What’s a Gradiometer?

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A gradiometer measures the rate of change that occurs in a known quantity, which can involve anything from temperature and pressure to a magnetic or gravimetric field. Gradiometers have numerous and widespread applications in science. They are used in everything from archeology to mapping the land surface and climate.
A gravitational gradiometer can be used to measure the density of subsurface earth layers for oil and mineral exploration. Miniaturized versions are being developed to detect subsurface oceans, such as Saturn’s moon Enceladus might have. Radio gradiometers have been installed on unmanned aerial vehicles (UAVs) that the US military uses to detect improvised explosive device (IED) lead wires under roads in Iraq, and are also employed to detect underground tunnels through Mexico to the US border that drug traffickers use. Since a gradiometer is also a type of inclinometer, it can also be used to measure angles to the horizon for construction and surveying equipment, aircraft flight paths, and cross-country sport cyclists.

Gravity gradiometry has various levels of sophistication for measuring different axes of acceleration, which depends on how many independent measuring units gradiometer or accelerometer are built into a device. All gradiometers, however, take the data produced and compare it to a standard quantity to determine the rate of change or steepness of the existing gradient. Gravity gradiometer technology is already being used in space in the Gravity Field and Steady State Ocean Circulation Explorer (GOCE), which was launched by the European Space Agency (ESA) into low Earth orbit in 2009.

The GOCE craft orbits in the outer atmosphere at an altitude of 162 miles (260 kilometers) to increase the resolution of onboard gradiometers, where it studies the behavior of ocean currents and volcanic activity. As of 2009, researchers at the University of Twente in the Netherlands are designing a miniature version of the gradiometer based on similar principles, which would weigh just 35 ounces (one kilogram) and could be added to space probes sent to explore the solar system . Two spring-loaded masses suspended by springs would measure comparable changes in gravitational pull at the scale of the picometer, or one-trillionth of a metre. These gradiometers could resolve lunar features of subsurface 124 miles (200 kilometers) in diameter or less.

Radio wave gradiometers, originally used in mining as portable units, were adapted in 2004 to fly UAV aircraft approximately 200 feet (61 meters) above the ground. They transmit a radio wave and detect reflections of the wave back that are altered by the presence of subsurface metal conductors or hollow structures. The original radio wave is filtered out as a kind of noise by the detectors, which makes it possible to see the much fainter variations of the wave due to gradient differences in the subsurface. The US government continued to sponsor the use and development of such radiogradometric systems with ongoing field tests beginning in 2007 and 2008.

Another type of gradiometer is the magnetic gradiometer used in archeology and related fields. It demonstrates the ability to be unaffected by fluctuations in the Earth’s magnetic field caused by magnetic storms and is used to locate very small anomalies near the surface that could indicate fossils or other deposits of ancient civilizations. The fluxgate gradiometer and cesium vapor sensor are used together to measure the magnetic field that the Earth imparts to buried walls, the remains of burnt objects, and so on over time. These readings are then compared to the Earth’s background magnetic field to pinpoint archaeological features at shallow depths.

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