Seismic observation at Nirano mud volcanoes, north Italy

Mud volcanoes are diapirical structures expression of cold overpressured fluidized fine sediments rising from depths of hundreds of meters. When depositional process was fast enough to hamper dehydratation of buried sediments, isolated geological reservoirs are generated marked by elevated fluids pressure also due to gas produced by decompositional processes affecting trapped animals.  Due to the density difference with respect to surrounding rocks and because of the high fluid pressure, those sediments move upwards by following faults or other mechanical discontinuities. In the last decades such Sedimentary Diapirism has increasingly interested scientific community as possible markers of hydrocarbon reservoirs, as responsible for explosive events and their close connection with regional seismotectonic activity. Many studies, in the last years, tried also to find a solid relationship between mud volcanoes and gases emissions, in particular CO₂ and CH₄, two of the most important greenhouse gases.

Among the Italian mud volcanoes, those of Nirano (north Italy), represent a typical example of mud volcanic field, with small and uneventful surface structures. This natural reserve is marked by three main lined up surface structures along the NE-SW direction, close to small pools with less thick clay materials, called “salse”.

The structure beneath Nirano mud volcanic field has been investigated by several methodologies, such as geoelectrical, gravimetrical and seismic surveys. In the present work, the study of dynamic behaviour of these structures is focused on aiming at monitoring gas outflow and locating eventual ducts and secondary reservoirs at shallow depth. Specifically, seismic signals possibly associated to gas outflow are investigated by deploying seismic arrays and three directional velocimetric stations.

Outcomes of these measurements show that subsonic seismic emissions of these structures present analogies with to those of active volcanoes, possibly due to similar dynamic mechanisms, probably associated to gas bubbling phenomena. Three kinds of seismic activity have been identified: background ambient vibrations, short periodic energy bursts (drumbeats) and high energy paroxysmic phases. All these observed events, compared to that of active volcanoes, present higher frequencies range.

The analysis of these signals, in particular of the drumbeats phases, allow the location of the sources. The final locations appear to be local (limited to a few tens of meters away from instruments) and shallow (around 5-10 m from the surface). If these emissions were actually associated to gas bubbling, this kind of outcomes could represent an effective tool for measuring gas outflow and monitoring outgoing mud volcanoes activity.