A gradiometer measures the rate of change that occurs in a known quantity, which can involve anything from temperature to pressure to a magnetic or gravimetric field. Gradiometers have numerous, widespread applications in science. They are used in everything from archeology to the mapping of the Earth's surface and climate.
A gravity gradiometer can be used to measure the density of layers of earth below the surface for petroleum and mineral exploration. Miniaturized versions of them are being developed to detect subsurface oceans, such as Saturn's moon Enceladus may have. Radio gradiometers have been put on unmanned aerial vehicles (UAVs) that the US military uses to detect the conducting wires of improvised explosive devices (IEDs) under roadways in Iraq, and they are also employed to detect tunnels underground across the Mexico-to-US border that drug smugglers use. Since a gradiometer is also a type of inclinometer, they can also be used to measure angles relative to the horizon for construction and surveying equipment, aircraft flight paths, and cross-country sport bicyclists.
Gravity gradiometry has varying levels of sophistication to measure different axes of acceleration, which depends on how many independent measuring gradiometer or accelerometer units are incorporated into a device. All gradiometers, however, take the data produced and compare it to a standard quantity to determine the rate of change or gradient slope that exists. Gravity gradiometer technology is already in use 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 a height of 162 miles (260 kilometers) to increase the resolution of the gradiometers on board, 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 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 variations in gravitational pull to the picometer scale, or one-trillionth of a meter. These gradiometers could resolve subsurface moon features 124 miles (200 kilometers) in diameter or smaller.
Radio wave gradiometers, originally used in the mining industry as handheld units, were adapted in 2004 to fly on UAV aircraft around 200 feet (61 meters) above the ground. They broadcast a radio wave and detect reflections of the wave back that are altered by the presence of metal conductors under the surface or hollow structures. The original radio wave is filtered out as a sort of noise by the detectors, which makes it possible to see the much weaker variations in the wave due to gradient differences underground. The US government has continued to sponsor the use and development of such radio gradiometer systems with ongoing field tests as of 2007 and 2008.
Another type of gradiometer is the magnetic gradiometer used in archeology and related fields. It demonstrates an ability to not be affected by fluctuations in the Earth's magnetic field caused by magnetic storms, and is used to locate very small anomalies near the surface which might indicate fossils or other deposits from ancient civilizations. The fluxgate gradiometer and cesium vapor sensor designs are used together to measure the magnetic field that the Earth imparts to buried walls, fired remains of objects, and so on over time. These readings are then compared to the Earth's background magnetic field to locate archeological features at shallow depths.