Arms control pacts can be verified - includes related article

country are monitored by ''over the horizon'' radars that seem todefy the physical law that all radio waves travel in a straight line. They bounce powerful beams at small grazing angles off the surface of the sea. These are reflected off the ionosphere, flooding the interior of the Soviet Union with radar waves. If missiles or aircraft happen to pass through them, the waves are disturbed in characteristic ways, so that when they're eventually recaptured by receivers at the other end of the continent -- by listening posts in the Aleutians, say -- they can provide information about the performance of the vehicles that caused the disturbances.

Beginning with Eisenhower, every American president except Reagan has tried to limit or to ban nuclear testing. John Kennedy agreed with the Soviet Union to stop all tests except those conducted underground. During the Nixon years, Washington and Moscow agreed to stop detonating nuclear explosives with a yield greater than the equivalent of 150,000 tons of TNT. Although Congress had urged the U.S. to join in the U.S.S.R.'s moratorium, the administration continued its test program. If we could in fact get a comprehensive test ban treaty, as the Carter administration had hoped, could the U.S. confidently detect a small, clandestine Soviet nuclear explosion?

When a nuclear explosive detonates underground, it releases a portion of its energy as earthquake-type waves. In addition to pinpointing the test site with standard seismic techniques, seismologists can calculate the size of the explosion. They start by measuring the amplitude of the waves, which are proportional to the amount of energy released. They then take into account what type of rock the pulses have traveled through, since the terrain will affect how quickly they lose their strength. (In general, higher-frequency waves are attenuated more rapidly than those of lower frequencies.) From these two measurements they can estimate the size of weapon yields with an accuracy of 10 to 20 per cent, with the uncertainty reduced further as their knowledge of the geology of the test site increases. In any event, seismometers are now so sensitive that the U.S. and the U.S.S.R. both claim to have recorded unannounced tests by the other with yields as low as one kiloton.

The detection technology consists of arrays of sensitive seismometers and the recording and computerized analysis of the signals they pick up. Such arrays are located in Norway, Montana, Turkey, and Japan, and two new ones may be operating now in China's Sinkiang province, only 300 miles from the U.S.S.R.'s underground test site at Semipalatinsk in Kazakhstan. To ensure that an underground test will be detected, the seismometers must be permanently on. Unfortunately for analysts, the earth's crust is a noisy place, shaken by numerous earthquakes, big and small, by the pounding of the oceans on shorelines, the rumbling of construction and mining operations, even the swaying of skyscrapers and large trees buffeted by high winds. As a result, there's always a risk that the waves caused by a detonation will be drowned out by the earth's other tremors, or that a nuclear test will be mistaken for an earthquake.