The Acoustics of Volcanic Explosions

This thesis consists of an analytical model for acoustic wave propagation in volcanoes and a comparison of the model's predictions with sounds recorded at Stromboli Volcano, Italy. The theoretical model yields the airborne sound field radiated by a magmatic conduit that is excited into resonance by an explosive source. The output of this model provides analytical solutions for the acoustic field in the magma and in the atmosphere as functions of frequency. At low frequencies the spatial structure of the field in the atmosphere is dominated by diffraction at the corner of the vent. At high frequencies, each mode is radiated into the atmosphere as a beam of sound, whose angle of elevation is determined by Snell's Law. The predicted acoustic field spectrum in the atmosphere has a peaked structure due to the horizontal and vertical modes of the magmatic conduit. The spectrum is transformed into a synthetic pressure pulse by numerical Fourier inversion. The second part of the thesis consists of a comparison between airborne acoustic data recorded at Stromboli Volcano and the predictions of the theory. By comparing the model's output with the acoustic data, estimates are obtained for the sound speed, void fraction and viscosity of the magma, as well as the length and radius of the conduits at Stromboli. Both theoretical and observed energy spectra exhibit (1) a concentration of energy in the infrasound region, which is associated with the first few longitudinal resonances; (2) a broad minimum corresponding to a suppressed longitudinal mode, which is explained by a source depth located near the antinodes for this mode; and (3) an abrupt rise in the spectral levels in the low audio region, which may be due to onset of radial resonances. In the time domain, the theoretical pressure pulses show a random character which is due to the propagation effect of conduit reverberation. Features of these synthetic pressure signals are consistent with those of the explosion transients. A separate analysis was made of Western and Eastern crater explosion signatures recorded at Stromboli. The results of the modeling suggest that W crater may be represented as a long and wide conduit with a high sound speed and viscosity. A superposition of synthetic pulses mimics well the explosion signatures of the W crater, implying these explosions are composed of many bursts of short duration. The E crater is modeled as a short and narrow conduit of low sound speed and viscosity. E crater explosive events are dominant in the infrasound region, but may be masked by degassing in the audio region. The difference between W and E crater explosions may be attributed to a higher gas content and temperature of the E crater magma. The success of the modeling indicates that the low-frequency temporal and spectral properties of volcanic sounds are determined by the normal modes of a resonant magmatic conduit, and that these sounds contain substantial information about the internal processes and structure of active volcanoes.