Passive seismic measurements to characterize gas reservoirs in a mud volcano field in Northern Italy

The presence of a minimum in the average Horizontal to Vertical Spectral Ratios (HVSR) of ambient vibrations has been suggested to be representative of hydrocarbon reservoirs in active oil and gas fields in the Middle East and Europe. Similar evidence has been also found in correspondence of active mud volcanoes in Italy. To explore the possibility of using ambient vibrations to identify and characterize reservoir corresponding to mud volcanoes vents, a dense network of velocimetric measurements (with average inter-distance of 30 m) has been deployed in the Nirano mud volcanoes area (Northern Italy) by considering single station and array configurations (at 59 and 8 sites respectively). Results obtained confirm that measurements with a clear HVSR minimum only characterize the area of emitting vents. In the assumption that this minimum depends on the characteristics of the reservoir responsible for fluid emissions, the Biot–Gassmann theory for seismic waves velocities of gas hydrate-bearing sediments has been considered to infer reservoir characteristics from Vs and Vp profiles obtained by the inversion of the HVSR curves. The satisfactory fit of model outcomes with observations testifies one more the effectiveness of ambient vibration measurements to characterize mud volcanoes and relevant subsoil configuration.