Explosion of a fuel depot in Hemel Hempstead, England

A strong explosion took place at around 7:01 (CET) on 11 December 2005 in a fuel depot in Hemel Hempstead around 40 km to the north of London, England. According to the reports made to date, only around 43 people were injured, which is considered minor given the explosion of around 60 million litres of petrol. The sonic signals from this explosion were detected in the Bavarian Forest at German IMS infrasound station I26DE. The infrasound measuring network IGADE north of Bremen in Germany also detected these signals.

The lower part of Figure 1 shows the results of the cross-correlation analysis, as well as the composite traces (above, middle and below) from the two stations I26DE and IGADE (above and below). Cross-correlation analysis is a method for determining the direction, and was originally developed to localise seismic signals. Direction finding here is analogous to the procedure used by a radar antenna: a plane-wave adjustment is carried out depending on the differential travel times of the recorded signals at each sensor of an array to determine the angle of incidence of the wave and the apparent speed. According to the two diagrams, the explosion signal reached station I26DE from a direction of 291 degrees and at apparent speeds of 345 to 380 m/s. The corresponding figures for IGADE are 247 degrees and 340 to 370 m/s. 

Cross-correlation analysis of infrasound records (upper and middle part of the diagram) at station I26DE, as well as the composite trace (lower part of the diagram). Source: BGR

Cross-correlation analysis of infrasound recordings (upper and middle diagram) at the IGADE station, as well as the composite trace (lower part of the diagram). Source: BGR

Cross direction finding of the source of the signal can be determined after determining the directions at both stations. Because the coordinates of the explosion are known in this case, this event can be used to investigate the ability of the infrasound recording systems to localise events. This is a difficult task because the temperature and wind conditions in the atmosphere fluctuate very strongly on a daily and seasonal basis, and affect the spread of the sound waves. Figure 2 shows the results of the cross-direction finding incorporating the directional figures provided by the Swedish station UPP (222 degrees and 345 m/s) and the French station FLERS (0 to 4 degrees and 330 to 350 m/s). The wind effects were corrected by calculating direction corrections for the relevant wave paths of the infrasound signals from the location of the explosion to each of the arrays. This was done using an average meteorological model which contains information on the wind and temperature profiles for the time of the explosion (11 December 2005, 7:00 (CET)). Thanks to the wind correction and many separate phases at the stations, localisation in this case was very precise. In total, 15 arrivals at four stations could be correlated with the explosion. The localisation error was only 8.2 km in terms of distance and 23 seconds in terms of the direction of the locus (see Figure 2). 

Localisation with the help of infrasound stations FLERS, UPP, IGADE and I26DE. These stations contributed 15 phases in total – the Swedish stations LYC, KIR and JAM failed to record any signals. Source: BGR