Gravimetry
It varies by about ±1000 nm/s2 (nanometers per second squared) at any location because of the changing positions of the Sun and Moon relative to the Earth.
The majority of modern gravimeters use specially designed metal or quartz zero-length springs to support the test mass.
The special property of these springs is that the natural resonant period of oscillation of the spring–mass system can be made very long – approaching a thousand seconds.
A test mass is allowed to fall freely inside a vacuum chamber and its position is measured with a laser interferometer and timed with an atomic clock.
Such instruments are capable of an accuracy of about 2 ppb or 0.002 mGal[1] and reference their measurement to atomic standards of length and time.
Their primary use is for calibrating relative instruments, monitoring crustal deformation, and in geophysical studies requiring high accuracy and stability.
Several types of gravimeters exist for making these measurements, including some that are essentially refined versions of the spring scale described above.
Most current work is Earth-based, with a few satellites around Earth, but gravimeters are also applicable to the Moon, Sun, planets, asteroids, stars, galaxies and other bodies.
A common type measures the acceleration of small masses free falling in a vacuum, when the accelerometer is firmly attached to the ground.
[6] The current standard for sensitive gravimeters are the superconducting gravimeters, which operate by suspending a superconducting niobium sphere in an extremely stable magnetic field; the current required to generate the magnetic field that suspends the niobium sphere is proportional to the strength of the Earth's gravitational acceleration.
In a demonstration of the sensitivity of the superconducting gravimeter, Virtanen (2006),[8] describes how an instrument at Metsähovi, Finland, detected the gradual increase in surface gravity as workmen cleared snow from its laboratory roof.
The largest component of the signal recorded by a superconducting gravimeter is the tidal gravity of the Sun and Moon acting at the station.
Because they have three axes, it is possible to solve for their position and orientation, by either tracking the arrival time and pattern of seismic waves from earthquakes, or by referencing them to the Sun and Moon tidal gravity.
Recently, the SGs, and broadband three-axis seismometers operated in gravimeter mode, have begun to detect and characterize the small gravity signals from earthquakes.
There is some activity to design purpose-built gravimeters of sufficient sensitivity and bandwidth to detect these prompt gravity signals from earthquakes.
MEMS gravimeters are currently variations on spring type accelerometers where the motions of a tiny cantilever or mass are tracked to report acceleration.
Precise GPS stations can be operated as gravimeters since they are increasingly measuring three-axis positions over time, which, when differentiated twice, give an acceleration signal.
Transportable relative gravimeters also exist; they employ an extremely stable inertial platform to compensate for the masking effects of motion and vibration, a difficult engineering feat.
The first transportable relative gravimeters were, reportedly, a secret military technology developed in the 1950–1960s as a navigational aid for nuclear submarines.
Subsequently in the 1980s, transportable relative gravimeters were reverse engineered by the civilian sector for use on ship, then in air and finally satellite-borne gravity surveys.
Microgravity investigations are carried out in order to solve various problems of engineering geology, mainly location of voids and their monitoring.
Currently, the static and time-variable Earth's gravity field parameters are determined using modern satellite missions, such as GOCE, CHAMP, Swarm, GRACE and GRACE-FO.
[13][14] The lowest-degree parameters, including the Earth's oblateness and geocenter motion are best determined from satellite laser ranging.
