IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_3E-P2

Date/Time: July 24 Poster

Infrasound of sustained volcanic eruptions, a laboratory and field study
Elizabeth E C Swanson, Theunissen Raf, Phillips C Jeremy, Rust Alison
University of Bristol, uk
E-mail: gleecs@bristol.ac.uk

Volcanic plumes represent a significant hazard both near to and at distance from source. Unfortunately, hightemporal resolution monitoring, especially in the case of more remote systems, is often lacking. Due to their
turbulent nature, these plumes are a substantial source of sub 10 Hz sound; meaning infrasonic methodology has
the potential to fill the monitoring void. Recent studies of volcano infrasound have drawn upon research from theaero-acoustics industry, fitting the large scale jet noise spectrum (LSS) to that observed during column generating
eruption events (Matoza et al 2011). However, due to the differences between volcanic plumes and the pure gas
jets, from which the aero-acoustics spectra are created, further laboratory studies are required to investigate thetrue source of the volcanic signals.
In the absence of industrial standard anechoic chambers, and indeed in terms of the field application ofexperimental findings, successful source localisation methods are paramount. Adaptive beamforming, as opposedto the commonly used delay and sum methodology, offers a means to achieve the required spatial resolution of thegenerated sound field.
In a set of experiments at the University of Bristol, an omni-directional microphone array was used to demonstrate
the ability of the adaptive method. The acoustic signatures of a series of subsonic jets were recorded using theGFAL 36 and 48 element GFAL Acoustic Camera arrays and subsequently analsyed using both GFAL and in house
built software. The experiments were housed in a purpose built test structure lined with 10cm pyramid foam. A flow
settling chamber and baffle box, upstream of the test section, were used to prevent the transmission of rig noise
to the recorded signals. A synchronous PIV dataset was also obtained enabling visualisation of the jet structure.
The level of PIV seeding was varied to enable assessment of the effect of particulate matter upon the acoustic
signature. Furthermore a range of different nozzle shapes have been test to better reproduce the fully turbulent
conditions of volcanic jets.
In addition to the laboratory dataset, the beamforming methodology has been applied in the field setting at
Santiaguito volcano, Guatemala. A 4 element infrasound array consisting of a 1 Chaparral Physics Model 25Vx
and 3 Hyperion Technology IFS-3114 Infrasound Sensors with an inter-element spacing of 100 m was installedat the Santiaguito Volcano Observatory 6 km from the vent and used to retrieve dominant source locations andpowers.
The adaptive methods employed here offer the potential to remove dominate acoustic sources, which preliminary
findings show to be related to the nozzle/vent exit. Such source removal has the potential to reveal any weakerflow related sources which may be present.

REFERENCES: Matoza,R., et al (2009) Geophysical Research Letters Vol, 36 L08303.

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