vs anomaly is observed below the town of Pozzuoli and is interpreted
as due to the presence of rocks that contain fluids in the liquid phase. A low Vp/Vs
body extending at about 3–4 km depth below a large part of the caldera is interpreted
as the top of formations that are enriched in gas under supercritical conditions
