milligal

The milligal, abbreviated mGal, is a unit of acceleration used in gravimetry and geophysics to express small variations in the Earth's gravitational field. One milligal is one thousandth of a gal, and a gal is defined as 1 centimeter per second squared. Therefore, 1 mGal equals 0.001 cm/s^2, or 1×10^-5 m/s^2. The magnitude of Earth's gravity at the surface is about 9.8 m/s^2, equivalent to roughly 980,000 mGal.

Milligals are used to measure gravity anomalies and vertical variations caused by subsurface density contrasts, topographic effects, ocean loading, and tides. They are commonly reported in gravity surveys, geodesy, and mineral exploration. Modern gravimeters, including absolute and relative instruments, can resolve signals at the milligal level, with data corrected for atmospheric pressure, tides, ocean loading, and instrumental drift.

The term derives from Galileo Galilei, with the base unit the gal (1 cm/s^2) and the prefix milli- indicating a factor of one thousandth. In practice, mGal is the standard unit for gravity data in field measurements and is widely used in geophysical interpretation and mapping of gravity anomalies.

See also: gal; gravimeter; gravity anomaly; geodesy; geophysics.