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.
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,
Nature - Measurement of the Earth tides with a MEMS gravimeter
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.”
SOURCES - Science Mag, Nature
The ability to measure tiny variations in the local gravitational acceleration allows, besides other applications, the detection of hidden hydrocarbon reserves, magma build-up before volcanic eruptions, and subterranean tunnels. Several technologies are available that achieve the sensitivities required for such applications (tens of microgal per hertz^(1/2)): free-fall gravimeters, spring-based gravimeters superconducting gravimeters, and atom interferometers. All of these devices can observe the Earth tides: the elastic deformation of the Earth’s crust as a result of tidal forces. This is a universally predictable gravitational signal that requires both high sensitivity and high stability over timescales of several days to measure. All present gravimeters, however, have limitations of high cost (more than 100,000 US dollars) and high mass (more than 8 kilograms). Here we present a microelectromechanical system (MEMS) device with a sensitivity of 40 microgal per hertz^(1/2) only a few cubic centimetres in size. We use it to measure the Earth tides, revealing the long-term stability of our instrument compared to any other MEMS device. MEMS accelerometers—found in most smart phones7—can be mass-produced remarkably cheaply, but none are stable enough to be called a gravimeter. Our device has thus made the transition from accelerometer to gravimeter. The small size and low cost of this MEMS gravimeter suggests many applications in gravity mapping. For example, it could be mounted on a drone instead of low-flying aircraft for distributed land surveying and exploration, deployed to monitor volcanoes, or built into multi-pixel density-contrast imaging arrays.