THEORY OF THE AIRBORNE SOUND FIELD GENERATED IN A RESONANT MAGMA CONDUIT

Explosive sources triggered inside a magma conduit may excite the cond uit into acoustic resonance. The acoustic field in the conduit can pro pagate into the atmosphere through an open vent and ensonify the overl ying atmosphere. The character of the airborne sound field is determin ed by a combination of propagation and source effects: the resonance o f the magmatic conduit and the diffraction of the sound field at the v olcanic vent are acoustic propagation effects, whereas the explosion p ressure signature and the firing rate of the explosions define the sou rce characteristics. For wavelengths larger than the conduit radius, o nly the longitudinal resonances of the magmatic conduit are relevant, and the open vent radiates like a piston surrounded by an infinite baf fle. In this case, the fluid particle velocities are directed along th e axis of the conduit and the sound field may propagate into the surro unding bedrock through the conduit wall displacement induced by the fl uid overpressure. This coupling may produce seismic signals with bande d spectra, such as volcanic tremor and long-period events. The airborn e 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 dri ven by the acoustic field in the magma and also contains this informat ion, although it may be filtered by propagation effects in the bedrock . The theoretical sound field in the magma conduit is used to interpre t 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 use d to monitor changes in the magmatic system. The results of the modell ing illustrate the difference between source effects and conduit reson ance, as well as the value of seismoacoustic measurements in volcanic environments. (C) 1997 Elsevier Science B.V.