Nuclear Listening Post

When the nuclear powers negotiated a test ban treaty in 1963, they were forced to confine their agreement to atmospheric testing. On-the-spot inspections of underground tests were politically unacceptable to the Russians, and remote monitoring by seismographs was considered unreliable. The trouble was, some explosions were likely to go un detected, and low-yield tests, when they were recorded, could not be reliably distinguished from earthquakes. But now, as negotiators are getting back to business again in Geneva, a new element has entered the argument. The U.S. is putting the finishing touches on an ultrasensitive seismic listening post that should enable scientists to refine their capability of detecting, locating and identifying underground bomb explosions.

Built on the prairies of Montana by the Defense Department's Advanced Research Projects Agency, the $10 million detection facility will go into full operation this fall. Spread out in a giant circle 125 miles in diameter near Miles City, the apparatus consists of 525 seismometers buried 200 ft. deep. They are arranged in 21 smaller circles, each 41 miles in diameter and each as sensitive as the best seismic array the U.S. has built to date.

Poolroom Processing. Modern seismometers have such good ears that they must be buried deep in relatively uninhabited areas to be as free as possible from the surface noises of wind, rain, traffic and grazing cattle. Known as LASA, for Large Aperture Seismic Array, the Montana system was laid out to get the best possible signal-to-noise ratio; it promises to provide a twentyfold improvement in the U.S.'s ability to detect seismic signals. With so many instruments spaced so far apart, it will also be possible to trace the direction and distance of an incoming signal because it will be received by all sensors at slightly different times. Though its potential detection capability is still unknown to scientists, the practical extent of the improvements will be checked in the next few months when LASA will be used to record and analyze signals from known earthquakes and known nuclear blasts.

In each cluster, the Montana seismometers are set like spokes in a wheel, and at the center of each wheel is a small vault housing instruments for collecting the seismic signals. After the signals are picked up and amplified, they are translated into digital data and transmitted over telephone lines and radio to a data-processing center in a converted poolroom 140 miles away in Billings. The signals are eventually sent to M.I.T.'s Lincoln Laboratory in Lexington, Mass., where computers are programmed to determine more precisely the source and direction of the vibrations and whether they were caused by an earthquake or a nuclear explosion.

Often on Sunday. Even with LASA, it may not always be easy to tell where every signal comes from; it would take several similar arrays positioned around the world to provide a system equal to the difficult task of accurately locating and identifying blasts. Neither does LASA guarantee that scientists can tell the difference between some earthquakes and some nuclear blasts. But the computers have a variety of valuable information built in to help them. They are set to label automatically as earthquakes any tremors coming from places with no nuclear capability. And a seismic wave definitely shown to originate from deeper in the earth than it would be practical for man to dig will also be classified as non-atomic.

The timing of the tremor can also be indicative; scientists have a habit of scheduling tests with clockwork precision. "The way to tell a bomb from an earthquake," says Lincoln Lab's Paul E. Green only half facetiously, "is if it goes on the even minute of an even hour. And if it's Sunday, you know it's either a Soviet or a Chinese bomb."

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