Explosions
in the Stratosphere Test Atmospheric Sound Propagation Models
P. Golden, E.T. Herrin, P. Negraru, H. Bass, W. Andre, M. McKenna
Southern Methodist University Department of Geological Sciences,
National Center for Physical Acoustics,
US Army Space and Missile Defense Command,
US Army Engineer Research and Development Center,
12 February 2007
- Introduction
- Historical Perspective
- Motivation
- The Experiments
- Future Directions
- Acknowledgements
- References
An infrasound
research team has been established to conduct large-scale experiments utilizing
atmospheric explosions to study infrasound propagation phenomenology and to
calibrate a network of infrasound arrays.
In the last two years three calibration experiments were carried out at
White Sands Missile Range (WSMR), during which explosions with yields of 70
pounds TNT were conducted at altitudes greater than 30 km. Infrasound data from the explosions were
collected by team members at many locations in the southwest US. The resulting waveforms represent a truly
unique infrasound dataset in terms of well-known source location and yield. The
primary goal of the project is to improve understanding of the fundamental
physics of the generation of infrasound from explosions and the atmospheric
dynamics affecting propagation of infrasound signals. Results indicate that the phase of an infrasound signal can be
strongly distorted by short-term (hours) dynamics of the atmosphere. Additionally, established period/yield
scaling relationships fail for sources above 40 Km.
From 1960 to the early
1970’s, NASA conducted a series of rocket grenade experiments to produce infrasound
as a means to study the mesosphere. In
the initial experiment 28 sounding rockets were launched from 1960 to
1963. Temperature, pressure, density
and wind measurements were derived from the received infrasound signals. During this early period, the grenades were
the equivalent of 1 or 2 pounds of high explosives. The charges were ejected and exploded at 4 to 6 km intervals to
profile the atmosphere from ~30 to ~90 km.
Radar and other ranging systems were used to measure the location of the
explosions. The time of the explosions
was recorded by photocells on the payload.
Later, charges from 5 to 10 pounds were used up to 50 km, and 2 to 3
pound grenades from 50 to 90 km.
The frequency of infrasound
signals received varied with altitude of the explosions, signals below 1 Hz
were received from the higher explosions.
These experiments were all reported in a series of NASA Technical
Reports from 1963 onward. The current
experiments utilize explosions at high altitude with small charges to produce
infrasonic waves for confirmation of the performance and calibration of
infrasound arrays. Small charges
exploded at high altitudes can produce infrasound very well.
The Columbia
disaster of February 1, 2003, was recorded at Lajitas TX when the space shuttle
generated infrasound as it traversed the continent to land in Florida. Previous passes had given researchers at SMU
distinct patterns of normal behavior. However,
on February 1, 2003, the recorded signals were vastly different from previous
passes. We concluded that the signals were multipath
arrivals from a single explosive event aboard Columbia.
It became
apparent that an investigation into similar sources, atmospheric explosions,
would be useful in understanding the event and modeling similar results
(McKenna and Herrin, 2006). In order to
estimate the size of the explosions we used an empirical period/yield formula
developed for surface explosions and corrected for altitude using the method of
Armstrong (1998). NASA data confirmed
that the Columbia event occurred at a height of just over 63 Km. Our analysis concluded that the acoustic
yield of the shuttle explosion was between 2 and 3 lbs TNT equivalent. This result prompted conversations with
current infrasound researchers concerning an effort to investigate the
possibility of conducting explosion experiments with well-controlled source
yield and location. The US Space and
Missile Defense Command undertook responsibility for the efforts and worked
with the Navy at WSMR to develop the explosive charges and rocket delivery
systems. A team of University, National
Laboratory, private corporation and Government researchers was put together to
plan and execute the experiments.
Team members
include the authors from Southern Methodist University, The University of
Mississippi, the US Space and Missile Defense Command and the Army Engineer Research and Development Center. Additional team members are The University
of Alaska Fairbanks, Dr. John Olson and Dr. Daniel Osborne; The University of
California San Diego, Dr. Michael Hedlin; The University of Hawaii Manoa, Dr.
Milton Garces; BBN Technologies, Dr. David Norris; Los Alamos National
Laboratory, Dr. Rodney Whitaker and the US Naval Research Laboratory, Dr.
Douglas Drob.
Atmospheric modeling played a key role in determining the schedules and locations of portable deployments of infrasound arrays for the explosion experiments. Figure 1 illustrates atmospheric modeling showing seasonal trends in effective sound speed profiles at the WSMR source site. These profiles are based entirely on climatology and attempt to resolve mean seasonal and diurnal atmospheric trends. In the Northern Hemisphere, strong westward ducts form during the summer months, with a peak in July. To take advantage of the possibility of strong ducting, which could provide increased distance for recording signals to the west, the third experiment was conducted in July 2006.
Figure1. Top: Map showing locations and
distances from the source for infrasound arrays and stations deployed during
the WSMR1 explosion experiment. Bottom: Effective sound speed profiles used in
selecting optimal test dates.
Based
on the expected climatic conditions, three infrasound calibration experiments
were conducted at White Sands Missile Range, New Mexico on September 9, 2005
(WSMR1), March 25 (WSMR2) and July 21, 2006 (WSMR3). During each experiment two rockets were launched at approximately
4 hour intervals. Although plans were
for detonations at 50 km altitude, WSMR1 detonations were limited to
approximately 30 km due to Range Safety concerns. WSMR2 detonations were allowed at an altitude of approximately
35 km after additional debris modeling was completed. WSMR3 detonations were at
42 and 49 km respectively. The main
goal of the experiments is to provide further understanding of the atmosphere
propagation of infrasound signals during different atmospheric conditions. A
second goal was validating the yield/dominant period scaling relationship.
Figure 1 shows the location of the permanent and temporary infrasound arrays
deployed for WSMR1 in September 2005. In total there were 19 arrays or stations
at ranges from 63 to 2049 km. For WSMR2, 22 stations were used, and 23 were
used for WSMR3 covering approximately the same distance ranges. The temporary infrasound arrays were placed
mostly west of the source for the WSMR1 and WSMR3 experiments, and east of the
source for the WSMR2 experiment. This
pattern was chosen in agreement with the direction of the zonal stratospheric
winds. The zonal winds are
predominantly westward at around 10 meters/second in July and September, while
at the end of March winds are predominantly eastward, with variable strength. However, observations suggest the second
experiment was carried out close to the time when the zonal winds were turning
to the west.
The
detection patterns of WSMR1 and WSMR3 were strongly dependent on the zonal
winds, as the preliminary modeling suggested (see Figure 1). The WSMR3 experiment yielded detections to
the west as far as Pinyon Flat CA (I57US) at a distance greater than 900
km. Station UM1 at 265 km was the
farthest station to the east to record signals for either WSMR1 or WSMR3.
Several
interesting observations have been made from the data. First, although the
explosions of the individual experiments were carried out approximately four
hours apart, the signals show significant waveform variations. This suggests
that dynamics of the atmosphere can change quickly and affect the amplitudes
and arrivals of signals. Figure 2 is an
example of waveforms from the first and second explosions recorded from WSMR1
recorded at Camp Navajo AZ. The first shot is clearly more impulsive than the
second one, while in the record for the second shot more arrivals can be
identified. It is important to note that the dominant periods of the signals
are almost the same. Therefore, only the phase of the signals appears to be
strongly distorted by the short-term dynamics of the atmosphere. Second, WSMR3 analysis indicates the
yield/dominant period relationship changes unexpectedly for explosions above 40
km.
Figure 2. Recordings of the
WSMR 1 experiment at Camp Navajo, Arizona. Although the shots were only 4 hours
apart, and the source is not believed to vary significantly, the differences in
the phase of the signal is significant.
Yield Determinations
To
estimate the dominant periods of short duration signals we calculated power
spectrum density (PSD) estimates using an autoregressive (AR) process of order
16 with Burg’s method (Burg, 1972). The
data were pre-filtered with a 1 Hertz high pass, zero phase, Butterworth filter
because the PSD is dominated by very long period noise. Figure 3 is the AR power spectrum density
estimate of the signal from the first WSMR1 explosion recorded at Camp Navajo,
AZ showing a clear predominant frequency.
Table 1 gives the altitude of the explosions, the predominant
frequencies/periods and calculated yields for each of the signals. The periods given in the table were derived
by calculating the mean predominant period for each array of stations (at least
four) and then the mean of all arrays.
The arrays included in the analysis were chosen based on good
signal-to-noise ratios. The yields were calculated using the same method as the
Columbia event. The table shows that
the dominant period is dependent on the altitude of the source as predicted by
the scaling relationship. However, the
first two sets of explosions, conducted between 30 and 35 Km. show similar
results in yield but those above 40 Km are significantly different. The scaling relationships appear to fail at
higher altitudes.
Figure 7. Left: An example
Power Spectral Density of the WSMR1 experiment showing the obvious predominant
frequency/period. Right: Launch of the Orion rocket with explosive on board for
detonation at altitude. Table 1 gives
the explosion altitudes, dominant frequencies/periods of signals and estimated
yields of all explosions.
Successful
calibration experiments were carried out at WSMR over the past two years for
the purposes of understanding the temporal dynamics of the atmosphere and for
the calibration of sensors and arrays. The calibration experiments involved
high altitude atmospheric sources for which the locations and yields were known
with a high degree of accuracy. The initial goal of the calibration experiments
was achieved, and detection patterns of the signals relative to the source
locations were in agreement with predicted atmospheric conditions. Future modeling will use atmospheric
observations at the time of each explosion, and will try to relate the observed
signals to the atmospheric variations. As a byproduct of the modeling
technique, the current infrasound modeling codes developed by BBN Technologies
will be tested and validated.
There are
established procedures for estimating the yield of an atmospheric explosion
from the recorded infrasound signal.
The relationship of energy partition into kinetic, heat and infrasound
is not well understood, but during these experiments, the period/yield scaling
relations appear consistent for sources below 40 Km. However, for explosions above 40 Km. the scaling relations
clearly fail. Additional experiments
with known source parameters conducted at altitudes above 40 Km may yield new
scaling relationships and validate new formulas for yield estimates at these
higher altitudes.
We
wish to thank the Naval Surface Warfare Center at WSMR for their efforts in
preparing the rockets and successfully conducting the launches including Mr.
John Winstead, Head, Test Planning Branch, Navy Project Engineer Ms. Kathie
Hoffman, Navy Flight Engineer Mr. Sal Rodriguez and, for Missile Systems, Mr.
Troy Gammill from New Mexico State University.
Armstrong
W.T., (1998), Comparison of infrasound correlation over differing array
baselines. Proceedings of the 20th Annual Seismic Research
Symposium, U.S. Department of Energy.
Burg J.P., (1972), The Relationship Between Maximum Entropy Spectra and Maximum Likelihood Spectra. Geophysics, Volume 37, Issue 2, pp. 375-376.
McKenna,
M. H., and E. T. Herrin (2006), Validation of infrasonic waveform modeling
using observations of the STS107 failure upon reentry, Geophys. Res. Lett., 33,
LXXXXX, doi:10.1029/2005GL024801.
Mutschlecner,
J. P., and R. W. Whitaker (2005), Infrasound from earthquakes, J. Geophys.
Res., 110, D01108, doi:10.1029/2004JD005067.
Author Information
Paul
Golden, E.T. Herrin and P. Negraru, Southern Methodist University, Department
of Geological Sciences, Dallas TX; E-mail: pgolden@smu.edu;
Henry Bass, National Center for Physical Acoustics; University of Mississippi,
University MS; William Andre, US Army Space and Missile Defense Command,
Huntsville AL; Mihan McKenna, US Army Engineer
Research and Development Center, Vicksburg MS