Forensic Seismology - A Weapon in the War on Terror


It all started back in World War I when Britain's War Minister, Winston Churchill, asked the academic community if there was a way to locate the exact positions of the big German guns. The slide-rule boys put their heads together and gave birth to a new science, Forensic Seismology.

Initially, the approach these guys used was about as simple as you can get. They placed three sound receivers along a baseline of several miles, and then recorded the arrival times through the ground of the bang from a large cannon shot. The difference in the arrival times between each set of pairs resulted in a line of position after suitable calculation. The three pairs produced three lines of position, which nailed down the precise position of the gun.

Well, sort of.

There were other factors: If the ground along each line of position was significantly different, say rock for one, sand for another, and dirt for the third, then one could no longer assume that the travel velocities were the same, which introduced an element of error. Furthermore, they had only a marginal ability to determine the arrival times precisely. The nature of trench warfare also introduced its own difficulties ranging from how to locate the receivers sufficiently far apart, to how to preserve the installation during a frontal infantry attack.

Relatively soon, the researchers began measuring the phase difference of the arriving acoustic energy, which solved the timing problem. They introduced specially trained personnel who attempted to determine the geology of the intervening terrain, and to the extent that they were successful, the calculations improved again.

The bottom line was that the Germans frequently found their guns under direct attack very soon after the first shot from a new location. I'm not sure they ever figured out exactly what was happening.

The situation is reminiscent of the World War II situation where the British had developed radar, and shortly thereafter, the Germans developed the ECM receiver - basically a radio receiver that could pick up transmitted radar signals. This allowed German submarines to dive after being detected by British radar, but before the British anti-submarine planes could get to their location.

When the German submarines could no longer be found at the detection sites within a few minutes of detection, the British correctly determined that they must be carrying some kind of radar receiver. Shortly thereafter, however, the British discovered that the German receivers were, themselves, broadcasting radio energy. So the British turned off their radar transmitters, and commenced using several sensitive receivers along a baseline to triangulate the location of surfaced German submarines using these "defective" receivers. This capability allowed the British to destroy nearly all the German U-Boats during the last year of the war.

Historically, the only German U-Boat skipper to figure out what was happening was Herbert A. Werner, later the author of Iron Coffins. He concluded that the only time British fighters appeared was when he used his ECM receiver, so he threw it overboard. As it turned out, he was one of only a handful of German submariners to survive, which he attributed to this insight.

Following World War II, Forensic Seismology came into its own with the commencement of underground nuclear tests by the United States and later Russia, France and China.

Prof. Terry Wallace of the Department of Geology at the University of Arizona, has spent the last 25 years following his graduation from New Mexico Tech and his doctoral studies at Cal Tech developing this technology. In 1989, as the world moved to digital, Wallace turned his efforts towards creating computer models that mimic how sound travels through the earth. He also was instrumental in generating a vast database of seismic "signatures."

These signatures accurately identify specific kinds of sounds and enable researchers to determine their sources. Currently, 550 stations feed his lab, and over 1,500 stations feed the seismic community.

With time, Wallace's interest has turned increasingly to developing ways for identifying and locating man-made sounds. Some of these are obvious, such as the big gun sounds from World War I and modern battlefield explosions. Many of these sounds, however, are much more subtle.

Most warriors are at least peripherally aware of the Sound Surveillance System (SOSUS) network that monitors all the world's oceans, tracking ship sounds with amazing precision. Perhaps the most interesting part of this technology is that the sounds being monitored are an order of magnitude or more below the level of the ambient noise of the ocean.

Wallace and his colleagues today are working on a similar technology to detect sounds in the Earth, sounds that are way below the natural level of ambient noise.

Where is this leading? Wallace explains that he probably will never be able to detect a special forces warrior landing by parachute, but he fully expects to have the capability to detect a Humvee arriving by chute, and to be able to identify the difference between a Humvee and an ATV by their characteristic signatures. He also expects to be able to count the number of vehicles in a column, and even determine what kind of vehicles they are.

Exploding C-4 produces a different signature than dynamite; and bombs are different still. How about determining whether the hand weapons being fired are Russian or Chinese, or whether the invisible moving group of men is 50 or 500 in number? Properly placed monitors can distinguish the man-made sounds emanating from a cave complex, even though they are "completely masked" by natural sounds like a rushing brook or other man-made sounds like traffic on a busy thoroughfare.

Here at home, Wallace and his colleagues are creating a seismic map of the entire country. Within 18 months they will have taken 400 coordinated measurements at 30-mile center-to-center seismometer positions. Over the next 10 years, the project will be completed. This detailed seismographic map of the United States will enable researchers to catalogue the signatures of literally millions of man-made events. They will also be able to fine-tune their forensic measurements with the detailed knowledge of the underlying terrain this vast study will generate.

As they develop their models, they are also creating automatic systems able to track and catalogue man-made events, and alert them when these events are out of the ordinary. An unexpected explosion in the hills of Idaho can be pinpointed for further investigation by the FBI. An excavation where one should not be taking place can trigger another investigation. An out-of-the-ordinary movement of trucks can be tracked as they converge - for example - on the Golden Gate Bridge. Even a sudden increase in ATV traffic can be analyzed for its potential impact.

The list of possible ways this information can be used in fighting the war on terror is almost endless. From the initial investigations under Churchill's World War I War Ministry to the state-of-the-art activities of Wallace and his colleagues, Forensic Seismology has become an increasingly valuable tool in our fight against terrorism.