Better Bomb Detection

The bitter and continuing argument about the possibility of detecting clandestine bomb tests took a turn last week in favor of the optimists--those scientists who believe a detection network to be feasible. The U.S. Defense Department's Advanced Research Projects Agency issued a four-page booklet stating that "there may be substantially fewer earthquakes that produce signals equivalent to an underground nuclear explosion of given yield than had been expected." All by itself, that brief statement represented quite a switch; the Defense Department has usually favored the attitude that secret Russian underground tests could not be distinguished from natural earthquakes except by dozens of seismic stations spaced a few hundred miles apart on Communist territory.

The change of attitude seems to have been brought about by recent improvements in seismic instruments and methods of using them. One new method is to lower instruments as much as 10,000 ft. into abandoned oil wells. At this depth they are sheltered from surface noise and are five to ten times more sensitive to earth waves from bomb tests or earthquakes. Another promising system is to spread out many instruments in cross-shaped array on the surface of the earth. When their readings are compared to eliminate the effects of noise, traces of earth waves be come much clearer.

The detailed results of official U.S. and British experiments with new seismic instruments have not yet been made public, but nonofficial seismologists state emphatically that earthquakes and bomb tests are quite different, that they send distinctly different wave patterns through the earth. Another important difference is that most earthquakes are caused by rock movements many miles below the surface. This vertical distance can be measured accurately; if it is too great, the waves almost surely will not have come from a test. It is not likely, say the seismologists dryly, that even the most industrious Communists will explode secret tests far down in the hot, semi-plastic depths of the earth.

Both U.S. and British statements about the new seismology were presumably timed to precede this week's renewal of disarmament discussions at Geneva. They may well mean that the West is prepared to consider a new test moratorium, confident that it can detect secret Soviet tests, perhaps without any instruments at all on Soviet territory.

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