The seismometer was built for bigger bangs. Housed in a Baghdad office, it was used to study how seismic waves caused by earthquakes differ from those caused by nuclear test explosions as they pass through the region's crust. But in 2006, the instrument was picking up the background sounds of an escalating war.
As they report online 22 December in the Bulletin of the Seismological Society of America, Ghassan I. Aleqabi and Michael Wysession of Washington University in St. Louis and their colleagues found a rich seismic record of car bombs, mortar explosions and most dramatically, the massive explosions related to a 10 October 2006 ammunition "cookoff" at U.S. Forward Operating Base (FOB) Falcon.
The data recovered from the Baghdad seismic observatory, located seven kilometers north of FOB Falcon, show that a number of different types of "military operations in urban terrain" or MOUTs can be identified by unique seismic signatures. Aleqabi and colleagues were able to distinguish the blast of a vehicle-borne improvised explosive device from mortar fire, for example. They also found the seismic traces of the helicopters that flew over the Falcon explosion site.
The information accidentally captured by the single seismometer offers a limited glimpse at how MOUT seismic profiles might be used by the military and others. MOUT seismic records obtained with a larger array of instruments might be used to identify the type, size and location of their source explosions, the scientists say.
The researchers used several aspects of the seismic signals, including their relative energy level and characteristics of their frequency amplitude variations over time, to identify and classify various MOUTs. To match their data with specific types of explosions, they compared their data with published reports of incidents, such as two separate car bombs--one that was detonated in June 2006 targeting a police patrol in Al-Karradah, Baghdad, and a December 2006 explosion caused by a bomber who drove a vehicle into a police checkpoint by the gate of Baghdad University.
The 10 October Falcon attack provided a wealth of seismic data for Aleqabi and colleagues to analyze. Mortar or possibly rocket rounds fired into FOB Falcon, an ammunition supply point, caused a series of explosions as the heat produced by the fired round caused the rest of the ammunition to fire off in a chain reaction. The explosion series lasted for hours and produced such huge mushroom-shaped clouds of debris that local residents thought at first that a small nuclear weapon had been detonated.
"I was amazed that we could recreate the sequences of events for this cookoff," Wysession said. "We could piece it all together, from the moment that the mortar hit. It was amazing to be able to take this bunch of squiggly lines [on the seismograms] and recreate this sequence of catastrophic destruction at the ammo depot."
Wysession said that the seismic signal from these explosions can vary depending on how the explosive material might have been shielded--inside a car, for instance--or the environment where the detonation took place, such as a narrow or wide street, or one with tall buildings around. To varying degrees, the energy of these explosions might be propagated into the airwaves, and then into the ground where the seismometer could pick up the secondary signal. In an ideal experiment to analyze MOUT seismic signals, Wysession noted, researchers would compare data from multiple seismometers, along with data from infrasound recorders that would capture the acoustic energy of a blast.
The Baghdad MOUT signals were recovered from the seismometer during a visit to Iraq, since the seismometer is not connected to others in the region. Aleqabi and Wysession have been studying data from the Baghdad seismometer to understand how fast seismic waves move and how fast they dissipate in the region's crust, as a part of a larger effort to develop mechanisms for distinguishing earthquakes and nuclear testing seismic sources.