Gravimeter

An Autograv CG-5 gravimeter being operated

For broader coverage of this topic, see Gravimetry § How gravity is measured.

A gravimeter is an instrument used in gravimetry for measuring the local gravitational field of the Earth. A vertical absolute gravimeter is a type of accelerometer, specialized for measuring the constant downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth. Though the essential principle of design is the same as in other accelerometers, gravimeters are typically designed to be much more sensitive in order to measure very tiny fractional changes within the Earth's gravity of 1 g, caused by nearby geologic structures or the shape of the Earth and by temporal tidal variations. This sensitivity means that gravimeters are susceptible to extraneous vibrations including noise that tend to cause oscillatory accelerations. In practice this is counteracted by integral vibration isolation and signal processing. The constraints on temporal resolution are usually less for gravimeters, so that resolution can be increased by processing the output with a longer time constant. Gravimeters display their measurements in units of gals (cm/s²), nanometers per second squared, and parts per million or parts per billion of the average vertical acceleration with respect to the earth.

Gravimeters are used for petroleum and mineral prospecting, seismology, geodesy, geophysical surveys and other geophysical research, and for metrology. Their fundamental purpose is to map

There are two types of gravimeters: relative and absolute. "Absolute" gravimeters measure the local gravity in absolute units, gals. Relative gravimeters compare the value of gravity at one point with another. They can be calibrated at a location where the gravity is known accurately, and then transported to the location where the gravity is to be measured. Or they can calibrated in absolute units at their operating location. They compare gravity at two or more locations. There are many methods for displaying acceleration fields, also called "gravity fields". This incudes traditional 2D maps, but increasingly 3D video. Since gravity and acceleration are the same, "acceleration field" might be preferrable, since "gravity" is an oft misused prefix.

Absolute gravimeters

Absolute gravimeters, which nowadays are made compact so they too can be used in the field, work by directly measuring the acceleration of a mass during free fall in a vacuum, when the accelerometer is rigidly attached to the ground.

The mass includes a retroreflector and terminates one arm of a Michelson interferometer. By counting and timing the interference fringes, the acceleration of the mass can be measured. A more recent development is a "rise and fall" version that tosses the mass upward and measures both upward and downward motion. This allows cancellation of some measurement errors, however "rise and fall" gravimeters are not in common use. Absolute gravimeters are used in the calibration of relative gravimeters, surveying for gravity anomalies (voids), and for establishing the vertical control network.

Atom interferometric and atomic fountain methods are used for precise measurement of the earth's gravity. And atomic clocks and purpose-built instruments can use time dilation (also called general relativistic) measurements to track changes in the gravitational potential and gravitational accelation on the earth.

"Absolute gravimeter" might better be labeled "gravimeters calibrated to international standards", and reserve the term "earth vertical field gravimeters" for high quality vertical only instruments. The term "absolute" does not convey the instrument's stability, sensitivity, accuracy, ease of use, and bandwidth. So it and "relative" should not be used when more specific characteristics can be given.

Relative gravimeters

Most common relative gravimeters are spring-based. They are used in gravity surveys over large areas for establishing the figure of the geoid over those areas. A spring-based relative gravimeter is basically a weight on a spring, and by measuring the amount by which the weight stretches the spring, local gravity can be measured. However, the strength of the spring must be calibrated by placing the instrument in a location with a known gravitational acceleration.

The most accurate relative gravimeters are superconducting gravimeters, which operate by suspending a liquid helium cooled diamagnetic 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 field. The superconducting gravimeter achieves sensitivities of 10⁻¹¹ms⁻² (one nanogal), approximately one trillionth (10⁻¹²) of the Earth surface gravity. In a demonstration of the sensitivity of the superconducting gravimeter, Virtanen (2006), 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 the superconducting gravimeter is the tidal gravity of the sun and moon acting at the station. This is roughly +/- 1000 nm/s2 (nanometers per second squared at most locations). The "SGs", as they are called, can detect and characterize earth tides, changes in the density of the atmosphere, the effect of changes in the shape of the surface of the ocean, the effect of the atmospheres pressure on the earth, changes in the rate of rotation of the earth, oscillations of the earth's core, distant and nearby seismic events, and more.

Many broadband, three axis, seismometers in common use are sensitive enough to track the sun and moon. When operated to report acceleration, they are properly rather useful gravimeters.

Recently, the SGs, and broadband three axis seismometers operated in gravimeter mode, have begun to detect and characterize the small gravity signals from earthquakes. These signals arrive at the gravimeter at the speed of gravity, so have the potential to improve earthquake early warning methods. (As a side note, the GW170817 gravitational wave event showed that the speed of light and the speed of gravity are identical for most practical applications.) There is some activity to design purpose-built gravimeters of sufficient sensitivity and bandwidth to detect these prompt gravity signals from earthquakes. Not just the magnitude 7+ events, but also the smaller, much more frequent, events.

Newer MEMS gravimeters, atom gravimeters - including atom chip gravimeters offer the potential for low cost arrays of sensors. 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. The change from calling a device an "accelerometer" to calling it a "gravimeter" occurs at approximately the point where it has to make corrections for earth tides.

The satellite borne gravimeters GOCE, GRACE, are mostly operating in gravity gradiometer mode. They yield detailed information about the earth's time varying gravity field The spherical harmonic gravitational potential models are slowly improving in both spatial and temporal resolution. Taking the gradient of the potentials gives estimate of local acceleration which are what is measured by the gravimeter arrays. The superconducting gravimeter network has been used to ground truth the satellite potentials. This should eventually improve both the satellite and earth-based methods and intercomparisons.

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 1950-60s 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.

See also

• Geophysical survey
• GOCE A modern satellite-bourne gradiometer containing pairs of gravimeters (accelerometers)
• Gravity gradiometry
• Elastogravity