ITW2008 Technical Program · NCPA, University of Mississippi [email protected] Ron Wagstaff NCPA,...

ITW2008

2008 Infrasound Technology Workshop

Grotto Bay Beach Resort Bermuda

November 3-7, 2008

Hosted by Hosted by Hosted by Hosted by The National Center for Physical AcousticsThe National Center for Physical AcousticsThe National Center for Physical AcousticsThe National Center for Physical Acoustics

The University of MississippiThe University of MississippiThe University of MississippiThe University of Mississippi

DedicationDedicationDedicationDedication This Workshop is dedicated to the memory of

Dr. Henry Ellis Bass 1944-2008

Table of ContentsTable of ContentsTable of ContentsTable of Contents Dedication ............................................................................................................................... 3 Table of Contents................................................................................................................... 5 List of Attendees .................................................................................................................... 9 Schedule................................................................................................................................. 13 Abstracts ................................................................................................................................ 19

Session 1: Sensors and Data Acquisition ...................................................................... 19 Chaparral Physics Research........................................................................................ 20 Development of a New Infrasound Microphone Technology.............................. 21 A Low Power and Low Noise Infrasound Sensor for Temporary Measurements....................................................................................................................................... 22 Efforts in Distributed Arrays for Infrasound Measurements................................ 23 PTS Portable Infrasound Array ................................................................................. 24 Development of a Portable, Controllable Infrasound Source ............................... 25 Application of an M-sequence Infrasonic Calibrator at the Camp Elliott OFIS Array.............................................................................................................................. 26

Session 2: Infrasound from Anthropogenic Sources .................................................. 27 Observations of Chemical Explosions ..................................................................... 28 Fast Arrivals at the Long Distances from Explosions............................................ 29 Infrasound Signals from Repeating Detonations at the Utah Test and Training Range Recorded in North America........................................................................... 30 Infrasound Calibration Experiment in Sayarim, Israel: Preparation and Test Shots.............................................................................................................................. 31 Low Frequency Airplane Noise Measurement ........................................................ 32 Infrasound Case Studies ............................................................................................. 33 The Effect of Wind Turbines Noise on the Infrasound Monitoring Stations .... 34

Session 3: The International Monitoring System......................................................... 35 The IMS Infrasound Network: current status and future plans............................ 36 From the IMS Installation and Certification Group to the IMS Engineering and Development Section: a new challenge ahead ......................................................... 37 Assessing the Detection Capability of the International Monitoring System Infrasound Network.................................................................................................... 38 Ground-Truth Events as Benchmark for Assessing the Infrasound Detection Capability ...................................................................................................................... 39 About results of the ISTC project # 2845 ............................................................... 40 The recently installed IMS Infrasound Station IS51, Bermuda ............................. 41 Preparing the Return of Infrasound Data Processing into IDC Operations....... 42 Enhancements to the CTBTO Operational Automatic Infrasound Processing System ........................................................................................................................... 43

Session 4: Wind Noise..................................................................................................... 45

Wind Noise Reduction at IMS Infrasound Stations ............................................... 46 Approaches to the Infrasound Signal Denoising by Using AR Method .............. 47 Infrasonic Noise Reduction Using Shelters/Windbreaks ...................................... 48

Session 5: Detection, Propagation and Modeling ........................................................ 49 Studies of Propagation from Atmospheric Sources Using Infrasound Arrays and Seismic Stations ........................................................................................................... 50 Towards an automatic detection and localization procedure: an European case study .............................................................................................................................. 51 Analysing Multiple Explosions in Europe: Signal Characteristics and Propagation Modelling...................................................................................................................... 52 Effects of Atmospheric Turbulence on Azimuths and Grazing Angles Estimation at the Long Distances from Explosions............................................... 53 Acoustic-Gravity Waves From Meteor Entry As Well As From Rockets And Missiles.......................................................................................................................... 54 Numerical Simulation of Infrasound Propagation at High Altitude, Including Attenuation and Non-linearity. .................................................................................. 55 Augmentation of IMS Infrasound Arrays for Near-field Clutter Reduction....... 56 InfraMonitor: Methodology for Automated Regional Monitoring....................... 57 InfraMonitor: Application to the Utah Infrasound Network................................ 58 Infrasound Assessment of a Railroad Bridge........................................................... 59 Recent Applications of the Time-Domain Parabolic Equation (TDPE) Model to Ground Truth Events ................................................................................................. 60 A Study on Characteristics of Seasonally Dependent Infrasound Propagation Based on the Ground-Truth Events from a Long-Term Experiment at a Quarry Mine............................................................................................................................... 61

Session 6: Infrasound from Geophysical Sources ....................................................... 63 A Canadian Program for Dedicated Monitoring of Meteor Generated Infrasound....................................................................................................................................... 64 An Estimate of the Terrestrial Influx of Large Meteoroids from Infrasonic Measurements .............................................................................................................. 65 Infrasonic Measurements of the Carancas Peru Meteorite Fall............................. 66 Infrasonic Observations of Meteoroid Entries........................................................ 67 Infrasound from 2008TC3 on 7 October 2008....................................................... 68 Microbarom signals recorded in Antarctica – A measure for sudden stratospheric warming?....................................................................................................................... 69 The IMS Infrasound Network and its potential for detections of a variety of man-made and natural events .................................................................................... 70 Acoustic-Gravity Wave Monitoring for Global Atmospheric Studies ................. 71 Infrasound from Atmospheric Vortices ................................................................... 72 Analysis of the Infrasound Signal from May 12 Earthquake Wenchuan China.. 73 Evaluation of the ASHE Project (Ecuador) ............................................................ 74

IMS Infrasound Station Observations of the Recent Explosive Eruptions of Okmok and Kasatochi Volcanoes, Alaska ............................................................... 75 Design of Monitoring and Early Warning System for Geological Hazards in Three Gorges Reservoir Area Using Infrasound..................................................... 76

Posters ............................................................................................................................... 77 Investigating regional industrial explosive events in Southern Ontario, Canada 78

List of AttendeesList of AttendeesList of AttendeesList of Attendees

Stephen Arrowsmith Los Alamos National Laboratory [email protected] George Atkins NCPA, University of Mississippi [email protected] Chris Bach 5-D Systems [email protected] Anatoly Baryshnikov National Institute of Pulse Techniques [email protected] Yochai Ben-Horin National Data Center of Israel [email protected] Elisabeth Blanc CEA/DASE [email protected] David Bowers AWE Blacknest [email protected] Roger Bowman SAIC [email protected] Nicolas Brachet PTS/CTBTO [email protected]

Rex Brontmeyer Bermuda Aviation Services/Serco [email protected] David Brown PTS/CTBTO [email protected] Peter Brown University of Western Ontario [email protected] Paola Campus PTS/CTBTO [email protected] Lars Ceranna BGR Hannover [email protected] Douglas Christie Australian National University [email protected] John Coyne PTS/CTBTO [email protected] Catherine de Groot-Hedlin University of California San Diego [email protected] Scott De Wolf University of California San Diego [email protected]

Stephane Denis CEA/DASE [email protected] Xiao Di NCPA, University of Mississippi [email protected] Kevin Dillion Miltec Research & Technology [email protected] Douglas Drob Naval Research Laboratory [email protected] Wayne Edwards University of Western Ontario [email protected] Milton Garces ISLA, University of Hawaii [email protected] Robert Gibson BBN Technologies [email protected] Kenneth Gilbert NCPA, University of Mississippi [email protected] Yefim Gitterman Geophysical Institute of Israel [email protected] Paul Golden Southern Methodist University [email protected]

David Green AWE Blacknest [email protected] Patrick Grenard PTS/CTBTO [email protected] Jay Helmericks University of Alaska, Fairbanks [email protected] Thierry Heritier CEA/DASE [email protected] Claus Hetzer NCPA, University of Mississippi [email protected] Daniel Higgins Air Force Research Laboratory [email protected] Wheeler Howard Miltec Research & Development [email protected] Sergey Kulichkov Institute of Atmospheric Physics [email protected] Alexis Le Pichon CEA/DASE [email protected] Hee-Il Lee KIGAM [email protected]

Ludwik Liszka Swedish Institute for Space Physics [email protected] Mihan McKenna US Army Engineer R&D Center [email protected] Ian Mills Bermuda Aviation Services/Serco [email protected] Tahahiko Murayama Japan Weather Association [email protected] Sue Nava ENSCO [email protected] Roger Oldfield Telecom Bermuda [email protected] John Olson University of Alaska, Fairbanks [email protected] Mark Pickens Space & Missile Defense Command [email protected] Ning Qiu China University of Geosciences [email protected] Douglas ReVelle Los Alamos National Laboratory [email protected]

Jere Singleton Space and Missile Defense Command [email protected] Stefka Stefanova PTS/CTBTO [email protected] Sun Haiyang Chinese Ministry of National Defense Curt Szuberla University of Alaska, Fairbanks [email protected] Carrick Talmadge NCPA, University of Mississippi [email protected] Ron Wagstaff NCPA, University of Mississippi [email protected] Kris Walker University of California San Diego [email protected] Wang Xiaohang North China Institute of Computing Technology [email protected] Roger Waxler NCPA, University of Mississippi [email protected] Jay Williams Miltec Research & Technology [email protected]

ScheduleScheduleScheduleSchedule

Monday, November 3

08:00-09:00 Registration and Sign-In, Palmetto Meeting Room

Session 0: Introduction and Administrative Matters

09:00-09:10 Introduction and Welcome The Premier of Bermuda, Dr. The Hon. Ewart Frederick Brown JP MP

09:10-09:20 Introductory Speech Patrick Grenard, PTS/CTBTO

09:20-09:30 Administrative Remarks

Session 1: Sensors and Calibration Chairs: Jay Helmericks and Carrick Talmadge

09:30-09:55 Chaparral Physics Research Jay Helmericks, University of Alaska

09:55-10:20 Development of a New Infrasound Microphone Technology Carrick Talmadge, University of Mississippi

10:20-10:35 Coffee Break

10:35-11:00 A Low Power and Low Noise Infrasound Sensor for Temporary Measurements Stephane Denis, CEA/DASE

11:00-11:25 Efforts in Distributed Arrays for Infrasound Measurements Kevin Dillion, Miltec Research & Development

11:25-11:50 PTS Infrasound Portable Array John Coyne & Patrick Grenard, PTS/CTBTO

11:50-13:30 Lunch

13:30-13:55 Development of a Portable, Controllable Infrasound Source Milton Garces, University of Hawaii

13:55-14:20 Application of an M-Sequence Infrasonic Calibrator at the Camp Elliott OFIS Array Kris Walker, University of California San Diego

Session 2: Infrasound from Anthropogenic Sources

Chairs: Sergey Kulichkov and Curt Szuberla

14:20-14:50 Observations of Chemical Explosions Ludwik Liszka, Swedish Institute of Space Physics

14:50-15:15 Fast Arrivals at Long Distances from Explosions Sergey Kulichkov, Oboukhov Institute for Atmospheric Physics

15:15-15:40 Infrasound Signals from Repeating Detonations at the Utah Test and Training Range Recorded in North America Roger Bowman, SAIC

15:40-16:00 Afternoon Tea

16:00-16:25 Infrasound Calibration Experiment in Sayarim, Israel: Preparation and Test Shots Yefim Gitterman, Geophysical Institute of Israel

16:25-16:50 Low Frequency Airplane Noise Measurement Xiao Di, University of Mississippi

16:50-17:15 Infrasound Case Studies Paul Golden, Southern Methodist University

17:15-17:40 Impact of Wind Generators on IMS Infrasound Stations Stefka Stefanova, PTS/CTBTO

18:30-20:30 Reception and Group Photo, Hibiscus South Dining Room

Tuesday, November 4

08:30-09:00 Breakfast

Session 3: The International Monitoring System of the CTBTO

Chairs: Nicolas Brachet and Paola Campus

09:00-09:25 The IMS Infrasound Network: Current State and Future Plans Paola Campus, PTS/CTBTO

09:25-09:50

From the IMS Installation and Certification Group to the IMS Engineering and Development Section: A New Challenge Ahead Patrick Grenard, PTS/CTBTO

09:50-10:15 Assessing the Detection Capability of the International Monitoring System Infrasound Network David Green, AWE Blacknest


10:35-11:00 Ground-Truth Events as Benchmark for Assessing the Infrasound Detection Capability Alexis Le Pichon, CEA/DASE

11:00-11:25 The recently installed IMS Infrasound Station IS51, Bermuda Paola Campus, PTS/CTBTO

11:25-13:30 Lunch

13:30-13:55 Preparing the Return of Infrasound Data Processing into IDC Operations Nicolas Brachet, PTS/CTBTO

13:55-14:20 Enhancements to the CTBTO Operational Automatic Infrasound Processing System David Brown, PTS/CTBTO


Session 4: Wind Noise Chairs: Douglas Christie and Stefka Stefanova

14:40-15:05 Wind-Noise Reduction at IMS Infrasound Stations Douglas Christie, Australian National University

15:05-15:30 Approaches to the Infrasound Signal Denoising by Using AR Method Takahiko Murayama, Japan Weather Association

15:30-16:00 Infrasonic Noise Reduction Using Shelters/Windbreaks Douglas ReVelle, Los Alamos National Laboratory

Wednesday, November 5


Session 5: Detection, Propagation and Modeling

Chairs: Douglas Drob and Roger Waxler

08:30-08:55 Studies of Propagation from Atmospheric Sources Using Infrasound Arrays and Seismic Stations Catherine de Groot-Hedlin, University of California San Diego

08:55-09:20 Towards an automatic detection and localization procedure: an European case study Lars Ceranna, BGR Hannover

09:20-09:45 Analysing Multiple Explosions in Europe: Signal Characteristics and Propagation Modeling David Green, AWE Blacknest


10:00-10:25 Effects of atmospheric turbulence on azimuths and grazing angles estimation at the long distances from explosions Sergey Kulichkov, Institute for Atmospheric Physics

10:25-10:55 Acoustic-Gravity Waves From Meteor Entry As Well As From Rockets And Missiles Douglas ReVelle, Los Alamos National Laboratory

10:55-11:20 Numerical Simulation of Infrasound Propagation at High Altitude, Including Attenuation and Non-Linearity Catherine de Groot-Hedlin, University of California San Diego

11:20-11:45 Augmentation of IMS Infrasound Arrays for Near-field Clutter Reduction Curt Szuberla, University of Alaska, Fairbanks

11:45-13:10 Lunch

13:10-13:35 InfraMonitor: Methodology for Automated Regional Monitoring Stephen Arrowsmith, Los Alamos National Laboratory

13:35-14:00 InfraMonitor: Application to the Utah Infrasound Network Stephen Arrowsmith, Los Alamos National Laboratory

14:00-14:25 Infrasound Assessment of a Railroad Bridge Mihan McKenna, US Army Engineering R&D Center


14:45-15:10 About Results of the ISTC Project #2845 Anatoly Baryshnikov, National Institute of Pulse Techniques

15:10-15:35 Recent Applications of the Time-Domain Parabolic Equation (TDPE) Model to Ground Truth Events Rob Gibson, BBN Technologies

15:35-16:00

A Study on Characteristics of Seasonally Dependent Infrasound Propagation Based on the Ground-Truth Events from a Long-Term Experiment at a Quarry Mine Hee-Il Lee, KIGAM

18:30-20:00 Reception hosted by His Excellency the Governor of Bermuda Sir Richard Gozney at Government House. Dress code: Ladies: Dress; Gentlemen: Jacket and Tie

Thursday, November 6


Session 6: Infrasound from Geophysical Sources Chairs: Milton Garcés and Alexis Le Pichon

09:00-09:25 A Canadian Program for Dedicated Monitoring of Meteor Generated Infrasound Wayne Edwards, University of Western Ontario

09:25-09:50 An Estimate of the Terrestrial Influx of Large Meteoroids from Infrasonic Measurements Douglas ReVelle, Los Alamos National Laboratory

09:50-10:15 Infrasonic Measurements of the Carancas Peru Meteorite Fall Peter Brown, University of Western Ontario


10:30-10:55 Infrasonic Observations of Meteoroid Entries Ludwik Liszka, Swedish Institute of Space Physics

10:55-11:25 Infrasound from 2008CT3 on 7 October 2008 Douglas ReVelle, Los Alamos National Laboratory

11:25-11:50 Microbarom signals recorded in Antarctica - A measure for sudden stratospheric warming? Lars Ceranna, BGR Hannover

11:50-13:30 Lunch

13:30-13:55 The IMS Infrasound Network and its potential for detections of a variety of man-made and natural events Paola Campus, PTS/CTBTO

13:55-14:20 Acoustic-Gravity Wave Monitoring for Global Atmospheric Studies Elisabeth Blanc, CEA/DASE

14:20-14:45 Infrasound from Atmospheric Vortices Ludwik Liszka, Swedish Institute of Space Physics


15:15-15:40 The Analysis of the Infrasound Signal from May 12 Earthquake Wenchuan China Wang Xiaohang, North China Institute of Computing Technology

15:40-16:05 Evaluation of the ASHE Project (Ecuador) Milton Garcés, University of Hawaii

16:05-16:30 IMS Infrasound Station Observations of the Recent Explosive Eruptions of Okmok and Kasatochi Volcanoes, Alaska John Olson, University of Alaska, Fairbanks

16:30-16:55 Design of Monitoring and Early Warning System for Geological Hazards in Three Gorges Reservoir Area Using Infrasound Ning Qiu, Chinese University of Geosciences

19:00-22:00 Banquet Dinner, Bayside Terrace

Friday, November 7


Session 7: Summary

09:00-11:00 Walking tour of I51GB Array Element and Optional Cave Tour 11:00-12:30 Summary of Sessions 1-6 and Farewell

Session Chairs 12:30-13:30 Lunch

AbstractsAbstractsAbstractsAbstracts

Session 1: Sensors and Data Acquisition Chairs: Jay Helmericks & Carrick Talmadge

Chaparral Physics Research

Jay Helmericks Chaparral Physics / University of Alaska Fairbanks

This talk will cover two areas of research that Chaparral Physics has been pursuing, both of which are of interest to the general Infrasound community. The first is an investigation of the linearity of Chaparral Physics sensors. The testing shows that there are three regions: with small-amplitude signals the sensor is fully linear; then, as the signal amplitude increases, there is a point where the linearity of the sensor depends on the shape of the incoming wave; and finally, as the signal exceeds 100 Pa peak-to-peak the sensor response completely departs from linearity. The second area of research looks at both the effectiveness and frequency response of wind noise reduction systems, from ~5 Hz to 100Hz. The effectiveness of wind noise reduction systems have been studied extensively, but little work has been done on the frequency response of such the systems. Preliminary results from this research will be presented.

Development of a New Infrasound Microphone Technology

Carrick L. Talmadge

National Center for Physical Acoustics, The University of Mississippi

A new class of infrasound sensors have been developed at the National Center for Physical Acoustics for use in the long-range detection of sound in the atmosphere. These sensors are designed to be low cost, ruggedized as well as to perform robustly in the field. This technology utilizes piezoceramic sensing elements that are very rugged, and hence ideal for outdoor infrasound measurement applications. The main drawback of this technology is the very low capacitance of the sensor elements (55 pF). In order for the “microphone”, the combined sensing element + conditioning amplifier, to be sensitive and infrasound frequencies, a very large input impedance for the active

electronics is required (typically 5 GΩ is used for our systems). We will report on comparisons between our new microphones and other existing designs, namely the Chaparral 50 (C50) and the B&K 4193. Our preliminary testing suggests a very flat frequency response for our sensors, compared to the C50 over the frequency range for which the C50 has a flat response (0.02 – 20 Hz). For frequencies below the nominal cut-off of the C50, a calibration chamber with a calibrated volume source is being developed to test the infrasound sensors at ultralow frequencies. We also are developing a higher frequency version of this microphone (0.1–1000Hz), and will be reporting on this microphone as well.

A Low Power and Low Noise Infrasound Sensor for Temporary Measurements

Damien Ponceau, Sebastien Peyraud, Patrick Dupont, and Stephane Denis CEA/DASE

Efforts in Distributed Arrays for Infrasound Measurements

Kevin Dillion1, Wheeler Howard1, Doug Shields1, Jere Singleton2 1MilTec Research and Technology

2U.S. Army Space and Missile Defense Command

Research has been done to measure the wind noise reduction by a wagon-wheel shaped array of distributed sensors at Piñon Flat Observatory in comparison to the IMS array currently in use there. The distributed array consists of 96 individual sensors spaced 4m apart along the wheels, and the aperture created is approximately 70m. The wagon-wheel shaped array very closely approximates the aperture of the Piñon array, and a test was completed to measure if the wind noise is reduced on both arrays over the same frequency ranges. Simultaneous data from a wireless porous hose sensor was also recorded for comparison to the above named arrays. The wireless porous hose sensor had a cross-like configuration with 90ft of hose on the four sections (each 75ft porous hose extended from 15ft of rubber (non porous) hose). This created approximately a 55m aperture for wind noise reduction. PSD’s for each of the 3 arrays were compared during times of moderate winds to illustrate wind noise reduction capabilites. PSD’s of the 3 arrays were also compared during quiet periods that contained sound sources, which illustrates sound reproduction capabilities.

PTS Portable Infrasound Array

John Coyne, Nicolas Brachet, David Brown, Pierrick Mialle, Patrick Grenard, Paola Campus, Pavel Martysevich, Stefka Stefanova, Alfred Kramer, Ekrem Demirovic

CTBTO PTS, Vienna International Centre, P.O. Box 1200, 1400 Vienna, Austria

Recognizing the direct relationship between optimizing measurement systems and data analysis techniques, and the need to have an integrated approach towards technology development, the PTS is implementing a cross divisional project for developing and deploying a portable infrasound array. The high level objective is to improve the contribution of infrasound detections at IMS stations into the REB. Progress in this area can be achieved in a variety of ways using the portable infrasound array. For example: temporary deployment of the array to a region with existing IMS station(s) to assist in improving the understanding of local and regional sources of infrasound; participate in field exercises with controlled sources to collect ground truth data and improve modeling of acoustic propagation; evaluate the impact of station configurations on data processing and the detection, characterization, and localization of acoustic sources; evaluate the positioning of an IMS station which is influenced by known infrasound sources. The PTS portable infrasound array is undergoing final system testing in Vienna, Austria. Over the last several months a number of remedial actions were necessary to facilitate field deployment. After final testing is completed, the array is planned to be deployed in the vicinity of the Llaima volcano in Chile, in cooperation with the University of Chile. This deployment will help to improve the understanding of local and regional sources of infrasound, and will provide some similarities with the future positioning of the infrasound station I40PG, which will also be located close to an active volcano. This effort is seen as an opportunity for the PTS to work closely with National Data Centres and other interested parties to carry out scientific projects that would help to identify and categorize sources of infrasound signals detected at IMS infrasound arrays. The PTS is looking forward to pursuing infrasound experiments in the future, along with scientific collaborations with other institutes in order to better understand infrasound signals and their sources.

Development of a Portable, Controllable Infrasound Source

Milton Garces and Joseph Park Infrasound Laboratory

HIGP, SOEST, University of Hawaii, Manoa

A commercial off-the-shelf rotary subwoofer intended for enhancing high-fidelity home theatre systems in presently being redesigned as a portable, controllable infrasonic source. This source has been used to acoustically excite deployed IMS and portable infrasound arrays and their wind noise reducing systems. A calibration signal can be broadcast to an entire array, a test signal could be projected to validate array operational status, or a simulated target signal can be propagated to evaluate system processing and localization performance. Preliminary results of recent tests are discussed within the context of nuclear monitoring applications.

Application of an M-sequence Infrasonic Calibrator at the Camp Elliott OFIS Array

Kris Walker, Mark Zumberge, Scott DeWolf, and Matthew Dzieciuch

Institute of Geophysics and Planetary Physics, University of California, San Diego

Because of the difficulty in generating repeatable, low-frequency acoustic signals with detectable power at distances of hundreds of meters, in situ calibration of array infrasonic sensors is difficult. With a coded pulse source, however, low acoustic power levels generated for a long averaging time can be extracted from sensor time series. In this method, a low-frequency carrier signal is applied to an array of speakers while data from a distant infrasound sensor is recorded. The phase of the carrier signal is modulated by a predetermined, random sequence of bits. Even though an acoustic signal at the carrier frequency may not be visible above the noise in the infrasound sensor recording, cross-correlating the recording with the known phase modulated broadcast waveform reveals a strong peak. Comparing the size of the peak observed by a calibrated receiver with the peak observed by an uncalibrated receiver allows the sensitivity of the latter receiver to be determined for that central frequency. An important application of this technique is to determine, for example, the level to which a wind filter attenuates an infrasound signal (the wind filter impulse response). We present results of using this technique with an array of 18-inch subwoofers. An eight-subwoofer array at a range of ~240 m yields a reliable calibration signal that smoothly decreases in power by a rate of 2.5 dB/Hz with decreasing frequency to 8 Hz. We show that the speaker array configuration does not affect the resulting calibration signal level. A comparison of a 60-m OFIS with a B&K microphone at a signal frequency of 50 Hz for a source ~240 m away shows that the OFIS reduced wind and other types of undesirable noise by 18 dB over the B&K microphone with a standard sponge wind screen. Generating the detectable 8 Hz signal was only possible when using 8 speakers. The lowest frequency at which we could generate a detectable signal with a single speaker was 20 Hz. We have confirmed the analytical expression for the impulse response of a liner OFIS as a function of angle and frequency between the OFIS length and the speakers. Finally, we collected 72 hours of calibration data and observed multiple calibration signal arrivals at night. These multiple arrivals are either refractions or reflections from impedance contrasts in the first 500 m of the atmospheric surface layer or from buildings several hundred meters away.

Session 2: Infrasound from Anthropogenic Sources Chairs: Sergey Kulichkov and Curt Szuberla

Observations of Chemical Explosions

Ludwik Liszka Swedish Institute of Space Physics

Infrasonic observations of chemical explosions in Northern Finland were carried on every summer since 2001 during the period August 15 – September 15 by stations belonging to the Swedish-Finnish Infrasound Network (SIN). The observations reveal the complexity of the propagation mechanism. A comparison with meteorological radiosonde measurements demonstrates, the earlier suggested, a close connection between the signal characteristics and the occurrence of atmospheric irregularities. There are possible indications of:

• Multiplicity of propagation paths due to atmospheric irregularities (both in wind and temperature).

• Reflections from large-scale gradients in the lower atmosphere

• Non-linear interaction of shock waves generated by explosions with the atmospheric turbulence.

A possibility of a statistical propagation model is discussed. Such model could be used for reliable predictions of infrasound propagation at regional distances (<500 km).

Fast Arrivals at the Long Distances from Explosions

Sergey Kulichkov1, Laslo Evers2, Elisabeth Blanc3, Lars Ceranna4, Alexis Le Pichon3 1 Oboukhov Institute of Atmospheric Physics RAS

2 Royal Netherlands Meteorological Institute (KNMI), Seismology Division 3 CEA/DAM

4 Federal Institute for Geosciences and Natural Resources

The mechanism of the formation of anomalously fast infrasonic arrivals at long distances from surface explosions is discussed. These arrivals have propagation speeds c ( c=r/T, r is the distance between source and receiver, and T is the propagation time) higher than 320 m/s and are observed in the absence of acoustic waveguides in the atmospheric boundary layer (in the presence of wind and temperature inversions) and in the troposphere (in the presence of jet streams). It is assumed that such signals correspond to infrasonic propagation along a slightly inclined ray path having a significantly extended horizontal portion at a height of the maximum effective sound speed within the upper stratosphere (z is about 50 km). Ray theory and the time domain parabolic equation code (TDPE) are used as theoretical models. The solutions obtained are compared with experimental data on acoustic signals recorded at a distance of 635 km from surface explosions (equivalent to about 500 t of TNT) and at a distances 435 km and 641 km from blast site at oil depot in the UK (oil depot in Buncefield, UK) December 11, 2005. A satisfactory agreement between the theoretical and experimental data is noted.

Infrasound Signals from Repeating Detonations at the Utah Test and Training Range Recorded in North America

J. Roger Bowman and Gordon Shields

Science Applications International Corporation

We investigate signals at infrasound stations in North America from 148 detonations at the Utah Test and Training Range. The detonations occurred in late March through early October during 2003 and 2007 and had net explosive weight of 20 to 40 tons. Signals are observed by at least one infrasound station in North America for 138 (93%) of the 148 detonations. Signals are observed by two stations for 45 events (30%), by three, for 28 events (19%), and by four or more, for 3 events (2%). Detection rates are highest for stations I56US at 860 km range (88%) and NVIAR at 550 km range (75%). Detection rates are lowest for stations I53US (3450 km, 2%) and I57US (890 km, 13%). Detection rates vary significantly by season. Stations to the west detect more detonations during the period May through early-September, and stations to the east detect more in March-April and mid-September and October. Seasonal trends are also seen in group velocity, azimuth residuals, and amplitude. The variation in detection rates and the observed trends are consistent with predictions made using zonal stratospheric winds in the Ground-to-Space-07 (G2S-07) atmospheric specification. Most stations observe more than one signal arrival for each detonation in characteristic sequences. These sequences allow comparisons of signal features among the commonly observed arrivals at a station. Modeling results using G2S-07 capture the basic seasonal variations of the observations, but generally fewer arrivals are predicted than observed and these are later than observed signals. We perform a sensitivity analysis on selected velocity profiles to determine which attributes most influence the predicted arrival times, number of arrivals, and time intervals between arrivals. We determine that increasing the magnitude of the stratospheric duct and the decreasing the tropospheric gradient most effectively aligns the predictions with the observations.

Infrasound Calibration Experiment in Sayarim, Israel: Preparation and Test Shots

Yefim Gitterman, Rami Hofstetter

Geophysical Institute of Israel (GII)

The GII started a new project of establishing a calibration infrasound dataset for Eastern Mediterranean/Middle East region for improvement of monitoring capabilities of international network infrasound stations. The dataset is intended to characterize the infrasonic wave propagation in the region: travel times, spectra, amplitudes, dependence on source yield and atmosphere conditions. To achieve these goals a series of experimental surface shots will be conducted at Sayarim Military Range (SMR), located in southern Israel, Negev desert, including the main calibration explosion of 80 tons (TNT equivalent). During the explosions numerous portable co-located infrasound and seismic systems will be deployed at near-source (0.1-1 km) (accelerometers), and local/near-regional (10 – 350 km) distances. Deployment of several infrasound arrays at far-regional distances (500 – 1500 km) in neighboring countries is planned during the main explosion, jointly with ISLA, UH. Preliminary test explosions of broad yield range and various design were conducted during the first project year, in different days, including outdate ammunition detonations, conducted regularly at Sayarim Range, thus providing a wide range of atmospheric conditions. Low-frequency electret condenser microphones (tripartite arrays and single sensors) were installed at close distances less than 100 km, and Chaparral sensors were used for infrasound observations at larger distances. The test explosions are intended to fulfill a number of tasks: testing of new infrasound recording systems; yield scaling of signal amplitude/energy; testing of charge design and preparations and conducting logistics for the big explosion; analysis of atmosphere effect on infrasound propagation in different azimuths based on collected meteo-data about wind direction and velocity in different months; validation of safety estimations/conditions for successive increasing charges. We collected GTI0 information and observations of acoustic phases at stations of Israel Seismic Network, and few infrasound portable stations for a series of 7 experimental single shots (in the range 0.4-1.2 ton) conducted by Israel Defense Forces at Sayarim in September 2007-February 2008. Just recently, in June-July 2008, we conducted jointly with the IDF another test series of 13 outdate ammunition detonations (in the range 0.2-10 ton) and 2 special experimental project explosions of 1 ton of pure different explosives (TNT and CompositionB). The two explosions, conducted close to an ammunition shot and 10 min afterwards, were intended to estimate ammunition actual yield, and also check feasibility of using as a charge element for the large calibration explosion. Analysis of signals recorded at seismic and acoustic channels of portable and permanent stations is presented, including charge scaling estimations, comparison of energy generation for different explosives, examination of wind direction influence on parameters of infrasound phases. Results obtained from the test series and collected atmospheric data will be used for elaboration of design and logistics of the main calibration explosion, modeling of infrasound propagation (ISLA, UH) and selection of optimal experiment conditions and infrasound system locations.

Low Frequency Airplane Noise Measurement

Xiao Di, Claus Hetzer, Ron Wagstaff, and Kenneth E. Gilbert National Center for Physical Acoustics, The University of Mississippi

It has been observed when aircraft lands at an airport, windows near airport rattle, even when there is no significant audible sound. This observation suggests that the landing aircraft produces infrasound that could be acoustically detected at long range. This hypothesis was tested by deploying infrasound sensors during the daytime approximately 2.5 miles from the Memphis airport. Two different types of infrasonic microphones were used in the acoustic measurement. The first microphone was a Chaparral 50, which has a useable bandwidth of .02 Hz to 50 Hz. The second microphone was a Chaparral 2.5 which has a useable bandwidth from .1 Hz to 200 Hz. In addition to the infrasonic microphones, B&K microphones were used to record audible sound. Data were collected before, during, and after the landings and take-offs of 75 aircraft. It was observed that approximately 11 seconds after an aircraft landed, a strong infrasound signal was received. The infrasound, associated only with landing, was not accompanied by audible sound. Infrasound was generated by each of the 75 aircraft landings. Representative data will be presented, and possible mechanisms for infrasound generation by landing aircraft will be discussed.

Infrasound Case Studies

Paul Golden and Eugene Herrin Southern Methodist University

We discuss three case studies of regional infrasound signals from natural and man made sources recorded at infrasound stations NVIAR and TXIAR in the western US. Case 1 Infrasound signals were recorded at the NVIAR infrasound array originating from a magnitude 6.0 earthquake located near the town of Wells NV, USA at a distance of 422 km from the station. An infrasound signal was detected using the cross correlation technique indicating a back-azimuth of 40 degrees, clearly from the earthquake location. We also calculated a reasonable celerity value indicating the earthquake epicenter as the source. Additional signals were also detected prior to the arrival of the earthquake infrasound signal, but after the origin time of the earthquake. After careful analysis of the back-azimuth estimates of these arrivals it was determined that these three arrivals were also associated with the earthquake. By estimating travel times of Rayleigh waves from the source to two small mountain ranges, one due north of the array and one at 40 degrees and calculating infrasound travel times from those ranges, we believe Rayleigh waves from the Wells earthquake caused enough motion to create an infrasound signal originating at each mountain. The third additional signal arrived shortly after the surface waves passed under the station. This signal arrived at the right time to indicate the surface waves triggered another infrasound signal from the Excelsior mountain range just south of the array. It appears that four infrasound signals from four locations were all related to the Wells earthquake. Case 2 An infrasound signal was recorded at the TXIAR array in south Texas from a 20,000 lb. surface chemical explosion in southern New Mexico at a distance of 546 km. from TXIAR. The calculated celerity and trace velocity indicated that the signal propagated through the troposphere. Additionally, the signal was disbursed, indicating it was a 'trapped wave' propagating in a low velocity wave guide. Subsequent modeling of the waveguide as a simple low velocity layer 2 km thick over a rigid plane surface properly fit the dispersion observed. The topography between the source and receiver is relatively flat as it includes the Rio Grand River valley from New Mexico into southwest Texas, allowing this simple model to explain the observed dispersion. Case 3 An infrasound signal was recorded at a temporary station during field experiments near the NVIAR array. This signal originated at a well controlled ordnance disposal site 157 km due south of the station. Calculated celerity and trace velocity indicated the signal propagated in the lower troposphere. This signal was also clearly dispersed. Atmospheric models obtained close to the time of the observations suggest the dispersion is affected by lateral inhomogeneities and multilayering. Attempts at modeling this dispersion using a simple one layer wave guide were not successful so a more complex multi-layer model must be used.

The Effect of Wind Turbines Noise on the Infrasound Monitoring Stations

Stefka Stefanova CTBTO IMS/ED/AM

The windfarms are known to be a complex source of electromagnetic, mechanical and acoustic noise affecting the performance of the infrasound monitoring stations. The electromagnetic interference triggered by the wind turbines may result in degradation of the radio link performance. The vibration of the entire structure of the windmills generates harmonic seismic signals transmitted through the ground. Finally, low frequency acoustic signals are created by the rotating blades and can be detected at considerable distances. This presentation gives some examples of IMS infrasound stations located in vicinity of wind generators. It shows the geometry of the installations, illustrates the noise observed in the recorded data, and discusses potential problems that might occur in future developments of the windfarms.

Session 3: The International Monitoring System Chairs: Nicolas Brachet & Paola Campus

The IMS Infrasound Network: current status and future plans

Paola Campus, Pavel Martysevich, Alfred Kramer, Ekrem Demirovic, Stefka Stefanova, and Andrew Forbes

PTS/CTBTO

The development of the IMS Infrasound Network has steadily progressed since one year ago. With more than 68% of the IMS infrasound stations installed around the world, new challenges for future installations are now showing up. A description of the current status of the IMS infrasound network, including future plans for installations is provided. The recent evolution of the Installation and Certification Group into the Engineering and Development Section has also prompted the Acoustic Monitoring Project with new challenges, aiming to the improvement of stations performance: an overview of the current and future ED/AM projects is presented.

From the IMS Installation and Certification Group to the IMS Engineering and Development Section: a new challenge ahead

Patrick Grenard CTBTO, Chief IMS/ED, Vienna, Austria

Since the end of 2006 the original structure of the International Monitoring System (IMS) Division has been revised, as new challenges, like the regular maintenance and operation of the IMS stations already installed and certified, as well as the obsolescence of equipment became urgent issues to be timely addressed by the Provisional Technical Secretariat (PTS). As part of the restructuring an Installation and Certification Group was formed within the IMS Division, with the special task of completing the pending installations and certifications of the four IMS technologies. At the beginning of 2008 the Installation and Certification Group evolved into the Engineering and Development Section in order to establish the engineering support processes required for the successful implementation of a sustainment programme. In addition a technology development programme has been initiated in order to progressively enhance and optimize the performance of the IMS network. An overview of the past and future activities and challenges faced by the former Installation and Certification Group as well as by the newly formed Engineering and Development Section is presented.

Assessing the Detection Capability of the International Monitoring System Infrasound Network

David Green and David Bowers AWE Blacknest, Brimpton, Reading, UK

Recently, with advances in understanding of infrasound propagation, a number of research groups have re-assessed the detection capability of the IMS infrasound network (e.g., Le Pichon et al., Assessing the performance of the International Monitoring System infrasound network: Geographical coverage and temporal variabilities, JGR, In Review). Here, we present our own findings using a probabilistic detection threshold model that extends previous work by incorporating the effects of seasonal and diurnal stratospheric wind variations on signal amplitude. A consequence of including the stratospheric wind variations is that the detection thresholds become time dependent, with the thresholds changing by up to an order of magnitude over the course of the year at some locations. However, for 79% of all locations the time dependent detection threshold is always lower than estimated in the windless case. In general, lower detection thresholds are expected in winter and summer, compared to the equinox periods, due to the stronger stratospheric winds at these times. In contrast to the apparent detection improvements when wind velocity is included, the location capability of the network is reduced by the high directionality of the stratospheric winds, which reduce the azimuthal range of the stations detecting an event. We also consider the frequency-dependence of the infrasonic noise distributions, and investigate the influence of persistent noise due to microbaroms which degrades detections of transient signals between 0.17 and 0.25Hz, a frequency band that is of importance when considering nuclear explosion monitoring. Important future improvements to the model will include incorporating variations of the noise with time and location.

Ground-Truth Events as Benchmark for Assessing the Infrasound Detection Capability

A. Le Pichona, J. Vergoza, D. Greenb, N. Brachetc, L. Cerannad, and L. Everse

aCEA/DASE, Bruyères-le-Châtel, France bAWE Blacknest, Brimpton, Reading, United Kingdom

cCTBTO PTS/IDC Cienna International Centre, Vienna, Austria dFederal Institute for Geosciences and Natural Resources, Hannover, Germany

eKNMI, De Bilt, The Netherlands

A global scale analysis based on available detection lists for all operating International Monitoring System (IMS) infrasound stations confirms that the primary factor controlling signal detectability is the seasonal variability of the stratospheric wind circulation. At most arrays, ~80% of the detections in the 0.2 to 2 Hz bandpass are associated with propagation downwind of the dominant wind direction. The seasonal transition in the bearings and number of detections between easterly and westerly directions is presented. The observed detection capability of the IMS network is compared ot the predicted one using near-real time atmospheric updates and station-dependent wind noise models. The influence of individual model parameters on the network performance is systematically assessed. At frequencies of interest for detecting atmospheric explosions (0.2 to 2 Hz), the simulations predict that explosions equivalent to ~500t of TNT would be detected by at least two stations at any time of the year. Comprehensive ground-truth databases provide a statistical approach for evaluating the potential of infrasound monitoring. Accidental large explosions within Europe, such as the Gerdec ammunition dump explosion (Albania, March 2008) are analyzed and used here as a benchmark for validating the calculated threshold maps. They are an important step towards understanding how the IMS network will provide a successful monitoring regime for atmospheric or surface events, and the potential of the network to act as an effective verification tool in any future enforcement of the CTBT.

About results of the ISTC project # 2845

Anatoly Baryshnikov Research Institute of Pulse Technique

The work under the ISTC Project 2845 (Investigation of simultaneous signal propagation from the sources of infrasound and seismic waves for improving the performance of the infrasound method of monitoring the conduct of nuclear tests ) has been carried out by the following Institutes and collaborators: leading research institute - Research Institute of Pulse Technique (RIPT); Institutes–participants - Institute of Solar and Terrestrial Physics (ISTP) of Siberian Branch of the Russian Academy of Sciences; Institute of Atmosphere Physics (IAP) of the Russian Academy of Sciences; Geophysical Survey of the Russian Academy of Sciences (GS RAS); Kola Regional Seismological Center (KRSC) of Geophysical Survey of the Russian Academy of Sciences; foreign collaborators - CEA / DAM / DASE (France), NORSAR (Norway), KNMI (The Netherlands); CTBTO (Austria), BGR (Germany). The performance of the Project allow the following to be developed and submitted: data archive of signals from sources generating seismic and infrasound waves; models of acoustic and seismic signals propagation; procedure of joint analysis of information about blast acquired via seismic and infrasound channels; software for graphical display of signals and their processing; infrasound array in Obninsk for recording quarry blasts in the area of Kursk Magnetic Anomaly. It was proposed in the Project to solve these tasks on the basis of joint analysis of the results of observations by seismic and infrasonic monitoring methods in the area of Apatity and Obninsk. Report in the ISTC format includes the description of the progress and the results of works carried out by the executives at the final stage of the Project. These results are as follows:

• Observation and data processing means manufacture, mounting and putting into operation;

• Collection and analysis of available experimental data on infrasound and seismic signals;

• Running experiments on recording infrasound and seismic signals from quarry blasts;

• Development of procedures, models and programs for joint processing of signals and archiving experimental data from selected signal sources;

• Summarizing of information acquired in all research areas, the results of its processing, analysis and executed estimates, preparation of materials for the Final Report

The recently installed IMS Infrasound Station IS51, Bermuda

Paola Campus1, Roger Oldfield2 1PTS/CTBTO

2Telecom Bermuda

One of the recent achievements of the IMS/ED/ Acoustic Monitoring Project has been the installation of the 4-elements infrasound array IS51 in the area of St. George, Bermuda. The station is currently under certification process. A number of events has been detected at the station. In particular, the implosion of the Club Med Facility has left a clear signature in the recorded data.

Preparing the Return of Infrasound Data Processing into IDC Operations

Nicolas Brachet, Pierrick Mialle, David Brown, and John Coyne CTBTO PTS/IDC, Vienna International Centre, P.O. Box 1200, 1400 Vienna, Austria

The International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) Preparatory Commission in Vienna is preparing the return of infrasound data processing into operations in 2009. After the decision of disconnecting infrasound from network processing was taken in 2004, significant work has been done by the IDC to find practical ways of re-introducing infrasound into the Reviewed Event Bulletin. Concurrently, work is also underway to further improve this process. Specific automatic and interactive tools have been developed to detect and categorize automatic infrasound detections (i.e. distinguish between phases and noise) at each individual IMS station, and finally to produce automatic event bulletin together with seismic and hydroacoustic technologies. A new configuration has been tested to ensure a high performance of the IDC detector for a broad frequency spectrum (11 frequency bands covering 0.07 to 4.0 Hz), while retaining a reasonable CPU load under either the Solaris or Linux Operating Systems. The offline review of automatic infrasound events helped find new criteria for associating infrasound phases while maintaining a low number of false alarms, which is a prerequisite for re-introducing infrasound into Operations. Our current objective with 37 operational stations is to produce 10 to 15 automatic events each day built with infrasound data. Of these about 10% would be real events. The current IDC processing can be enhanced by modeling the infrasound propagation in the atmosphere and then labeling the phases in order to improve the event localization. In 2008, the IDC acquired WASP-3D Sph (Virieux et al. 2004), a 3-D ray tracing software which takes into account the heterogeneity of the atmosphere. Once adapted to the IDC environment, WASP has been used to improve the understanding of infrasound wave propagation and is being compared with the 1D ray tracing Taupc software (Garcés and Drob, 2007) at the IDC. Different atmospheric models are available at the IDC: ECMWF, HWM93 or the latest HMW07 (Drob, 2008), used in their initial format or interpolated into G2S. The IDC infrasound reference database is used for testing and validation of the propagation software and atmospheric specifications.

Enhancements to the CTBTO Operational Automatic Infrasound Processing System

David Brown, Nicolas Brachet, and Ronan J. Le Bras. International Data Centre, Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO)

The Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) is exploring methods to enhance the current operational infrasound processing system at the International Data Centre (IDC) for infrasound data recorded by the International Monitoring System (IMS). Several enhancements are under development and are currently being tested. The first enhancement is the incorporation of methods for determining signal amplitude. The following signal amplitudes are being determined for each Infrasound detection: Peak-to-Peak, RMS amplitude, and Instantaneous amplitude as revealed by the Analytic Trace via the Hilbert Transform. Initial efforts consider a variety of frequency bands, with the utility in network processing being of primary importance. A second enhancement is the incorporation of station noise characterization in terms of the Power Spectral Density (PSD). This determines the noise field at each station for various times of day for each month. This information is useful in determining network detection capability.

Session 4: Wind Noise Chairs: Douglas Christie & Stefka Stefanova

Wind Noise Reduction at IMS Infrasound Stations

Douglas R. Christie The Australian National University

Wind-generated turbulence continues to be the most serious source of background noise at IMS infrasound monitoring stations. Almost all established stations in the 60-station IMS network are subject at certain times of day and during certain seasons of the year to unacceptably high levels of wind-generated background noise that can seriously reduce detection capability. Good detection capability at all times is a feature found only at a relatively small number of IMS stations located inside tall dense forests. Many stations in the IMS network, such as stations installed on remote barren islands, in open deserts, and in the bleak ice-covered wastes of the Artic and Antarctic, are located out of necessity in high-wind environments with virtually no shelter from the ambient winds. The detection capability at these unsheltered stations may be substantially reduced during periods of high wind. All infrasound stations in the global IMS network, including stations located inside forests, have state-of-the-art wind-noise reducing systems installed at each array element. These noise-reducing systems are based on the use of a pipe array to collect and average micropressure samples over a limited area surrounding the infrasound detector. Pipe arrays are very effective in low and modest winds, but the degree of wind-noise reduction in winds of more than about 2.5 m/s may not be sufficient to guarantee reliable detection of signals from small nuclear explosions. The design of wind-noise-reducing pipe arrays has essentially reached practical limits. Further improvements in pipe array design will result only in a marginal increase in wind-noise reduction capability. This approach will not resolve wind-noise problems at stations located in high wind environments. Other wind-noise reducing techniques need to be developed for use either with or without pipe arrays. The design and performance of a variety of screened enclosures that can be used to reduce the level of wind-generated turbulence was described briefly at the 2006 and 2007 Infrasound Technology Workshops in Fairbanks and Tokyo, respectively. These turbulence-reducing enclosures can be used to provide effective wind-generated background noise reduction by enhancing the performance of a spatially distributed pipe array system, or, in some cases, as a stand-alone system with only a single inlet port. The results of further experiments with turbulence-reducing enclosures will be presented in detail, along with a comparison of the performance of these systems with pipe array systems at a typical unsheltered IMS station, a critical assessment of the combined use of turbulence-reducing enclosures and pipe arrays, and recommendations for the design of turbulence-reducing enclosures for use at existing IMS stations.

Approaches to the Infrasound Signal Denoising by Using AR Method

N. Arai, T. Murayama and M. Iwakuni and Solution Dept. Japan Weather Association

At the CTBT IMS infrasound monitoring station IS30 in Japan, The observed data is analyzed regularly by using of the PMCC (Progressive Multi-Chanel Cross-Correlation) software provided by CEA/France. Until now, some interesting infrasound signals were detected and their sources were estimated to be large earthquakes, rockets, eruptions of volcano and so on. And according to the three and half years observation at IS30, the amplitude of the noise caused by wind seems to be little bit larger than the level which we imagined before the start of the observation. Actually, when wind speed at IS30 is 2 m/s or more, wind noise appears prominently in the observed data, whose band may be 0.1 to 1.0 Hz. The dominant frequency of some infrasound signals may be also in these frequency ranges. Therefore, the adoption of filter in frequency domain would not be adequate way to remove noise to extract signals from noisy data. In order to remove wind and other background noise from the observed data, we tried to make time series of noise by using of the AR model and extract signals by removing them from observed data. In this workshop, we would like to introduce our approaches and discuss them.

Infrasonic Noise Reduction Using Shelters/Windbreaks

D.O. ReVelle EES-17, Geophysics Group, Los Alamos National Laboratory

We have performed very detailed numerical (finite difference) solutions of the steady state, incompressible, linearized Navier-Stokes equations in a homogeneous fluid (with no air density gradients), including the nonhydrostatic form of the vertical momentum equation in a fully viscous fluid, combined with the equation of continuity, on a two-dimensional (2D) x,z grid of ~105 points (starting from the early vestiges of porous flow behvaior presented in Counihan, Hunt and Jackson, J. Fluid Mechanics, 64, 529-563, 1974). These calculations were accomplished in order to properly identify the behavior of airflows through a uniformly porous wind shelter and thus determine its sheltering efficiency, etc. with respect to possible wind noise reduction for infrasonic pressure sensors. Turbulence has been modeled using gradient transport theory for linear momentum (a first order turbulent closure scheme). Previous calculations by others have shown only quite small sensitivity to the turbulence model chosen in the overall shelter modeling process (~ 5 % variability in wind speed predictions can occur depending on the turbulence closure model utilized). An upstream influence effect has been allowed in our calculations and computations have been made well upstream and downstream of the windbreak as well as from very near the lower boundary to up to 5 times the height of the shelter itself. This has been done as a function of the computed height of the local Prandtl (surface) boundary layer (itself a function of the aerodynamic roughness of the lower boundary and of the speed of the geostrophic wind for an assumed 10 % reduction of the turbulent surface stress, etc.). These calculations have also been performed as a function of the locally autobarotropic meteorological conditions through the ratio z/L, where z is height above the lower boundary and where L is the famous Monin-Obukhov-Lettau length scale for values of this ratio from 3 > z/L > -2.2. The assumed L values allow for both stable (intermittent internal gravity wave regime) as well as unstable (convective) meteorological situations so that we could readily examine their behavior on the resulting wind flow. In this talk we will present systematic results as a function of several of the most important parameters encountered for realistically modeling flow at the IMS infrasound array sites. This work includes new concepts for realistically computing upstream surface layer winds in regions where the physical shelter height either exceeds or is less than the local aerodynamic roughness length. This includes the possibility of reliable sheltering calculations within either a heavily wooded forest as well as within an open grassland setting, etc.

Session 5: Detection, Propagation and Modeling Chairs: Douglas Drob & Roger Waxler

Studies of Propagation from Atmospheric Sources Using Infrasound Arrays and Seismic Stations

M. Hedlin,1 C. D. de Groot-Hedlin,1 A. Le Pichon,2 J. Vergoz2, D. Drob3

1Scripps Institution of Oceanography, La Jolla CA 92093-0225 2 Commissariat à l'Energie Atomique, Centre DAM – Ile de France , Bruyères-le-Châtel, France

3Naval Research Laboratory

Studies of infrasound events must contend with the temporally and spatially variable structure of the atmospheric temperature and the winds that govern atmospheric propagation. Unfortunately, the global array of infrasound stations remains sparse, which complicates efforts to understand acoustic propagation, and ultimately, to use infrasound stations to characterize source position and source type. A potential solution to this problem is to use recordings of coupled infrasound-to-seismic energy made at the relatively densely space seismic stations of the USArray. The utility of the USArray has already shown to be of value for studies of the shuttle Atlantis as it passed over southern California in 2007. In that study, we limited our analysis to arrival times of acousto-seismic waveforms generated by the shuttle, which were recorded by over 100 stations in the USArray. In this talk, we report on preliminary findings from our study of a large bolide that exploded over Oregon and was recorded by over 100 seismic stations in the USArray and four infrasound arrays. The purpose of this study is to determine how much information on infrasound arrivals can be obtained from recordings at seismic stations. We analyze the polarization of three-component seismic data and compare this to the azimuthal information provided by the infrasound arrays. We also compare the arrival times and signal-to-noise ratios at infrasound arrays and nearby seismic stations.

Towards an automatic detection and localization procedure: an European case study

Lars Ceranna1, Alexis Le Pichon2 & Julien Vergoz2 1 BGR, Hannover, Germany

2 CEA/DASE, Bruyères-le-Châtel, France

The infrasound network of the International Monitoring Network (IMS) for the compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) is currently not fully established. However, it has demonstrated its capability for detecting and locating infrasonic sources like meteorites, as well as volcanic eruptions on a global scale. Such ground-truth events are rare; therefore regions providing a dense network of infrasound stations have to be considered to test and to calibrate detection and location procedures. In central Europe, several years of continuous infrasound waveform data are available for eight stations in Sweden, France, and Germany, whereas only one of them is part of the IMS. This exquisite setting with an average inter-station distance below 500 km allows the analysis of natural and artificial infrasonic activity in Europe. The results of the association of multiple arrays demonstrate the need of continuous infrasound monitoring on a regional scale to advance the development of automatic location procedures. Beside the seasonal variation of the network’s detection capability, which is dominated by the prevailing stratospheric winds, dominant source regions showing repeating events will be discussed in detail considering the next recording stations. An automatic procedure for re-location will also be presented, which is based on a robust grid-search algorithm and which makes use of ECMWF profiles for estimating appropriate celerity values and back-azimuth corrections. Moreover, the applicability of the double-difference approach will be discussed for infrasound.

Analysing Multiple Explosions in Europe: Signal Characteristics and Propagation Modelling

David Green1, Julien Vergoz2, Alexis Le Pichon2, Lars Ceranna3, Douglas Drob4, Laslo Evers5

1AWE Blacknest, Brimpton, Reading, UK 2CEA/DASE, Bruyeres-le-Chatel, Arpajon, France

3Federal Institute for Geosciences and Natural Resources, Hannover, Germany 4Naval Research Laboratory, Washington DC, USA

5KNMI, De Bilt, Netherlands

Since 2005 there have been four large ground level explosions within Europe that have generated infrasound signals which have been recorded at multiple arrays: the Buncefield vapour cloud explosion (December 2005), and the ammunition dump explosions at Novaky, Slovakia (March 2007), Gerdec, Albania (March 2008) and Chelopechene, Bulgaria (July 2008). The Gerdec and Chelopechene explosions are especially interesting case studies for infrasound analysis because multiple explosions occurred within a couple of hours of each other at the same location, with the origin times constrained by local seismic recordings. The majority of the signals (>90%) recorded at ranges greater than 500 km exhibit celerities (0.25-0.36 km/s) and frequency content (>2Hz) consistent with stratospheric propagation paths, rather than thermospheric returns. The spatial pattern of returns in each case is, as expected, highly influenced by the dominant stratospheric wind direction, although unexpected arrivals at I48, Tunisia, from the Gerdec explosions appear to have propagated upwind. Small signals (<0.01 Pa peak amplitude on the beam in the 2-4Hz bandpass) have been retrieved, using array processing to suppress incoherent noise. The similarities and differences between the signal characteristics are assessed, both between different explosion series and individual explosions in each series. Previous work showed that the stratospheric infrasound propagation from the Buncefield explosion could be modelled satisfactorily with a 1-D atmospheric acoustic and wind velocity model, derived from the 4-D high-resolution ground to space (G2S) models. Here we show that such 1-D models cannot explain the arrivals observed from the Gerdec and Chelopechene explosions, and explore how incorporating 2-D and 3-D atmospheric structure into the models influences the results. Combining both the signal characteristic studies and the atmospheric models we compare the recorded signals with the modelled network capability, and study the consistency of yield estimates across the network of recording stations.

Effects of Atmospheric Turbulence on Azimuths and Grazing Angles Estimation at the Long Distances from Explosions

Sergey Kulichkov1,Gregory Bus1, Vladimir Asming2 ,Elena Kremenetskaya3 , Anatoly

Baryshnikov3 , Yurii Vinogradov 2 1 Oboukhov Institute of Atmospheric Physics RAS

2 Kola Regional Seismology Center Geophysical Department RAS 3 Research Institute of Pulse Technique, MAE RF

The effect of the atmospheric fine structure on the azimuth and grazing angle of infrasonic signals recorded at long distances from surface explosions is studied both theoretically and experimentally. The data on infrasonic signals recorded at a distance of about 300 km from surface explosions. There were explosions with equivalent to 20—70 t of TNT to destruction of soviet medium range missiles and 31 Finnish explosions to destruction of old weapon and armament. The experiments were carried out during different seasons. Variations in the azimuths and grazing angles of infrasonic signals are observed in all experiments. A theoretical interpretation of the experimental results is proposed on the basis of the theory of anisotropic turbulence in the atmosphere. The theoretical and experimental results are compared, and a satisfactory agreement between these results is noted.

Acoustic-Gravity Waves From Meteor Entry As Well As From Rockets And Missiles

D.O. ReVelle EES-17, Geophysics Group, Los Alamos National Laboratory

Shock waves produced by hypersonic entry conditions for meteors as well as for supersonic entry conditions for rockets and missiles can be detected at close as well as great ranges from their respective sources. At sufficiently great range these waves will be diminished in their amplitude so that the structure no longer resembles a shock wave at all, but will have the character more of a small amplitude dispersed wave train. At sufficiently close range for sufficiently “small” sources, we can identify an infrasonic shock wave arrival followed by atmospheric waves. At great ranges from “large” sources, either a dispersed low frequency Lamb wave train (with low frequencies arriving first followed by higher frequencies) or a Bessel function type wave train with high frequencies arriving first followed by low frequencies as well as possible ducted waveguide arrivals (whose detailed properties depend on the atmospheric sound speed and horizontal wind speed structure) may all be present. The size or magnitude of the source can be characterized through the blast wave radius with either a spherical solution for the initial, stationary point source explosion type or a cylindrical solution for the hypersonic line source limit. Ducted wave solutions are determined separately for the Stratospheric or the Thermospheric waveguides (or both for downwind conditions for example). These solutions are determined using separate waveguide constraints that depend on the complex temperature or equivalently upon the adiabatic, thermodynamic sound speed below about 85 km and also the horizontal wind speed structure as well as upon the phase angle between the wave and the wind as a function of height. These constraints are applied at both the upper and lower boundaries and demand that the both incident angle = reflected angle and that the vertical gradient of the wave amplitude will vanish (so that the wave frequency and amplitude were unchanged after reflection from either boundary). We have also assumed that there is a near-integral number of hops between the source and the receiver given the range and source height of the event to the infrasonic pressure wave array while allowing for a realistic “miss” distance between predicted and observed arrivals. Here we have utilized Gaussian beam theory in order to predict the “ray” miss distance as a function of range (see for example, Porter and Bucker, JASA, 42, 1349-1359, 1987). In this talk we will examine examples of bolide and of rocket reentry and predict the AGW waveforms that we would expect to arrive.

Numerical Simulation of Infrasound Propagation at High Altitude, Including Attenuation and Non-linearity.

C. D. de Groot-Hedlin

Scripps Institution of Oceanography, La Jolla CA 92093-0225

Infrasound propagation within the thermosphere is associated with a variety of phenomena not observed at stratospheric and tropospheric altitudes. For instance, acoustic absorption increases with altitude and frequency, so that only low frequency infrasound is refracted from the thermosphere. Furthermore, infrasound propagation through an attenuating atmosphere is dispersive, so that sound velocity increases with increasing frequency. This suggests that the altitude of refraction depends on the frequency, with higher frequencies refracting at lower altitudes than lower frequencies. Here I report on the equations governing infrasound propagation in an absorptive medium, and show how these equations are incorporated in a finite-difference time-domain (FDTD) simulation of infrasound propagation. I show simulations that illustrate the combined effects of wind and attenuation on infrasound propagation, and compare the results with those computed when attenuation is ignored. Furthermore, upward propagation of infrasound to high altitudes can become non-linear as the sound wave pressure increases relative to the very low ambient pressure. The equations governing non-linear propagation at high altitude are presented and I show initial steps in solving problems in non-linear acoustic propagation.

Augmentation of IMS Infrasound Arrays for Near-field Clutter Reduction

C. A. L. Szuberla, J. V. Olson and K. M. Arnoult Wilson Infrasound Observatories

Geophysical Institute University of Alaska Fairbanks

The analysis of data recorded at IMS infrasound arrays is complicated by the presence of near-field clutter, both from anthropogenic and geophysical sources. Ideally, the IDC would like to exclude events that arise from within ~100 km of an array from its analysis pipelines. Previous work by our group made use of a signal processing technique to identify signal sources arising from within about ~20 km of a single IMS array, using only that array for the identification. Subsequently, we have explored the use of several, small arrays for precise localization of infrasound sources. This work has begun to be extended to the augmentation of an existing IMS array in an effort to push that near-field identification zone farther out. We present an introduction to this work and preliminary results of numerical simulations.

InfraMonitor: Methodology for Automated Regional Monitoring

Stephen J. Arrowsmith Los Alamos National Laboratory

An integrated suite of algorithms is presented for the generation of event catalogs from regional infrasound networks. First, a new detector is outlined that adaptively accounts for temporally variable correlated noise, which often arises from continuous sources and can swamp detection bulletins. Next, a simultaneous association and event location method is introduced. Association and location is implemented using a grid search algorithm. Location polygons are computed by placing bounding constraints on group velocities and backazimuth deviations. This approach avoids the requirement for an atmospheric model, which is important because current models remain to be comprehensively validated at regional scales. The full suite of algorithms presented here is implemented into a Matlab toolbox and GUI program called InfraMonitor.

InfraMonitor: Application to the Utah Infrasound Network

Stephen J. Arrowsmith Los Alamos National Laboratory

We report on the application of InfraMonitor (outlined in a separate presentation) to a regional infrasound network in Utah. A total of 287 events are detected, associated, and located using three infrasound arrays during a period of approximately 1 month. A comprehensive analyst review of all 287 events suggests a network-based false detection rate of <25%. Four ground-truth rocket motor explosions during the period of study are all detected and located within an average distance of ~5 km from the ground-truth location. A ground-truth survey using Google Earth has allowed us to associate events with the Utah Test and Training Range (UTTR), the Dugway Proving Ground, open-pit mines, and other potential sources of infrasound.

Infrasound Assessment of a Railroad Bridge

Mihan McKenna, Sarah McComas, Alanna Lester and Paul Mlakar U.S. Army Engineer Research & Development Center

Large infrastructure, such as bridges, emits such signals at their natural or driven frequencies of vibration. These frequencies can provide an indication of the structural condition. Infrasound may represent a means of remotely detecting the condition of large structures. The feasibility of this was recently evaluated in an in-service test of a railroad bridge. Infrasound experimental results are compared with those from operational modal testing, propagation modeling and finite element analysis.

Recent Applications of the Time-Domain Parabolic Equation (TDPE) Model to Ground Truth Events

Robert Gibson and David Norris

BBN Technologies, Arlington VA, USA

Recent progress has been made in the development of full-wave infrasound propagation modeling techniques, such as the Time-Domain Parabolic Equation (TDPE), which can be implemented together with state-of-the-art atmospheric specifications. Three-dimensional ray tracing is widely used to predict travel times and azimuth deviations; however, some shortcomings exist with these techniques, including the prediction of strong shadow zones that are contrary to many observations. Studies are conducted of high-quality ground-truth events to evaluate modeling capabilities through comparison of observations with predicted waveforms. Examples are presented wherein the TDPE modeling technique is used to predict certain observed arrival features (or phases) that are not predicted using conventional propagation modeling techniques. Characterization of fine-scale atmospheric structure, using a spectral model of wind perturbations induced by gravity waves, frequently enables the prediction of observed signals in shadow zones. The development of full-wave modeling techniques for infrasound, such as the TDPE, and higher-fidelity atmospheric specifications contribute to improved waveform prediction and more accurate phase identification.

A Study on Characteristics of Seasonally Dependent Infrasound Propagation Based on the Ground-Truth Events from a Long-Term Experiment at a Quarry

Mine

Il-young Che, Hee-il Lee Korea Institute of Geoscience & Mineral Resources (KIGAM)

The purpose of this study is to describe the characteristics of infrasound propagation and detectability of the infrasound events depending on the seasons at multiple seismo-acoustic network in Korea. Infrasound sources used in this study are industrial surface explosions at an open-pit quarry mine generating both active seismic and infrasonic signal nearly every day. The charge size varies from 2 to 10tons of ANFO. To estimate ground-truth explosion information such as exact source time and yield, we have installed two temporary seismo-acoustic stations inside the mine and operated them over the past one and half year. The long-term experiment covering all seasons could help clarify the seasonal variation of celerity in the mid-latitude Northern Hemisphere around the Korean peninsula. In addition, these ground-truth events could be used with meteorological data to study the effects of local atmospheric conditions on infrasound propagation, such as detectability, phase identification, and amplitude variation of infrasound signals at regional distances.

Session 6: Infrasound from Geophysical Sources Chairs: Milton Garces & Alexis Le Pichon

A Canadian Program for Dedicated Monitoring of Meteor Generated Infrasound

Wayne N. Edwards, Peter G. Brown

University of Western Ontario

Infrasound from meteors has long been an established signal source in the infrasound “zoo”. However, systematic recording of meteor infrasound has been elusive as many other sources produce meteor-like infrasonic signals. The late 1960's and 1970's saw significant strides forward in this regard as several programs developed showing how collaborations between infrasound networks and visual meteor networks could help isolate meteor infrasound signals. In this presentation these early experiments and their results are discussed along with a new experiment currently underway in southern Ontario, Canada as part of the Southern Ontario Meteor Network (SOMN). This ongoing experiment, now completing its third year of continuous operation, combines a network of automated all-sky low light level cameras, the Canadian Meteor Orbit Radar (CMOR) and the Elginfield Observatory Infrasound Array (ELFO) to monitor for meteor infrasound from centimetre-sized and larger meteoroids. This experiment is detecting meteor infrasound at a rate of ~1 event/month allowing the first statistical comparisons from well constrained meteor trajectories to early theoretical models. The results of these observations and their implications will also be discussed.

An Estimate of the Terrestrial Influx of Large Meteoroids from Infrasonic Measurements

E. A. Silber1, D. O. ReVelle2, P. G. Brown1, W. N. Edwards1

1University of Western Ontario 2EES-17, Geophysics Group, Los Alamos National Laboratory

The influx rate of meteoroids hitting the Earth is most uncertain at sizes of ~10 meters. From the early 1960’s to the mid 1970’s, the Air Force Tactical Applications Center (AFTAC) operated the global infrasound network designed to monitor nuclear explosions, and during that period recorded ten events which were later recognized to be bolides. For the first time ever, this historic infrasonic bolide data set was fully digitized, corrected for the instrument response function, and analyzed using modern approach and methods. Several independent energy yield relations were applied to these airwave data to calculate bolide kinetic energies and refine the terrestrial influx rate at sizes of ~ 10 meters. The South African bolide of August 3, 1963, originally estimated at 1,100 kT, is of particular interest, because it plays a crucial role in determining the frequency of energetic impacts, such as the Tunguska 1908 event. At low energies our flux results are in agreement with previous estimates. However, for 5-20 m diameter objects our measurements of the cumulative number of Earth impacting meteoroids are as much as an order of magnitude higher than estimates from telescopic surveys of near-Earth objects and satellite detected bolides impacting the Earth. The South African bolide remains an energetic event, with the source energy estimated between 0.67 – 2.01 MT.

Infrasonic Measurements of the Carancas Peru Meteorite Fall

P.G. Brown1, W. N. Edwards1, A. Le Pichon2, K. Antier2, D.O. ReVelle3, G. Tancredi3, S. Arrowsmith3

1University of Western Ontario 2CEA/DASE

3Los Alamos National Laboratory

On September 15, 2007 at 16:40 UT, a bright fireball was witnessed close to the southern tip of Lake Titicaca near the Peru – Bolivia border. Shortly thereafter, a 14 m diameter crater was discovered near the town of Carancas, Peru and fragments of an H4-5 chondrite recovered. Airwaves from the Carancas fireball were detected at infrasound stations in Bolivia (I08BO) and Paraguay (I41PY) and at least at five seismic stations in Bolivia. The closest seismic station was less than 50 km distant from the crater and its record show clear P and S wave arrivals from the crater impact. This is the first unambiguous seismic recording of a meteorite impact on Earth. From modeling of the airwave arrivals, the fireball trajectory is found to have an approximate East to West orientation and relatively steep (~50 degree) entry angle. Using various source energy estimations from infrasound records we estimate an explosion yield for the crater of order a few tons of TNT equivalent. The seismic coupling efficiency of the impact is found to be ~10-4. The initial energy of the fireball is found to be ~0.1 kT TNT with an entry velocity below 17 km/s and most likely at the lowest end of possible entry speeds (~12 km/s), suggestive of initial masses of <10 metric tonnes for the pre-impact meteoroid. The most remarkable aspect of the event, production of the crater by a small stony meteorite, remains a puzzle; some potential solutions will be offered.

Infrasonic Observations of Meteoroid Entries


The presentation deals with two subjects: Extraction of orbital parameters from infrasonic observations

The method seems to work when events are observed at distances of the order of 100 km, or shorter. The method is based on sorting of recorded wave packets with respect to the corresponding trace velocity.

A search for unknown meteoroid entries in the database containing 14 years of infrasonic data recorded by the Swedish Infrasound Network.

The search method is a statistical technique, the Multiple Indicator Model. As the multiple indicators distributions of different signal characteristics were used. The model was calibrated with a number of known meteoroid entries. The calibrated model was used to scan through the entire database to search for similar events. A number of possible events were detected. In one case it was possible to confirm the detected event as the meteor event of January 17, 2002 over the North Sea.

Infrasound from 2008TC3 on 7 October 2008

D.O. ReVelle1, P.G. Brown2, W.N. Edwards2 and E.A. Silber2 1EES-17, Geophysics Group, Los Alamos National Laboratory

2University of Western Ontario, Physics and Astronomy Department

At 3:08 am MDT on October 6, 2008, a team of scientists in Italy (NEODys team: Andrea Milani, Maria Eugenia Sansaturio, Fabrizio Bernardi, and Giovanni B. Valsecchi) announced by email that a very small asteroid (or a very large meteorite) with its designations, STA9D69 or 2008TC3, would impact the atmosphere at 02:47 UT on October 7th over the northern Sudan (at latitude 20.855 N and longitude 31.697 E) with a very high predicted probability of ~99.8 to 100.0 %. For a spherically shaped, 2 m diameter chondritic (rocky) object, computed initial kinetic energies range from 1.35 to 5.4 kt for initial velocities from 30-60 km/s (and from 10.8 to 43.2 kt for a 4 m diameter object for the same velocity range). Thus, for the first time scientists have given our modern human civilization advanced warning of an impending bolide impact. The object was originally detected at Mount Lemon in Arizona by the Catalina Sky Survey and later confirmed by scientists at JPL (Pasadena) and elsewhere. A team of scientists led by Professor Peter G. Brown at the University of Western Ontario’s Department of Physics and Astronomy also produced telescopic images of 2008TC3 prior to its entry. Raw unprocessed measurements taken at 01:41:03 UT using the 1.22 m Elginfield Observatory telescope resulted in an excellent view of the very small asteroid against the background star field with a 9 sec exposure that showed just how fast the object was moving. With a field of view of about 1/3 of a degree, they managed to track the object for almost 1.5 hours and were able to do some multi-band ECAS spectrophotometry, albeit near the SNR limit. Evidence was found for a 3-4 Hz rotation rate of the body in the last few images just before the object crossed into the Earth’s shadow at their location at 01:49 UT. Subsequent to the entry, weak infrasonic waves could readily be detected in Kenya at the 7

element infrasonic array, I32. These relatively small amplitude waves (~ ±0.025 Pa peak to peak) with an arrival time of about 05:10 UT were both self-consistent with an infrasonic Stratospheric arrival delay time with an observed signal velocity of 0.28 km/s as well as with an observed 5-6 second wave period (at maximum signal amplitude) with a duration of several minutes at a reasonable level of cross correlation. Using the semi-empirical AFTAC yield-period relationship that has been reported earlier upon by ReVelle, a kinetic energy near the end height of 1.1 to 2.1 kt is predicted. This is the kinetic energy at the point on the trail that could reach the ground location at I32. In addition, the computed plane wave back azimuths from I32 very nearly intersect the predicted impact location in N. Sudan (with angular deviations of < ~3 degrees over a horizontal range of ~2500 km). An additional and much more robust detection has also just been made for the I31 Kazakhstan infrasound array at ~4000 km range. It completely confirms the Kenya detection and also intersects the N. Sudan impact location. Future analyses will benefit from a careful examination of the proposed impact region for a search for small meteorites. Great benefits may also be made by a search for optical as well as infrared satellite images for this rather unique event.

Microbarom signals recorded in Antarctica – A measure for sudden stratospheric warming?

L. Ceranna1, A. Le Pichon2, E. Blanc2, and L. Evers3

1 BGR, Hannover, Germany 2 CEA/DASE, Bruyères-le-Châtel, France

3 KNMI, De Bilt, The Netherlands

Germany is operating one of the four Antarctic infrasound stations to fulfill the compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). I27DE is a nine element array which in continuous operation since its deployment in January 2003. Using the PMCC detection algorithm coherent signals are observed in the frequency range from 0.0002 to 4.0 Hz covering a large variety of infrasound sources such as low frequent mountain-associated wave or high frequency ice-quakes. The most prominent signals are related microbaroms (mb) generated by the strong peri-Antarctic ocean swells. These continuous signals with a dominant period of 5 s show a clear trend in the direction of their detection. This trend is well correlated to the prevailing stratospheric wind direction and speed. Although harsh weather conditions have often been faced at the station the infrasound array is capable to detect mb-signals up to wind speeds of 15 m/s. Therefore, I27DE is 85 % of the time operational. For mb-signals a strong increase in trace velocity along with a decrease in the number of detections were observed during the Austral summer 2006 indicating strong variations in the stratospheric duct. However, ECMWF profiles at the stations give no evidence for such anomaly. Nevertheless, strong events of sudden stratospheric warming (SSW) at latitude ranges of the peri-Antarctic belt occurring during Austral winter 2006 provide a potential explanation for the abnormal sound propagation. This will be demonstrated computing 2-D numerical simulations for sound propagation from the ocean swell to I27DE using ECMWF profiles.

The IMS Infrasound Network and its potential for detections of a variety of man-made and natural events

Dr. Paola Campus CTBTO, IMS/ED/AM, Vienna, Austria

One of the biggest challenges faced when the International Monitoring System (IMS) Network of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) was designed consisted in the construction of an Infrasound Network comprising 60 stations: in fact, ten years ago, none of these 60 infrasound stations existed. Since then, a significant effort has been made to establish the Infrasound Network, which now counts more than 68% of its stations operational. The increased number of stations, located all around the world, has permitted to detect several types of events, both man-made and natural, in very different geographical areas, opening the way to a broader analysis of the performance of the Infrasound Network. Beside the main mandate of the CTBT IMS Infrasound Network, data recorded by this network can play an important role in a wide variety of international geophysical hazard warning systems, ranging from the monitoring of local and global volcanic activity, including areas where other types of monitoring networks are not available, to the monitoring of global warming, by observing, for example, the rate of cracking of icebergs and the occurrence of avalanches and landslides. A number of examples of events detected by the IMS Infrasound Network is presented.

Acoustic-Gravity Wave Monitoring for Global Atmospheric Studies

E. Blanc1, A. Le Pichon1, L. Ceranna2, T. Farges1 1Commissariat Energie Atomique Laboratoire de Detection et de Geophysique,

CEA/DAM Ile de France Bruyères le Chatel, 91297 Arpajon France 2Federal Institute for Geosciences and Natural Ressources,

Section B3.11 Seismology Stilleweg 2, Hannover, 30655 Germany

The infrasound network developed for the verification of the Comprehensive nuclear Test Ban Treaty, provides an unique opportunity to monitor continuously pressure waves in the atmosphere. Such infrasonic waves propagate in the channel formed by the temperature and wind gradients of the atmosphere. Long term observations provide information about the evolution of the propagation conditions and then of atmospheric parameters. The monitoring of continuous sources, as ocean swell, gives the characteristics of the stratospheric wave channel submitted to stratospheric warming effects. Large scale gravity waves, which are also observed by the network, produce a forcing of the stratosphere at low and middle latitudes and long-lived changes in the stratospheric circulation towards high latitudes, leading to fluctuations in the strength of the polar vortex. These fluctuations move down to the lower stratosphere with possible effects on the tropospheric temperature. Gravity wave monitoring in Antarctica reveals a gravity wave system probably related to the wind effect over mountains. At mid latitudes an additional main sources of disturbances is the thunderstorm activity. The infrasound monitoring system allows a better knowledge of the atmospheric wave systems and of the dynamics of the atmosphere. In return this better knowledge of the wave systems allow a better identification of the possible explosion signals in the background of the atmospheric waves and then to improve the discrimination methods.

Infrasound from Atmospheric Vortices


Infrasonic arrays belonging to the Swedish-Finnish Infrasound Network frequently detect peculiar signals with very high horizontal trace velocity (HTV), i. e. arriving nearly simultaneously to all microphones. For a long time these signals were considered as some kind of electromagnetic interference and discarded. The signals are nearly saw-tooth shaped, reach in harmonics. It has been found that the HTV signals are most common when a weather front passes the infrasonic array. The HTV signals are likely to be emitted by atmospheric vortices (Stenflo, L.: Acoustic solitary vortices. Phys. Fluids, 30 (10), 3297, 1987).which are expected to generate infrasonic waves with just such frequency spectrum as that observed for HTV waves. It has been found, measuring the HTV pressure amplitude, that these waves carry a considerable energy, up to 40 Watts/m2 sec. HTV waves have another interesting property: they seem to be correlated with radar echoes from low troposphere. Observations of atmospheric vortices may be of value for understanding the energy balance in the tropospheric boundary layer.

Analysis of the Infrasound Signal from May 12 Earthquake Wenchuan China

Wang Xiaohang

North China Institute of Computing Technology

May 12 2008, 14:28, a strong motion earthquake shocked the area of Wenchuan Sichauan China. This disaster caused a terrible death and properties destroy at the local areas. During the two hours following the earthquake, our experimental infrasound array over thousand kilometers away from the epicenter recorded two series of infrasound signals apart each other thousands of seconds. The signals are better accord with the propagation theory in arrival time. But the first series of the signals were not from the azimuth of the epicenter, they were from almost opposite and variational directions. We analyzed these phenomena and found that if we attributed it to the terrain about our experimental infrasound array, the phenomena could get a reasonable explanation. So we thought that all of these signals were caused by this earthquake. They looked different only because they arose from different mechanisms.

Evaluation of the ASHE Project (Ecuador)

M. Garcés1, D. Fee1, and A. Steffke1; D. McCormack2; R. Servranckx3; H. Bass4 and C. Hetzer4; M. Hedlin5 and R. Matoza5; H. Yepes6 and P. Ramon6

1Infrasound Laboratory, University of Hawaii at Manoa 2Geological Survey of Canada, Ontario, Canada

3Montréal Volcanic Ash Advisory Centre, Meteorological Service of Canada 4University of Mississippi

5University of California, San Diego 6 Instituto Geofísico, Escuela Politécnica Nacional (EPN), Quito, Ecuador

Volcanic eruptions may produce powerful infrasonic signals that may be detected at global scales and be used for International Monitoring System (IMS) evaluations. Furthermore, volcano monitoring is one of various natural hazards applications of the IMS. The International Civil Aviation Organization (ICAO) has expressed a strong interest in the potential of global and regional infrasound networks to provide eruption notifications to the aviation community through the existing Volcanic Ash Advisory Center (VAAC) framework. Since data distribution and latency issues may complicate the release of IMS data for practical operational use, international groups are collaborating in the design of prototype low-latency infrasound surveillance systems specifically tailored to the needs of the aviation community. The Acoustic Surveillance for Hazardous Eruptions (ASHE) proof-of-concept project seeks to develop and evaluate the potential for robust, operational infrasonic remote sensing of volcanic eruptions. As part of this effort, the ASHE team presently produces automated notification products on a test basis to a participating ICAO-designated VAAC for comparison against and possible integration with their existing warning systems. These notifications are coupled with more detailed real-time data products (presently provided in designated web pages), and could be used by volcano observatories to disseminate updated information. Based on the acoustic records captured during the Vulcanian to Plinian eruptions of Tungurahua volcano, source parameters that may be estimated during large eruptions include (but may not be limited to) the height probability, start time, and duration of an ash cloud injection that could pose a hazard to international carriers at cruising altitudes. The possibility of inferring these source parameters even at greater distances was suggested by the recognizable acoustic fingerprints of the Summer 2008 eruptions of Okmok and Kasatochi Volcanoes in the remote Aleutian Arc of Alaska. These eruptions were recorded at distances of 1700-4500 km by infrasound arrays in Fairbanks, Alaska, Washington state, Hawaii, Russia, and Japan. As a result of the ASHE system evaluation during the 15-19 September 2008 International Airways Volcano Watch Operations Group Fourth Meeting in Paris, France, IAVWOPSG Members from Canada and the United States are tasked to continue to assess the feasibility of using ASHE and CTBT IMS-type arrays to automatically identify eruptions with a Volcano Explosivity Index (VEI) of 2 or greater at regional distances and prepare a report in time for consideration by the IAVWOPSG/5 Meeting. In addition, a request was made to coordinate with the CTBTO and the Toulouse VAAC to continue to assess the feasibility of using infrasound data to automatically identify ash-producing volcanic eruptions.

IMS Infrasound Station Observations of the Recent Explosive Eruptions of Okmok and Kasatochi Volcanoes, Alaska

J. V. Olson1, K. Arnoult1, C. A. L. Szuberla1, C. Wilson1, M. Garces2, M. Hedlin3, S. McNutt4

1Wilson Infrasound Observatory, Geophysical Institute, University of Alaska 2ISLA, University of Hawaii

3L2A, IGPP, University of California San Diego 4Alaska Volcano Observatory, Geophysical Institute, University of Alaska

Infrasonic signals produced by the 2008 eruptions of Okmok and Kasatochi volcanoes, Alaska, demonstrated the potential for acoustic remote sensing of eruptions. During its recent period of eruptive activity, Kasatochi Volcano produced at least five explosive eruptions that were recorded by seismometers and infrasound arrays. These eruptions ejected significant ash into the stratosphere. Seismometers located along the Aleutian Islands recorded explosive episodes with multiple start times. Infrasound arrays located in Fairbanks, Alaska (I53US, 2104 km from Kasatochi), in Kona, Hawaii (I59US, 3996 km from Kasatochi) and in Newport, Washington (I56US, 4060km from Kasatochi) also recorded these explosions at times consistent with the seismic observations. The volcanic signals recorded by I53US and I59US had high signal to noise ratios and reached pressure levels of 2.0 Pa peak-to-peak at both arrays during the second explosive eruption. A single infrasound microphone located on Mt. Shishaldin (818 km from Kasatochi) recorded the onsets of explosions 1, 4, and 5 at approximately the times expected; however, due to technical problems, it did not record explosions 2 and 3. The eruptive events of Okmok Volcano that occurred 12-13 Jul 2008 were also recorded by infrasound arrays I53US and I59US. Although a large ash cloud was produced by Okmok Volcano, the pressure levels of the Okmok infrasound signals, relative to those from Kasatochi, were much less (< 0.5 Pa peak-to-peak at I53US). Three groups of infrasound signals were observed at I53US with arrival times of 21:44 UT on 12 July 2008, and 01:14 and 05:41 UT on 13 July 2008. Their durations were 43, 95, and 29 minutes, respectively. The data from other IMS infrasound arrays will be used to study source and propagation effects. Of particularly interest will be the data from I56US in Newport, Washington (4060 km from Kasatochi, 3547 km from Okmok) and I57US in Pinon Flat, California (5067 km from Kasatochi, 4585 km from Okmok). The relationships between the infrasound signals and the physical characteristics of the ash injections will be investigated.

Design of Monitoring and Early Warning System for Geological Hazards in Three Gorges Reservoir Area Using Infrasound

Ning Qiu1, Zuo-xun Zeng1,2, Yi-Chun Yang,3 1 China University of Geosciences

2 Huazhong Tectonmechanical Research Center 3 Institute of Acoustics, Chinese Academy of Sciences

With the progress of the Three Gorges Dam Project, geological disasters have become increasingly prominent. The reservoir area prone to landslides, collapses, cracks, and earthquake disaster because the complex terrain and geological conditions. It is of significance to monitor and foresee geological hazards in the reservoir area. Here we introduce our design of Monitoring and Early Warning System for Geological Hazards in Three Gorges Reservoir Area Using Infrasound. Infrasound may be abnormal during geological disasters, such as debris and earthquake occurred. The formation a d movement of debris flow in its basin will generate infrasound, and spread to the surrounding air medium. Velocity of infrasound is much larger than that of debris flow, so we can monitor and forecast debris flow using infrasound. The sudden vertical displacement brought about by Earthquake will generate acoustic-gravity wave which can be observed in distance to monitor earthquake, especially to monitor earthquake precursors. So we try to monitor the geological disasters for the Three Gorges reservoir area in China.by design a infrasound array system.. The infrasound monitor system is comprised of two observation stations arranged in Badong County inside the reservoir area and in Wuhan City, respectively. Each station has respectively arranged a kind of augmentable linear array in the form of quasi-uniform linear array and additional amending direction sensors. The linear array comprises eight sensors arranged in several different uniform intervals along a line. The amending direction sensor is situated at certain point in mid-perpendicular of linear array in order to reduce multiplicity in determine the direction of arrival. The sensors used in the system are CDC-2B capacitances infrasonic receiver which can observe frequency ranging 0~20Hz. The, measurement resolution is 750mV/LPa. Infrasonic wave signal collected by sensor is transferred from observation stations to Data Processing Center in China University of Geosciences, using telecommunication networks, and then we can process and analyze the infrasonic signals. Observations all-weather infrasound near the reservoir area can be achieved on monitoring geological disasters in Yangtze River Three Gorges. There are still problems to be considered in the system: (Ⅰ) in the conditions of using few sensors, this system need more rational array layout mode; (Ⅰ) array sensors in field can lasting stability in some way; (Ⅰ) the amount of data acquisition, real-time data transmission, storage and backup need higher demands in hardware and software. This work is supported by Chinese "985 Project" - National Level Innovation Projects Platform of China.

Posters

Investigating regional industrial explosive events in Southern Ontario, Canada

Wayne N. Edwards1, Peter G. Brown1, David McCormack2 1University of Western Ontario 2Geological Survey of Canada

Since its inception in January 2005, the Elginfield Observatory Infrasound array (ELFO) located in South Western Ontario, Canada some 30 km from the city of London has continuously monitored the infrasonic and lower audible band between 0.1 – 50 Hz. Although its primary research goal is to record impulsive signals from meteors, on occasion it has recorded explosive sources. Some of these events have established provenance allowing for calibration of yield and propagation models, while others remain uncertain. Here we present signal records and modeling associated with a number of well known sources including an oil refinery explosion in Sarnia, Ontario in 2006, a methamphetamine lab explosion near Ilderton, Ontario in April 2008, and most recently a propane storage facility explosion in Toronto on August 10, 2008. An anomalous series of mysterious loud explosion sounds reported by residents of Kincardine, Ontario have also been recorded, but their origin remains uncertain. Details of these event investigations, their infrasonic signal propagation and source characterization will be presented.

ITW2008 Technical Program · NCPA, University of Mississippi [email protected] Ron Wagstaff NCPA,...

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