Volcano & Ice

New military techniques are where you find them. Last week Drs. M. Ewing and Frank Press, Columbia geophysicists, told how they discovered a practical method of measuring from an airplane the thickness of Arctic ice. They started by studying Krakatoa Volcano, near Java, which had its most famous explosion almost 70 years ago.

Ewing and Press had long believed that disturbances in the air cause waves in the land and sea. In search of proof, they selected as a "laboratory test" the 1883 Krakatoa eruption, whose mighty bang sent air waves seven times round the earth. One chapter in a fat report by Britain's Royal Society recorded the progress of the volcano's air wave. Another clocked the sea wave, which swept across the Indian Ocean and was measured by tidal gauges as far away as Britain. Oddly enough, both waves were reported in many distant places, including the English Channel, Panama and San Francisco, at the same time.

To Drs. Ewing and Press this looked interesting. The air wave and the sea wave did not travel at the same speed or by the same route. Why did they arrive together? Further study of the records showed that only the air wave could be a direct effect of the volcano's explosion. In distant places like England, the sea wave recorded on the tide gauges must have been "induced" by the air disturbance. Here was dramatic proof that events in the atmosphere do affect the sea below.

Next step was to test this theory by a controlled experiment. Financed by an Air Force grant, Drs. Ewing and Press went to frozen Lake Superior last winter and exploded blasting caps above the ice. With "geophones" (small seismographs) they listened for waves in the ice.

As they suspected, the slight explosion in the air shook the ice for considerable distances, sending to the geophones long trains of vibrations. The frequency of the vibrations, they found, varied with the thickness of the ice: the thicker the ice, the slower the vibrations.

The Air Force is now using their discovery in a practical way. Flying over the ice on the Arctic Ocean, an airplane first drops a geophone fitted with radio apparatus to report what it hears. Then the airplane drops a small bomb, which explodes above the ice. By measuring the frequency of the ice waves picked up by the geophone, the airplanes's crew can tell how thick the ice is. This technique, the Air Force hopes, will help it find stretches of ice that are strong enough for radar or weather stations.

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