Waveform Modeling and Comparisons with Ground Truth Events David Norris BBN Technologies 1300 N. 17 th Street Arlington, VA Nov 2007

Motivation Quantify terrain effects on predicted travel times and waveform parameters Provide seamless integration between terrain and atmospheric specifications Improve overall propagation modeling capabilities and suite of tools that can be utilized by analysts and researchers InfraMAP – Infrasonic Modeling of Atmospheric Propagation

Terrain Modeling Approaches Goal is to quantify the lower boundary condition for propagation modeling over complex terrain General approaches: Stair-stepping or Terrain Masking –Terrain realized as a series of stair steps –Results in the terrain being modeled as a series of knife-edge diffractors –All surface reflections approximated by tip diffraction –Assumes perfectly reflecting surface

Terrain Modeling Approaches (Cont.) Piecewise Conformal Mapping –Propagation coordinates are transformed into terrain-following arcs –Each arc is applied over a defined piece of the total terrain profile –Helmholtz equation and fundamental form of PE solution do not change –The mapping in implemented by applying a modified index of refraction over a uniform grid

Terrain Modeling Approaches (Cont.) Piecewise Linear Shift Mapping –New coordinate system based on shifting height to follow terrain –Recomputed at each range step –Results in rederivation of Helmholtz equation –Additional terms must be numerically addressed in PE solution r z T(r) r U(r) = z – T(r)

Terrain Masking Approach Mt. Everest Mt. Fuji Mt. Everest Mt. Fuji Terrain masking integrated into Parabolic Equation (PE) and Time- Domain Parabolic Equation (TDPE) models Test case of PE predictions with 10 km wedge 2.0 Hz 0.2 Hz

Hypothetical Scenario Source just East of Mt. Fuji 400 km propagation to West PE predictions at 2 Hz –Significant interference pattern near source due to nearby terrain features –Effects at range not obvious No Terrain Terrain

Hypothetical Scenario (cont.) PE predictions 1 km above sea level at 2 Hz –First arrival with terrain 30 km shorter in range –First arrival structure more complex with terrain TDPE predictions over 1-3 Hz at 207 km range –Waveform with terrain more disperse than without terrain –Decrease in peak amplitude in terrain case Terrain

Henderson Event Two major chemical explosions occurred in Henderson, Nevada on May 4, 1988 Explosions resulted from plant fire which ignited stores of ammonium perchlorate Estimated surface burst yields of 0.7 and 1.8 kT “Infrasonic Signals from the Henderson, Nevada, Chemical Explosion,” Mutschlecner, J. and R. Whitaker, LA-UR , 2006

Henderson Event

Detection at SGAR Infrasound detections –St. George, Utah (SGAR), range 159 km –Los Alamos, New Mexico (LANL), range 774 km SGAR –Both major explosions observed with similar waveforms –Two arrivals First arrival consistent with surface wave (group velocity 332 m/s) Second arrival consistent with stratospheric or thermospheric path (group velocity 275 m/s)

No Terrain Terrain PE Prediction: 0.2 Hz PE prediction at 0.2 Hz through climatology (HWM/MSIS) No energy predicted to reach SGAR due to lack of stratospheric duct Terrain results in deeper shadow zone

Small-Scale Atmospheric Variability Mean Zonal and Meridional Winds specified using NRL-G2S Horizontal Wind perturbations specified using Spectral gravity wave model –Range-dependent Horizontal Correlation Length: 50 km Total wind field along propagation path realized from sum of mean and perturbed components NRL-G2S MeanTotal RealizationGravity Wave Perturbation Wind along propagation Path

No Terrain Terrain PE Prediction: 0.2 Hz PE prediction at 0.2 Hz through climatology (HWM/MSIS) Horizontal wind perturbations introduced by gravity waves model Terrain results in more distinct scattering paths

Waveform Comparison Bandpass filter 1-3 Hz Terrain introduces more spreading in waveform arrival No Terrain Terrain Data

Interface with Atmospheric Specifications How terrain/atmospheric boundary condition is handled may have significant effect on prediction performance –Masking of atmospheric profiles near terrain –Mesoscale atmospheric effects over regions such as mountains Terrain Flat Earth Assumption Terrain Blocking Assumption Effective Sound speed Terrain matching Assumption

Conclusions and Future Research Preliminary Conclusions –Terrain appears to disperse down-range waveforms in cases where significant terrain features are in proximity of source Issues for further research –Comparison of terrain masking, conformal mapping, and linear shift mapping approaches –Sensitivity to ground impedance specification and its variability over typical propagation paths –Required Topographic database resolution