ALBERT

Description: Air-ground and ground-air elastic wave coupling are key processes in the rapidly developing field of seismo-acoustics and are particularly relevant for volcanoes. During a sustained explosive volcanic eruption, it is typical to record a sustained broadband signal on seismometers, termed eruption tremor. Eruption tremor is usually attributed to a subsurface seismic source process, such as the upward migration of magma and gases through the shallow conduit and vent. However, it is now known that sustained explosive volcanic eruptions also generate powerful tremor signals in the atmosphere, termed infrasonic tremor. We investigate infrasonic tremor coupling down into the ground and its contribution to the observed seismic tremor. Our methodology builds on that proposed by Ichihara et al. (2012) and involves cross correlation, coherence, and cross-phase spectra between waveforms from nearly collocated seismic and infrasonic sensors; we apply it to datasets from Mount St. Helens, Tungurahua, and Redoubt volcanoes.

Description: We obtained an unprecedented view of the acoustic radiation from persistent strombolian volcanic explosions at Yasur volcano, Vanuatu from the deployment of infrasound sensors attached to a tethered aerostat. While traditional ground-based infrasound arrays may sample only a small portion of the eruption pressure wavefield, we were able to densely sample angular ranges of ~200 o in azimuth and ~50 o in take-off angle by placing the aerostat at 38 tethered loiter positions around the active vent. The airborne data joined contemporaneously collected ground-based infrasound and video recordings over the period 29 July to 1 August 2016. We observe a persistent variation in the acoustic radiation pattern with average eastward-directed root-mean squared pressures more than 2 times larger than in other directions. The observed radiation pattern may be related to both path effects from the crater walls, and source directionality.

Description: [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). However, jet noise studies since the 1990s have largely abandoned some key ideas used by Woulff and McGetchin (1976), e.g., that jet noise can be represented as an equivalent distribution of dipoles or quadrupoles. In addition, the directionality of jet noise is now better characterized and understood. Based upon detailed laboratory studies, new empirical scaling laws have been proposed that take into account the temperature and directional dependence of jet noise (Viswanathan, 2006). We reexamine the approach of Woulff and McGetchin (1976) within the context of the present day understanding of jet noise. We use laboratory jet, full-scale military jet aircraft, and full-scale rocket motor noise data to highlight the complications in inferring volcanic gas exit velocity from infrasound data. Accurate estimates of acoustic power require good spatial sampling of the jet noise directionality as a function of angle from the jet axis; this is not usually possible in volcano acoustic field experiments. Typical volcano acoustic data, which have limited angular sampling of jet noise directionality, better represent point measurements of acoustic intensity ineI ( θ ) at a particular angle rather than acoustic power. For pure-air jet flows, the velocity scaling laws currently proposed for acoustic intensity differ from those for acoustic power and are of the form , where c is the ambient sound speed and n θ varies nonlinearly from ~5 to 10 as a function of temperature ratio and angle θ from the jet axis. Volcanic jet flows are more complex than the pure-air laboratory case, which suggests that we do not currently know how the exponent n θ varies for a volcanic jet flow. This indicates that the original formulation of Woulff and McGetchin (1976) can lead to large errors when inferring eruption dynamics from acoustic data and 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 column parameters.

Description: The two major explosive phases of the 22–23 April 2015 eruption of Calbuco volcano, Chile produced powerful seismicity and infrasound. The eruption was recorded on seismo-acoustic stations out to 1,540 km and on 5 stations (IS02, IS08, IS09, IS27, and IS49) of the International Monitoring System (IMS) infrasound network at distances from 1,525 to 5,122 km. The remote IMS infrasound stations provide an accurate explosion chronology consistent with the regional and local seismo-acoustic data, and with previous studies of lightning and plume observations. We use the IMS network to detect and locate the eruption signals using a brute-force, grid-search, cross-bearings approach. After incorporating azimuth deviation corrections from stratospheric cross-winds using 3D ray-tracing, the estimated source location is 172 km from true. This case study highlights the significant capability of the IMS infrasound network to provide automated detection, characterization, and timing estimates of global explosive volcanic activity. Augmenting the IMS with regional seismo-acoustic networks will dramatically enhance volcanic signal detection, reduce latency, and improve discrimination capability.