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IVERSIDADE DOS AÇ

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URORA SCIENTIA LUCE

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Ponta Delgada 2018

The use of infrasound in volcano monitoring.

Contribution for future application in the Azores Islands

Dissertação de Mestrado

Sandro Branquinho de Matos

Vulcanologia e Riscos Geológicos

Mestrado em

UN

IVERSIDADE DOS AÇ

OR

ES

SIC

UT A

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The use of infrasound in volcano monitoring.

Contribution for future application in the Azores Islands Dissertação de Mestrado

Orientadores

Doutor Nicolau Maria Berquó de Aguiar Wallenstein Professor Auxiliar, Universidade dos Açores Doutor Maurizio Ripepe Professor in Geophysics, UniFi -Università degli Studi di Firenze

Dissertação de Mestrado submetida como requisito parcial para obtenção do grau de Mestre em Vulcanologia e Riscos Geológicos

Sandro Branquinho de Matos

Table of Contents

I

TABLE OF CONTENTS

LIST OF FIGURES …………………………………………………………..……………………………………………………… V

LIST OF TABLES …..………………………………………………………………………………………………………….….. X

LIST OF PHOTOGRAPHIES ………………………………………………………………………………………………..… XI

LIST OF ACRONYMS ……………………………………………………………………………………………………….… XII

ACKNOWLEDGEMENTS ………………………………………………………………………………………………….. XIV

ABSTRACT ………………………………………………………………………………………………………………………. XVI

RESUMO …………………………………………………………………………………………………………………….…. XVII

CHAPTER 1. INTRODUCTION

1.1. Framework and objectives of the work ……………………………….……………………………………………...... 1

1.1.1. Motivation ………………………………………………………………………………………………………………….……………….….. 1 1.1.2. Study relevance …………………………………………………………………………………………………………………………..….. 1 1.1.3. Research objectives …………………………………………………………………………………………………………………………. 2 1.1.4. Dissertation structure and organization …………………………………………………………………………………………..… 2

CHAPTER 2. INFRASOUND

2.1. Introduction …………………………………………………………………………………………………..………………………………….. 4

2.1.1. What is Infrasound …………………………………………………………………………………………………………………….….... 5 2.1.2. Infrasound characteristics ………………………………………………………………………………………………………….…….. 5

2.2. Origin of Infrasound …………………………………………………………………………………………………………………….…… 6

2.2.1. Introduction ...……………………………………………………………………………………………………………………….……….. 6 2.2.2. Infrasound Natural sources ………………………………………………………………………………………………………..…….. 7 2.2.3. Anthropogenic sources ………………………………………………………………………………………………………….……. 11

2.3. Properties and propagation of sound waves ………………………………………………………………………………… 14

2.3.1. Physical properties of sound waves ……..………………………………………………………………………………………..… 14

2.3.1.1. Sound Attenuation ……………………………………………………………………………………………………………….. 16 2.3.1.2. Sound Dispersion ……………………………………………………………………………………………………………...….. 16 2.3.1.3. Sound Absorption ……………………………………………………………………………………………………………..…. 16

2.4. Atmosphere and Infrasound propagation ……………………………………………………………………………..…..….. 17

2.4.1. General concepts …………………………………………………………………………………..……………………………….…….. 17

2.4.1.1. Chemical composition of the atmosphere …………………………………………………………………….……….. 17 2.4.1.2. Thermal structure of the atmosphere ……………………………………………………………………………………. 18

2.4.2. Effects of winds in the propagation of infrasound …………………………………………………………………………….. 20 2.4.3. Waveguides ………………………………………………………………………………………………..……………………………….. 22 2.4.4. Infrasonic phases ……………………………………………………………………………..…………………………………….…….. 23

Table of Contents

II

CHAPTER 3. IMS INFRASOUND STATIONS

3.1. The Background ………………………………………………….…………………………………………………………………………... 26

3.1.1. Historical Review: Comprehensive Nuclear-Test-Ban Treaty (CTBT) …………………………………………………... 26

3.2. Introduction to the IMS infrasound stations …………………………………………………………..………………….…. 28

3.3. Basic components of an infrasound monitoring station ………………………………………………….…………... 30

3.3.1. Infrasound station array ……………………………………………………………………………………………………………….... 30 3.3.2. Array design ………………………………………………..………………………………………………………………………………... 32 3.3.3. Spatial aliasing ………………………………………………….………………………………………………………………………….... 33 3.3.4. Wind noise ………………………………………………………………………………….………………………………………………... 34 3.3.5. Data communication …………………………………………………………………………………………………………………….. 39

3.3.5.1. The Global Communications Infrastructure ………………………………………………………………………….... 39

3.4. General concepts in Infrasound data processing ……………………………………..…………………………………… 40

3.4.1. Time domain and Frequency domain …………………………………………………………………………………………...... 40 3.4.2. Progressive Multi-Channel Correlation algorithm - PMCC ……………………………………………………………..….. 43

3.4.2.1. Introduction …………………………………………………………………………………………………............................. 43

CHAPTER 4. METHODOLOGY

4.1. Introduction ……………………………………………………………………………………………………………………………..….….. 48

4.2. IMS Station IS42, Graciosa, Azores, Portugal ………………………………………………………………………………… 50

4.2.1. Location ……………………………………………………………………………………………………………………………………….. 50 4.2.2. Geological settings ……………………………………………….........................................………………………………….…… 52 4.2.3. Instrumentation ……………………………………………………………………………………….…………………………………… 53

4.2.3.1. The Infrasonic array …………………………………………………………………………………………………………... 54 4.2.3.1.1. Equipment vault …………………………………………………..……………………………………………….. 55

4.2.3.1.1.1. Microbarometer …………………………………………………………..……………………….….. 56 4.2.3.2. The Central recording Facility (CRF) ……………………………………………………..…………………………..….. 57

4.3. The ETN infrasound station ……………………………………..……………………………………………………..…………….… 57 4.3.1. The ETN infrasonic array ……………………………………………………………………………………………………………….... 58

4.4. Data Processing ……………………………………………………………………………………………………………………….…….… 58

4.4.1. Data preparation …………………………………………………………………………………………………………………………... 59 4.4.2. Processed data files organization ………………………………………………………..………………………………………...… 59 4.4.3. The use of GPMCC and MATLAB software in data analysis …………………….……………………………………….… 59

4.4.3.1. Graphical Progressive Multi-Channel Correlation (GPMCC) ……………..………………………………….... 59 4.4.3.1.1. PMCC data processing …………………………………………………………………………………………... 62

4.4.3.2. MATLAB scripts ……………………………………………………………………………………………………………..….. 64 4.4.3.2.1. Data preparation and organization ……….………………………………………………………………... 64 4.4.3.2.2. Data processing ……………………………………………………….………………………………………..….. 65

Table of Contents

III

CHAPTER 5. Long –Range Infrasound detections of volcanic activity by IS42 Station

5.1. General introduction to volcanic monitoring with infrasound resources ……………………………..……. 67

5.1.1. Chronology …………………………………………………………………………………………………………………………………... 68 5.1.2. Volcanic infrasound terminology …………………………………………………………………………………………………….. 69

5.2. Mount Etna activity ……………………………………………………………………………………………….………………………… 73

5.2.1. Introduction …………………………………………………………………………………………………………………………….….... 73

5.2.1.1. Geographic location ………………………………………………………………………………………………………….….. 74 5.2.1.2. Geological Framework ………………………………………………………………………………………………………….. 75 5.2.1.3. The Mount Etna Volcano …………………………………………………………………………………………….………... 75

5.2.2. The 2011 May – August lava fountaining activity detections …………………………………………………………...... 78

5.2.2.1. Data collection ……………………………………………………………………………………………………………………… 78 5.2.2.2. Framework of volcano and local infrasonic activity …………………………………………..………………….….. 79 5.2.2.3. Data analysis ………………………………………………………………………………………………………………………... 83

5.2.3. The 16th to 26th May 2016 eruptive activity ………………………………………………………….…….…………………….. 92

5.2.3.1. Data collection ……………………………………………………………………………………………………………….…….. 92 5.2.3.2. Framework of volcano and local infrasonic activity …………………………………………………………………... 92 5.2.3.3. Data analysis …………………………………………………………………………………………………………………….….. 96

5.3. Grímsvötn Volcano activity …………………………………………………………………………………………………..……...... 99

5.3.1. Introduction ………………………………………………………………………………………………………………………………….. 99

5.3.1.1. Geographic location …………………………………………………………………………………………………..…………. 99 5.3.1.2. Geological Framework ……………………………………………………………………………………………………...... 101 5.3.1.3. The Grímsvötn volcano ………………………………………………………………………………..……………….…..... 105

5.3.2. The 22nd to 30th May 2011 eruptive activity …………………………………………………………………………..….…. 106

5.3.2.1. Data collection ……………………………………………………………………………………………………………………. 107 5.3.2.2. Framework of volcano activity ……………………………………………………………………………………..………. 107 5.3.2.3. Data analysis ………………………………………………………………………………………………………………………. 108

CHAPTER 6. DISCUSSION OF THE RESULTS …………………………………………………….………………………………..……. 114

FINAL CONSIDERATIONS ……………………………………………………………………………….……………………….………………….. 126

BIBLIOGRAPHICAL REFERENCES ………………………………………………………………………………………….…………………… 129

WEBGRAPHICAL REFERENCES ………………………………………………………………………….……………………………………… 147

Table of Contents

IV

APPENDICES APPENDIX 01 - Some Infrasound sources ......................................................................................................................... …….. A-1

APPENDIX 02 - CTBTO IMS Stations Map Overview ................................................................................................................ A-3

APPENDIX 03 - Physical characteristics of the MB2005 equipment ...................................................................................... A-5

APPENDIX 04 - PMCC_Flow Chart .............................................................................................................................................. A-7

APPENDIX 05 - PMCC_conf_file used in GPMCC ...................................................................................................................... A-9

APPENDIX 06 - Some infrasound studies about volcanoes ...................................................................................................... A-13

APPENDIX 07 - 2011 volcanic activity news updates (INGV) ..................................................................................................... A-15

APPENDIX 08 - May-August 2011 Mt. ETNA activity bulletins ................................................................................................. A-24

APPENDIX 09 - May 2016 Mt. ETNA activity news update (INGV) ......................................................................................... A-31

APPENDIX 10 - May 2016 Mt. ETNA activity bulletins .............................................................................................................. A-36

APPENDIX 11 - 2011 IMO Icelandic Meteo Office bulletins ..................................................................................................... A-41

APPENDIX 12 - The 12th May paroxismal eruptive episode ..................................................................................................... A-51

APPENDIX 13 - The 9th July paroxismal eruptive episode ......................................................................................................... A-55

APPENDIX 14 - The 19th July paroxismal eruptive episode ....................................................................................................... A-59



APPENDIX 17 - The 5th August paroxismal eruptive episode ................................................................................................... A-72

APPENDIX 18 - The 12th August paroxismal eruptive episode ................................................................................................. A-75



APPENDIX 21 - May 2016 Mount Etna explosive activity ......................................................................................................... A-87

APPENDIX 22 - 2011 Grímsvötn eruptive activity ...................................................................................................................... A-96

List of Figures

V

LIST OF FIGURES

Fig. 2.1 -1883 Krakatoa eruption (VEI 6) in the Dutch East Indies (actual Indonesia) ………………….……………………….………….. 4

Fig. 2.2 - Sound Wave propagation (physics.tutorvista.com, adapted). …………………………………………………………….……….. 4

Fig. 2.3 - Sound Wave propagation (CTBTO, 2009). ……………………………………………………………………..………………………..…. 5

Fig. 2.4 - Infrasound sources (Christie & Campus, 2010). …….…………………………………………………………………….…………..….. 6

Fig. 2.5 - Lightning storm types and Transient Luminous Event (TLE) (www.nasa.gov) . ………………………………………..….... 10

Fig. 2.6 - a) Two explosions recorded at IS42 (elements H1-H4); and b) Spectrogram of the two explosions (elements H1 to H4) (Wallenstein & Campus, 2017). ……………………………………………………………………………..………………………………...…….. 12

Fig. 2.7 - Diagram illustrating that compressions and rarefactions are respectively regions of higher and lower than normal atmospheric pressure (https://www.physicsforums.com, adapted). .………………………………...…………..……………………….. 14

Fig. 2.8 - Temperature profile of a standard atmosphere (http://pangeaplusgeo.weebly.com/a-layers.html). ………... 18

Fig. 2.9 - a) The sound speed in the atmosphere that results from the temperature profile is shown vs. Altitude. b) Northern hemisphere typical atmospheric profile with the primary layers of the atmosphere and (b, c) their seasonal variation (adapted from Waxler, 2016). …………………………………………………………………………………………………….……....…….………….. 21

Fig. 2.10- Directions of zonal wind in North Hemisphere (e.g.over Europe) depending on the season (Mialle, 2007). …. 21

Fig. 2.11- Waveguide is any region in which a layer of smaller sound velocity is bounded above or below by regions of higher velocity and the Earth’s surface (CTBTO, 2009). ………………………………….………………………………………..……………………..….. 22

Fig. 2.12 - Simulations of the propagation of infrasound in the atmosphere (CTBTO, 2009). ……………………………...…..….. 25

Fig. 3.1 - CTBT treaty book and text on the establishment of a Preparatory Commission for the CTBTO (image from https://www.ctbto.org/the-treaty/treaty-text/). …………………………………………………………………..……..…………………….….. 27

Fig. 3.2 - IMS infrasound stations network on 01 October 2018 (adapt. from https://www.ctbto.org). …………….…….….. 28

Fig. 3.3 - Basic Infrasound station elements (CTBTO, 2017). See text for description of the different components of the station. ………………………………………………………………………………………………………………………………………………...………….….. 30

Fig. 3.4 - Possible configurations of Infrasound arrays. They depend on local conditions and logistical issues. Stations located in high wind areas or on islands often require more array elements to improve their detection capability (CTBTO, 2009). …………………………………………………………………………………………………..……………………………………………………………....…….….. 32

Fig. 3.5 - Array layout and array response of a 4-Element station (1st and 2nd images from the left), and an array layout and response for an eight-element pentagon main array with a triangular subarray (3rd and 4th images from the left) (CTBTO, 2009). ……………………………………………………………………………………………………………………..…………………………...………….….. 34

Fig. 3.6 - IS42 Geometry and response (CTBTO, 2017). ………………………………………………………………………..………………….. 34

List of Figures

VI

Fig. 3.7 - Temperature profiles during the day (BCD; inversion between CB) and night (AB; inversion). The temperature increase during the day in the surface layer is one mechanism for driving wind (Walker & Hedlin, 2010). ………..…….…... 36

Fig. 3.8 - Regional influences affect the wind in the interior of the continents during the day, due to variations of solar heating near the surface, (Walker & Hedlin, 2010). …………………………………………………………………………………………………….…...….. 36

Fig. 3.9 - Regional influences on coastal winds during the day lead to convective systems at night. (Walker & Hedlin, 2010). ………………………………………………………………………………………………………………………………………………………………………..…….. 37

Fig. 3.10 - Examples of some of the wind-noise-reducing systems (WNRS) used at stations in the IMS infrasound network (adapted from Christie & Campus, 2010). ……………………………………………………………………………………………………………….. 38

Fig. 3.11 - GCI Global Communications Infrastructure (CTBTO 2017). …………………………………………….……………………….... 40

Fig. 3.12 - Time and frequency domain (https://www.quora.com). …………………………………………...........……………….…….. 41

Fig. 3.13 - Depicts time domain and frequency domain measurements. Any signal can be represented in this three-dimensional axis viz. time, amplitude and frequency. As shown complex waveform can be obtained as sum of multiple sine waves of different amplitude, phase and frequency (image by ©T&M World. www.test-and-measurement-world.com). 41

Fig. 3.14 - Example of a signal pictured in time-domain graphic (left) with amplitude (vertical axis) versus time (horizontal axis) (https://www.erzetich-audio.com/knowledgebase-05-time-vs-frequency.html) …………………………..…………….….... 42

Fig. 3.15 - Mathematical Fourier Transform functions. ………………………………………………..……………………….………………….. 42

Fig. 3.16 - PMCC post processing connection of close-enough pixels into a family (Le Pichon & Cansi, 2003). …………… 46

Fig. 4.1 - Geographical location of the Azores archipelago (from ESRI, adapted by Wallenstein et al., 2011). ……..……….. 50

Fig. 4.2 - Graciosa Island and I42 array area (yellow) (Wallenstein, 2011). ..……………………………………………………...…..…... 51

Fig. 4.3 - Azores region main tectonic structures (adapted from Carmo et al., 2015). ………………….………………………...……... 53

Fig. 4.4 - Configuration of infrasound sites and access roads. ……..………………………………………………………….…………….….. 54

Fig. 4.5 - MB2005 Microbarom sensor (CEA-DASE, 2017). ……..…………………………………………………………………….………….. 56

Fig. 4.6 - Frequency response of the MB2005 sensor (CEA-DASE, 2017). …..…………………………………………….………….….. 57

Fig. 4.7 - A) ETN position map (white star), geometry (inset) of the array and B) back-azimuth of the five main ative vents and effusive fracture (EF) of 2008 (Uliveri et al., 2013). ..………………………………………………………………….………………….………….. 58

Fig. 4.8 - DTK-GPMCC main window display: (1) toolbar, where can access to general tools; (2) Station Tabs (can be more than one); (3) Detection Atributte Pixels panels, where processed PMCC detection atributes are displayed; (4) Waveform and Arrival Picks, where recorded waveform and detected arrival time are displayed. ………………………………………………. 62

Fig. 4.9 - Fiftheen-bands with logarithmically spaced filter parameters configuration (0.07 Hz-4 Hz) with variable window length. On the left side the frequency bands attributes are displayed. On the right side the frequency response (magnitude and phase) filters are shown. …………………………………………………………………………………………..…………………………………….. 63

List of Figures

VII

Fig. 4.10 - List of mat files examples of that resulted from the conversion of wfdisk files to be readable by the MatLab software (in this case related to Grímsvötn activity). …………………………………………………..…………..……………………………….. 65

Fig. 4.11 - a) m-file script example for H4, H7 and H8, and b) final image plot correlation signal between H4, H7 and H8 IS42 array sensors recorded on 19/05/2016. ……………………………………………………………………………………………………………….... 66

Fig. 4.12 - Example of plot back-azimuths detections on 17-23 May 2016 from station IS42 filtered between 50° and 150 ° degrees. ……………………………………………………………………………………………………………………………….……………………………... 66

Fig. 5.1 - Grimsvötn and Mt. Etna locations relative to IS42 (Matos et al., 2017). ………………………………………………………. 67

Fig. 5.2 - Sicily Island location (Esri®). …………………………………………………………………….……………………………………………..…. 74

Fig. 5.3 - Mount Etna location and Central Mediterranean geological-structural scheme: 1) Regional overthrust of the Sardinia-Corsica block upon Calabride units; 2) Regional overthrust of the Kabilo-Calabride units upon the Apenninic- Maghrebian chain; 3) External front of the Apenninic-Maghrebian chain upon the Foreland units and the External Thrust System; 4) Thrust front of the External Thrust System; 5) Main normal and strike-slip faults (Branca et al., 2011a). ....….. 75

Fig. 5.4 - a) Sicily island and Etna region (red square); b) Schematic map of Mt Etna main faults (modified from Azzaro et al., 2012): PPFS - Pernicana Provenzana Fault System; TFS - Timpe Fault System; VdB - Valle del Bove; c) The central craters (orange square on b): NEC - Northeast Crater; BN - Bocca Nuova crater; VOR - Voragine crater; SEC - Southeast crater; NSEC - New Southeast crater (PIT crater); and the new Southeast 3 (SEC3 or Cono della sella). The yellow dots indicate the location of degassing apertures of BN, VOR, NSEC and SEC3 (INGV, 2017); d) Eruptive fissures and pyroclastic cones distribution on Mt. Etna Volcano (Azzaro et al., 2012). ……………………………………………………………………………..……………………………………. 76

Fig. 5.5- IS42 detections on 11th and 12th May, based on the expected travel time and back-azimuth. …..…………….……. 83

Fig. 5.6 - Infrasonic parameter (IP) timeline on 11th and 12th May. …………………………………..……………………………………….. 83

Fig. 5.7 - IS42 detections on 9th July, based on the expected travel time and back-azimuth. ………………………….………..….. 84

Fig. 5.8 - Infrasonic parameter (IP) timeline on 9th July. …………………………………………….………………………………………..…….. 84

Fig. 5.9 - IS42 detections on 19th and 20th July, based on the expected travel time and back-azimuth. …………….…….…….. 85

Fig. 5.10 - Infrasonic parameter (IP) timeline on 19th July. ………………………………………………………………….…………….……….. 85

Fig. 5.11 - IS42 detections on 24th and 25th July, based on the expected travel time and back-azimuth. …………………..….. 86

Fig. 5.12 - Infrasonic parameter (IP) on 24th and 25th July. …………………………………………………………..…………………….……….. 86

Fig. 5.13 - IS42 detections on 30th July, based on the expected travel time and back-azimuth. ………………………………..….. 87

Fig. 5.14 - Infrasonic parameter (IP) on 30th July. ………………………………..…………………………………………………………………….. 87

Fig. 5.15 - IS42 detections on 5th and 6th August, based on the expected travel time and back-azimuth. .……..………….….. 88

Fig. 5.16 - Infrasonic Parameter (IP) from 5th to 6th August. ……………………………………………………………………………………….. 88

Fig. 5.17 - IS42 detections on 12th August, based on the expected travel time and back-azimuth. ………….…………..……….. 89

List of Figures

VIII

Fig. 5.18 - Infrasonic parameter (IP) on 12th August. ……………………………………………………..………………………………………….. 89

Fig. 5.19 - IS42 detections on 20th August, based on the expected travel time and back-azimuth. ………………………..…….. 90

Fig. 5.20 - Infrasonic parameter (IP) on 20th August. ……………………………………………………………..………………………..….…….. 90

Fig. 5.21 - IS42 detections on 29th August, based on the expected travel time and back-azimuth. ………..…………..…..…….. 91

Fig. 5.22 - Infrasonic parameter (IP) on 28th and 29th August. ……………………………………….…………………………………….…….. 91

Fig. 5.23 - PMCC back-azimuths results between 16th and 26th May 2016. …………………….………………………………………….. 96

Fig. 5.24 - IS42 detections on 17th and 18th May, based on the expected travel time and back-azimuth. …………….…….….. 96

Fig. 5.25 - IS42 detections on 17th and 18th May, associated to selected back-azimuths and clusters of detections. ……... 97

Fig. 5.26 -ETN IP parameter on 17th and 18th May, associated to selected back-azimuths and clusters of detections. ……. 97

Fig. 5.27 - IS42 detections on 19th May, based on the expected travel time and back-azimuth. ………………………………….. 98

Fig. 5.28 - a) IS42 detections on 19th May, related to selected back-azimuths and b) ETN IP parameter on 19th May. ….... 98

Fig. 5.29 - Iceland main topographic and batimetric map features (Esri®). …………………………………………………………..…….. 99

Fig. 5.30 - Orographic map of Iceland with the main glaciers. Smaller glaciers are on the icecaps. 1-Vatnajökull (8.160 km2); 2 - Langjökull (950 km2); 3 - Hofsjökull (925 km2); 4 - Mýrdalsjökull (596 km2); 5 - Eyjafjallajökull (77 km2); 6 - Drangajökull (160 km2); 7 - Snæfellsjökul (7 km2). lnlet: Geological map showing the active volcanic zone and the central volcanoes. Adapt from Bjornsson & Pálsson, 2008. ………………………………………………………………………………………………………………………………….. 100

Fig. 5.31 - Iceland Basalt Plateau lies at the junction between submarine segments of the Middle Atlantic Ridge, the Reykjanes Ridge to the south and the Kolbeisey Ridge to the north. The line with the small circles shows the progressive position of the Iceland mantle plume from 65 million years until the actuality. Modified by Sauders et al., 1997 and adapted from Thordarson and Larsen, 2007. ……………………………………………………………………………………………………………..……….. 101

Fig. 5.32 - Geological map of Iceland with 30 volcanic systems. Main geological formation divisions and rift zones. RR, Reykjanes Ridge; KR, Kolbeinsey Ridge; RVB, Reykjanes Volcanic Belt; SISZ, South Iceland Seismic Zone; WVZ, West Volcanic Zone; MIB, Mid-Iceland Belt; NVZ, Northern Volcanic Zone; ÖVB, Örœfi Volcanic Belt; EVZ, Eastern Volcanic Zone; SVB, Snœfellsnes Volcanic Belt; TFZ, Tjörnes Fracture Zone. The Grímsvötn volcanic system (19) is located near the vertical axis of the current mantle plume. (Based on Jóhannesson and Sæmundsson (1998b) and Thordarson & Hoskuldsson, 2008). .. 103

Fig. 5.33 - 2011 IS42 detections between 21th and 28th May, based on the expected travel time and back-azimuth. …... 108

Fig. 5.34 - IS42 detections 21st May, based on the expected travel time and back-azimuth. …………………………………….… 108

Fig. 5.35 - 21st May detections between 21:59 UTC and 23:59 UTC, with 19 families selected, a speed of 372 m/s, a mean back-azimuth of 18° and a mean frequency of 0.70 Hz b) polar plot according to back-azimuth (polar angle) and trace velocity (polar radius) related to detections time; c) polar plot of pixels families detections related to back-azimuth, speed and frequency. ……………………………………………………………………………………………………………………………………………..……….….. 109

Fig. 5.36 - IS42 detections on 22nd May, based on the expected travel time and back-azimuth. ……………………..……...…... 109

List of Figures

IX

Fig. 5.37 - 22nd May detections between 21:59 UTC and 23:59 UTC, with 19 families selected, a speed of 372 m/s, a mean back-azimuth of 18° and a mean frequency of 0.70 Hz b) polar plot according to back-azimuth (polar angle) and trace velocity (polar radius) related to detections time; c) polar plot of pixels families detections related to back-azimuth, speed and frequency. ……………………………………………………………………………………………………………………………………………..…………... 110

Fig. 5.38 - IS42 detections on 23rd May, based on the expected travel time and back-azimuth. …..…………...................….. 111

Fig. 5.39 - 23rd May detections between 00:00 UTC and 23:05 UTC, with 21 families selected, a speed of 375 m/s, a mean back-azimuth of 14.82° and a mean frequency of 0.74 Hz b) polar plot according to back-azimuth (polar angle) and trace velocity (polar radius) related to detections time; c) polar plot of pixels families detections related to back-azimuth, speed and frequency. ………………………………………………………………………………………………………………………………………………...….. 111

Fig. 5.40 - IS42 detections on 24th May, based on the expected travel time and back-azimuth …………………………....…….. 112

Fig. 5.41 - 24th May detections between 00:00 UTC and 23:05 UTC, with 4 families selected, a speed of 375 m/s, a mean back-azimuth of 14.82° and a mean frequency of 0.74 Hz b) polar plot according to back-azimuth (polar angle) and trace velocity (polar radius) related to detections time; c) polar plot of pixels families detections related to back-azimuth, speed and frequency. ……………………………………………………………………………………………………………………………………………….….. 112

Fig. 5.42 - IS42 detections on 25th May, based on the expected travel time and back-azimuth. ……….…………………....….. 113

Fig. 5.43 - May 25th detections between 00:00 UTC and 23:05 UTC, with 4 families selected, a speed of 375 m/s, a mean back-azimuth of 14.82° and a mean frequency of 0.74 Hz b) polar plot acording to back-azimuth (polar angle) and trace velocity (polar radius) related to detections time; c) polar plot of Pixels Families detections related to back-azimuth, speed and frequency. …………………………………………….……………………………………………………………………………………………….….... 113

Fig. 6.1 - Volcanic activity in REB (IDC-CTBTO) on 19th and 25th July 2011 in the Sicily region. 19th - Red line: stations that recorded the 00:51 UTC event; 25th - Yellow line: stations that recorded the 04:25 UTC event (Matos et al., 2017). ………………………………………………………………………………………………………………………………………………………..……………….…. 115

Fig. 6.2. - a) I42PT back-azimuth detections between 16th and 25th May 2016 showing the Mount Etna back-azimuth (red line) and the three analysed clusters of detections; b) Early-Warning System principal stages; c) ETN array IP-Infrasonic parameter timeline: Strombolian phase (1), Lava-fountain activity (2,3,5), strong explosive activity (4) (Matos et al., 2017). …………………………………………………………………………………………………………………………………………………………………….….….. 119

Fig. 6.3 - Volcanic activity in REB (IDC) on 21st to 22nd May 2011 in Grímsvötn region activity. Day 21st, Orange line: stations that recorded the 19:16 UTC event; Blue line: stations that recorded the 21:25 UTC event; Day 22nd , Green line: stations that recorded the 01:40 UTC event; Red line: stations that recorded the 04:49 UTC event; Yellow line: stations that recorded the 05:49 UTC event (Matos et al., 2017). ………………………………………………………………………………………………………………...... 122

Fig. 6.4 - Example of microbaroms recorded at IS42 on 2017/09/08 (Wallenstein & Campus, 2017). ………………………… 124

Fig. 6.5 – Signal recorded during a strong geomagnetic storm, showing a large perturbation with frequencies between 0.01 and 0.1 Hz (compatible to aurora-genetated infrasound (Wallenstein & Campus, 2017). …………………………………………… 125

Fig. 6.6 – Example of an event with a local magnitude of 5.9 occurred in Azores Archipelago showing seismic arrivals at IS42 recorded on 2013/04/30 and compatible with surface waves propagation. Data was filtered between 0.4 and 4 Hz (Wallenstein & Campus, 2017). …………………………………………………………………………………………………………………………… 125

List of Tables

X

LIST OF TABLES

Table 2.1 - Sound propagation velocity in various types of materials. …………………………………………………………………….. 15

Table 2.2 - Composition of the Atmosphere near the Earth´s surface. ……………………………..…………………………………….. 18

Table 2.3 - Phases designations, maximum infrasound refraction altitudes to the ground by phase type and characteristic values of celerity. …………………………………………………………………………………….……………………………………………………….... 23

Table 4.1 - Name and sources locations of study volcanoes. ………………………………………..………..……………………………... 49

Table 5.1 - Mount Etna activity cronology from May to August 2011 used in this study. .……………………………………...… 78

Table 5.2 - IS42 main detection parameters on 11th and 12th May. ……………….…………………………..…………………….…….. 83

Table 5.3 - IS42 main detection parameters on 9th July. …………………………………………………..…………………………...…..….. 84

Table 5.4 - IS42 main detection parameters on 19th July. …………………………………………………………………………….…....….. 85

Table 5.5 - IS42 main detection parameters on 24th and 25th July. …………..…………………………..……………………………..….. 86

Table 5.6 - IS42 main detection parameters on 30th July. ……………………………………….……………………………..………...…….. 87

Table 5.7 - IS42 main detection parameters on 5th and 6th August. ………………..………………………………………………...…….. 88

Table 5.8 - IS42 main detection parameters on 12th August. …………………………………..……………………………………..…….... 89

Table 5.9 - IS42 main detection parameters on 20th August. ………………………..………………………………………………….……. 90

Table 5.10 - IS42 main detection parameters on 29th August. ………………………………………………………………………..…….... 91

Table 5.11 - Etna activity chronology in May 2016 used in this study. …………………………………………………………………….. 92

Table 5.12 -ETN IP parameter on May 17th and 18th related to selected back-azimuths. ………………………………………..... 97

Table 5.13 - 30 volcanic systems of Iceland. ModifiedThordarson & Höskuldsson (2008). ……………………………………..….. 104

Table 5.14 - Grimsvötn activity chronology in May 2011 used in this work. ………………………………………………………….. 107

Table 5.15 - IS42 main detection parameters on 22nd May (Grimsvötn activity). ….…..……………..……………………….…... 110

Table 5.16 - IS42 main detection parameters on 23rd May (Grimsvötn activity). ……………….…………………………..…….... 111

Table 5.17 - IS42 main detection parameters on 24th May (Grimsvötn activity). …….…………………………………….……….. 112

Table 6.1– Resume of the comparison between 2011 May – August detections on IS42 with ETN station and UniFi/ Protezione Civile Nazionale bulletins. ………………………………………………………………………………………………….…………...… 116

Table 6.2 –Resume of the comparison between May 2016 detections on IS42 with ETN array and UniFi/ Protezione Civile Nazionale bulletins. ……………………………………………………………………………………………………………………………………..…... 120

Table 6.3 - May 2011 resume table of comparison between IS42 detections with IMO bulletins …………………………... 123

List of Photographies

XI

LIST OF PHOTOGRAPHIES

Photo 1.1 - Frank Perret (1867-1943) at the Campi Flegrei in Pozzuoli, Italy, with an improvised "geophone” in hopes of detecting magma’s subterranean movements. Here, using a microphone to amplify the sounds from the earth interior, he connected a cable from the geophone to the loudspeaker on his ear. …………………………………………………………………..…... 1

Photo 3.1 - IS42 Infrasound station in Graciosa Island, Acores, Portugal, I42H7 element. ……………………..…………………... 26

Photo 4.1 – Detail of I42H1 (Photo IVAR). …………………………………………………………………………………..……………………….….. 48

Photo 4.2 – IS42 was installed in heavily forested area. Pico do Timão scoria cone in the background (photo from Nicolau Wallenstein). ………………………..……………………………………………………………………………………………………………………….…….. 51

Photo 4.3- WNRS pipes and manifold. ………………………..…………………………………………………………………………………..….….. 55

Photo 4.4- I42H8 station element before and after putting gravel on stainless steel pipes and on inlet ports. …………………………………………………………………………………………………………………………………………………………………………………55

Photo 5.1 - Night eruption of Mt. Etna taken from the town of San Teodoro. © Fernando Famiani. ……………………..…. 73

Photo 5.2 - Grimsvötn volcano 2011 eruption photo. © Institute of Earth Science ES- Un. of Iceland. ……………………….. 105

Photo 5.3 - Photo of the 2011 Grimsvötn volcano eruption (By Mathew J Roberts). ……………………..……………………….... 106

List of Acronyms

XII

LIST OF ACRONYMS

ABL - Atmospheric Boundary Layer

ARISE2 – Atmospheric dynamics Research InfraStructure in Europe

CEA/DASE – Commissariat à l’energie atomique et aux energies alternatives, Departement Analyse,

Surveillance, Environnement

CMSCG - Santa Cruz da Graciosa Municipality

CRF - Central Recording Facility

CTBT - Comprehensive Nuclear-Test-Ban Treaty

CTBTO - Preparatory Commission of the Comprehensive Nuclear-Test-Ban Treaty Organization

CVARG - Centre of Volcanology and Geological Risks Assessment

DFT - Discrete Fourier Transform

DTFT - Discrete-Time Fourier Transform

DTK-GPMCC – Dase Tool Kit - Graphical Progressive Multi-Channel Correlation

EU - European Union

EUp- Eurasian plate

FD / TD - Frequency Domain / Time-Domain

FFT - Fast Fourier Transform

GCI - Global Communication Infrastructure

GPS - Global Positioning System

GVS - Grímsvötn Volcanic System

IDC - International Data Centre

IFFT - Inverse Fast Fourier Transform

IMS - International Monitoring System

INGV - Istituto Nazionale di Geofisica e Vulcanologia

IP - Infrasonic Parameter

List of Acronyms

XIII

IS – Infrasound Station

IVAR - Instituto de Investigação em Vulcanologia e Avaliação de Riscos

JTA - Triple Junction of the Azores

LKO - Outside temperature long period instrument channel code

LP - Long Period

LVDT - Linear Variable Displacement Transducer

MAR – Mid-Atlantic Ridge

NAp - North American plate

NDC - National Data Centre

NUp – Nubia/African plate

OSI - On-Site Inspections

PBL - Planetary Boundary Layer

PC – Personal computer

PDC - Pyroclastic Density Current

PMCC - Progressive Multi-Channel Correlation

PSD - Power Spectral Density

PTBT - Partial Test Ban Treaty

PTS - Provisional Technical Secretariat

SOFAR - SOund Fixing And Ranging

TLE - Transient Luminous Event

ULP - Ultra Long Period

UPS - Uninterruptible Power Supply

US –United States (of America)

VPN - Virtual Private Network

VSAT - Very Small Aperture Terminal

WNRS - Wind-Noise-Reducing System

Acknowledgements

XIV

ACKNOWLEDGEMENTS

Throughout the preparation of this Master's Dissertation, I relied on the trust and support of several people and

institutions, as well as the generous support of friends and family members who, in a more direct or indirect

way, contributed to its conclusion. In this regard, I would like to express a few words of thanks and deep

appreciation, in particular:

- to the University of the Azores and the Faculty of Science and Technology for the facilities granted;

- to the Geosciences Department of the University of the Azores and to the Institute of Research

in Volcanology and Risk Assessment for the possibility of accomplishing this master's degree and the means

granted to carry out the work;

- to Professor Nicolau Wallenstein, for his supervision, for his willingness to guide this work, from

help in defining the object of study to the critical revision of the text, by the requirement of method and

rigor, by the proficient comments, clarifications, opinions, suggestions and advice, cordiality and sympathy

demonstrated by the trust he has always given me and by the constant encouragement in the preparation

of this work, as well as by the interest and accompaniment evidenced throughout my academic career;

- to Professor Maurizio Ripepe, for his supervision, for his willingness to guide this work, help,

collaboration and support during this work elaboration. Thanks for hosting me at Università Degli Studi di

Firenze – Laboratório Geofisica Sperimentale, as well my deep appreciation for all the suggestions and

advices;

- to Dr. Emanuelle Marchetti, for the support provided in my trip to UniFi in Florence, for sharing

their precious knowledge and lessons in solving problems of data analysis, as well as the transfer of data

from Mount Etna for analysis. For all this my sincere gratefulness for his friendly words and time spent with

me;

- to the teachers of the various disciplines, by sharing knowledge and support throughout the

master's course;

Acknowledgements

XV

- to Infrasound laboratory colleagues, Eng. Arturo Montalvo for his teachings and his readiness to

clarify doubts, as well as to Flávio Soares for solving logistical problems, for their fellowship and friendliness;

- to Dr. Alexis le Pichon, Dr. Pierrick Mialle and Dr. Robin Matosa by the clarification of doubts in

the analysis of data and by suggestions and advice;

- to all employees and researchers of CIVISA, IVAR and FGF, who directly or indirectly contributed

to the conclusion of this work;

- to colleagues of the MSc, for solidarity, friendship and mutual support provided throughout the

course.

- to everyone in Florence- Italy who, in one way or another, helped me during my stay at the

Università Degli Studi di Firenze I express my gratitude;

- to the relatives who have always supported and encouraged me in taking hard decisions. For

them all my respect and consideration;

- To the people I protect most in the world, Catarina Batista, for her unconditional support in the

critical hours, for her love, affection and above all for the immense patience she had, and to Guilherme, who

still does not know how to read but, without knowing it is the strength of all my dedication and effort. Thank

you both for understanding the long absences and for offering me long hugs;

- to the ARISE2 project (H2020) for all support in my participation in Arise2 Workshops;

- to the Preparatory Commission of the Comprehensive Nuclear-Test-Ban Treaty Organization-

CTBTO for all support in my participation in Workshops and Trainnings;

- to the University of Reunion Island and NEXA, a regional agency for development, investment

and innovation, for support my participation in ARISE2 Workshop and GRIF-RUN 2017;

- My deep and heartfelt thanks to all the people who contributed to the accomplishment of this

dissertation.

Acknowledgements

XVI

ABSTRACT

Infrasound is an atmospheric pressure perturbation associated both to natural or man-made sources. Among

the natural causes are the atmospheric extreme events like severe weather, avalanches, earthquakes and

volcanic eruptions. The major sources of anthropogenic sources are explosions related with mining, chemical

production and nuclear tests. Due its low frequency (<20 Hz) they can travel long distance in atmospheric

waveguides and be recorded several thousand kilometres from the source.

Eruptions are processes in which volcanoes restore the equilibrium perturbed by rising magma in a chamber

deep in the crust. When a volcano erupts, it releases energy in the form of pressure waves into the atmosphere,

generally with low frequency, below the audible range of human hearing. The use of infrasound provides a

valuable working tool for monitoring volcanic activity, both in the near- and far-field.

There are a variety of eruptive styles, and each one produces unique and different infrasound signals, most

commonly related to explosions, tremor, eruptive column and degassing. Infrasound also enhances the efficacy

of the volcanic hazard monitoring once the infrasonic pressure field may be directly associated with the flux rate

of gas released.

Recently, infrasound studies from active volcanoes have permitted successful advances in volcanic hazards

mitigation and in the understanding of volcanic source. On Etna volcano, infrasound array monitoring allowed

to suggest a gas flow regime transition from slug to churn flow, driving lava fountains, with the two phases being

reflected by different infrasonic signature.

The Instituto de Investigação em Vulcanologia e Avaliação de Riscos - IVAR operates an infrasound station (IS42)

located in Graciosa island, which integrates the International Monitoring System (IMS) that operationalise the

verification regime of the Comprehensive Test-Ban Treaty (CTBT). It also integrates the European Union ARISE

project consortium, with the objective to develop collaborative research on infrasound regarding extreme

events.

The work presented within the scope of this thesis has as objectives:

- (1) the verification of the capacity of the I42PT station to detect, locate and characterize infrasound waves

originated from volcanic activity and propagated over long distances, showing as examples the eruption of the

Grímsvötn volcano, Iceland, from 21st to 30th May 2011, and Mt. Etna volcano (Italy) paroxysmal eruptive

episodes from May-August 2011 and eruptive activity between 16th to 22nd May2016.

-(2) to check the ability of the I42PT station to detect and characterize infrasonic waves originating from other

extreme events in the North-Atlantic area.

Resumo

XVII

RESUMO Infrassons resultam de perturbações na pressão atmosférica associadas quer a fontes naturais quer

antropogénicas. Entre as causas naturais estão eventos extremos que ocorrem na atmosfera, tais como

tempestades, avalanches, sismos e erupções vulcânicas. As principais origens das fontes antropogénicas são

explosões relacionadas com a atividade mineira, produção química e testes nucleares. Devido á sua baixa

frequência (<20 Hz), podem percorrer longas distâncias através de correntes atmosféricas e serem registados a

vários milhares de quilómetros de distância da fonte.

Erupções vulcânicas são processos através dos quais os sistemas vulcânicos restauram o equilíbrio perturbado

pela ascensão de magma de um reservatório na profundidade da crosta. Quando um vulcão entra em erupção,

liberta energia sob a forma de ondas de pressão para a atmosfera, geralmente de baixa frequência, abaixo da

faixa audível para o ouvido humano. O uso de infrassons proporciona uma valiosa ferramenta de trabalho para

monitorização da atividade vulcânica, tanto a nível local como a nível global.

Existe uma variedade de estilos eruptivos, e cada um produz inequívocos sinais de infrassons, normalmente

relacionados com explosões, tremor, desenvolvimento da coluna eruptiva e desgaseificação. A utilização de

infrassons aumenta a eficácia na monitorização da perigosidade vulcânica, uma vez que o campo de pressão

dos infrassons pode estar diretamente relacionado com a taxa de fluxo de gás libertado.

Recentemente, o estudo de vulcões ativos através de infrassons, tem permitido, com sucesso, avanços na

mitigação do risco vulcânico e na compreensão dos processos de origem vulcânica. No vulcão Etna, em Itália, a

monitorização com recurso a um array de microbarómetros permitiu sugerir uma transição de fluxo de gás

lento a impulsivo na génese de fontes de lava, com as duas fases a serem associadas a diferentes assinaturas

dos sinais infrassónicos.

O Instituto de Investigação em Vulcanologia e Avaliação de Riscos - IVAR, opera uma estação de infrassons

localizada na ilha da Graciosa, IS42, que integra o Sistema Internacional de Monitorização (IMS) que

operacionaliza o regime de verificação do Tratado de Proibição Total de Ensaios Nucleares (CTBT). Integra

também o consórcio do projeto ARISE da União Europeia, com o objetivo de desenvolver trabalhos de

colaboração sobre a utilização de infrassons na observação de eventos extremos.

Inserido no tema, o trabalho apresentado no âmbito desta tese tem como objetivos, (1) a verificação da

capacidade da estação I42PT de detetar, localizar e caraterizar ondas infrassónicas originadas por atividade

vulcânica a longas distâncias, mostrando como exemplos a erupção do vulcão Grímsvötn, na Islândia, de 21 a

30 de maio de 2011, e episódios eruptivos paroxísticos de maio a agosto de 2011 e atividade eruptiva entre 16

a 22 de maio de 2016 do vulcão Mt. Etna em Itália; e (2) a verificação da capacidade da estação I42PT de detetar

localizar e caraterizar ondas infrassónicas originadas por outros eventos extremos na área do Atlântico Norte.

CHAPTER 1- INTRODUCTION 1.1-Framework and objectives of the work

1

CHAPTER 1. INTRODUCTION

1.1. Framework and objectives of the work

1.1.1. Motivation

This work arises from the need to understand, analyse and characterize the potential of the IS42 infrasound

station, for remote volcanic monitoring and to establish the rationale, routines and procedures to use

infrasound as a relevant tool for the Azores seismovolcanic monitoring programme.

Located on the Graciosa Island, in the Azores archipelago, this station is part of an international network of 60

infrasound stations belonging to the International Monitoring System (IMS) of the Preparatory Commission of

the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO).

Based on a collaboration with the University of Florence under the ARISE2 project (EU - H2020 Program), this

work aims to verify the capacity of detection of volcanic eruptions based on infrasound long distance

observations, and the possibility of comparing data recorded near origin by a local infrasound station, with

detections obtained by the IS42 station at identical time periods.

1.1.2. Study relevance

Due to its geodynamic location, the Azores islands play an important role in understanding the tectonics,

magmatism and volcanism of North Atlantic tectonic triple junction, being a natural laboratory where

geophysical processes are materialized by seismic and volcanic activity.

Photo from "The Day's Work of a Volcanologist." The World's

Work, V. 25, November 1907 (image in public domain).

Photo 1.1- Frank Perret (1867-1943) at the Campi

Flegrei in Pozzuoli, Italy, with an improvised

"geophone” in hopes of detecting magma’s

subterranean movements. Here, using a

microphone to amplify the sounds from the earth

interior, he connected a cable from the geophone

to the loudspeaker on his ear.


2

Due to its geographic location, the IS42 station covers a key region for the Comprehensive Nuclear-Test-Ban

Treaty (CTBT) monitoring purposes, complementing the coverage of other infrasound stations in the North

Atlantic. At the same time, it has a key position in the Atlantic for monitoring various types of infrasound sources

(volcanic eruptions, explosions or extreme weather events).

1.1.3. Research objectives

The main objective of this work is to verify the capability of volcanic eruptions detections, based on long-

distance observations of the IS42 infrasound station. In this context the present work aims to verify the I42PT

station capability to support the location of sources and the characterisation of infrasound waves originated

from volcanic activity over long distances. This work uses as examples the Mount Etna volcano (Italy)

paroxysmal eruptive episodes from May-August 2011, as well the eruptive activity between 2016 May 16 to 22,

and the eruption of the Grímsvötn volcano, in Iceland, from 21st to 30th May 2011.

1.1.4. Dissertation structure and organization

In order to reach the proposed objectives, the present work is structured accordingly with the following format:

Chapter 1. Introduction: chapter referring to the objectives and structure of the work, which describes the

reason for its elaboration, its purpose, as well as its organizational structure.

Chapter 2. Infrasound: introduction chapter to the infrasound definition. Starts with an introductory chapter of

the concept and characteristics of infrasound. The various types of sources and physical properties of infrasound

are described. A subchapter is also included referring to the atmosphere, where its structure, its properties and

its effects in the infrasound propagation are characterized.

Chapter 3. IMS Infrasound Stations: this chapter is divided into 4 subchapters. The first sub-chapters starts with

a historical review of Comprehensive Nuclear-Test-Ban Treaty (CTBTO) and an introduction to IMS infrasound

stations The third sub-chapter describes the basic components of an infrasound monitoring station and the later

one present the general concepts in infrasound processing.

Chapter 4. Methodology: this chapter is divided into 4 subchapters. The first sub-chapter is related to

introduction methodology. The second one describes the international station IS42, located in Graciosa Island,

Azores archipelago, which data is the central base of the study of this work, and characterizes the geographical

and geological framework of the area where the station was established. Next subchapter describes the ETN

infrasound array near Etna Volcano and its characteristics. The later subchapter characterizes the data

acquisition system and the data preparation, organization and processing.


3

Chapter 5. Results of long–range infrasound detections of volcanic activity by IS42 station: being one of the

objectives of this work, this chapter is presented through the results obtained in the analysis of the detections.

This chapter begin to make a brief introduction to the topic of volcanic monitoring using infrasound technics,

followed by two subchapters corresponding to Mount Etna and Grímsvötn volcanoes, in which an introduction

to the theme is presented in the first part and the framework of the study area is characterized. In the second

part of each subchapter, results of the IS42 station are presented, in the time period corresponding to the

eruption of the Mount Etna volcano between 2011 May - August lava-fountaining activity, on 16th- 22nd May

2016 Etna eruptive activity, and from Grímsvötn volcano, between May 21th to 28th, 2011 respectively.

Chapter 6. Discussion of the results: in this chapter we present the results of the detections potentially related

to the Mount Etna volcano and the Grímsvötn volcano as well as the analysis and discussion of these same

results.

Final considerations: resume the essential ideas retained and achieved by this project as well as the presents

the perspectives of the work to be developed in the future.

the use of infrasound in volcano monitoring : contribution ...€¦ · the use of infrasound in...

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