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

Truth Events 

Robert Gibson and David Norris

BBN TechnologiesArlington, VA, USArgibson@bbn.com

Infrasound Technology Workshop, Bermuda

November 2008

Motivation

• Calculation of infrasound propagation paths is necessary for event identification, phase association, source location

• Ray tracing techniques are widely used to predict travel times and azimuth deviations; however shortcomings exist:

– High frequency approximation – Strong shadow zones predicted, contrary to many observations– Broadband waveform predictions are not computed

• Models are needed to predict apparent scattering from coherent structures, as reported by Kulichkov and others

• Recent progress has been made in the development of full-wave propagation modeling techniques

• Full-wave models such as the Time-Domain Parabolic Equation (TDPE) can be readily used with state-of-the-art atmospheric characterizations

– Mean atmospheric specifications (global or regional)– Perturbation estimates based on physics of gravity waves

Enabling Capabilities and Tools

• Infrasound Propagation Modeling – Fourier-synthesis TDPE model implementation (Norris)– Absorption and dispersion models (Sutherland and Bass)

• Mean Atmospheric Characterization– NRL-G2S Ground-to-Space global specification (Drob)– Climatology of upper atmosphere (Hedin, Picone, Drob)

• Fine-Scale Atmospheric Structure Characterization– Gravity Wave spectral model (Gardner)– Technique to generate height-dependent, range-dependent

realizations of horizontal wind perturbation (Norris and Gibson)

• Observations and Ground Truth Metadata– Infrasound databases– Station operations and prior event data analyses

Prior Comparison Studies • TDPE Model has been used to predict shadow zone arrivals

– Watusi HE event (controlled test at Nevada Test Site, 2002) to SGAR (St. George, Utah). Ref. Norris, ITW 2005, Tahiti.

– Henderson, Nevada, event (PEPCON plant explosion, 1988) to SGAR (St. George, Utah). Ref. Norris, ITW 2006, Fairbanks.

• Summary of previous findings– Conventional propagation modeling failed to predict arrival– Scattering introduced via model of gravity wave wind perturbations– TDPE used to identify propagation mode from scattering in stratopause

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Watusi • Blue: SGAR

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• Red: TDPE prediction

Travel Time (s)Ref. Norris, ITW 2005

Predictions for Watusi Event

Top:

G2S Profile, with no perturbation,

PE Model at 0.5 Hz

Bottom:

Perturbed G2S Profile, based on gravity wave spectra,

PE Model at 0.5 Hz

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Explosions in Northern Finland

• Multi-year dataset of ordnance disposal explosions– Ref. Gibbons, Ringdal and Kvaerna, JASA Express Letters, Nov

2007

• Repeated daily explosions at same location in Finland– Source yield estimated at 20 tons– Source believed to be repeatable

• Signals observed at ARCES (ARC) in Norway – Range approx. 178.9 km– Signals also observed at other locations

• Arrival structure observed to vary from day to day• Select two days that have markedly different arrivals

– 2005 September: 2nd, 3rd selected– Origin times at 11:00 UT, for both events

• TDPE modeling, including effects of wind perturbation due to atmospheric gravity waves

Ray Trace Model Showing Shadow Zone

Effective Sound Velocity Profile Ray Tracing Results: Fan of Rays

Ray Types Modeled at Receiver Range:None

Finnish Ordnance Explosion site to ARCES, 2-Sep-2005Ref. Gibbons et al., JASA, 2007

PE Model w/ and w/o Perturbation

PE: Relative Amplitude, 2.0 Hz PE: Relative Amplitude, includingGravity Wave Perturbation, 2.0 Hz

Finnish Ordnance Explosion site to ARCES, 2-Sep-2005Ref. Gibbons et al., JASA, 2007

Explosions in Northern FinlandFinnish Ordnance Explosion site to ARCES (Data Ref. Gibbons and Kvaerna, NORSAR)

3-Sep-2005 2-Sep-2005

TDPE: Signal Amplitude, including absorption and Gravity Wave Perturbation, 2.0-5.0 Hz

Early Arrival

Only

LateArrival

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Tropospheric arrival

Scattered stratospheric

arrival

Buncefield Explosion at Flers

• 11-Dec-2005 event in England

• Infrasound recorded throughout Europe

• Event analyzed by Ceranna, Green, Le Pichon, others

• Propagation modeled using ray trace (example at right), PE, TDPE

– Frequency content of source modeled over 0-4 Hz bandwidth, based on seismic analyses by Green, ITW 2006

– Assumed 30 ton yield

• Modeled to Flers, France

• Path to Flers– 334 km range– 0.6 deg back azimuth

Buncefield Explosion at Flers

TDPE synthetic waveform,Computed over 0-4 Hz,

using blast wave source, NRL-G2S mean atmosphere,

absorption model, and gravity wave

perturbation model

Bottom plot,TDPE synthetic waveform,

as above,shown with expanded

vertical axis

Ref. Ceranna et al. (2007), The Buncefield Explosion: A benchmark for infrasound Analysis in Europe, ITW 2007, Tokyo

Ghislenghien Explosion at Flers

• 30-Jul-2004 event in Belgium

• Infrasound recorded throughout Europe

• Event analyzed by Evers, Ceranna, Le Pichon, others

• Propagation modeled using PE (example at right), TDPE

– Frequency content of source modeled over 0-4 Hz bandwidth

– Assumed 40 ton yield, per Evers/ Whitaker analysis (BSSA , April 2007)

• Modeled to Flers, France

• Path to Flers– 379 km range– 54.3 deg back azimuth

PE, 1.0 Hz, absorption, no wind perturbation

Ghislenghien Explosion at Flers

ItIsIsIs

Observation: Ref. Evers and Haak (2006), Seismo-acoustic analysis of explosions and evidence for infrasonic forerunners, ITW 2006, Fairbanks.

TDPE synthetic waveform,Computed over 0-4 Hz,

using blast wave source, NRL-G2S mean atmosphere,

absorption model, and gravity wave

perturbation model

Note: observed event

likely shows effect of

burning fuel, following initial

blast

Conclusions

• TDPE modeling techniques can be used effectively to model infrasound waveforms

– Multiple phases of ground truth events are predicted– TDPE phase identification more robust than ray tracing– Full-wave modeling allows for frequency-dependent features

• 3-d ray tracing techniques are still useful to predict azimuths and travel times, but full-wave models are essential for understanding events more fully

• Introduction of gravity wave wind perturbations frequently enables prediction of observed signals in shadow zones

– Existing perturbation technique models the effects of coherent atmospheric structures

– Additional physics should be incorporated in perturbation model

• Further work is needed to refine amplitude predictions

Plans and Recommendations

• Investigation of gravity wave phenomena in greater detail, and development of higher fidelity gravity wave model

– Incorporate additional physics in model– Amplitude scaling, Geographic dependence, Correlation lengths– Investigation of other classes of fine-scale atmospheric

inhomogeneities

• Further investigation of observed events, for example:– Amplitude comparisons for Flers observations– Additional event studies for NORSAR observations

• Additional full-wave model validation with ground truth events, especially over regional ranges, to include:

– High-resolution regional atmospheric specifications– Variable terrain effects– Effects of absorption and dispersion in thermosphere