Overview of the 2009 and 2011 Sayarim Infrasound Calibration Experiments

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 118, , doi:1.12/jgrd.5398, 213 Overview of the 29 and 211 Sayarim Infrasound Calibration Experiments David Fee, 1 Roger Waxler, 2 Jelle Assink, 2 Yefim Gitterman, 3 Jeffrey Given, 4 John Coyne, 4 Pierrick Mialle, 4 Milton Garces, 5 Douglas Drob, 6 Dan Kleinert, 2 Rami Hofstetter, 3 and Patrick Grenard 7 Received 13 December 212; revised 5 March 213; accepted 8 April 213; published 21 June 213. [1] Three large-scale infrasound calibration experiments were conducted in 29 and 211 to test the International Monitoring System (IMS) infrasound network and provide ground truth data for infrasound propagation studies. Here we provide an overview of the deployment, detonation, atmospheric specifications, infrasound array observations, and propagation modeling for the experiments. The experiments at the Sayarim Military Range, Israel, had equivalent TNT yields of 96., 7.4, and 76.8 t of explosives on 26 August 29, 24 January 211, and 26 January 211, respectively. Successful international collaboration resulted in the deployment of numerous portable infrasound arrays in the region to supplement the IMS network and increase station density. Infrasound from the detonations is detected out to ~35 km to the northwest in 29 and ~63 km to the northeast in 211, reflecting the highly anisotropic nature of long-range infrasound propagation. For 29, the moderately strong stratospheric wind jet results in a well-predicted set of arrivals at numerous arrays to the west-northwest. A second set of arrivals is also apparent, with low celerities and high frequencies. These arrivals are not predicted by the propagation modeling and result from unresolved atmospheric features. Strong eastward tropospheric winds (up to ~7 m/s) in 211 produce high-amplitude tropospheric arrivals recorded out to >1 km to the east. Significant eastward stratospheric winds (up to ~8 m/s) in 211 generate numerous stratospheric arrivals and permit the long-range detection (i.e., >1 km). No detections are made in directions opposite the tropospheric and stratospheric wind jets for any of the explosions. Comparison of predicted transmission loss and observed infrasound arrivals gives qualitative agreement. Propagation modeling for the 211 experiments predicts lower transmission loss in the direction of the downwind propagation compared to the 29 experiment, consistent with the greater detection distance. Observations also suggest a more northerly component to the stratospheric winds for the 29 experiment and less upper atmosphere attenuation. The Sayarim infrasound calibration experiments clearly demonstrate the complexity and variability of the atmosphere, and underscore the utility of large-scale calibration experiments with dense networks for better understanding infrasound propagation and detection. Additionally, they provide a rich data set for future scientific research. Citation: Fee, D., et al. (213), Overview of the 29 and 211 Sayarim Infrasound Calibration Experiments, J. Geophys. Res. Atmos., 118, , doi:1.12/jgrd Introduction [2] The International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) is a global network consisting of seismic, hydroacoustic, infrasound, and radionuclide stations built to monitor for nuclear test explosions. Currently 45 of 6 infrasound arrays have been completed and certified into the IMS. The CTBT itself makes no reference to explosion 1 Wilson Infrasound Observatories, Alaska Volcano Observatory, Geophysical Institute, University of Alaska, Fairbanks, AK, USA. 2 National Center for Physical Acoustics, University of Mississippi, University, MS, USA. Corresponding author: D. Fee, Wilson Infrasound Observatories, Alaska Volcano Observatory, Geophysical Institute, University of Alaska, Fairbanks, AK, USA. (dfee@gi.alaska.edu) 213. American Geophysical Union. All Rights Reserved X/13/1.12/jgrd Seismology Division, The Geophysical Institute of Israel, P.O.B. 182, Lod 711, Israel. 4 International Data Centre, CTBTO, Vienna, Austria. 5 Infrasound Lab, HIGP, SOEST, University of Hawaii at Manoa, Manoa, HI, USA. 6 Upper Atmospheric Modeling Section, Naval Research Laboratory, Washington, DC, USA. 7 International Monitoring System Division, CTBTO, Vienna, Austria. 6122

2 Table 1. Explosion Characteristics Date UTC Time Weight (t) Yield (t TNT equivalent) Latitude ( N) Longitude ( E) Elevation (m) 26 August 29 6:31: January :17: January 211 7:17: magnitude, but for practical reasons the IMS infrasound network was designed to detect and locate atmospheric explosions of at least 1 kt TNT-equivalent yields with at least two stations [Christie and Campus, 21]. The development of the IMS infrasound network has helped stimulate research and development in the field of infrasound. Numerous studies have shown that the IMS network routinely detects large infrasonic events at multiple stations, such as bolides [Silber et al., 211], chemical explosions [Ceranna et al., 29], large volcanic eruptions [Dabrowa et al., 211], earthquakes [Le Pichon et al., 26], tsunamis [Le Pichon et al., 25], and microbaroms [Garces et al., 24]. Several studies have modeled the detection capability of the global IMS infrasound network. Le Pichon et al. [29] focused on the influence of the stratospheric winds and found that the majority of IMS infrasound detections are in the downwind stratospheric wind direction. They estimate that the IMS network is capable of detecting a ~5 t explosion at any time of the year with at least two stations, but that much smaller explosions (e.g., 5 t yields) could be detected under favorable propagation conditions. Green and Bowers [21] used a probabilistic model accounting for stratospheric winds and found that chemical explosions >21 t will be detected at over 95% of the Earth s surface at any time of the year by at least two stations of the full IMS network. [3] The middle and upper atmosphere is dynamic and difficult to sample evenly and consistently, contributing to large uncertainties in long-range acoustic propagation modeling and often resulting in less than satisfactory agreement between modeling and observations [Norris et al., 21]. In addition, the relatively low station density of the IMS (~2 km between stations) only permits coarse comparisons between modeling and observations. Higher station density allows a more detailed view of acoustic propagation and atmospheric structure. Large anthropogenic sources such as chemical explosions [e.g., Ceranna et al., 29] and natural events such as volcanic explosions [e.g., Matoza et al., 211] have provided more detailed studies of the atmosphere and signal identification. However, these events are somewhat rare and difficult to predict, thereby complicating installation of higher-density networks to validate acoustic propagation modeling and middle- and upper-atmospheric specifications. Furthermore, accurate source constraints, such as explosive yield and source waveforms, are difficult to impossible to obtain for unexpected events. Some previous large-scale infrasound calibration experiments have taken place [Herrin et al., 28], using rocket-launched low-yield explosions in the middle atmosphere. The use of ground-coupled airwaves on seismic networks has led to studies with higher station density [e.g., Hedlin et al., 21], but these studies are limited to travel time information alone. Therefore, it is crucial to have a large, well-constrained source and high-density infrasound network to fully test the IMS network and atmospheric propagation models. [4] In 29 and 211, three large-scale infrasound calibration experiments were performed. Each calibration experiment consisted of a singular surface detonation of chemical explosives with charge weights of 82, 1.24, and 12.8 t [Gitterman et al., 211] on 26 August 29, 24 January 211, and 26 January 211, respectively, at the Sayarim Military Range, Israel (Table 1). These explosions produced significant infrasound detected by numerous permanent and temporary infrasound arrays deployed across the region. The goal of the calibration experiments was to test the IMS infrasound network and evaluate atmospheric specifications and propagation models using large, well-constrained infrasound sources and infrasound networks of unprecedented station density. This manuscript provides an overview of the calibration experiments, including deployment (section 2), data processing and methods (section 3), seasonal winds and atmospheric specifications (section 4), explosion characterization (section 5), infrasonic signal detection and identification (section 6), and a discussion of the results and implications (section 7). The Sayarim calibration experiments represent a considerable and successful collaboration between the CTBTO and other international groups and will provide a rich ground truth data set for more detailed infrasound studies in the future. 2. Deployment [5] For the calibration experiments, numerous pressure gauges and temporary infrasound arrays were deployed to supplement the existing IMS infrasound network. Arrays were deployed to the north and west of the source (Figure 1a) for the 29 experiment, due to the predicted summer westward stratospheric winds expected to promote long-range propagation. IMS arrays in the region include IS26 (Germany), IS31 (Kazakhstan), IS43 (Russia), and IS48 (Tunisia). Fifteen temporary arrays were deployed in total. The temporary arrays were deployed by numerous groups and consisted of variable array configurations and equipment summarized in Table 2. A five-element infrasound array (IMA) was deployed by the National Data Center of Israel (Soreq) in northern Israel, Mt. Meron (colocated with IMS seismic array MMAI). The University of Mississippi National Center for Physical Acoustics (NCPA) and University of Alaska Fairbanks (UAF) deployed sensors in Israel as well. A linear array of three sensors was deployed by the NCPA and Geological Survey of Cyprus in Cyprus (CYPR). The University of Hawaii Infrasound Lab and National Observatory of Athens deployed three arrays in Greece: Rhodes (RHOD), Crete (CRET), and the Peloponnese (PELO). The University of Florence deployed one array in Southern Italy (CALA), and the CTBTO deployed arrays in North Italy (I62) and Austria (I63). In France, the French Alternative Energies 6123

3 Figure 1. Overview map of infrasound arrays for the (a) 26 August 29 and (b) January 211 Sayarim experiments. Black circles denote arrays that detected the respective explosion, while open circles indicate arrays that did not detect the explosion. Gray squares denote IMS arrays that detected the respective explosion, while open gray squares indicate IMS arrays that did not detect the explosion. Red diamonds indicate the explosion location. The 29 explosion was detected extensively the northwest to a distance of ~344 km, while the 211 explosion was detected out to ~63 km to the northeast. A more detailed map of the regional arrays in 211 is given in Figure 15c. andatomicenergycommission(cea/dase)deployedan array in Provence (PROV), as well as operating research arrays near Paris (CEA) and Flers (FLERS). In Netherlands, the Royal Netherlands Meteorological Institute (KNMI) operates three research arrays in the region (DBN, DIA, and TEX). All arrays consisted of at least three sensors with flat ( 3 db) frequency responses in the band of interest (.1 2 Hz) and were sampled at >2 Hz. The majority of the sensors were either Chaparral Physics Model 25 or Martec Tekelec MB25 [Ponceau and Bosca, 21]. Coordination for the 29 temporary deployments was led by Dr. Milton Garces of the University of Hawaii and sponsored by the U.S. Army Space and Missile Defense Command (SMDC). [6] The 211 experiments aimed to take advantage of the typically strong eastward tropospheric and stratospheric winds at this location and date; therefore, arrays were deployed primarily to the east of the source (Figure 1b). Table 3 lists the array locations as well as azimuth and distance to the explosion source. IMS arrays to the north and east are IS31 (Kazakhstan), IS43 (Russia), IS46 (Russia), and IS34 (Mongolia). The aforementioned IMS arrays to the west and northwest of Sayarim (IS26 and IS48) also recorded data during the period. NCPA deployed three arrays in northern Israel (IN1, IN2, and IN3) with assistance from the Geophysical Institute of Israel (GII) and Soreq and two lines of single-sensors near the source running ~4 km east and south of the explosion site (with the assistance of GII, UAF, and U.S. Army Research Lab 6124

4 Table 2. List of Stations for 29 Experiment as a Function of Distance From the Source Station Code Latitude ( N) Longitude ( E) Back Azimuth ( N) Range (km) Detections? Sensor Description Sayarim IMA Y MB2 x 5 CYPR Y NCPA IS Mic x 3 RHOD Y Chaparral 2.2 x 6 CRET Y Chaparral 2.2 x 6 PELO Y Chaparral 2.2 x 6 CALAB Y MBAR-D-4V x6 IS Y MB25 x 7 IS Y MB2 x 4 IS Y MB25 x 4 IS Y MB2 x 5 IS N MB2 x 4 IS N MB2 x 4 PROV Y MB25 x 7 IGA Y MB2 x 4 DIA Y KNMI x 13 CEA Y MB25 x 4 DBN N KNMI x 6 I N MB2 x 8 TEX N KNMI x 6 FLERS N MB25s x 4 (ARL)). The CTBTO deployed one array in Djibouti (DJ) with assistance from Centre d Etudes et de Recherche de DjiboutandtwoinOman(OM_NandOM_S)withSultan Qaboos University. Joint teams from the NCPA and CTBTO deployed one array in Kuwait (KU) and Qatar (QA) and four arrays in Jordan (JO_NE, JO_NW, JO_S, JO_M) with assistance from the Kuwait Institute for Scientific Research, the National Committee for the Prohibition of Weapons in Qatar, ARL, the Japan Weather Association (JWA), and the Natural Resources Authority in Jordan. UAF and Ilia State University of Georgia installed an array in southern Georgia (GE). ARL deployed an array in Iraq. Note that some sites from the 29 experiment were also reoccupied due to the potential for westward stratospheric winds in the winter, as well as for comparison between easterly and westerly arrays. The reoccupied sites include: Cyprus (CYPR) by NCPA and Greece (RHOD and PELO) by the CTBTO and NCPA. The majority of the temporary arrays in 211 (14/17) used the newly designed and constructed NCPA digital infrasound microphones [Alberts et al., 213]. These sensors have a flat frequency response between ~.1 and 4 Hz and low noise level, and the vast majority performed well during the experiment. The remaining sensors deployed all have flat frequency responses in the band of interest (~.1 2 Hz). Noise reduction techniques varied between array locations and Table 3. List of Stations for 211 Experiment as a Function of Distance From the Source Station Code Latitude ( N) Longitude ( E) Azimuth ( N) Range (km) Detections? Sensor Description Sayarim INRE_ Y NCPA IS Mic x 1 INRE_ Y NCPA IS Mic x 1 INRS_ Y NCPA IS Mic x 1 INRS_ Y NCPA IS Mic x 1 JO_NR Y NCPA IS Mic x 1 JO_S Y NCPA IS Mic x 4 JO_M Y NCPA IS Mic x 4 JO_NW Y NCPA IS Mic x 4 IN Y NCPA IS Mic x 5 IN Y NCPA IS Mic x 5 IMA Y MB2 x 5 IN Y NCPA IS Mic x 6 JO_NE Y NCPA IS Mic x 4 CY N NCPA IS Mic x 3 RH N NCPA IS Mic x 4 IR_ Y NCPA IS Mic x 1 KU Y NCPA IS Mic x 4 PE N NCPA IS Mic x 4 GE Y NCPA IS Mic x 3 QA Y NCPA IS Mic x 4 DJ N MB25 x 5 OM_S N MB25 x 4 OM_N Y MB2 x 4 IS N MB2 x 4 IS Y MB2 x 8 IS Y MB2 x 4 IS Y MB2 x

5 experiments, but careful site selection in low wind and forested areas was a priority for all installations. Porous hose installation was standardized for the 211 experiments to reduce noise levels. The 211 temporary deployments were coordinated by a team from the NCPA and CTBTO. 3. Data Processing and Methods [7] To identify infrasound signals from the Sayarim explosions, array data are processed in the following manner. Data are first band-pass filtered and split into discrete windows. Due to the expected variation in infrasound arrival characteristics at the arrays, variable frequency bands were chosen to capture the peak signalto-noise ratio (S/N). The frequency band.3 5 Hz is chosen for the majority of the arrays in 211, except for the Jordan and Israel arrays where.5 5 Hz is selected. For the 29 array data,.5 5 Hz is chosen to reduce interference from the microbarom peak [Bowman et al., 25]. The peak explosion periods are about 1 2 s so that most signal phases should have significant energy in the.5 5Hz band. Window durations were typically 1 s, except for JO_S (Jordan) which was 5 s. After filtering, the trace velocity (component of the signal velocity in the plane of the array) and azimuth (apparent geographic bearing from the array to the source) are found for each time window using a least squares solution for plane waves traversing the array [Szuberla and Olson, 24]. S/N characterization is done using the Fisher Statistic [Melton and Bailey, 1957], a common infrasonic detection technique which performs a comparison of the signal and uncorrelated noise variances to estimate the signal-to-noise power ratio (P S/N ) of correlated signals across an array [Olson and Szuberla, 28]. P S/N is estimated by: P S=N ¼ F 1 (1) n where F is the Fisher statistic and n is the number of sensors in the array. We apply 75% overlap between data segments [Blandford, 1974] and further restrict signals of interest to data segments originating from 1 of the theoretical azimuth to the explosion location and have an acoustic trace velocity ( km/s). In this manuscript, we focus on the general identification and characterization of likely signals from the calibration experiments; therefore, we do not set a specific P S/N thresholdtoidentify detections. We do note that the majority of signals have high Fisher ratios. Furthermore, other detection techniques (e.g., PMCC) [Cansi, 1995] could be used and should produce similar results. To increase the S/N of the array data, delay and sum beamforming [Johnson and Dudgeon, 1992] is performed. [8] Propagation modeling in this study is performed using two different methods. The first is a high-mach number, planar approximation parabolic equation (PE) method [Lingevitch et al., 22]. Here we use the PE to predict the transmission loss as a function of range and height. Range-dependent PE simulations are run for each experiment at.5 Hz out to 35 km for 26 August 29 and 24 January 211 and 7 km for 26 January 211. Absorption is accounted for using estimates from Sutherland and Bass [24]. This attenuation model does well in the troposphere and stratosphere, however, thermospheric absorption is greatly overestimated by the Sutherland and Bass [24] model compared to observations [Fee et al., 21; Norris et al., 21]. Attenuation in the thermosphere is thus currently not well-understood, so we will use the Sutherland and Bass [24] model with the caveat that thermospheric arrivals are not captured by the model. The model also assumes a rigid ground boundary condition and no topography. Ray tracing is also used to predict arrival times and visualize propagation paths. The ray-tracing method here follows from a range-dependent planar approximation of the full classical Hamilton ray tracing equations (3-D spherical coordinates) found in Gossard and Hooke [1975]. Propagating rays that best connect the source and receiver to within a specified tolerance (2 km), termed eigenrays, are selected. 4. Seasonal Winds and Atmospheric Specifications [9] Long-range sound propagation is primarily determined by the horizontal wind and temperature gradients in the atmosphere [Drob et al., 23]. Sound from a surface source primarily radiates upward from the ground due to the generally decreasing sound speed with height. In a stationary atmosphere, upward propagating sound refracts back down to the ground when the static sound speed (c) at altitude exceeds that at the source, creating a waveguide or duct. However, winds in the atmosphere also influence sound propagation. The effective sound speed approximation, valid for shallow angle propagation, gives a qualitative representation at all angles. It is assumed that the propagation is in-plane and that the horizontal winds are small compared to the adiabatic sound speed (Mach number). The effective sound speed c eff is given by the static sound speed plus the horizontal wind component in the direction of propagation: p C eff ¼ ffiffiffiffiffiffiffiffi!!!! grt þ v n ¼ c þ v n (2) where g is specific heat ratio, R is universal gas constant, T is temperature, v is horizontal wind vector, and n is horizontal projection of the ray normal. Thus, to a first order, a sound duct in a moving atmosphere is created where c eff at altitude exceeds that at the source, and sound is preferentially guided downwind. Quantitative propagation modeling using this approximation leads to slightly underpredicted travel times and shadow zone locations, effects that are magnified when the Mach number increases (stronger winds) [Assink et al., 211]. [1] Seasonal stratospheric wind jets focused at roughly 55 (winter hemisphere) and 7 km (summer hemisphere) height have been shown to strongly influence the propagation of infrasound [e.g., Donn and Rind, 1971; Drob et al., 23; de Groot-Hedlin et al., 21] and hence the number and location of IMS infrasound array detections [e.g., Le Pichon et al., 29]. In spring and summer, midlatitudes have a relatively stable stratospheric wind jet flowing west. Fall and winter are characterized by a reversal of this wind jet, as it flows predominantly to the east. During this period, the jet is often stronger in magnitude and slightly lower in altitude than during spring and summer but is less stable. Global-scale disruption of the Northern Hemispheric wintertime stratospheric wind jet can occur as a result of 6126

6 a) Velocity (m/s) 5 5 b) c) Altitude (km) Velocity (m/s) Date Zonal Winds (m/s) d) Altitude (km) Zonal Winds (m/s) Figure 2. Zonal winds above Sayarim G2S zonal winds are shown at (a) 15 (black) and 5 (red) km height and (b) between and 1 km height. Clear seasonal trends are observed, with the stratospheric wind jet blowing eastward (positive, warmer colors) in the winter and predominantly westward (negative, cooler colors) in the summer. The tropospheric wind jet (the jet stream) always blows to the east and increases in magnitude in winter. Zonal winds in January 211 are indicated in Figures 2c and 2d and show a typical midwinter pattern. A SSW occurred in early January with westward blowing stratospheric winds, followed by a return to more typical conditions by the end of the month. The timing of the two calibration experiments is marked by vertical gray lines on 24 and 26 January. sudden stratospheric warming (SSW) events [e.g., Charlton and Polvani, 27]. Westward stratospheric infrasound ducting in the Northern Hemisphere is possible during winter times as the result of westward winds associated with the polar stratospheric vortex splitting. The cause of SSW events is likely related to planetary waves propagating vertically from the troposphere into the stratosphere [Holton, 24]. Evers and Siegmund [29] showed how a major SSW event clearly affected detections at multiple mid- to high-latitude IMS infrasound arrays. In addition to the stratospheric winds, at these latitudes a tropospheric wind jet centered at ~1 km (the jet stream) flows east during spring and summer and generally increases in magnitude and becomes more stable during fall and winter. The consequences of these dominant wind jets are that long-range sound at midlatitudes is preferentially guided in a stable stratospheric duct to the west during spring and summer and in a stronger but less consistent stratospheric duct to the east during fall and winter. If the jet stream is strong enough to overcome the negative lapse rate, sound is ducted to the east in the troposphere as well. In addition to the circulation pattern variability mentioned above, solar heating causes diurnal, semidiurnal, and terdiurnal migrating solar tides in the upper atmosphere that affect refraction in the stratosphere [Donn and Rind, 1972; Green et al., 212] and thermosphere [Garces et al., 22; Assink et al., 212]. [11] There are several operational atmospheric data assimilation specifications that can be used to compute the instantaneous (or historical) characteristics of infrasound propagation. These specifications (e.g., NOAA GFS [Kalnay et al., 199]; NASA GEOS-5/MERRA [Rienecker et al., 28]) are derived from available global satellite- and ground-based observations and are limited to altitudes 6127

7 a) b) c) 26 Aug 29 6 UTC 24 Jan UTC Jan UTC Altitude (km) Velocity (m/s) Velocity (m/s) Velocity (m/s) d) e) f) Altitude (km) Sound Speed (m/s) Sound Speed (m/s) Sound Speed (m/s) Figure 3. G2S and ECMWF atmospheric profiles above the sources for the three calibration experiments. (a-c) Winds, with the zonal winds in blue and meridional winds in red. G2S winds are solid lines, while ECMWF are dotted lines. (d-f) Sound speed (c, dashed line) and effective sound speed (c eff, solid line). G2S sound speeds are in black, and ECMWF in green. Effective sound speed profiles are shown for propagation to 31,6, and 6, respectively. Stratospheric ducts are predicted for all three experiments, with the duct in 211 being particularly strong. Tropospheric ducts are also predicted for 211. below 45 and 75 km, respectively. In order to account for thermospheric modes in the infrasound propagation modeling, these available lower and middle atmospheric specifications are typically extended with the NRLMSISE- [Picone et al., 22] and HWM7 [Drob et al., 28] empirical climatological models using the approach described in Drob et al. [23]. The resulting Ground-to-Space (G2S) atmospheric specifications provide global estimates of the atmospheric state variables, winds, and temperature up to 17 km altitude at 6 h intervals that are specifically tailored for infrasound propagation calculations. Both the European Centre for Medium-Range Weather Forecasts (ECMWF) [Moltenietal., 1996] and G2S specifications based on the NOAA-GFS and NASA GEOS5 have been used extensively in long-range infrasound propagation studies. Neither the G2S nor ECMWF specifications can explicitly resolve small-scale variability (e.g., subgrid-scale gravity waves) in the atmosphere and both must still rely on climatological models above ~7 km. Both small-scale variability and details of the upper atmospheric state can affect infrasonic propagation [Kulichkov, 21]. [12] Figure 2 focuses on the zonal wind velocities (east-west, positive east) above Sayarim (~3. N, 34.8 E) between 23 and 211, taken from the 4 times daily G2S models for this period. Figure 2a is the zonal winds at 12 km (black) and 5 km (red) height. The aforementioned zonal wind magnitude and direction trends are clearly seen. Note that the peak zonal wind velocities occur during the winter, but that the direction is highly unstable during this period. Between the months of November to February , the zonal winds at 5 km blow east 72% of the time. For the period May to August , winds at 5 km are going west 99.2% of the time. Zonal wind velocities between and 1 km for the same period and location are 6128

8 a) 12 FEE ET AL.: SAYARIM INFRASOUND OVERVIEW 26 Aug 29 6: UTC, Az= Height (km) C eff Ratio b) Jan : UTC, Az= Height (km) C eff Ratio.8 c) Jan 211 6: UTC, Az=6 1.2 Height (km) C eff Ratio Distance (km).8 Figure 4. Effective sound speed ratios (c eff_ratio ) for (a) 26 August 29, (b) 24 January 211, and (c) 26 January 211 at 31,6, and 6, respectively. Sound is predicted to be ducted where c eff_ratio > 1. Clear range dependence exists for all three scenarios. shown in Figure 2b, yellow to red colors indicating eastward propagating wind and blue colors westward. Again, the temporal changes are clearly seen. Note however that the zonal wind variability during the winter in the stratosphere jet peaking near 55km is not reflected in the westward summer time zonal middle atmospheric wind jet that forms near ~65 75 km and is inherently more stable. Figures 2c and 2d display the zonal winds for the three weeks surrounding the 211 Sayarim experiments. A SSW occurs prior to the 211 explosions, causing a change in the stratospheric winds and temperature. If the experiments would have taken place during this period, sound could have propagated to the west being ducted at ~4 5 km and to the east being ducted at ~7 km height. The jet stream magnitudes are also noteworthy, sometimes reaching 7 m/s. The meridional winds (north-south, positive north) are much smaller in magnitude and do not affect stratospheric propagation as significantly as the zonal winds. [13] The G2S, denoted by solid lines, and ECMWF, denoted by dotted lines, derived zonal and meridional winds as a function of height above the explosion source for the three Sayarim calibration experiments are shown in Figures 3a 3c. The static and effective sound speeds are also shown (Figured 3d 3f). The c eff profiles are displayed for sound directed 31 for the 29 experiment and 9 for the 211 experiments, corresponding to the general direction of deployed arrays (Figure 1). The black line represents c eff the effective sound speed at the source, and ducting heights are located where c eff exceeds c eff. During the 29 experiment, the winds are characterized by a moderately strong westward zonal wind jet, peaking at ~44 m/s at 62 km altitude (Figure 3a). Meridional winds are mostly minor until the thermosphere. Tropospheric winds have a positive component in both the north and east directions, indicating a weak tropospheric jet to the northeast (~13 m/s peak). For propagation to the west, sound is predicted to be ducted in the stratosphere beginning at ~4 km height (Figure 3b). The wind specifications during the 211 experiments are dominated by two main features: (1) a strong eastward stratospheric jet, peaking at ~86 m/s at ~64 km height on 24 January and ~79 m/s at ~63 km on 26 January and (2) a strong eastward tropospheric jet (the jet stream), peaking at ~5 m/s at ~14 km on 24 January and ~7 m/s on 26 January (Figures 3b and 3c). Meridional winds are mostly weak, with a slight northerly component. The c eff profile for propagation to the east largely reflects these two wind jets, with significant stratospheric waveguides predicted for both experiments beginning at ~4 km. 6129

9 1 8 6 FEE ET AL.: SAYARIM INFRASOUND OVERVIEW a) b) Pressure (Pa) Time (s) Figure 5. Pressure signal (a) and photo (b) of the 26 January 211 explosion. The pressure signal was recorded 5.6 km from the source and the x axis represents time since the explosion. The photo was taken immediately after the explosion from approximately 5.7 km distance with an unknown zoom and shows a hemispherical shock wave emanating from the source. Furthermore, strong tropospheric waveguides are predicted on both 24 and 26 January (Figures 3e and 3f). There is general agreement between the G2S and ECMWF wind and sound speed profiles below ~4 km. However, significant differences exist above 4 km, particularly in the wind profiles on 26 August 29. These differences are significant enough to have qualitative impact on the effective sound speed profiles. The detailed propagation modeling and data analyses required to determine whether these differences can be resolved in the acoustic data collected in these experiments will not be undertaken here but will be considered in future work. The GII launched radiosondes near the explosion site and measured atmospheric variables up to 35 km. These data are broadly consistent with the G2S specifications, with the major difference being the slight underestimation of the strength of the jet stream. [14] The atmosphere (and thus infrasound propagation) also changes as a function of range. To examine the range dependence, we calculate the effective sound speed ratio (c eff_ratio ), defined as the effective sound speed as a function of range and altitude divided by the sound speed at the source: c eff_ratio = c eff (r,z)/c(,). Sound ducting is predicted where c eff_ratio > 1. Figure 4 shows the effective sound speed ratio for the three Sayarim experiments out to 3 km for azimuths of 31,6,and6 respectively for the 29 and two 211 explosions. Red colors indicate ducting conditions. For the 29 experiment, the two stratospheric turning heights are present at ~45 and 65 km, with the 65 km duct increasing initially and then both weakening after ~15 km, indicating a marginal stratospheric duct at this range. Range dependence also exists in 211, and stratospheric ducting is much stronger. The c eff_ratio in the stratosphere is well above 1. on both 24 and 26 January between ~4 and 75 km. Although range dependence for the stratospheric duct exists, it remains well established out past 3 km for these profiles. Changes in the jet stream clearly affect the c eff_ratio for both experiments and weakens the duct after ~1 km. Although the c eff_ratio for north-south propagation is not shown, it also contains significant range-dependent variations. The range dependence of the lower atmosphere over the ranges of interest are driven by land-sea contrasts, topography, and other localized sources of vertical convection; in contrast to the stratosphere and upper atmosphere where the in situ dynamical forcing terms are much more homogeneous and globally connected, with the exception of some possible range dependence created by buoyancy waves that propagate upward from the troposphere [Holton, 24]. 5. Explosion Characterization [15] Charge design and TNT yield estimations are presented in detail in Gitterman and Hofstetter [212]. All three explosions were successfully conducted by the GII on the ground surface, at the Sayarim Military Range in southern Israel (Figure 1). The GII recorded all explosions with near-source high-pressure gauges (in the 2 6 m distance range), accelerometers, and high-speed video cameras. [16] The first Sayarim experiment consisted of ~82 t of cast chemical explosives (higher explosive energy density than TNT) placed in large barrels and assembled in a compact pyramidal shape. Successful detonation of the explosives occurred on 26 August 29 6:31:54 UTC at N, E[Gitterman, 21]. The majority of the explosion energy was directed upward by the preferential placement of mines in the booster to focus the energy into the air rather than the ground (the same upward detonation concept was applied for the 211 shots). High-pressure gauges recorded the airblast waves, and explosive yield estimation based on these data give an equivalent TNT yield of ~96 t, an ~17% increase over the actual weight [Gitterman, 21]. High-speed video also recorded the explosion. [17] The January 211 explosions were conducted at a nearby location on the Sayarim Military Range and consisted of two separate explosions, using bulk ANFO explosives in big bags and assembled in a roughly hemispherical shape. The first was a 1.24 t shot detonated 613

10 IMA (339 km, 19 ) CYPR (576 km, 161 ) Fisher ratio 2. Pa.7 Pa Celerity [km/s] Travel time [s] Trace velocity [km/s] Back azimuth [deg] RHOD (941 km, 135 ) CRET (188 km, 119 ) Fisher ratio.7 Pa.7 Pa Celerity [km/s] Travel time [s] Trace velocity [km/s] Back azimuth [deg] 19 PELO (1375 km, 12 ) CALA (1944 km, 116 ) Fisher ratio.5 Pa.1 Pa Celerity [km/s] Travel time [s] Trace velocity [km/s] Back azimuth [deg] 16 Figure 6. Array processing results for the 26 August 29 experiment out to 2 km. Clear arrivals with stratospheric celerities (~ km/s) are apparent on all displayed arrays. Detections are colored by the Fisher ratio. See text for more details on processing parameters. on 24 January :17:54 UTC at N, E[Gitterman et al., 211]. The larger explosion consisted of 12.1 t of explosives detonated on 26 January 211 7:17:43 UTC at 3.64 N, E. The 24 January explosion was conducted as a test explosion and to provide further comparison between events, in particular one with a comparable but different atmosphere. Explosion yields were determined from the high-pressure gauges in a similar manner as the 29 experiment and are estimated to be 7.4 and 76.8 t of TNT equivalent [Gitterman and Hofstetter, 212], lower than the charge weights. Gitterman et al. [211] suggest that the lower yields resulted from energy loss from nonhemispherical and nonhomogenous charges and air voids between the charge units. Peak periods for the explosion signals are between ~1 and 2 s. Figure 5 shows the pressure signal recorded at 5.6 km from the 26 January explosion (a) as well as a photo of the explosion and association shock wave (b). More detailed analysis and discussion of the near-field pressure data and explosion source are discussed in Bonner et al. [212] and Gitterman and Hofstetter [212] 6. Results [18] The signals from the explosions recorded at each arrayareshowninfigures6 8 for 29 and Figures 1 and 11 for 211. Each figure subplot shows the received waveform, travel times, celerities (defined as range to the source divided by travel time), trace velocities, and back azimuths along the waveform. The frequency bands, time windows, and step sizes used for these computations are listed in section 3 and were chosen specifically for the different arrivals to maximize the visible energy. The celerities and absolute travel times of the different portions of the wave train are indicated on the axis below the waveform. Trace velocities and observed back azimuths along the wave train have been determined and are plotted below the wave train in gray scale. The gray scale indicates the Fisher ratio for the corresponding time window and is a measure of the statistical significance of the detection. The solid horizontal lines correspond to the sound speed on the ground and to the actual back azimuths. [19] Celerities, trace velocities, and apparent back azimuths can be used to infer properties of a signal phase and its propagation path. The celerity is a measure of the horizontal propagation speed and reflects both signal propagation speed and path length. As a consequence, the closer the propagation path is to horizontal, the closer the celerity is to the ground sound speed. The various phases of an infrasonic wavetrain recorded by an array on the ground tend to cluster into groups determined by the type of propagation path. Signals traveling along tropospheric 6131

11 Figure 7. Array processing results for the 26 August 29 experiment at ranges >2 km. Numerous stratospheric arrivals are apparent, with generally decreasing Fisher ratio as you increase in distance. See text for more details on processing parameters. paths tend to arrive first, with celerities closer to the ground sound speed, followed by stratospheric arrivals with smaller celerities and then by thermospheric arrivals. Typical stratospheric celerities range between.28 and.31 km/s while typical thermospheric celerities are km/s [e.g., Brown et al., 22]. The trace velocity is equal to the cosine of the grazing angle times the sound speed at the ground and thus the closer the trace velocity is to the ground sound speed, the closer the signal grazing angle is to. In addition, the trace velocity is equal to the effective sound speed at the altitude at which the propagation path turns back toward the ground. Signal phases with trace velocities significantly higher than the ground sound speed are either from an elevated source or have returned to the ground along a path which has ascended to an altitude at which the (effective) sound speed is approximately equal to the trace velocity. Finally, the bearing deviation, defined as the difference between the apparent back azimuth and the actual back azimuth to the source, is a measure of the cross winds encountered by the signal along its propagation path [Diamond, 1963]. In identifying signal phases and their propagation paths the bearing deviation can be used as a consistency check. To produce a significant bearing deviation, the signal must pass through a wind jet flowing perpendicular to the propagation direction Infrasound Array Detections [2] Infrasound from the 29 explosion is detected at 13 arrays to the west and north of the detonation site, out to 3428 km (CEA array). Figure 1a shows a map of the arrays, with filled markers representing arrays that detected the explosion using the detection methods outlined in section 3. Individual array detection are shown in Figures 6 (out to 2 km) and 7 (>2 km). Beamformed array data as a 6132

12 IMA CYP RHOD CRET PELO Range (km) CALA IS48 IS62 IS63 IS PROV IGA CEA FLERS.24 km/s 4.34 km/s.3 km/s.27 km/s 7: 8: 9: 1: 11: UTC Time Figure 8. Beamed waveforms as a function of range for the 26 August 29 experiment. Data are filtered between.5 and 5 Hz, and celerity lines of.34,.3,.27, and.24 km/s are shown by dotted lines. Amplitude in Pa is listed to the left of each waveform. The first set of stratospheric arrivals is seen at most arrays with celerities between ~.28 and.31 km/s. The second set of arrivals is also apparent between ~1 and 2 km with celerities between.21 and.24 km/s. Signal durations generally increase as a function of distance until decreasing signal-to-noise levels eventually reduce them. a) b) Back Azimuth Pressure (Pa) F stat Celerity (km/s) Power (db//2x1 6 Pa/Hz) c) :41:3 7:52: 8:6: 8:24:3 7:3: 7:4: 7:3 7:4 7:5 8: 8:1 8:2 8:3 8:4 26 Aug 29 UTC Time Frequency (Hz) Figure 9. Detailed plot of PELO array detections, with (a) back-azimuth colored by the Fisher ratio, (b).5 5 Hz beamed waveforms, and (c) power spectral density. Black line represents the first set of arrivals, red line the second set of arrivals, and the blue line background noise. The two sets of arrivals are clear. Note the relatively high frequency of the second set between ~8:6: and 8:24:3 UTC. 6133

13 Figure 1. Array processing results for the 24 January t explosion. (a) The Israel arrays clearly recorded multiple arrivals, likely consisting of multiple stratospheric and one thermospheric arrival. (b) The Jordan arrays recorded strong tropospheric arrivals, and all but JO_S (c) have stratospheric arrivals as well. Arrays are grouped by geographic region and only arrays that detected the explosion are shown. function of distance from the source out to 4 km are shown in Figure 8. Array IMA (19 azimuth to explosion, 339 km distance from the source) has two clear sets of arrivals, with the first being impulsive and highest amplitude. The detections here likely correspond to tropospheric and stratospheric arrivals, based on the observed celerities (Figure 6). Arrays RHOD, PELO, and CALA ( , km) all show similar characteristics (Figure 6). Detections at these arrays consist of two sets of arrivals: the first being higher amplitude with celerities between 6134

14 .5 Pa KW (1253 km, 276 ) 7.4 t Celerity [km/s] Travel time [s] Trace velocity [km/s] Back azimuth [deg] Pa QA (1688 km, 294 ) Fisher ratio Celerity [km/s] Travel time [s] Trace velocity [km/s] Back azimuth [deg] 5.1 Pa OM N (242 km, 292 ) Celerity [km/s] Travel time [s] Trace velocity [km/s] Back azimuth [deg] Figure 11. Array processing results for the 24 January t explosion. The Kuwait array likely recorded a tropospheric arrival, while all three shown arrays have clear stratospheric arrivals. Arrays are grouped by geographic region and only arrays that detected the explosion are shown..33 and.27 km/s and the second of lower amplitude with celerities between ~.24 and.2 km/s. CYPR appears to record both sets of arrivals, but due to its configuration (line array), we are not able to resolve the azimuth, hence its resolution is fixed at an azimuth of 162 and we solve for trace velocity only. Processing in a more defined band (~1-5 Hz) at CRET also shows evidence of later arrivals. Figure 9 details the signals from the low-noise PELO array located ~1375 km NW of Sayarim. The aforementioned two sets of arrivals are clear. The first consists of multiple pulses of energy between ~7:41:3 and 7:52: UTC, corresponding to celerities between ~.33 and.29 km/s. The second set of arrivals are lower amplitude between ~8:6: and 8:24:3 UTC, with celerities ranging from.24 to.2 km/s (Figures 9a and 9b). Also of note is the frequency content of the arrivals. The spectra (Figure 9c) shows high-frequency energy (up to 6 Hz) above the background noise for both sets of arrivals, although the first set of arrivals is centered at lower frequency (~.3 Hz compared to ~1 3 Hz for the second set). Beyond 1943 km, only a single set of arrivals are detected at stations IS48, IS62, IS63, IS26, PROV, IGA, and CEA (Figures 7 and 8). Signal durations generally increase as a function of range until ~275 km, where higher noise levels (and thus lower S/N) begin to reduce observed durations. The IMS arrays IS26 and IS48 clearly detect the explosion as well. No detections are made beyond the CEA array (3428 km) at FLERS, DBN, DIA, TEX, or IMS arrays to the south or east of Sayarim Infrasound Array Detections [21] The 211 explosions are clearly detected on multiple arrays to the east and north of Sayarim, with the most distant station being IS34 (~63 km) (Figure 1b). Figures 1 13 show the waveforms and array processing results from regional and remote stations that detected the 24 and 26 January explosions. The plots are grouped, roughly, by range and geographic region. The middle and northern arrays in Jordan (JO_M, JO_NW, JO_NE) lie roughly on the same azimuth and are grouped together. Similarly, the arrays in northern Israel (IN1, IN2, IN3) are roughly at the same azimuth and are grouped together. The array in southern Jordan (JO_S) is an outlier, both in range (at 14 km, it was squarely in the classical shadow zone) and azimuth. The Kuwait (KU), Qatar (QA), and Georgia (GE) arrays are grouped together by virtue of being at ranges intermediate between the regional network and the IMS stations. Finally, the IMS arrays in Kazhakstan, Russia, and Mongolia (IS31, IS46, IS34) are also grouped together. The northern Oman array (OM_N) detected both the 24 and 26 January explosions with low signal noise while the southern Oman array (OM_S) did not detect either, partially due to high noise levels. [22] The 24 January explosion is clearly recorded by the regional arrays to the north and east of Sayarim in Israel and Jordan (Figures 1 and 11), extending to OM_N at 242 km. No detections are made at any IMS stations or any of the stations to the west. Based on celerity and trace 6135

15 Figure 12. Array processing results for the 26 January t explosion. (a) The Israel arrays again recorded multiple arrivals, likely consisting of stratospheric and one thermospheric arrival. (b) Strong tropospheric arrivals are recorded on the Jordan arrays. Stratospheric arrivals are present on all Jordan arrays except for JO_S (c). JO_NW recorded a possible thermospheric arrival. Arrays are grouped by geographic region and only arrays that detected the explosion are shown. velocity, all of the Jordan arrays (JO_S, JO_M, JO_NW, JO_NE) and the Kuwait array (KU) appear to have detectible and, in most cases strong, tropospheric arrivals. With the exception of JO_S, which was well within the classical stratospheric shadow zone [e.g., Gutenberg, 1926], all these arrays appear to record stratospheric arrivals as well. The signal detected in Qatar (QA) was not very strong, but stratospheric returns appear to have been detected, and possibly tropospheric as well, but with little S/N. Stratospheric returns and a single thermospheric return were detected on the two arrays in northern Israel (IN1, IN2). 6136

16 Figure 13. Array processing results for the 26 January t explosion. (a) GE, KW, and QA all appear to record tropospheric and stratospheric arrivals, noteworthy considering their range from the source. (b) The IMS arrays recorded low signal-to-noise stratospheric arrivals typical of long range propagation. Arrays are grouped by geographic region and only arrays that detected the explosion are shown. [23] Detection of the 26 January explosion was widespread to the east and north of Sayarim (Figures 1b, 12, and 13). Signals from the event were detected at all the arrays deployed in northern Israel and Jordan; at the arrays in Qatar, Kuwait, and Georgia; and at three IMS stations, IS31 in Kazakstan, IS46 in Russia and IS34 in Mongolia. [24] Conditions had changed between 24 and 26 January (Figures 2c and 2d). By the time of the 76.8 t explosion on 6137

17 26 January, a low altitude (about 8 m) wind jet flowing toward the southeast had developed. This wind jet was of great significance for the near-field [Bonner et al., 212]. At higher altitudes, the jet stream had increased to about 75 m/s, and the stratospheric jet had turned slightly to the north. In addition, there were rainstorms along the Persian Gulf leading to high noise levels at the stations in Kuwait, Qatar, and Oman. [25] All three arrays in northern Israel (IN1, IN2, IN3) show what appear to be stratospheric arrivals with celerities ranging from.27 to.31 km/s (Figure 12a). There appears to be a thermospheric arrival detected on all three arrays with celerities between.24 and.26 km/s. A possible second thermospheric arrival is seen on IN3 with celerity of about.225 km/s. We do not make any claims on the U-shaped signal observed at celerities of about.286,.292, and.295 km/s on IN1, IN2, and IN3, respectively. Strong tropospheric arrivals, characterized by celerities of about km/s and trace velocities close to.34 km/s, are seen on the three arrays along the northeast line in Jordan: JO_M, JO_NW, JO_NE (Figure 12b). On JO_NW and JO_NE somewhat weaker stratospheric arrivals follow the tropospheric phases with celerities ranging from.29 to.31 km/s and trace velocities of.36 km/s and higher. A possible thermospheric arrival is visible in the record from JO_NW. A very large tropospheric signal was recorded on the JO_S array (Figure 12c). [26] As pointed out above, noise levels along the Persian Gulf were high on 26 January. A weak detection was made in northern Oman, and those signals detected in Qatar and Kuwait were not much larger than the background atmospheric noise (Figure 13a). Despite this, what appear to be tropospheric arrivals were detected on both QA and KU, despite the ranges being well in excess of 1 km. Stratospheric arrivals appear to have been detected at QA as well. Similarly, in Georgia, at GE, signals with both stratospheric and tropospheric celerities are observed over the atmospheric noise. The signals detected at the three IMS stations, IS31, IS46 and IS34, consist of extended waveforms typical of very long range detections and appear to be stratospheric in origin (Figure 13b). 7. Discussion [27] As expected, the 29 experiment showed enhanced propagation to the west of the explosion source due to the westward stratospheric wind jet. However, the full azimuthal dependence of the propagation is not well resolved as the vast majority of stations were deployed to the northwest of the explosion source. Most of the array detections have celerities corresponding to stratospheric arrivals, and multiple energy pulses at more distant stations likely correspond to multiple stratospheric arrivals. Stratospheric arrival durations generally increase with distance, as expected from multipathing (multiple stratospheric arrivals reaching the ground). Eventually, decreased amplitudes from greater attenuation and higher noise levels reduce the observed durations. The eastward flowing stratospheric jet during the 211 experiment is significantly stronger than the westward flowing jet during the 29 experiment. In addition, on both 24 and 26 January, there was a significant eastward flowing jet stream at about 15 km altitude, reaching about 5 m/s on 24 January and about 7 m/s on 26 January. The jet stream produced large tropospheric arrivals to the east while the stratospheric jet enabled detections at very long range. A tropospheric jet blowing to the northeast also exists for the 29 experiment, but the lack of infrasound arrays in this region leaves the infrasound propagation unconstrained August 29 Propagation [28] Ray tracing and PE modeling to the PELO array are shown in Figure 14a. Predicted stratospheric celerities for the PELO array from ray tracing are ~.34 km/s (~453 s travel time), in decent agreement with the observed celerities of the first arrivals ( km/s), particularly the highest amplitude arrivals at PELO. Only eigenrays being refracted around 6 km are predicted to arrive at PELO. However, the gradient of the c eff in the stratosphere during this time period is relatively weak (Figure 3d), which would result in sound energy being refracted over a range of heights and therefore a broad range of celerities. This is apparent in the PE modeling in Figure 14a. We also note the discrepancies between the G2S and ECMWF profiles at these heights (Figures 3a and 3d). Small-scale variability not captured by the atmospheric specifications may also contribute [Green et al., 211]. The second set of 29 observed arrivals at PELO have celerities (.24.2 km/s) on the low end of that expected for thermospheric arrivals. Ray tracing predicts thermospheric arrivals (Figure 14a) with celerities ~.235 km/s (~5855 s travel time), consistent with the fastest portion of the second set of arrivals. Low-celerity (<.23 km/s celerity) arrivals are not predicted by the ray tracing or PE modeling, and thermospheric arrival amplitudes are predicted to be very low (Figure 14a). Although the second set of arrivals have celerities similar to typical thermospheric arrivals, the frequency content is unexpected. Nonlinear propagation effects and acoustic attenuation are both very strong in the thermosphere. Typically, thermospheric arrivals have longer periods and lack energy at higher frequencies due to nonlinear propagation effects and severe absorption, compared to stratospheric arrivals [Rogers and Gardner, 198; Kulichkov, 22]. The second set of arrivals have higher-frequency content compared to the first set (Figure 9c). Although thermospheric arrivals are not unexpected, the relatively low celerities (down to ~.19 km/s), relatively high amplitudes, and high-frequency content are not predicted by the propagation model results. These arrivals are thus likely due to features not resolved by the atmospheric specifications [Chunchuzov et al., 211; Drob et al., 213]. For example, sharp wind jets in the mesosphere have been proposed as a potential reflector of sound energy [Kulichkov, 21] and would effectively high-pass filter thesoundenergy. Asharpmesosphericwind jet may therefore be responsible for the second set of arrivals in 29 and helps explain the abundant high-frequency energy and long travel times. The travel times (and low celerities) are likely due to longer path lengths in low velocity zones such as the mesosphere. During this period the c eff in the mesosphere along this path is quite low (down to 21 m/s at 92 km, Figure 3d), indicating slow propagation velocities. Mesospheric wind jets at ~9 km would not be well constrained by the G2S models due to their use of climatological versus measurement-based models at this altitude. Other studies 6138

18 Figure 14. Ray tracing and.5 Hz PE out to 14 km for (a) 26 August 29 at 3 and (b) 26 January 211 at 6. Stratospheric and very weak thermospheric arrivals are predicted in 29. PE amplitudes in the thermosphere are very low due to the very high absorption predicted. For 211, tropospheric, stratospheric, and thermospheric arrivals are all predicted. Note the higher ground amplitudes predicted in 211 versus 29 due to the strong tropospheric and stratospheric ducts, consistent with the greater detection distance. have reported mesospheric arrivals not predicted by propagation calculations in conjunction with available atmosphere specifications [Kulichkov, 21;Assink et al., 212]. Higher than predicted amplitudes from upper atmosphere arrivals have also been documented elsewhere [Fee et al., 21; Norris et al., 21], suggesting a potential overestimation of absorption in the mesosphere and thermosphere by Sutherland and Bass [24]. Lastly, we note that although the two sets of arrivals have different frequency content and celerities, their qualitatively similar amplitude envelopes in the filtered waveforms (Figure 9b) is likely a consequence of some similarities in their propagation paths January 211 Propagation [29] Acoustic propagation for the 211 explosions was better than expected, with infrasound being recorded out to nearly 63 km. Although the yield of the 26 January 211 explosion was less than the 26 August 29 explosions (76.8 versus 96. t TNT equivalent), the stronger stratospheric wind jets and deeper sound ducts during this time permitted longer range detection. Clear tropospheric and stratospheric arrivals are observed on numerous stations, and complexity in the arrival structures exists due in part to the multiple wind jets and range dependence. The closely spaced Israel arrays to the north have a qualitatively similar, yet complicated arrival structure (Figures 1a and 12a) with multiple arrivals likely corresponding to stratospheric and thermospheric phases (based primarily on the celerities, trace velocities, and examination of the atmospheric profiles). These arrays also have long-duration codas after the main arrivals, possibly due to scattering from atmospheric inhomogeneities. The Jordan arrays along a line to the northeast have a complicated arrival structure as well. Multiple tropospheric and stratospheric arrivals are observed, with the tropospheric jet filling the classic shadow zone between ~5 and 25 km. The arrival structure at these arrays suggests complex sound propagation in both the tropospheric and stratospheric ducts. Figure 14b shows the ray tracing and.5 Hz PE modeling for propagation to the northeast on 26 January. Strong tropospheric and stratospheric ducting is predicted, consistent with the numerous tropospheric and stratospheric arrivals observed. Weak thermospheric ducting is also predicted and observed. Note the lower TL predicted along the ground for the 211 experiment versus 29, consistent with the greater detection range. Due to the large number of arrays and variability in propagation and infrasound arrivals, detailed arrival identification and analysis for the 211 experiments will be examined further in other studies. [3] Upper atmospheric arrivals are also not as prevalent in 211 compared to 29. The deep waveguides in 211 will duct a higher fraction of sound energy in the troposphere and stratosphere (compared to 29), decreasing the amount of energy available to refract in the thermosphere (Figure 14). Unlike the 29 experiment, arrays were also deployed opposite the direction of the 6139

19 Figure 15. Transmission loss (TL) maps for all three experiments from the.5 Hz PE modeling. Arrays that detected the explosions have a filled in marker versus an open marker for stations that did not detect the respective explosion. (a) TL map for 26 August t explosion showing the dominant propagation to the west and northwest from the stratospheric jet, along with a tropospheric duct to the northeast. Numerous arrays to the northwest of the explosion (e.g., IS62, IS63, IGA) detected the explosion even though they are predicted to be in regions of high TL, suggesting a stronger northerly component to the winds. (b) TL map for the 24 January t explosion. Sound is strongly ducted to the east and detected out to northern Oman (OM_N). (c) Regional map for the 26 January t explosion, again illustrating the sound strongly directed to the east. (d) Map for the 26 January t explosion out to ~63 km. The strength of the stratospheric wind jets permitted widespread detection to the east, southeast, and northeast out to IS34. stratospheric wind jet for the 211 explosions. These westerly stations did not detect the explosions, again verifying the strongly anisotropic propagation. Note that upper atmospheric returns were not detected at these westerly stations, despite their unambiguous detection during the 29 experiment. [31] It is clear from this study and previous work that for surface explosions with yields >75 t at nonequatorial 614