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IT COULD soon become much easier to detect underground bunkers or sinkholes,
say researchers in Massachusetts. The trick is to measure variations in the
gravitational field.

For decades, variations in gravity have been used to ferret out underground
structures. The gravitational force should be slightly weaker above a
hollowed-out region, so people look for places where gravity is weakest. But
this 鈥済ravimetry鈥� method often fails鈥攚hen the vast mass of a mountain,
say, overwhelms the weak change near a bunker. Now there鈥檚 a way around this,
says James Battis of the Air Force Research Lab (Journal of Physics D,
vol 34, p 433). Rather than measuring the absolute strength of gravity, he looks
at the gravity gradient鈥攖he rate at which the strength of the field
changes as the observer moves from one spot to the next.

Using a gravity gradiometer, the AFRL team successfully detected an
underground missile complex at Vandenberg Air Force Base in California. Their
results were far more accurate than those obtained with a simple gravimeter.
鈥淲e鈥檝e demonstrated gravity gradiometry can detect underground structures,鈥�
Battis says. 鈥淣ow someone has to decide what to do with it.鈥�