Aeroacoustics of volcanic jets: Acoustic power estimation and jet velocity dependence

Abstract

[1] A fundamental goal of volcano acoustics is to relate observed infrasonic signals to the eruptive processes generating them. A link between acoustic power˘and volcanic gas exit velocity V was proposed by Woulff and McGetchin (1976) based upon the prevailing jet noise theory at the time (acoustic analogy theory). We reexamine this approach in the context of the current understanding of jet noise, using data from a laboratory jet, a full-scale military jet aircraft, and a full-scale rocket motor. Accurate estimates of˘require good spatial sampling of jet noise directionality; this is not usually possible in volcano acoustic field experiments. Typical volcano acoustic data better represent point measurements of acoustic intensity I( ) at a particular angle  from the jet axis rather than˘. For pure air jet flows, velocity-scaling laws currently proposed for acoustic intensity differ from those for acoustic power and are of the form I( ) (V/c) n  , where c is the ambient sound speed and n  varies nonlinearly from 5 to 10 as a function of temperature ratio and angle  . Volcanic jet flows are more complex than the pure air laboratory case, which suggests that we do not currently know how the exponent n  varies for a volcanic jet flow. This indicates that the formulation of Woulff and McGetchin (1976) can lead to large errors when inferring eruption parameters from acoustic data and thus requires modification. Quantitative integration of field, numerical, and laboratory studies within a modern aeroacoustics framework will lead to a more accurate relationship between volcanic infrasound and eruption parameters.

Acoustic Characterization of Explosion Complexity at Sakurajima, Karymsky, and Tungurahua Volcanoes

“…This highlights the complexity in explosion mechanisms at intermediate composition (andesitic) volcanoes. The source mechanism associated with sustained gas and/or ash venting likely departs significantly from a monopole approximation (Woulff and McGetchin, 1976;Matoza et al, 2013).…”

Automated detection and cataloging of global explosive volcanism using the International Monitoring System infrasound network

“…Thus, there may be significant variability in acoustic source power between eruptions with the same assigned integer VEI value. The relationship between acoustic power and gas exit velocity was reviewed by Matoza et al [2013a].…”

“…2. Large sustained explosive eruptions typically produce long-duration volcanic jet noise signals (continuous infrasound signals lasting from hours to days [e.g., Matoza et al, 2013a]) or sequences of multiple shorter duration signals (e.g., a series of explosions over several days lasting a few minutes to hours in duration each [e.g., Matoza et al, 2011a]). At a given array, we therefore expect a large number of signal detections coming from the same backazimuth (varying slightly due to atmospheric propagation effects) throughout an eruption (Figure 2).…”

Seismic-acoustic energy partitioning during a paroxysmal eruptive phase of Tungurahua volcano, Ecuador

“…The importance of these observations on volcanic jets has been highlighted by Matoza et al (2013) who show that the estimation of total power may be subjected to large errors if both directivity and temperature parameters are not taken into account. Following their suggestion, we write the time-dependent acoustic intensity I a (t) for a given station as…”

“…In particular, Woulff & McGetchin (1976) presented power laws, using dimensional analysis, for the radiated-acoustic power of eruptions where their possible sources are monopolar (due to changes in mass fluxexplosions), dipolar (fluid-solid interaction) or quadrupolar (fluidfluid interactions). Matoza et al (2013) modified the quadrupolar power law to account for the directivity and temperature of the jets. The implication of these concepts, field observations and analyses of far-field infrasound records on Tungurahua, including the case here studied and other volcanoes, have been presented by Garcés et al (2008), Matoza et al (2009 and Matoza & Fee (2014).…”