Air-blast data for Sayarim calibration explosions ... · tunnels in the 2,200 m tall Mt. Mantap...
Transcript of Air-blast data for Sayarim calibration explosions ... · tunnels in the 2,200 m tall Mt. Mantap...
SnT2013, Vienna, Austria, 17-21 June 2013 Air-blast data for Sayarim calibration explosions facilitate new method
of source identification and TNT yield estimation
Yefim Gitterman
CONCLUSIONS
1) a new simple and cost-effective method was developed for yield estimation of chemical (calibration) explosions based on a novel scaling relationship for the air-blast Secondary Shock delay;
2) Spectral analysis of teleseismic P-waves from N. Korea nuclear tests at ISN stations revealed spectral minima, interpreted as P+pP interference, that correspond to both cases: vertical borehole of the depth ~2 km, or
horizontal shaft in the mountain of the height ~2 km; the result should be verified by other data and methods.
Acknowledgements The research was supported by the Israel Ministry of Immigrant Absorption.
Evaluation of source depth for N. Korea nuclear tests from ISN teleseismic data
Two underground nuclear explosions conducted by North Korea in 2009 and 2013 were recorded by several stations of Israel Seismic Network. Pronounced minima (spectral nulls) at 1.25 Hz were
revealed in the spectra of teleseismic P-waves. For a ground-truth explosion with a shallow source depth (relatively to an earthquake), this phenomenon can be interpreted in terms of the
interference between the down-going P-wave energy and the pP phase reflected from the Earth's surface. Based on the null frequency dependency on the near-surface acoustic velocity and the
source depth, the depth of the both N. Korea tests was estimated as ~2 km, different from the value ~1 km informed by USGS. Abstract Large on-surface explosions were conducted by the Geophysical Institute of Israel at Sayarim: 82 tons of strong HE explosives in August 2009, and 10&100 tons of ANFO explosives in January 2011 (initiated and supported by the
CTBTO). The main goal was to provide strong controlled sources in different wind conditions, for calibration of IMS infrasound stations. High-pressure gauges were deployed at 100-600 m to record air-blast properties and provide
reliable yield estimation. The rarely reported Secondary Shock (SS) phenomenon was clearly observed at the gauges, and numerous seismic and acoustic sensors. Empirical relationships for peak pressure, impulse, and SS time
delay were developed and analyzed. The parameters, scaled by the cubic root of estimated TNT equivalent charges, were found uniform for all explosions, except of SS delays, clearly separated for 2009 and 2011 shots, thus
demonstrating clearly dependence on the type of explosives with different detonation velocity. Additionally air-blast records from non-Sayarim shots, were used to extend the charge and distance range for the SS delay relationship, and
showed consistency with Sayarim data. Obtained results evidence that measured SS delays can provide important information about an explosion source character, and can be used as a new simple cost-effective yield estimator.
Classic method of yield estimation from air-blast basic parameters
Accurate and reliable TNT yield is an important Ground Truth (GT) parameter of a calibration explosion.
Free-field high-pressure records and basic air-blast parameters (Pm, I+, +) were used for accurate yield estimation.
Preparation of the infrastructure at the SMR: leveling and cleaning
the site, trench for pressure gauge cables, recorder concrete bunker
Sample air-blast
pressure record
and calculated
impulse
The closest gauge G1 before and after
the 2009 explosion
This high-cost and time-consuming method needs 2-3 weeks of infrastructure preparation, gauge
calibration, and protection of expensive equipment from air-blast impact at close distances
New method of yield estimation from air-blast Secondary Shock delay
Based on these SMR observations of high-pressure gauges, acoustic sensors,
accelerometers and seismometers, and using the charge cubic root scaling law a novel
empirical relationship was developed for the scaled delay Dt versus the scaled
distance R (for the TNT equivalent charge W):
Δt = tSS – tMS , Dt = Δt/W1/3 (s/kg1/3), R = r/W1/3 (m/kg1/3) ANFO
Linear RMS fit regression curve was obtained for two Sayarim surface
hemispherical ANFO explosions, for the range 0.1-37 km (56 points):
Dt = 0.0057565log(R) + 0.0032
ANFO
IMI-TNT
A good agreement is found of non-Sayarim ANFO shots with Sayarim ANFO fit curve.
SS delay dependence on explosives (energy, VOD) is revealed
The data for Sayarim 82-ton shot (2009) with IMI explosives, are clearly separated
from the ANFO data, showing also a linear relationship, but significantly lower (with
much smaller SS delays).
The possible reason: IMI explosives are stronger than ANFO, and have
a higher VOD (7.5 km/s vs 4 km/s).
Secondary Shock delay as a new yield estimator
Observations of on-surface Sayarim explosions and previous experiments demonstrate a special character
of the SS delay, as a reliable, stable air-blast feature, different from other basic parameters (Pm, +, I+):
• easily measured, by any simple low-cost acoustic or seismic sensor, not calibrated; even video;
• a sensor can be deployed at remote locations, in the low over-pressure range, and should not be
protected from the blast impact (like the expensive high-pressure gauge system);
• it is a differential parameter, and not necessary to know detonation time, it is critical in a blast accident;
• on the same reason – doesn’t depend on atmospheric conditions - speed and direction of the wind;
• depends on the explosives VOD, thus serving in some cases as a blast source indicator.
The obtained results evidence that this air-blast parameter - SS delay – can be used
as a new yield estimator, based on the developed scaled relationship – for surface
chemical explosions (ANFO, TNT, gas, etc.).
Secondary Shock delay and nuclear explosions
Nuclear explosions do not appear
to produce secondary shocks.
It could be explained by a different
source phenomenology, because a
nuclear test provides an
instantaneous and point-like source
of energy release.
Therefore theis technique could not
be used for the yield estimation.
Nevertheless, some hypothetic
cases can be imagined, when the
second shock can be used for
identification of the source:
nuclear or chemical.
The source depth is an important parameter of a nuclear explosion, conducted
discreetly, exhibiting a violation of the CTBT. Its accurate estimation contributes
to understanding of test design and technological features.
Usual location procedures based on regional and teleseismic records show
large errors for shallow tests.
Some nuclear explosions demonstrate at short-period teleseismic P-wave
records pronounced spectral minima (nulls) near 1-2 Hz resulting from strong
destructive interference between down-going P-wave energy and the pP-wave
reflected from the Earth’s surface.
P+pP interference
The interference produces a spectral modulation
minimum at the frequency
f1 = Vp/(2h)
where Vp(m/s) is the compressional (P-wave)
velocity of the medium above the source and
h (m) is the depth
Spectral null data for previous nuclear explosions
Pronounced spectral minima near 1 Hz
were found at teleseismic records of
Nevada tests (Kulhanek, 1971), 1.5-1.8
Hz for Semipalatinsk tests (Kulhanek,
1973), and ~1.7 Hz for a Pakistan test
(Gitterman et al., 2002).
From paper:
Gitterman, Pinsky, Hofstetter, 2002.
Signal Processing for Indian and
Pakistan Nuclear Tests Recorded at
IMS Stations Located in Israel.
Pure Appl. Geophys., Vol. 159,
no. 4, pp. 779-801.
Seismograms (vert.) and P-wave spectra at ISN stations
from the first Pakistan nuclear test on 28 May 1998
North Korea nuclear tests: observations at Israel Seismic Network (ISN)
The 2006 test was too
small and P-waves were
not observed at ISN
stations, but two larger
explosions in 2009 and
2013 were recorded
Seismograms of the 2nd
North Korea nuclear
explosion in 2009
recorded at ISN stations
(vertical), band-pass
filtered 1-3 Hz
P-waves were observed at some stations after a narrow BP filtering; at some stations signals were not revealed
2009 test: f1~1.2-1.3 Hz 2013 test: f1~1.2-1.3 Hz vertical seismograms are BP filtered 0.7–2 Hz, teleseismic P-waves are aligned
Clear semblance in spectral minima at ~1.2-1.3 Hz
was found for 2009 and 2013 tests, supposedly due to
P+pP interference
Acoustic (P-wave) velocity for granites of the test site is
Vp~5.1 km/s (Bonner et al., 2008, BSSA, V.98, No.5)
Then the depth of both tests is estimated roughly as
h = Vp/(2f1) ~ 2 km
Radionuclides (krypton-85 and xenon-133) were found in atmosphere after the 2006 test, but not
revealed after much stronger shots in 2009 and 2013, thus indicating possible much deeper sources.
For nuclear explosions of this size (2-10 kT) a source depth of less 1 km (~0.5-0.8 km) is sufficient to
provide full containment. A larger depth can be suspected in order to prevent exit of radioactive gases
to the atmosphere that can be detected by IMS stations and provide sensitive information about design
of a clandestine explosion.
Most of underground nuclear explosions were conducted at depths less than 1 km. However, there were
a number of tests in the USSR and USA (23) at the depths 1.4-2 km, and even ~2.5-2.8 km.
The waveform similarity for 2009 and 2013 tests
supposedly indicates the same source depth
The GII depth estimation is based only
on one method and several closely
placed teleseismic stations and should
be verified by other data and methods
North Korea nuclear tests – supposed horizontal shaft case
Satellite imagery of the test site showing horizontal
tunnels in the 2,200 m tall Mt. Mantap
(YONHAP news agency, 2013/02/04 17:46 KST)
Hypothetical reflection of pP wave from the mountain surface on
the height h relatively to the source can interfere with the straight
P-wave and produce the same spectral nulls effect.
Some optimal directions for the reflected pP-wave can be sugested.
Seismology Division, Geophysical Institute of Israel
Calibration surface explosions at Sayarim Military Range
conducted by GII in collaboration with IDF, supported by US Army SMDC and PTS CTBTO (2011)
Different explosives were used
explosives
strong cast IMI bulk ANFO
VOD, m/s
7500 ~4000
density, g/cm3
1.6 0.8
TNT equivalent, tons
96.0 76.8
IMI ANFO
26 Aug. 2009, 82 tons 26 Jan. 2011, 102 tons
Observations of air-blast Secondary Shocks
In all Sayarim surface explosions, distinct air-blast secondary shocks (SS) were observed at high-pressure gauges
(the known effect), and also at acoustic and seismic sensors and even at video-records (new interpretation):
Secondary
shock
delay
Main
shock
Δt = 0.35 s
at near-source distances (0.1-0.6 km) at local distances (up to 37 km)
82-ton (2009) by a high-pressure gauge (0.4 km) 102-ton ANFO (2011) by accelerometer (0.3 km) 102-ton ANFO (2011) by 3C seismo-acoustic station (WGC, USA) (2 km)
102-ton ANFO (2011) Visualization of audio-channel from a home video-camera at 9 km
A new special air-blast
parameter is proposed –
the secondary shock delay
The delay was found increased for
larger charges & distances
Secondary Shocks for non-Sayarim shots We extended charge and distance ranges, including WSMR extra-large shots Distant Image (1991)&Minor Uncle (1993) > 2000 tons ANFO, recorded at 28-60 km
Distant Image
Minor Uncle Distant Image, 20 June 1991, 2,210 tons ANFO, station Rimfire
(pressure sensor and 2 seismic channels RADIAL and VERTICAL)
TNT equivalent:
WTNT = 0.82W=1,812,200kg
Distance r = 28,000m,
SS delay: dt = 2.024 sec
Sayarim experiment layout
SS delay data for Sayarim ANFO shots
SS delay data for all shots and different explosives