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Tiny gravity sensor could detect drug tunnels, mineral deposits

A postage stamp–sized device (on platform) is the heart of a new gadget that can measure minuscule changes in Earth’s gravitational field.

Giles Hammond

Tiny gravity sensor could detect drug tunnels, mineral deposits

A new device the size of a postage stamp can detect 1-part-per-billion changes in Earth’s gravitational field—equivalent to what the gizmo would experience if it were lifted a mere 3 millimeters. The technology may become so cheap and portable it could one day be mounted on drones to spot everything from hidden drug tunnels to valuable mineral deposits.

Gravity’s force is nearly the same everywhere on Earth. But there can be minute fluctuations, based on the density of the rock or other material below. Distance from Earth’s core, which varies according to altitude, also affects the magnitude of our planet’s gravitational attraction.

Most devices that measure these gravitational differences, called gravimeters, are based on two principles: They either measure the time it takes an object to fall a certain distance, or they measure the distance that a certain weight stretches a spring. (The stronger the force of gravity, the faster an object will fall, and the farther it will stretch a mass hanging by a spring.) In either case, state-of-the-art gravimeters cost more than $100,000 and are the size and weight of a car battery or larger—all of which severely limits their uses, says Giles Hammond, a physicist at the University of Glasgow in the United Kingdom. Although portable, current devices—some of which weigh as much as 150 kilograms—can’t easily fit in many places scientists would like to use them or be readily carried to remote locations or mounted on small drones.

So Hammond and his colleagues set out to build a smaller, cheaper spring-based gravimeter. The heart of their device is a postage stamp–sized bit of silicon; it’s carved so that in its center there’s a 25-milligram bit of material left suspended by three stiff, fiberlike structures that are each about 5 micrometers across (less than one-third the diameter of the finest human hair). Together, these act as the spring. As the gravitational field surrounding the device changes—such as it would if it passed over a large underground cavern or a dense deposit of minerals, because of the sudden change of density in the underlying rocks—the tiny bit of silicon bobs up and down in response to that change,  Hammond says. Those movements are tracked by monitoring the silicon’s shadow as it moves across a light detector.

The team’s gravimeter is so sensitive it can track the up-and-down motions of Earth’s surface caused by the changing positions of the sun and moon, the researchers report online today in Nature. (These so-called “Earth tides” occur and are measurable, but they are much smaller than those seen in the seas because rock is stiffer than water.)

For now, Hammond’s team has proven the device’s worth in the lab. Doing so in the field will be challenging, says Hazel Rymer, a volcanologist at the Open University in Milton Keynes, U.K. But if successful, the availability of gravimeters that are cheaper and much more portable than today’s equipment “will be a game-changer,” she notes. Researchers could deploy networks of the tiny gravimeters to monitor the movements of magma within and underneath volcanoes, possibly discerning the magnitudes and patterns of flows in advance of an eruption, for example. Or they could mount them on drones and use them to search for underground voids that could eventually evolve into sinkholes, or for humanmade structures such as tunnels used to smuggle drugs.

They could also help prospect for mineral deposits that are denser than the surrounding rock, thus affecting the local gravitational field, says Tim Niebauer, a physicist and president of Micro-g LaCoste, a Lafayette, Colorado–based company that manufactures a variety of gravimeters. Or, he notes, a string of the devices—especially ones that had parts-per-billion accuracy and could withstand high temperatures and pressures—could be fed down a borehole to monitor widespread changes in the amount of water in an aquifer or petroleum in a surrounding oilfield, possibly yielding information about how quickly such reservoirs might run dry. Those sorts of data can be gathered at Earth’s surface now, he adds, but “the closer you are to the reservoir, the better the measurements can be.”

Many of the potential applications for such devices “have been science fiction for so long,” Rymer says. “We’ve just been waiting for the technology to catch up with our ideas.”