Infrasonic observations of chemical explosions

Infrasonic observations of chemical explosions

Ludwik Liszka

Swedish Institute of Space Physics

Swedish-Finnish Infrasound Network

Geographical coordinates of infrasound stations

Name Latitude (Degs) Longitude (Degs)

Kiruna 67.8?N 20.4?E

Sodankylä 67.42N 26.39E

Jamton 65.87?N 22.51?E

Lycksele 64.61?N 18.71?E

Uppsala 59.85?N 17.61?E

Explosions:

Since 2001, during the period August 15 – September 15, destruction of explosives takes place in Northern Finland at the location 67.934N 25.834E 1 or 2 times per day. Each explosion equivalent to 10 tons TNT

Explosion site:

Directions and distances to the explosion site

Name Direction (Degrees)

Distance (km)

Kiruna 83.5 231

Sodankylä 337.9 62

Jämtön 30.8 280

Lycksele 37.3 492

Uppsala 20.8 978

This presentation will be mainly limited to two stations: Kiruna and Jämtön, located favourable with respect to the explosion site, at typical regional distances

Scalograms of typical signals at both stations, August 22, 2008

Kiruna Jämtön

Multifrequency measurements of the angle-of-arrival

List of wavelet filters Frequency band # Frequency, Hz Type of filter Centre frequency, Hz

1 <1 Low-pass .75

2 1 – 2 Band-pass 1.5

3 2 – 3 “ 2.5

4 3 – 4 “ 3.5

5 4 – 5 “ 4.5

6 >5 High-pass 5.5

Angle-of-arrival as a function of frequency, August 22, 2008

Kiruna Jämtön

Angle-of-arrival and trace velocity of the broad-band signal, September 1, 2008

Angle-of-arrival for different frequencies, September 1, 2008

Trace velocity for different frequencies, September 1, 2008

Source localization, September 1, 2008

Hilbert-Huang transform

A combination of the Hilbert transform and an empirical procedure decomposing the signal into Intrinsic Mode Functions (IMF) It has been found that the IMF1 derived from the original, broad-band signal may be used for localization purposes Reference: Huang, et al. "The empirical mode decomposition and the Hilbert spectrum for nonlinear and non-stationary time series analysis." Proc. R. Soc. Lond. A (1998) 454, 903–995

First four IMF:s on Sep 1, 2008

IMF1

IMF2

IMF3

IMF4

8 Hz

0

UT, minutes

IMF1

Hz

UT, minutes

IMF2

IMF3

IMF4

The fine structure of IMF1

Angle-of-arrival derived from the IMF1

Kiruna Jämtön

Source localization, September 1, 2008. Comparison with localization using the HHT IMF1

Standard deviations of derived latitude and longitude as a function of day no., Aug 13 – Sep 11,2008

Sodankylä radiosonde data Aug 13 – Sep 11,2008

Sound velocity EW wind NS wind

Sound velocity

EW windpositive towards W

NS windpositive towards S

A possible propagation model

Infrasound propagates in the form of discrete wave packets

Scalograms of typical signals at both stations, August 22, 2008

Kiruna Jämtön

A possible propagation model

Infrasound propagates in the form of discrete wave packets Each recorded wave packet may be described by:

Amplitude Dominating frequency Angle-of-arrival Horizontal trace velocity

At a given distance and direction from the source probability distributions of above parameters may be constructed. When the distance from the source increases the probability of observation of a wave packet decreases with the squared distance. It is why, even for distant sources short bursts of the signal may occasionally be observed.

A possible propagation model