Modeling the Acoustic Flux Inside the Magmatic Conduit by 3D‐FDTD Simulation
Giorgio Lacanna, Maurizio Ripepe
Abstract
Abstract Infrasound measurements at open‐vent volcanoes are crucial parameters for monitoring and exploring the explosive source dynamics. Volcano infrasound depicts the pressure variation within the volcanic conduit generated by the sudden expansion of volcanic gases during the fragmentation process, and it can provide important constraints on the explosive source parameters (i.e., volumetric flux and exit velocity). The acoustic source of volcanic explosions is modeled by infrasonic signals measured near the volcano vent (<3 km) assuming linear theory of sound and considering the effects of topography and atmosphere on the acoustic wavefield. However, little is known on the initial conditions within the volcanic conduit. In linear acoustics, the wavefield in a cylindrical duct propagates as a plane wavefront, which becomes spherical outside the vent. The acoustic impedance at the open end of the duct is a function of the vent radius and the pressure wavelength, and it controls the acoustic wavefield radiated outside the duct in terms of amplitude and radiation pattern. We present here a 3D‐Finite Difference Time Domain (FDTD) numerical method to evaluate the scattering effects on the infrasound signal produced by topography around the crater and along the source‐receiver path in terms of Green's function. Once the effects of topography are removed, we show how pressure perturbation is largely affected by the impedance contrast at the vent which, when not considered, is introducing errors in the way we quantify explosive dynamics.