Soviets visit U.S. for mock nuclear blasts.

Soviets visit U.S. for mock nuclear blasts

Nine Soviet scientists and a private U.S. group plan to detonate a pair of simulated nuclear explosions later this month aimed at laying the scientific foundations for a ban on underground nuclear testing. The visiting scientists and their U.S. colleagues are currently at field stations in Nevada preparing for the chemical blasts that are part of an agreement between the Soviet Academy of Sciences and the Natural Resources Defense Council (NRDC), a Washington, D.C.-based environmental group.

The explosions, located at two spots in Nevada, will help detail how high-frequency seismic waves travel away from the official U.S. nuclear weapons testing site in the southern part of the state. The scientists involved in the project also will be assessing state-of-the-art seismometers that may be the watchdog instruments used to monitor a future ban or limitation on nuclear testing.

"The objective is to do some good science," says Thomas B. Cochran, an NRDC staff scientist. Last September, he and a team of U.S. experts visited the Soviet Kazakh testing site, where they witnessed similar chemical explosions.

In the 1970s, the United States and the Soviet Union negotiated two treaties that limit the size of underground nuclear tests to the explosive equivalent of 150 kilotons of TNT. Both countries signed these agreements, but they have yet to ratify the treaties, primarily because they have failed to agree on how to verify compliance with the testing limits. The verification is issue also stands in the way of any future total ban on testing.

To measure the size of nuclear explosions, scientists have traditionally relied on seismometers -- instruments that record waves traveling through the earth from explosions and earthquakes. U.S. officials, however, have suggested certain evasion scenarios by which a country might "fool" a seismic system.

Some have suggested that it would be possible to hide an explosion by detonating the device during an earthquake. Another scenario involves setting off the explosion in a hollow underground cavern. This technique, called decoupling, would muffle the blast.

Seismologists now believe, however, that by monitoring high-frequency seismic waves, they can reliably protect against both evasion scenarios.

"The decoupling effect is weaker at high frequencies," says James Brune, director of the seismological laboratory at the University of Nevada at Reno, which is participating in the NRDC experiments. Moreover, says Brune, scientists are now confident that they can distinguish between earthquakes and explosions by using high-frequency seismometers. These devices can measure waves up in the realm of 20 to 50 hertz. Traditional earthquake seismometers record at about 1 hertz.

The experiments in Nevada will involve a 10-ton and a 15-ton explosion, which the scientists will monitor from three permanent seismic stations located approximately 100 to 120 miles from the test site. The NRDC and Soviet Academy built these stations and similar ones in the Soviet Union during an earlier phase in their agreement (SN: 7/4/87, p.6). The Soviets also have brought with them a ton of their own seismic equipment for an additional temporary station.

The federal government has criticized the NRDC agreement in the past. But now, for the first time, the State Department has granted unrestricted visas to Soviet scientists connected with the project, which allow them to travel to the field stations. "We're really pleased with that because without that permission, we could not carry out the experiment," says S. Jacob Scherr, an NRDC attorney.

Officially, the two governments are preparing for a set of experimental nuclear explosions expected this summer (SN: 1/30/88, p.71). The countries will compare several monitoring techniques in an effort to agree on a verification scheme for the unratified treaties.