On the volcanic waveguide

Abstract

An analytical solution for the sound field in a resonant magma conduit with depth‐dependent magma properties provides a versatile interpretative tool for the analysis of seismoacoustic signals generated by fluid processes in active volcanoes. Expressions are given for the acoustic field in the atmosphere and the seismic field in the ground radiated through the magma conduit vent and walls, respectively. A new source model allows the resonance in the conduit to be excited by the vertical fluid velocity at the bottom of the conduit. This source model represents acoustic excitation induced by low‐frequency fluid oscillations or large volume fluctuations originating at depth. The magma conduit is modeled as a three‐section duct, with each section having a different density, sound speed, viscosity, and dimension. The modal structure of the coupled resonant system is retained as the sound field in the conduit propagates into the surrounding bedrock and the overlying atmosphere. The source‐time function permits estimates of the total mass injected or removed from the magmatic system, and the source spectrum acts as a band‐pass filter on the resonant modes of the conduit. Theoretical or empirical source functions can be used to drive the seismoacoustic wave field, and the amplitude of the source velocity may be constrained by the ballistics of pyroclasts and the height of ash plumes. The acoustic properties of the melt may be predicted from the temperature, composition, and flow conditions of the magma as a function of depth. Comparisons of synthetic waveforms and spectra with those of seismic and acoustic signals recorded on active volcanoes permit estimates of the conduit geometry, density, sound speed, and attenuation properties of the melt, and characterize the temporal and spectral features of the source function. The ability to extract this information accurately and efficiently from seismic and acoustic data streams may permit near real time modeling of fluid‐driven processes in active volcanoes.