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Detecting Stowaways

January 1, 1997

Had the ancient Greeks tried to use their Trojan horse to sneak into the Department of Energy’s Y-12 facility in Oak Ridge, Tenn., they would almost certainly have given themselves away by a sound they couldn’t control: the beating of their own hearts. Engineers at the national laboratory have developed a technology so sensitive it can pick up the sound of a heartbeat even when a person is well hidden inside a large vehicle.

The aim of the heartbeat detector is to catch terrorists attempting to gain entry to secured sites, prisoners trying to escape from jail, illegal immigrants crossing borders, even endangered animals being smuggled into the United States. So successful are early versions of the technology that it is being tested in maximum security prisons in California and in Tennessee.

The centerpiece of the technology is software that allows an ordinary portable computer to register the shock wave that the heart creates when it beats. The measurement of this wave, which is known medically as a ballistocardiac effect, is the mechanical equivalent of an electrocardiogram, which measures the intensity of the heart’s electrical signals.

That the mechanical force of a heartbeat can be measured has been known since 1877, when Englishman J.W. Gordon correlated it to the jiggling readings he noticed while standing on a sensitive scale. What has not been clear until now is that the ballistocardiac effect can be measured as it induces slight vibrations in much larger objects such as cars and trucks.

In fact, tests show that a beating heart can perturb a tractor-trailer’s walls, says Richard Pack, a design engineer at Oak Ridge. But the challenge was in trying to distinguish those vibrations from confounding factors such as wind blowing and doors opening and closing. Oak Ridge researchers conducted vibrational analysis of the ballistocardiac signal and discovered a pattern that featured several distinctive harmonic peaks and that varied from about 50 to 150 beats per minute. Given these characteristics, they devised a detection algorithm based on a common mathematical technique called wavelet analysis that is well known for its ability to distinguish impulse signals, even those spaced irregularly.

The detection algorithm in the Oak Ridge system receives data from either of two vibration-sensing techniques. One, undergoing testing at Centinela Maximum Security Prison near El Centro, Calif., places several motion detectors called geophones (which are traditionally used by geologists to hunt for seismic signatures that reveal underground pockets of minerals) on the roof, bumper, or other flat surface of a vehicle. The supersensitive devices contain a weight suspended in an electromagnetic field that when moved will generate an electrical signal. These signals are, in turn, relayed to a computer running the heartbeat-detection algorithm, which reveals a beating heart’s vibrational characteristics and provides unmistakable evidence that someone is present inside a vehicle.

To test the efficacy of its technology, the Oak Ridge group has gone to great lengths to muffle the heart’s characteristic sound. “We have rolled people in bubble wrap, in mattresses, in comforters, put them in tractor trailers and in garbage trucks three-quarters full of garbage, and the technology still works,” says Leo Labaj, a technical operations manager at Oak Ridge.

The Tennessee researchers are also working on a microwave-based vibration-detection system that would eliminate the need for geophones, which must be placed directly on a vehicle. This technology would measure heartbeat-induced vibrations in a manner analogous to that used by a police radar gun tracking a speeding car. The detector would fire a microwave beam at the wall of a vehicle and calculate vibrations according to the subtle changes in the time, and therefore the distance, it takes for the beam to travel back. The Oak Ridge group plans to have a test microwave facility operating in a year.

In the meantime, Houston-based Geovox Security Systems is conducting further trials of the geophone-based system at Pelican Bay Maximum Security prison in California and Riverbend Maximum Security Institution in Tennessee. As part of the test, prison guards and other workers have been trying to smuggle themselves out of the compound. But so far, say Oak Ridge researchers, no one has evaded detection.
To install a geophone-based system at a prison entrance, from which prisoners often try to escape (hidden, for example, in a truck), would cost somewhere between $55,000 and $65,000 dollars, says Geovox president, former Texas governor Mark White. But the price is reasonable, he says, because even an escape in which the prisoners are caught only a few miles from the prison can cost the state $100,000 in terms of manpower and equipment.

The Oak Ridge scientists do not know exactly how sensitive the heartbeat detector is at detecting fainter heartbeats. So far it has been able to reveal stowaway dogs and cats. But in the next round of tests, it will try to pick up a bird’s heartbeat, an important milestone if the device is to be used to thwart smuggling of endangered parrots and other small animals.

The geophone-based detector is not without shortcomings. The main one is that it can be used only on a vehicle that is cushioned from the ground-for instance, by shock absorbers, springs, and rubber tires. If it is not, the earth itself serves as a kind of vibrational damper. That is, the vehicle and the earth virtually become one solid body. “Vibrations from heartbeats are strong enough to move a truck, but not strong enough to move the earth,” explains Pack. This means that ships, which are essentially one with the water they rest in, and railroad cars, whose rigid steel wheels ride on steel tracks, will not vibrate strongly enough for heartbeats to be detected by the geophones.

To deal with this limitation, a container could be taken off a ship or a train and then put on a truck so a heartbeat inside it could be measured. The Oak Ridge group is hopeful that the microwave technology, which in initial testing appears more sensitive, might be able to pick up the faint heartbeat vibrations on boats and trains. If so, it may prove useful in detecting illegal immigrants attempting to enter ports or cross border checkpoints.

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