Scientists are monitoring sound waves generated by underground nuclear tests to help them make more accurate maps of the Earth’s crust.
The echo-sounding technique is called reflection seismology. It produces detailed “pictures” of formations 10 miles to 20 miles below the Earth’s surface.
“The technique is analogous to ultrasonic imaging of the human body, except that sound waves of lower frequency are needed to penetrate the Earth,” said Dr. George Thompson, chairman of the Geophysics Department at Stanford University.
“It is essentially an echoing device. Each type of stratum in the Earth sends back its own distinctive echo.”
On a seismic map five yards long, Thompson pointed to shimmering lines of dots, each representing an echo. Darker lines swirled across the map, indicating particular types of strata.
“With the aid of modern techniques,” he explained, “imaging is now being carried out to depths of 40 kilometers (about 25 miles). With the reflecting technique, it is possible to ‘see’ what is limestone, shale, granite, so on.”
Lines on the seismic map that break off sharply indicate faults caused by earthquakes, Thompson said.
“Still deeper exploration of the Earth’s crust and underlying mantle must rely on larger sound sources, such as earthquakes or nuclear explosions,” he said. “When tests of nuclear devices are made, they are useful for geophysicists.”
“They are more useful to us than earthquakes, for sometimes we know in advance exactly where they will be and at what precise time. With an earthquake we have to find out after the event,” Thompson said.
“As a result of the new technology, we have learned accurately, for example, the thickness of the Earth’s crust, which varies from place to place.
“We also look for ‘hot’ areas where the rocks are softer because of the heat from the interior of the planet and in which sound waves travel more slowly than through cooler rocks. You find these under volcanic and geothermal areas.
“Even nuclear tests carried out in the Soviet Union are of use to geophysicists--they help us define the whole interior of the Earth.”
Thompson is nationally known for research that has shed light on how mountains are formed and how the Earth’s crust is being pulled apart in areas of the Western United States.
He is currently studying how the tremendous pressures and heat in the Earth’s mantle produce, according to the conditions involved, not only mountains but high plateaus and rift valleys.
In his recent work, he has shown that stresses between opposing forces of the Earth’s crust have undergone major changes in direction.
His efforts also have demonstrated that major mountains such as the Sierra Nevada are the result of thermal processes in and below the Earth’s crust.
As an indirect result of his team’s work, scientists are gaining information about underground water resources and possible oil and other mineral deposits, Thompson said.