Theory of the airborne sound field generated in a resonant magma conduit

Explosive sources triggered inside a magma conduit may excite the conduit into acoustic resonance. The acoustic field in the conduit can propagate into the atmosphere through an open vent and ensonify the overlying atmosphere. The character of the airborne sound field is determined by a combination of propagation and source effects: the resonance of the magmatic conduit and the diffraction of the sound field at the volcanic vent are acoustic propagation effects, whereas the explosion pressure signature and the firing rate of the explosions define the source characteristics. For wavelengths larger than the conduit radius, only the longitudinal resonances of the magmatic conduit are relevant, and the open vent radiates like a piston surrounded by an infinite baffle. In this case, the fluid particle velocities are directed along the axis of the conduit and the sound field may propagate into the surrounding bedrock through the conduit wall displacement induced by the fluid overpressure. This coupling may produce seismic signals with banded spectra, such as volcanic tremor and long-period events. The airborne pressure field retains the modal structure of the sound field in the magmatic conduit, which contains information on the conduit geometry and gee-acoustic properties of the magma. The seismic wavefield is driven by the acoustic field in the magma and also contains this information, although it may be filtered by propagation effects in the bedrock. The theoretical sound field in the magma conduit is used to interpret seismic tremor signals recorded at Mt. Spurr Volcano, Alaska. Rapid variations in the acoustic impedance of the magma conduit terminations can create systematic changes in the tremor spectra, which can be used to monitor changes in the magmatic system. The results of the modelling illustrate the difference between source effects and conduit resonance, as well as the value of seismoacoustic measurements in volcanic environments. (C) 1997 Elsevier Science B.V.