Milton Garces

Global morphology of infrasound propagation

[i] Atmospheric sound waves in the 0.02-10 Hz region, also known as infrasound, exhibit long-range global propagation characteristics. Measurable infrasound is produced around the globe on a daily basis by a variety of natural and man-made sources. As a result of weak classical attenuation (∼0.01 dB km -1 at 0.1 hz), these acoustic signals can propagate thousands of kilometers in tropospheric, stratospheric, and lower thermospheric ducts. To model this propagation accurately, detailed knowledge of the background atmospheric state variables, the global winds and temperature fields from the ground to ∼170 km, is required. For infrasound propagation calculations, we have developed a unique atmospheric specification system (G2S) that is capable of providing this information. Using acoustic ray tracing methods and detailed G2S atmospheric specifications, we investigate the major aspects of the spatiotemporal variability of infrasound propagation characteristics.

Magma acoustics and time-varying melt properties at Arenal Volcano, Costa Rica

The similarity of acoustic and seismic spectra recorded during Strombolian activity of Arenal Volcano provides conclusive evidence that pressure waves are generated and propagated within the magma-gas mixture inside volcanic conduits. These pressure waves are sensitive to the flow velocity and to small changes in the gas content of the magma-gas mixture, and thus can provide useful indicators of the time-varying properties of the unsteady flow regime and the chemical composition of the melt. The dominant features of the observed explosion and tremor signals are attributed to the source excitation functions and the acoustic resonance of a magma-gas mixture inside the volcanic conduit. We postulate that explosions are triggered in the shallow parts of the magma conduit, where a drastic pressure drop with depth creates a region where violent degassing can occur. Tremor may be sustained by unsteady flow fluctuations at depth. Equilibrium degassing of the melt creates a stable, stratified magma column where the void fraction increases with decreasing depth. Disruption of this equilibrium stratification is thought to be responsible for observed variations in the seismic efficiency of explosions and enhanced acoustic transmission from the interior of the conduit to the atmosphere.

Theory of the airborne sound field generated in a resonant magma conduit

Abstract Explosive sources triggered inside a magma conduit may excite the conduit into acoustic resonance. The acoustic field in the conduit can propagate into the atmosphere through an open vent and ensonify the overlying atmosphere. The character of the airborne sound field is determined by a combination of propagation and source effects: the resonance of the magmatic conduit and the diffraction of the sound field at the volcanic vent are acoustic propagation effects, whereas the explosion pressure signature and the firing rate of the explosions define the source characteristics. For wavelengths larger than the conduit radius, only the longitudinal resonances of the magmatic conduit are relevant, and the open vent radiates like a piston surrounded by an infinite baffle. In this case, the fluid particle velocities are directed along the axis of the conduit and the sound field may propagate into the surrounding bedrock through the conduit wall displacement induced by the fluid overpressure. This coupling may produce seismic signals with banded spectra, such as volcanic tremor and long-period events. The airborne pressure field retains the modal structure of the sound field in the magmatic conduit, which contains information on the conduit geometry and geo-acoustic properties of the magma. The seismic wavefield is driven by the acoustic field in the magma and also contains this information, although it may be filtered by propagation effects in the bedrock. The theoretical sound field in the magma conduit is used to interpret seismic tremor signals recorded at Mt. Spurr Volcano, Alaska. Rapid variations in the acoustic impedance of the magma conduit terminations can create systematic changes in the tremor spectra, which can be used to monitor changes in the magmatic system. The results of the modelling illustrate the difference between source effects and conduit resonance, as well as the value of seismoacoustic measurements in volcanic environments.

Acoustic propagation and atmosphere characteristics derived from infrasonic waves generated by the Concorde

Infrasonic signals generated by daily supersonic Concorde flights between North America and Europe have been consistently recorded by an array of microbarographs in France. These signals are used to investigate the effects of atmospheric variability on long-range sound propagation. Statistical analysis of wave parameters shows seasonal and daily variations associated with changes in the wind structure of the atmosphere. The measurements are compared to the predictions obtained by tracing rays through realistic atmospheric models. Theoretical ray paths allow a consistent interpretation of the observed wave parameters. Variations in the reflection level, travel time, azimuth deviation and propagation range are explained by the source and propagation models. The angular deviation of a rays azimuth direction, due to the seasonal and diurnal fluctuations of the transverse wind component, is found to be approximately 5 degrees from the initial launch direction. One application of the seasonal and diurnal variations of the observed phase parameters is the use of ground measurements to estimate fluctuations in the wind velocity at the reflection heights. The simulations point out that care must be taken when ascribing a phase velocity to a turning height. Ray path simulations which allow the correct computation of reflection heights are essential for accurate phase identifications.

Canonical model of volcano acoustics

A full wave-theoretical model is developed for the acoustic field generated by an explosive point source embedded in a magma column that is open to the atmosphere. The Greens functions for the field in the magma and the atmosphere are derived on the basis of several simplifying assumptions concerning the geometry of the conduit, the boundary conditions, and the geoacoustic properties of the magma. A kinematic model for the acoustic signature of the explosive source is proposed, which, when combined with the Greens functions, provides full analytical expressions for the acoustic field in the magma and in the atmosphere as (complex) functions of frequency. By Fourier inversion the airborne pressure spectrum is transformed into a pressure time series. The predicted sound pulse in the atmosphere and its energy spectrum are highly dispersive, showing complicated structure that arises from the coherent addition of many normal modes of oscillation (i.e. depth and radial resonances) in the magma column. At high frequencies, for which the aperture of the vent is many wavelengths across, each mode is launched into the atmosphere as a parallel-sided beam of sound with a characteristic angle of elevation, which, through Snells law, is determined by the speed of sound in the magma relative to that in air. At somewhat lower frequencies, the modal beams of sound undergo angular spreading due to diffraction at the edge of the vent. In the lowest-frequency regime, where the wavelength is comparable with the aperture, the airborne field shows little angular structure. A comparison between airborne acoustic data that we recorded in July 1994 at the western vent of Stromboli Volcano and the predictions of the theory, using parameters that are characteristic of Stromboli, show compelling agreement. The theoretical and observed power spectra both display the following features: (1) a concentration of energy below 20 Hz, associated with the first four longitudinal resonances; (2) radial resonances between 35 and 65 Hz; and (3) a broad minimum around 30 Hz, arising because the source lies near nulls in longitudinal modes that would otherwise be excited. The conclusion is that the airborne sound signature from an explosive volcanic event may be inverted to provide estimates of the depth and radius of the magma conduit, the depth, spectral shape and peak shock-wave pressure of the source, and the viscosity of the magma.

Capturing the Acoustic Fingerprint of Stratospheric Ash Injection

More than 100 separate incidents of interactions between aircraft and volcanic ash were documented between 1973 and 2003. Incidents on international flight paths over remote areas have resulted in engine failures and significant damage and expense to commercial airlines. To protect aircraft from volcanic ash, pilots need rapid and reliable notification of ash- generating events. A global infrasound array network, consisting of the International Monitoring System (IMS) and other national networks, has demonstrated a capability for remote detection of Vulcanian to Plinian eruptions that can inject ash into commercial aircraft cruise altitudes (approximately 12 kilometers) near the tropopause. The identification of recurring sound signatures associated with high- altitude ash injection implies that acoustic remote sensing can improve the reliability and reduce the latency of these notifications.

Infrasonic precursors to a Vulcanian Eruption at Sakurajima Volcano, Japan

The May 1998 eruption sequence of Sakurajima Volcano was monitored by ten infrasonic stations, ten seismometers, and a video camera. During this seismo-acoustic experiment, we recorded hundreds of infrasonic tremor and long-period events associated with seismic signals, and observed a progression from relative quiescence to a Vulcanian eruption. The number of infrasonic events increased with escalating volcanic activity, and the dominant character of the infrasonic signals changed from impulsive to emergent. At 22:17 of May 19, Sakurajima released ash and gases to a height of 2 km above the vent, an event that was recorded continuously by one infrasonic and two seismic stations. We present the experimental setup as well as a procedure through which infrasonic signals may be incorporated into future eruption monitoring and forecasting algorithms for open-vent volcanic systems. In addition, our recordings suggest that infrasonic signals are more representative of processes occurring within the volcanic interior than are seismic signals, which are strongly altered by diffraction and scattering in the volcanic edifice.

Infrasonic Observations of Open Ocean Swells in the Pacific: Deciphering the Song of the Sea

Ocean waves produced during severe marine weather may generate infrasonic signals in the 0.1–0.5 Hz frequency band. Theory suggests that the source mechanism for these infrasound signals, known as microbaroms, is the nonlinear interaction of ocean surface waves. Multiple swells coexisting at any given point are able to radiate infrasonic waves if the ocean-wave spectrum contains swell components that are almost opposite in direction and of a nearly identical frequency. Global ocean-wave spectra provided by the National Oceanic and Atmospheric Administration’s (NOAA’s) Wavewatch 3 (WW3) model can be used to estimate the acoustic source pressure spectra induced by nonlinear ocean-wave interactions. Comparison of microbarom observations with surface weather, ocean-wave charts, and WW3-produced acoustic sources suggests that microbarom source regions occur in locations that contain opposing wave trains, instead of exclusively from regions of marine storminess.

Listening to the secret sounds of Earth's atmosphere

A new global network is breathing life into a dormant branch of geophysics. The study of infrasound, or long-period acoustic signals in the atmosphere was bustling in the 1950s and 1960s. Prior to 1963, almost all nuclear tests occurred in the atmosphere. After 1963, the USSR and U.S. signed the Limited Test Ban Treaty (LTBT), which eliminated all atmospheric nuclear tests. During the era of atmospheric nuclear testing, infrasound research was in demand, since the massive explosions produced strong, long-period acoustic waves that were globally observed and could be used to locate and describe the nuclear tests. Interest in this branch of geophysics waned with the end of atmospheric testing.

On the volcanic waveguide

An analytical solution for the sound field in a resonant magma conduit with depth-dependent magma properties provides a versatile interpretative tool for the analysis of seismoacoustic signals generated by fluid processes in active volcanoes. Expressions are given for the acoustic field in the atmosphere and the seismic field in the ground radiated through the magma conduit vent and walls, respectively. A new source model allows the resonance in the conduit to be excited by the vertical fluid velocity at the bottom of the conduit. This source model represents acoustic excitation induced by low-frequency fluid oscillations or large volume fluctuations originating at depth. The magma conduit is modeled as a three-section duct, with each section having a different density, sound speed, viscosity, and dimension. The modal structure of the coupled resonant system is retained as the sound field in the conduit propagates into the surrounding bedrock and the overlying atmosphere. The source-time function permits estimates of the total mass injected or removed from the magmatic system, and the source spectrum acts as a band-pass filter on the resonant modes of the conduit. Theoretical or empirical source functions can be used to drive the seismoacoustic wave field, and the amplitude of the source velocity may be constrained by the ballistics of pyroclasts and the height of ash plumes. The acoustic properties of the melt may be predicted from the temperature, composition, and flow conditions of the magma as a function of depth. Comparisons of synthetic waveforms and spectra with those of seismic and acoustic signals recorded on active volcanoes permit estimates of the conduit geometry, density, sound speed, and attenuation properties of the melt, and characterize the temporal and spectral features of the source function. The ability to extract this information accurately and efficiently from seismic and acoustic data streams may permit near real time modeling of fluid-driven processes in active volcanoes.