Tornado Sensor May Become Reality

April 16, 1997

HOUGHTON, MI--Residents of tornado-prone areas across the country may soon be able to purchase an inexpensive form of protection that will warn them when a tornado is approaching.

Just as scientists in the recent movie Twister worked to develop a warning system to allow people adequate time to seek shelter, real-life scientists are now working on a novel approach to accomplish that goal. Researchers will soon field test a two-part seismic device designed to detect ground vibrations given off by an approaching tornado in time to allow its users to seek shelter. The "sensor" portion of the device would be positioned outdoors. When it received vibrations from a nearby tornado, it would transmit an "alarm" to the second part of the unit that would be located in the home or other building much like a smoke detector. The apparatus would be affordable for just about everyone.

The idea for a sensor was spawned by Frank B. Tatom, president of Engineering Analysis, Inc. of Huntsville, Alabama after he heard of people feeling ground vibrations prior to the touchdown of a devastating tornado that killed 29 persons in Huntsville in 1989. But it was in conversations with Stanley J. Vitton, then a civil engineering professor at the University of Alabama and now at Michigan Technological University, that the idea was crystallized. At the time, Vitton was working in Huntsville studying sinkholes, landslides, and other geologic hazards.

"I called Frank to talk with him about a computer program he had designed to determine the distance at which explosions such as that at the Oklahoma City Federal Building can cause damage," recalls Vitton. "I deal in soil dynamics and he is an expert on turbulence and fluids systems. During the course of our conversation, he wanted to know if it were conceivable that energy from a tornado could be transferred to the ground; that's how we began working on this problem. It turned out there had been little or no study on winds causing ground vibrations."

Tatom and Vitton began investigating the mechanisms that transfer energy. They found that when a tornado passes over the landscape, the swirling motion of the wind creates pressure fluctuation with tremendous energy and much of that energy can be transferred to the ground. "This confirmed the statements of several witnesses in the Huntsville area who said they actually felt vibrations in the ground before the tornado of 1989 was sighted," says Vitton. "We then decided that it should be possible to create a seismograph-type instrument that could pick up the vibrations of a tornado and serve as a warning device for human populations that might be in danger."

Tatom made some calculations to determine how much energy could be transferred to the ground by different size tornadoes. His figures showed that large tornadoes were capable of transferring energy to the ground that is equivalent to a thousand pounds of TNT per second, with the largest tornadoes producing up to ten thousand pounds of TNT per second of energy. Vitton then started modeling the type and magnitude of the seismic signals given off by tornadoes.

"Normally signals given off by a tornado would be at a higher frequency than traditional seismographs are programmed to detect," he explains. "Seismographs are usually designed to detect earthquake tremors, so scientists generally filter out any other type of signal so nothing will interfere with the earthquake data."

A second aspect of tornadoes identified by Vitton was that tornadoes also cause a very slight tilt to the earth's surface as they make contact with the ground. Vitton and Tatom, along with their colleague, meteorology professor Kevin Knupp of the University of Alabama at Huntsville, believe that a tornado works like a plunger trying to lift up or press down on the earth's surface and it is this action that can cause the earth's crust to tilt slightly. While this tilt is too small for humans to detect naturally, very precise instruments would be able to detect the change.

Vitton says a seismologist from Oxford, MS recorded what was thought to be a seismic signal, but had an unusually low frequency even for earthquake-type waves. "When I reviewed his data, we were able tie in those signals with a tornado that had passed through near Oxford on the same date and time the recordings were made," says Vitton. "This was the first real evidence of vibration or tilt signals from a tornado being confirmed by data."

Tatom and Vitton are now working on instrument packages that will be placed in the field by "storm chasers" to monitor both seismic vibrations and tilt from tornadoes. Storm chasers currently place other instruments called "Turtles" for a number of scientific organizations such as the Severe Storm Lab in Norman, Oklahoma.

"Our sensors are smaller and we call the package they're in 'Snails,'" explains Vitton. "We'll be passing them out this month (April) to storm chasers in states like Oklahoma and Texas that have high tornado frequency. The storm chasers will try to get ahead of approaching storms that could be harboring tornadoes and place the devices strategically in the path of the storm to see how well they work."

How much warning the sensor gives will depend on the strength of the signal and the sensitivity of the unit, says Vitton. "We hope the device will give as much as five minutes warning during which people could seek safety."

If the device works as well as the scientists hope, they predict the sensors could eventually be available for purchase by homeowners for as little as $40-$50.
-end-
For more information, contact Stan Vitton at 906-487-2527 or e-mail: vitton@mtu.edu.

Michigan Technological University

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