Infrasound produced by degassing of shield-volcanoes: Hawaii and Galapagos, and its potential for atmospheric monitoring

Omar E. Marcillo and Jeffrey B. Johnson

Acoustic waves with frequencies below 20 Hz, or infrasound, propagate long distances with minimal intrinsic attenuation or dispersion. These characteristics make infrasound a powerful remote sensing tool. Sources of infrasound, natural and man-made, are mostly sporadic, including hurricanes, avalanches, earthquakes, and most volcanic eruptions. How-ever, some volcanic eruptions, such as the one at Kilauea Caldera, are active for years and produce high amplitude and continuous infrasound that can be recorded locally and regionally. This type of activity and its corresponding infrasound can be used to study the characteristics and evolution of the source vent. Also, this infrasound radiation provides a unique opportunity to probe the conditions of the near-surface atmo-sphere.

Volcanoes located on hotspots (e.g.: Hawaii, Galapagos) usually erupt ef-fusively releasing significant amounts of material and gas phases to the at-mosphere. The seismic signature of this type of activity has been used to infer the characteristics of the underground plumbing system and to monitor its evolution. The atmospheric acoustic response of this activity can provide important information regarding energy partitioning and the evolution of surface processes.

1. Infrasound and its potential for volcanic and atmospheric studies

References

5. Conclusions and future work

Fee, D., M. Garces, M. Patrick, B. Chouet, P. Dawson and D. Swanson (2010). "Infrasonic harmonic tremor and degassing bursts from Halema'uma'u Crater, Kilauea Volcano, Hawaii." Journal of Geophysical Research 115(B11): B11316.Marcillo, O. and J. B. Johnson (2011). "Implementation, characterization, and evaluation of an inexpensive low-power low-noise infrasound sensor based on a micro-machined differential pressure transducer and a mechanical filter.” In preparation.Marcillo, O. and J. B. Johnson (2010). "Tracking near-surface atmospheric conditions using an infrasound network." The Journal of the Acous-tical Society of America 128(1): EL14-EL19.

The signals recorded with a dense sensor network around Halema’uma’u crater are used to characterize the infrasonic field produced by the degassing of this active vent. The ability to use infrasound to estimate source parameters (e.g.: vent dimensions, precise vent loca-tion, or released gas amount) depends on our understanding of the effects of topography, atmospheric conditions, and energy attenua-tion. This study shows the influence of the atmospheric conditions in infrasound propagation by recovering atmospheric parameters (temperature and wind field) from variations in infrasound time arrivals. Island volcanoes can remain active for long periods of time and provide an ideal setting for continuous studies related to the evolution of volcanic eruptions. Volcanoes in the Galapagos Islands as well as Hawaiian volcanoes can provide an ideal laboratory to study the aerial response of volcanic activity. Already in-place seismic, and deformation networks can benefit from infrasound networks by con-straining the energy that is released in the atmosphere during volcanic unrest. Infrasound can also provide an invaluable tool for vol-cano surveillance in situations with poor visibility conditions.

4. Inverting phase delay times for temperature, wind direction and speed.Relative phase delay times, calculated by cross-correlation, can be inverted for temperature, wind direction and speed (Marcillo and Johnson, 2010). The inversion assumes that waves travel in straight lines from the vent to the sensor. The inversion is based on the fol-lowing system of equations:

The inversion of the delay times from the period with the highest cross-correlation (night hours) results in temperature and wind speed

and direction within reasonable values (see figure on the right). In the figure, ground based weather measurements are included for compari-

son

2. Kilauea volcano summer 2010: recording volcano infrasound with a dense local networkThe Halema`uma`u crater at the summit of Kilauea is a particularly loud and persistent infrasound source (Fee et al., 2010), which became active on March 12, 2008. During the summer of 2010, we deployed 47 infrasound sensors (red dots) around this vent in three different network topologies (see two of the topologies in the maps below). Each topology was on the ground and fully operational for 4 days; maximum radial distances for each configuration were 20, 10, and 5 km. The sensors were deployed in vegetated areas, when possible, to reduce the influence of wind and human noise. The instrumentation for this deployment was based on the infraNMT infra-sound sensor (see box on the right) and the single channel RefTek Texan datalogger.

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3. Infrasound from Halema’uma’u crater

Detectability of infrasound across the network is controlled primarily by the background noise and specific acoustic site-response char-acteristics. Night and day hours are characterized by low and high acoustic background noise, respectively. In addition, we found that the background noise changes spatially, i.e., the sensors located south and southeast of the vent(SE line), descending in the direction to the ocean, have permanent high background noise that masks almost completely the signals from the volcano. On the other hand, sta-tions located northwest of the vent, ascending the Mauna Loa road (ML line), feature a very low background noise that make the signal visible even during day hours.

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Example of signals recorded for the sensor on the crater rim

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Infrasound signals recorded at different stations

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The figure on the left shows two minutes of highly correlated infra-sound tremor (scaled at 1 km radial distance). Stations recording the tremor (located 2-4 km from the vent) are illustrated on the map. This tremor detail is a subset of the one hour tremor shown at bottom left. The figure below shows the corresponding Power Spectral Density (PSD) of the one hour tremor.

The infraNMT sensor (developed at New Mexico Tech) uses a piezoelectric transducer (AllSensors 0.5-INCH-D-MV) and incorporates a pair backing volume-capillary tube to filter out low frequency barometric fluctuations. The table on the right and the figure below show the characteristics of the sensor.

Low-cost infrasound sensor: infraNMT

10 cmINPUT FOR SIGNAL CABLE

INPUT TO THE PRESSURE SENSOR

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infraNMT SELF−NOISE RESPONSE

500−pound ANFO explosionrecorded 5.5 Km from the source

ALNM [Bowman et al., 2005]infraNMT self−noise response

Parameter UnitValue

Sensitivity μV/Pa45.1 +/- 0.23

Linear Range Pa+/-124.5

Noise(0.1-20 Hz) mPa rms5.47

Unit Price USD<1000

Power Consump. mW27

Sensor’s characteristics

Robust relative phase delay times can be calculated for periods of low back-ground noise. The figure on the left shows the cross-correlation of two sta-tions, one in ML line and the other in SE line (with the vent located between them). Cross-correlation with the signals filtered between 0.3-4 Hz (red) and spectrally whitened (blue). Pre-whitening reduces the correlation coeffi-cient value but permits the emergence of small signals. In this case, a coher-ent signal coming from the direction of the Pu’u O’o vent can be identified.

Infrasound recorded in a dense network around an active vent in Kilauea volcano illuminates source characteristics and details of near-surface atmospheric conditions. During the summer of 2010, 45 sensors were deployed for 10 days around the active Halema'uma'u vent to capture the atmospheric response of degassing processes at Kilauea volcano. The activity is characterized by continuous in-frasonic tremor with amplitudes of up to 10 Pa (when scaled at 1 km). Relative phase delay times are determined and inverted for air temperature and wind conditions of the near-surface atmosphere. Shield-volcanoes like the ones in Hawaii and Galapagos can remain active for long periods and provide an ideal setting for studies related to degassing processes and the evolution of the source vent. In these studies, infrasound can complement other techniques, such as, seismic, deformation, and gas monitoring by constraining the amount of acoustic energy released in the atmosphere.

This vent hosts an active convecting lava pond with prodigious continuous degassing and intermittent explosive events. This vent, active since 2008, generates high amplitude infrasonic tremor with amplitudes of up to 10 Pa ( when scaled at 1 km). This activity can last con-tinuously for tens of hours and is characterized by a main energy peak located between 0.3 and 0.4 Hz. Other energy peaks can be seen up to 4 Hz. The continuous activity is only interrupted by short duration intervals (< 5 hours) with undetectable infrasonic energy.

10−1 100 10110−7

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102Power Spectral Density Estimate

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Halema’uma’u energy peaks: 0.35, 0.55, 1.55,3.5 Hz.

microbarom energy (0.2 - 0.3 Hz) is masked by wind and volcanic energy

Signal coherency across the network

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0.3−3.5 Hz filtered signalspectrally whitened signal

Pu’u O’o

Halema’uma’u pit crater

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INVERSION OF RELATIVE DELAY TIMES August 2, 2010 WIND (m/s)

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ML line

SE line

The figure on the right shows two coher-ency maps (night and day hours) of the sig-

nals recorded by the network. The coher-ency is measured by cross-correlating a

controlled station to a target station, and using the maximum cross-correlation coef-

ficient for the color code. For each sensor line, the controlled station is the closest to

the VENT

Δti−base =di

C +r u ⋅

r n i

−dbase

C +r u ⋅

r n base

Δti−base =ux

C2 (dbase hx − diax ) +uy

C 2 (dbase hy − diay ) +1C

di − dbase( )

relative delay time equation: low-wind velocity approximation:

where is the unit vector to the wavefront, is the mean wind vector, C the sound velocity, d the distance from vent to the sensors, a and h the components of the unit vector of the i-th station and base station

r n r

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infraNMT( G model)

Support for this work was made possiblethrough the National Science Foundation ( EAR 0738802)