Airborne acoustics of explosive volcanic eruptions

A recently developed theoretical model of the airborne acoustic field from an explosive volcanic eruption of the Strombolian type is described in this article. The magma column is assumed to be a circular cylinder, which is o pen to the atmosphere at the top, and which opens into a large magma chambe r below. The magma itself is treated as a fluid, and the surrounding bedroc k is taken to be rigid. An explosive source near the base of the magma colu mn excites the natural resonances of the conduit. These resonances result i n displacement of the magma surface, which acts as a piston radiating sound into the atmosphere. The source is modeled in much the same way as an unde rwater explosion from a high-explosive chemical such as TNT, although in th e case of the volcano the detonation mechanism is the ex-solution of magmat ic gases under extremely high hydrostatic pressure. The new theory shows co mpelling agreement with airborne acoustic signatures that were recorded in July 1994 at a distance of 150 m from the western vent of Stromboli volcano , Italy. The theoretical and observed power spectra both display the follow ing features: (1) four energetic peaks below 20 Hz, identified as the first four longitudinal resonances of the magma column; (2) a broad minimum arou nd 30 Hz, interpreted as a source-depth effect, occurring because the sourc e lay close to nulls in the fifth and sixth longitudinal resonances and thu s failed to excite these modes; and (3) radial resonance peaks between 35 a nd 65 Hz. On the basis of the theory, an inversion of the acoustic data fro m Stromboli yields estimates of the depth (approximate to 100 m) and radius (approximate to 16 m) of the magma column as well as the depth (approximat e to 83 m), spectral shape and peak shock-wave pressure (approximate to 1 G Pa) of the explosive source. Most of the parameters estimated from the acou stic inversion compare favorably with the known geometry and source charact eristics of Stromboli.