Detection of infrasonic events using a sparse global network of infrasound stations
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Detection of Infrasonic Events Using a Sparse Global Network of Infrasound Stations. Michael O’Brien 1 , Manoch Bahavar 1 , Eli Baker 1 , Milton Garces 2 , Claus Hetzer 2 , Hans Israelsson 1 , Charles Katz 1 , Colin Reasoner 1 , and Jeff Stevens 1

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Detection of infrasonic events using a sparse global network of infrasound stations

Detection of Infrasonic Events Using a Sparse Global Network of Infrasound Stations

Michael O’Brien1, Manoch Bahavar1, Eli Baker1, Milton Garces2, Claus Hetzer2, Hans Israelsson1, Charles Katz1, Colin Reasoner1, and Jeff Stevens1

1.Science Applications International Corporation

2.University of Hawaii

Infrasound Technology Workshop

University of California, San Diego

October 27-30, 2003

Outline of Infrasound Stations

  • Challenges

  • Approach

  • Data sets

  • Detector optimization

  • Clutter classification

  • Network detection simulation

  • Ongoing work

  • Summary

Challenges in infrasound monitoring
Challenges in Infrasound Monitoring of Infrasound Stations




Detection approach
Detection Approach of Infrasound Stations





data set
















ID rules

Clutter ID




Noise spectra

and PDFs










Project network
Project Network of Infrasound Stations

  • All stations available in June 2003

Canonical detection data set
Canonical Detection Data Set of Infrasound Stations

  • Focuses on signals of interest:

    • Surrogates for atmospheric nuclear explosions

  • Waveforms and arrivals for

    • Bolides (7)

    • Chemical explosions (2)

    • Scaled nuclear explosions embedded in ambient noise

Fixed data set
Fixed Data Set of Infrasound Stations

  • Objective

    • Provide a standard reference for assessing processing in absence of signals of interest (e.g., false-alarm rates)

  • Contents

    • Contiguous 7-day time interval in April 2003 of all seismic, hydroacoustic and infrasound data currently received

      • April 1-7 selected based on total percentage data coverage

    • Seasonal 3-day contiguous time intervals of all infrasound data currently received

      • May 5-7, 2002: northern spring

      • August 5-7 , 2002 : northern summer

      • November 5-7, 2002: northern autumn

      • February 5-7, 2003: northern winter

Detector optimization approach
Detector Optimization Approach of Infrasound Stations

  • Detector used

    • Detection and Feature Extraction (DFX / libinfra)

    • Same as former PIDC detector

  • Processing parameters (libinfra)

    • Governed by inter-spacing of group of array elements

    • Frequency band from 0.1 - 4.5 Hz depending on diameter of element group

    • Data window for correlation between 30 and 90 sec

Detector optimization approach 2
Detector Optimization Approach (2) of Infrasound Stations

  • Key detector parameters:

    • Thresholds for correlation and signal energy

    • Lengths of data windows

  • Parameters tuned against scaled waveforms implanted into real station data (ambient noise for this purpose)

  • Explosion implants (about 100 per array):

    • Derived from recordings of historical atmospheric explosions

    • Scaled to different yields and distances < 2000 km

  • Receiver Operator Characteristic (ROC) curves computed for various parameter settings

  • Parameters chosen to maximize detection of implants at an acceptable overall detection rate (100-200 / day)

  • Parameters validated with data from different seasons

Detection of scaled events i33mg
Detection of Scaled Events: I33MG of Infrasound Stations

  • ~ 80% of implants detected

  • Smaller sources have detections at higher frequencies

  • Larger sources have detections at lower frequencies

Detections Made by DFX/libinfra on I33MG Implants (~100)

# of detections

# of detections / implant

Equivalent Pressure (PA)

I33mg detection efficiency
I33MG Detection Efficiency of Infrasound Stations

  • Derived from previous plot (scaled embedded explosion waveforms)

fraction of implants detected

equivalent pressure (Pa)

Receiver operator characteristic roc
Receiver Operator Characteristic (ROC) of Infrasound Stations

  • Stations fall in performance categories

    • High:DLIAR, I08BO, I55US

    • Intermediate:I33MG, IS07

    • Low :I24FR, I57US

  • Local clutter limits detection ability

fraction of implants detected (Probability of Detection)

detection rate on raw data [per day] (False Alarm Rate)

Detector optimization
Detector Optimization of Infrasound Stations

  • Performance of detector:

    • Depends critically on correlation threshold

    • Much less sensitive to energy threshold

  • Performance improved by use of robust estimator of array correlation:

    • One accidentally correlated site pair can strongly contaminate arithmetic mean of all correlation pairs for array

    • False alarms can be reduced significantly by computing median instead

Detector optimization 2
Detector Optimization (2) of Infrasound Stations

  • Different length processing windows are sensitive to different explosion signals

    • Shorter windows sensitive to shorter, more impulsive signals

    • Longer windows sensitive to longer, less compact signals

  • Probability of detection improved significantly by processing data sequentially with multiple data windows of different length

  • Multiple detections often made on the same signal

    • Similar slowness vector, closely spaced in time

    • Primary detector not sophisticated enough to recognize these as the same signal

False alarm rate reduction
False Alarm Rate Reduction of Infrasound Stations

  • Too many detections can complicate downstream analysis

  • Detection clustering

    • Primary detection processing generates multiple detections of the same signal

    • If we can associated them with one another, we can replace multiple detections with one

  • Clutter rejection

    • Most detections relate to energy in which we are not interested

    • If we can identify them as clutter we can remove them from further consideration

Detection clustering
Detection Clustering of Infrasound Stations

  • Look for similar features among detections:

    • Slowness, azimuth and frequency

    • Apply threshold on the difference for each feature separately (tuning parameters)

    • Assumes features of a given signal will at worst vary slowly

  • Look for temporal proximity of detections

    • Compute a weighted integral of the past time during which a signal with similar features was detected

    • Use a weighting function that decays exponentially from present

    • Threshold, characteristic time and look back are tuning parameters

Clutter rejection
Clutter Rejection of Infrasound Stations

  • Based on repeatable nature of clutter signal features

    • Slowness, azimuth, frequency and duration

  • Approach

    • Tabulate/compute probabilities of occurrence of features for:

      • Signals of interest (theoretical)

      • Station-specific clutter (empirical)

    • Implement a post-detection process that labels likely clutter detections – formulate a statistic based on probability functions of clutter and signal on the feature space:

    • Chose threshold to optimize false alarm rate

Clutter characterization approach
Clutter Characterization Approach of Infrasound Stations

  • Pass data through PMCC for detection and estimation of features for each detection

  • Compare detection feature sets to known clutter sources (if any) and tabulate results, subsequently using detections associated with known sources to refine features of those sources

  • Record unmatched detections and bin similar signals in feature space (e.g. azimuth-velocity-frequency)

  • Define new clutter sources based on contents of bins (number of detections per unit of time)

Clutter characterization i59us
Clutter Characterization: I59US of Infrasound Stations

Clutter characterization i57us
Clutter Characterization: I57US of Infrasound Stations

Network detection simulation
Network Detection Simulation of Infrasound Stations

  • Ambient noise estimation

  • Receiver Operator Characteristic (ROC) curves

  • Preliminary simulations using NetSim

Ambient noise estimation
Ambient Noise Estimation of Infrasound Stations

  • Objectives

    • Characterize all current infrasound stations using consistent approach

    • Provide quantitative noise estimate for network detection simulations

  • Spectra estimated for all 19 stations of project network

    • ~ 8 months of data

      • January 12 – August 31, 2003 (data on spinning disk archive)

    • 4 times/day (0100, 0700, 1300, 1900 local time)

    • 21 consecutive non-overlapped 180 second spectra for each hour

    • 0.03 – 10 Hz

  • Probability Density Functions (PDF) estimated for each processing band

Ambient noise for i07au warramunga
Ambient Noise for I07AU (Warramunga) of Infrasound Stations

Ambient noise for project network
Ambient Noise for Project Network of Infrasound Stations

  • All times of day

  • All stations with spectra for both months and no apparent quality problems

  • No compensation for southern/northern seasons

Noise probability density function
Noise Probability Density Function of Infrasound Stations

  • Objective

    • Provide time-domain estimation of noise in same passbands used for signal detection

    • Provide non-Gaussian representation of noise

0.5-1.0 Hz

0.5-1.0 Hz

Number of noise intervals

Receiver operator characteristic roc curves
Receiver Operator Characteristic (ROC) Curves of Infrasound Stations

  • Good illustration of effect of clutter on Pd

  • Notice ~10 dB lower implant SNR for same Pd in the 0.5-1.0 Hz band

(0.5-1.0 Hz)

(0.1-0.5 Hz)

Preliminary network detection simulation
Preliminary of Infrasound Stations Network Detection Simulation

  • NetSim program

  • Gaussian approximation of noise

  • 2 station detection 90% probability

  • Winds not yet included

Contours in kilotons

Ongoing work
Ongoing Work of Infrasound Stations

  • Detector tuning

    • Tune for mine explosions using observed signals recorded at I07AU, I10CA PDIAR and TXIAR

    • Quantify detector performance using reference data set

  • Clutter classification

    • Refine classification approach

    • Extend to additional station

    • Complete methodology to suppress selected clutter sources

  • Network simulations

    • Interpret ambient noise estimates

    • Include non-Gaussian noise estimates

    • Include seasonal winds

    • Extend to full IMS network by using noise estimates for geographically analogous stations

Summary of Infrasound Stations

  • Detector tuning

    • Assembled canonical detection data set for assessing effectiveness of detector

    • Improved detection performance for current stations (DFX/libinfra)

  • Clutter classification

    • Developed methodology for classifying clutter based on measured features

    • Applied new methodology to classify clutter for several stations

  • Network simulation

    • Estimated ambient noise levels for all current stations

    • Estimated detectable SNRs using ROC curves

    • Performed initial simulation for current network