Experimental investigations on degassing behavior and related seismo-acoustic markers: the effect of complex conduit geometries

Parma

One of the main challenge in monitoring active volcanoes is understanding gas-magma dynamics in
volcanic conduits and relating them with their geophysical markers at the surface. Accordingly, we combine
here two main approaches used to investigate conduit dynamics via indirect observations, i.e. analogue
laboratory experiments and seismo-acoustic measurements, and address a crucial, yet unexplored, subject: the
irregularity (i.e. the departure from an ideal smooth cylindrical shape) of the conduit surface. To this aim, we
developed a protocol to assemble epoxy conduits with different fractal dimensions (Dc; i.e. irregularity) of the
internal surface, and used silicone oil as a proxy for magma. We investigated different degassing patterns, from
bubbly to slug and churn-annular flow, by varying systematically: 1) injected gas flux (5 to 180x10-3 l/s); 2)
analogue magma viscosity (10 to 1000 Pas); 3) fractal dimension (Dc) of the conduit surface (i.e. Dc=2, Dc=2.18 and Dc =2.99). The experiments were monitored by tracking the temporal evolution of sample expansion and outgassing periodicity through a video-camera and investigating the relative seismo-acoustic fingerprint by means of a set of dedicated sensors (i.e. microphone, piezo-films, accelerometer). Results show that viscosity strongly influences the transition among degassing patterns and the frequency of slug bursts at the surface.
Furthermore, we noticed an increase of the exponent of the power law equation linking squared seismic
amplitude to gas flow rate with conduit roughness; whereas the opposite trend was observed increasing
the viscosity of the liquid phase. These results have fundamental implications for linking eruption source
parameters such as the volume discharge rate to seismic data (i.e. volcanic tremor) at different volcanoes or for
investigating their temporal evolution at a single vent.