Recent Applications of the Time-Domain Parabolic Equation (TDPE) Model to Ground Truth Events Robert Gibson and David Norris BBN Technologies Arlington, VA, USA 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 Watusi Blue: SGAR observation 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

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 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: 2 nd, 3 rd 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 ProfileRay Tracing Results: Fan of Rays Ray Types Modeled at Receiver Range: None Finnish Ordnance Explosion site to ARCES, 2-Sep-2005 Ref. Gibbons et al., JASA, 2007

PE Model w/ and w/o Perturbation PE: Relative Amplitude, 2.0 Hz PE: Relative Amplitude, including Gravity Wave Perturbation, 2.0 Hz Finnish Ordnance Explosion site to ARCES, 2-Sep-2005 Ref. Gibbons et al., JASA, 2007

Explosions in Northern Finland Finnish Ordnance Explosion site to ARCES (Data Ref. Gibbons and Kvaerna, NORSAR) 3-Sep Sep-2005 TDPE: Signal Amplitude, including absorption and Gravity Wave Perturbation, Hz Early Arrival Only Late Arrival Only 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