Detection, source location, and analysis of volcano infrasound

Thesis (Ph.D.) University of Alaska Fairbanks, 2017The study of volcano infrasound focuses on low frequency sound from volcanoes, how volcanic processes produce it, and the path it travels from the source to our receivers. In this dissertation we focus on detecting, locating, and analyzing infrasound from a number of different volcanoes using a variety of analysis techniques. These works will help inform future volcano monitoring using infrasound with respect to infrasonic source location, signal characterization, volatile flux estimation, and back-azimuth to source determination. Source location is an important component of the study of volcano infrasound and in its application to volcano monitoring. Semblance is a forward grid search technique and common source location method in infrasound studies as well as seismology. We evaluated the effectiveness of semblance in the presence of significant topographic features for explosions of Sakurajima Volcano, Japan, while taking into account temperature and wind variations. We show that topographic obstacles at Sakurajima cause a semblance source location offset of ~360-420 m to the northeast of the actual source location. In addition, we found despite the consistent offset in source location semblance can still be a useful tool for determining periods of volcanic activity. Infrasonic signal characterization follows signal detection and source location in volcano monitoring in that it informs us of the type of volcanic activity detected. In large volcanic eruptions the lowermost portion of the eruption column is momentum-driven and termed the volcanic jet or gas-thrust zone. This turbulent fluid-flow perturbs the atmosphere and produces a sound similar to that of jet and rocket engines, known as jet noise. We deployed an array of infrasound sensors near an accessible, less hazardous, fumarolic jet at Aso Volcano, Japan as an analogue to large, violent volcanic eruption jets. We recorded volcanic jet noise at 57.6° from vertical, a recording angle not normally feasible in volcanic environments. The fumarolic jet noise was found to have a sustained, low amplitude signal with a spectral peak between 7-10 Hz. From thermal imagery we measure the jet temperature (~260 °C) and estimate the jet diameter (~2.5 m). From the estimated jet diameter, an assumed Strouhal number of 0.19, and the jet noise peak frequency, we estimated the jet velocity to be ~79 - 132 m/s. We used published gas data to then estimate the volatile flux at ~160 - 270 kg/s (14,000 - 23,000 t/d). These estimates are typically difficult to obtain in volcanic environments, but provide valuable information on the eruption. At regional and global length scales we use infrasound arrays to detect signals and determine their source back-azimuths. A ground-coupled airwave (GCA) occurs when an incident acoustic pressure wave encounters the Earth's surface and part of the energy of the wave is transferred to the ground. GCAs are commonly observed from sources such as volcanic eruptions, bolides, meteors, and explosions. They have been observed to have retrograde particle motion. When recorded on collocated seismo-acoustic sensors, the phase between the infrasound and seismic signals is 90°. If the sensors are separated wind noise is usually incoherent and an additional phase is added due to the sensor separation. We utilized the additional phase and the characteristic particle motion to determine a unique back-azimuth solution to an acoustic source. The additional phase will be different depending on the direction from which a wave arrives. Our technique was tested using synthetic seismo-acoustic data from a coupled Earth-atmosphere 3D finite difference code and then applied to two well-constrained datasets: Mount St. Helens, USA, and Mount Pagan, Commonwealth of the Northern Mariana Islands Volcanoes. The results from our method are within ~<1° - 5° of the actual and traditional infrasound array processing determined back-azimuths. Ours is a new method to detect and determine the back-azimuth to infrasonic signals, which will be useful when financial and spatial resources are limited

Introduction to the special issue on three-dimensional underwater acoustics

Author Posting. © Acoustical Society of America, 2019. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 146(3), (2019): 1855-1857, doi:10.1121/1.5126013.This special issue focuses on compelling three-dimensional (3D) volumetric and boundary effects on underwater sound propagation and scattering in complex and time-varying (thus four-dimensional) underwater environments. It consists of 24 papers covering analytical, numerical, and experimental studies and presents a collection of up-to-date research on this active and relevant topic.2020-03-3

Identification of snow avalanche release areas and flow characterization based on seismic data studies

[eng] The main objectives of this PhD thesis are the identification of the snow avalanche release areas and the snow avalanche flow regime characterization with the use of seismic data. We aim to search for new methods for conducting detailed studies of the seismic signal generated by snow avalanches, considered as a moving seismic source. One of the main bases for achieving these objectives is to use widely tested seismological methods designed for the study of single seismic sources. In order to adapt them to a record of a moving source generating seismic vibrations (snow avalanches), we decided to apply windowing methods to the seismic signal so that the seismic signal could be processed in small portions. Over each time window of the seismic signal, we perform the study of ground particle motion polarization (3D) and obtain the frequency content information (Power Spectral Density). In the seismic signal produced by snow avalanches can be identified sections linked to the relative position of the avalanche front with respect to the position of the seismic sensor. The evolution of the total envelope of the seismic signal (At(t)) allows us to establish a criterion for the identification of the different sections based on the seismic signal amplitude. Furthermore, we are able to identify the moment when the snow avalanche mass starts to move (first vibrations generated by the avalanche) using an appropriate configuration of the STA/LTA algorithm adapted for our purposes. For each of the sections of the seismic signal, we apply a different methodology to obtain different information about the snow avalanche flow in each avalanche part. We seek to identify the snow avalanche release area by analysing the signal produced at the beginning of the snow avalanche mass movement (Signal Onset - SON). The study of the polarization of the ground particle motion (3D, ZNE coordinate system) makes it possible to identify the area where the first vibrations are generated, and by extension link it to the start of the snow avalanche mass movement. By analysing the seismic signal section corresponding to when the snow avalanche mass is flowing over the seismic sensor (Signal Over the sensor - SOV), we characterize the snow avalanche flow and identify the different regions in the snow avalanche body. The seismic signal is rotated to the QLT coordinate system in order to better link the information of the seismic signal to the flow progression direction. Work on this PhD thesis has also enabled us to establish a homogenization of the seismic data processing steps for studying snow avalanches. We automated these processes for all the data acquired in the Vallée de la Sionne test site between winter seasons 2013 and 2020, thereby creating a database consisting of more than 420 seismic events (source: automatic data acquisition system activations at VDLS). From all these events we managed to identify the snow avalanches and to test the procedures designed in this thesis for the release area identification and the flow characterization. Although the designed methods are subject to some limitations, we consider that the contribution to novel approaches for the study of the seismic signal produced by moving sources is proved. The release area identification has a very good success rate (78%) in a supervised application. The automated execution could be improved with a better isolation process for the seismic signal SON section. The method for the flow characterization provides new information regarding the interaction of the flow with the ground and with the snow cover. We consider that future studies in this direction may yield information about the snow avalanche basal friction as an indirect measurement.[cat] En aquesta tesi els objectius principals són la identificació de les zones d’alliberament d’allaus de neu i la caracterització del seu tipus de flux a partir de l’ús de dades sísmiques. Perseguim noves maneres de realitzar una caracterització detallada del senyal sísmic generat per allaus de neu. Ens basem en l’ús de mètodes sismològics àmpliament provats per a l'estudi de fonts sísmiques puntuals. Per tal de poder implementar aquests mètodes al registre sísmic d’una font sísmica en moviment (allaus de neu), utilitzem procediments de tractament del senyal en finestres de temps. En cada finestra del senyal sísmic, realitzem l’estudi de la polarització del moviment de la partícula (3D) i n’obtenim la informació del contingut en freqüències. L’evolució de l’evolvent total del senyal sísmic (At(t)), ens permet establir un criteri per a la identificació de diferents seccions en el senyal, relacionades amb la posició relativa de l’allau respecte el sensor sísmic. També podem identificar l’instant en què s’inicia el moviment de l’allau mitjançant una configuració adequada de l’algorisme STA/LTA. En cadascuna d’aquestes seccions hi apliquem una metodologia diferent per tal d’obtenir informació específica. Amb l'estudi de la polarització del moviment de la partícula sísmica en l’anàlisi del senyal sísmic produït per l’inici del moviment de l’allau de neu (secció SON), és possible identificar l'àrea des d'on es generen les primeres vibracions (78% de taxa d’èxit). Analitzant el senyal sísmic corresponent al pas de l’allau per sobre el sensor sísmic (secció SOV), caracteritzem el tipus de flux i identifiquem les regions en el cos de l’allau a partir de l’evolució del moviment de la partícula sísmica i l’evolució del contingut en freqüències. L'elaboració d'aquesta tesi també ens ha portat a homogeneïtzar i automatitzat els passos en el processament de dades sísmiques per a l’estudi de les allaus, creant una base de dades amb més de 420 esdeveniments sísmics al lloc experimental de Vallée de la Sionne (2013-2020). Tot i que trobem algunes limitacions, considerem que queda demostrada la contribució en nous enfocaments per a l’estudi del senyal sísmic produït per fonts sísmiques en moviment

Efficient localization in a dispersive waveguide : applications in terrestrial continental shelves and on Europa

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 211-225).Methods are developed for passive source localization and environmental parameter estimation in seismo-acoustic waveguides by exploiting the dispersive behavior of guided wave propagation. The methods developed are applied to the terrestrial continental shelf environment and the Jovian icy satellite Europa. The thesis is composed of two parts. First, a method is derived for instantaneous source-range estimation in a horizontally-stratified ocean waveguide from passive beam-time intensity data obtained after conventional plane-wave beamforming of acoustic array measurements. The method is advantageous over existing source localization methods, since (1) no knowledge of the environment is required except that the received field should not be dominated by purely waterborne propagation, (2) range can be estimated in real time with little computational effort beyond plane-wave beamforming, and (3) array gain is fully exploited. Second, source range estimation and environmental parameter inversion using passive echo-sounding techniques are discussed and applied to Europa. We show that Europa's interior structure may be determined by seismo-acoustic echo sounding techniques by exploiting natural ice fracturing events or impacts as sources of opportunity.(cont.) A single passive seismic sensor on Europa's surface may then be used to estimate the thickness of its ice shell and the depth of its subsurface ocean. To further understand the seismo-acoustic characteristics of natural sources on Europa, a fracture mechanics model is developed for the initiation and propagation of a crack through a porous ice layer of finite thickness under gravitational overburden. It is found that surface cracks generated in response to a tidally induced stress field may penetrate through the entire outer brittle layer if a subsurface ocean is present on Europa. While Europa's ice is likely highly porous and fractured, our current caculations show that porosity-induced scattering loss of ice-penetrating radar waves should not be significant.by Sunwoong Lee.Ph.D

Blind deconvolution in multipath environments and extensions to remote source localization

In the ocean, the acoustic signal from a remote source recorded by an underwater hydrophone array is commonly distorted by multipath propagation. Blind deconvolution is the task of determining the source signal and the impulse response from array-recorded sounds when the source signal and the environment’s impulse response are both unknown. Synthetic time reversal (STR) is a passive blind deconvolution technique that accomplishes two remote sensing tasks. 1) It can be used to estimate the original source signal and the source-to-array impulse responses, and 2) it can be used to localize the remote source when some information is available about the acoustic environment. The performance of STR for both tasks is considered in this thesis. For the first task, simulations and underwater experiments (CAPEx09) have shown STR to be successful for 1.5-4 kHz broadcast signal. Here STR is successful when the signal-to-noise ratio is high enough, and the receiving array has sufficient aperture and element density so that conventional delay-and-sum beamforming can be used to distinguish ray-path-arrival directions. Also, an unconventional beamforming technique (frequency-difference beamforming) that manufactures frequency differences from the recorded signals has been developed. It allows STR to be successful with sparse array measurements where conventional beamforming fails. Broadband simulations and experimental data from the focused acoustic field experiment (FAF06) have been used to determine the performance of STR when combined with frequency-difference beamforming. For the source localization task, the STR-estimated impulse responses may be combined with ray-based back-propagation simulations and the environmental characteristics at the array into a computationally efficient scheme that localizes the remote sound source. These localization results from STR are less ambiguous than those obtained from conventional matched field processing in the same bandwidth. However, when the frequency of the recorded signals is sufficiently low and close to modal cutoff frequencies, STR-based source localization may fail because of dispersion in the environment. For such cases, an extension of mode-based STR has been developed for sound source ranging with a vertical array in a dispersive underwater sound channel using bowhead whale calls recorded with a 12-element vertical array (Arctic 2010).PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/102443/1/shimah_1.pd

Highly-sensitive measurements with chirped- pulse phasesensitive OTDR

Distributed optical fiber sensing is currently a very predominant research field, which perceives optical fibers as the potential nervous system of the Earth. Optical fibers are understood as continuous densely-packed sensing arrays, able of retrieving physical quantities from the environment of the fiber. Some of the most prominent distributed sensing implementations nowadays rely on performing interferometric measurements using the Rayleigh backscattered light, resorting to a technique called Phase-sensitive Optical Time-Domain Reflectometry (CP-ϕOTDR). A variant to this technique has been recently proposed in 2016, known as Chirped-Pulse Phase-Sensitive OTDR, which allowed to overcome most of the limitations of traditional ϕOTDR implementations while retaining a simple setup, yielding remarkably high sensitivities. In this thesis, we aim to optimize the stability and performance of chirped-pulse ϕOTDR systems over long-term measurements, and develop novel paradigm changing applications benefiting from the high sensitivity provided by the technique. We reach a mK-scale long-term stability in ϕOTDR systems, and perform highly sensitive strain, temperature, and refractive index measurements, demonstrating new photonic applications such as distributed bolometry, electro-optical reflectometry, or distributed underwater seismology. We discuss how these applications might be able of increasing the efficiency in the energy field, paving the way towards the development of self-diagnosable grids (smart-grids), and also of revolutionizing next-generation seismological networks, allowing to overcome some of the greatest limitations faced in modern seismology today.Distributed optical fiber sensing is currently a very predominant research field, which perceives optical fibers as the potential nervous system of the Earth. Optical fibers are understood as continuous densely-packed sensing arrays, able of retrieving physical quantities from the environment of the fiber. Some of the most prominent distributed sensing implementations nowadays rely on performing interferometric measurements using the Rayleigh backscattered light, resorting to a technique called Phase-sensitive Optical Time-Domain Reflectometry (φOTDR). A variant to this technique has been recently proposed in 2016, known as Chirped-Pulse Phase-Sensitive OTDR, which allowed to overcome most of the limitations of traditional φOTDR implementations while retaining a simple setup, yielding remarkably high sensitivities. In this thesis, we aim to optimize the stability and performance of chirped-pulse φOTDR systems over long-term measurements, and develop novel paradigm changing applications benefiting from the high sensitivity provided by the technique. We reach a mK-scale long-term stability in φOTDR systems, and perform highly sensitive strain, temperature and refractive index measurements, demonstrating new photonic applications such as distributed bolometry, electro-optical reflectometry, or distributed underwater seismology. We discuss how these applications might be able of increasing the efficiency in the energy field, paving the way towards the development of self-diagnosable grids (smart-grids), and also of revolutionizing nextgeneration seismological networks, allowing to overcome some of the greatest limitations faced in modern seismology today. We finally conclude and summarize the objectives achieved in this thesis, commenting on the potential of the novel applications shown, and proposing future lines of research based on the results

Ocean Noise

Scientific and societal concern about the effects of underwater sound on marine ecosystems is growing. While iconic megafauna was of initial concern, more and more taxa are being included. Some countries have joined in multi-national initiatives to measure, monitor and mitigate environmental impacts of ocean noise at large, trans-boundary spatial scales. Approaches to regulating ocean noise change as new scientific evidence becomes available, but may also differ by country. The OCEANOISE conference series has provided a platform for the exchange of scientific results, management approaches, research needs, stakeholder concerns, etc. Attendees have represented various sectors, including academia, offshore industry, defence, NGOs, consultants and government regulators. The published articles in the Special Issue cover a range of topics and applications central to ocean noise