Posted on 01/16/2017 12:22:48 PM PST by doug from upland
Tiny gravity sensor could detect drug tunnels, mineral deposits
By Sid PerkinsMar. 30, 2016 , 1:00 PM
A new device the size of a postage stamp can detect 1-part-per-billion changes in Earths gravitational fieldequivalent 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.
Gravitys 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 Earths core, which varies according to altitude, also affects the magnitude of our planets 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 largerall of which severely limits their uses, says Giles Hammond, a physicist at the University of Glasgow in the United Kingdom. Although portable, current devicessome of which weigh as much as 150 kilogramscant easily fit in many places scientists would like to use them or be readily carried to remote locations or mounted on small drones. Sign up for our daily newsletter
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So Hammond and his colleagues set out to build a smaller, cheaper spring-based gravimeter. The heart of their device is a postage stampsized bit of silicon; its carved so that in its center theres 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 changessuch 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 rocksthe tiny bit of silicon bobs up and down in response to that change, Hammond says. Those movements are tracked by monitoring the silicons shadow as it moves across a light detector.
The teams gravimeter is so sensitive it can track the up-and-down motions of Earths 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, Hammonds team has proven the devices 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 todays 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, Coloradobased company that manufactures a variety of gravimeters. Or, he notes, a string of the devicesespecially ones that had parts-per-billion accuracy and could withstand high temperatures and pressurescould 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 Earths 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. Weve just been waiting for the technology to catch up with our ideas.
“I imagine they could used for navigation, too once accurate gravity maps are established.”
A replacement for when the magnetic poles go berserk?
Way beyond me.
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I spent several years working on development of micromachined silicon microsensors -- including suspended-mass motion sensors like these.
Believe me, the sensors discussed in this article would be far more effective seismometers than they are gravity sensors. The problem is in nulling out seismic or other vibration signals in order to detect micro-gravity variations...
Perhaps one approach might be to encapsulate them in a controlled-viscosity damping fluid -- perhaps a non-Newtonian one... Even so, temperature effects on fluid viscosity might have to be controlled or compensated...
A seismograph is sensitive enough for Moore...
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