Gravimeter (or Gravity Meter, Gravitometer) is an instrument used in gravimetry for measuring the local gravitational field of the Earth. A 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. Gravimeters are used for petroleum and mineral prospecting, seismology, geodesy, geophysical surveys and other geophysical research, and for metrology.

The principle of design of Gravimeter 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, instead of ordinary units of acceleration.

Types of Gravimeters

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 must be calibrated at a location where the gravity is known accurately, and then transported to the location where the gravity is to be measured. They measure the ratio of the gravity at the two points.

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 extraordinary sensitivities of one nanogal, one thousandth of one billionth of the Earth surface gravity.

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.