November 12, 2012
Atomic Clocks Could Help Survey The Earth’s Insides
Lee Rannals for redOrbit.com - Your Universe Online
Scientists say they are planning to use portable atomic clocks to help identify mineral deposits and concealed water resources inside the Earth.
An international team believes that atomic clocks could already have reached the necessary degree of precision to be useful for geophysical surveying. They said that these clocks will provide the most direct measurement of the geoid, which is the Earth's true physical form.
The Earth's geoid can only be determined indirectly, and can be calculated on continents by tracking the altitude of satellites in orbit.
Using atomic clocks to help determine the geoid is an idea based on Einstein´s theory of general relativity that has been discussed for the past 30 years.
An atomic clock uses an electronic transition frequency in the microwave, optical or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its time keeping.
Clocks located at different distances from a heavy body like our Earth tick at different rates. The closer a clock is to a heavy underground structure, the slower it ticks.
"In 2010 ultraprecise atomic clocks have measured the time difference between two clocks, one positioned 33 centimeters above the other," Ruxandra Bondarescu from the University of Zurich said in a prepared statement. "Local mapping of the geoid to an equivalent height of 1 centimeter (0.4 inches) with atomic clocks seems ambitious, but within the reach of atomic clock technology."
Bondarescu explained that if an atomic clock is placed at sea level, a second clock could be positioned anywhere on the continent as long as it is synchronized with the first clock. The connection between the clocks can be made with fiber optics cable or through telecommunication satellites.
The second clock would tick faster or slower, depending on whether it is above or beneath the geoid, which is at sea level. The local measurement of the geoid can then be combined with other geophysical measurements like those from gravimeters, which measures the acceleration of the gravitational field.
Atomic clock surveying is possible at great depths provided the heavy underground structure to be studied is large enough to affect the tick rates of clocks in a measurable manner.
The smallest structure that atomic clocks accurate to 0.4 inches in geoid height can be determined is a buried sphere with a radius of about 0.6 miles buried at 1.2 miles below the surface, provided it has a density contrast of about 20 percent with the surrounding upper crust.
However, scientists believe that the same clocks could be sensitive to a buried sphere with a radius of about 2.5 miles at a depth of about 18.6 miles for the same density contrast.
The most precise atomic clocks currently only work in labs, and are not able to be transported. However, various research institutes are working on the development of portable versions of these "ultraprecise" atomic clocks.
"By 2022 at the earliest, one such ultraprecise portable atomic clock will fly into Space on board an ESA satellite," Professor Philippe Jetzer, the Swiss delegate for the STE-Quest satellite mission aimed at testing the general relativity theory very precisely, said in a statement.