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Science: Atomic Earth Study

4 minute read

Man-made atomic explosions can help scientists explore the center of the earth, 4,000 miles down. This is the hope of Professor K. E. (for Keith Edward) Bullen of Australia’s University of Sydney, who told a Toronto meeting of geophysicists last week that he has already used waves from A-and H-bomb explosions to refine his theories about the earth’s deep metallic core.

Most information about the earth’s interior, Mathematician Bullen pointed out, has been gained by recording and measuring the several kinds of waves sent out by earthquakes. As the waves travel through the earth, they are bent and reflected in complicated ways. Some waves move faster than others; some are absorbed entirely. By disentangling the jiggly lines made by instruments recording many earthquakes, seismologists have determined that the earth is formed of concentric layers of different materials, with iron-nickel at the center and stony oxides nearer the surface.

No Warning. Natural earthquakes, said Dr. Bullen, are not ideal as tools for earth study. Their waves often start from a large region, which makes them leave fuzzy records, like the shadows cast by a bonfire. Even worse, they give no warning, so seismologists have no time to start up the expensive, sensitive instruments they use when they want to record events of special interest.

Nuclear explosions are not nearly as powerful as major earthquakes, but even old-style A-bombs can send waves strong enough to pass right through the earth. They come from a small area whose position is accurately known, and since the time of the explosion is under human control, warning can be given when the man-made waves are about to start through the earth.

In 1955 Professor Bullen and a group of colleagues tried to persuade the U.S., Britain and Russia to explode at least one smallish atom bomb for scientific purposes during the International Geophysical Year (1957-58). The effort got nowhere, partly because of public revulsion against all nuclear explosions, but in 1956 the British and Australian governments gave in advance the time and place of four military test shots in Australia’s central desert. Seismologists in many countries were all set for the waves, and they gained new information from their highly detailed records.

Tests on the Minute. Other knowledge has come from studying seismograph records of the U.S.’s great H-bomb tests in the Pacific in 1954. The U.S. Atomic Energy Commission gave the seismologists no help at all, but Dr. Bullen figured out the exact times of all four blasts. Apparently the AEC is a creature of habit: it exploded all its H-bombs at an exact multiple of five minutes after 6 p.m. Greenwich mean time. According to Bullen’s figuring, Test Bravo (which killed the Japanese fisherman with radioactive fallout) exploded at 45 minutes, zero seconds past 6 p.m. on Feb. 28.

Bullen thinks that much better results could be obtained from smallish A-bombs exploded at places selected by seismologists. If they were placed below the earth’s surface or under the sea, much more of their energy would turn into useful earth waves. The exact time would be told in advance, so scientists all over the earth could have their instruments tuned to concert pitch. A radio signal might start abreast of each burst of waves. When the earth’s gentle, controlled trembling finally quieted down, the scientists would have data for a new understanding of its mysterious interior.

At Toronto, Bullen renewed his plea for scientific explosions. This time he got a part of what he wanted. Dr. Willard F. Libby, scientist-commissioner of the U.S. Atomic Energy Commission, wired him that the AEC will explode this week a smallish atomic charge 800 ft. below the surface of a Nevada mountainside. Libby told the time (Sept. 14, 10 a.m. Eastern daylight-saving time) and place (North 890600 and East 635000 on the Nevada state grid system). If there is a delay of more than two minutes in firing the shot. the test will be postponed for 24 hours.

Dr. Bullen expressed gratitude. There would not be time, he said, to set up special apparatus in remote places, but the world’s 600-odd established seismic stations will be listening. The waves from Nevada will surely be recorded all over the earth.

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