Gravimetre

A gravimeter, or gravimètre in French, is a scientific instrument designed to measure the local gravitational acceleration, denoted g, or spatial variations in the Earth's gravity field. Gravity is approximately 9.8 m/s^2 near the surface, with the standard gravity defined as 9.80665 m/s^2, and it varies with location, altitude, and time.

There are two broad classes of gravimeters: absolute and relative. Absolute gravimeters measure g directly by observing the motion of a free-falling test mass; common implementations use laser interferometry to track displacement or corner-cube reflectors whose position is measured with light. Relative gravimeters compare gravity against a stable reference and include spring-based devices, quantum or quartz-based sensors, and portable variants.

Principles and operation vary by type. Absolute devices deduce g from the measured acceleration of a reference mass while correcting for air resistance and rotation. Relative devices detect changes in gravity by monitoring the deflection of a suspended mass, the frequency of a resonant oscillator, or a continuous signal’s phase.

Applications span geodesy, geophysics, and environmental monitoring. Gravimeters map gravity anomalies, monitor tectonic and volcanic processes, track groundwater or magma movement, and calibrate regional gravity fields. They contribute to studies of sea level change and crustal density contrasts, and aid mineral prospecting. Modern approaches include atom interferometry, which uses interference of ultra-cold atoms to measure g with high precision, and superconducting gravimeters that provide long-term stability.

Historical development includes 19th-century pendulum- and spring-based instruments, with 20th-century advances in free-fall optical methods and superconducting technologies. Accurate gravimetry requires careful calibration and correction for tidal effects, atmospheric pressure, and ocean loading.