Eötvös effect

A gravimeter carried by a moving vehicle measures acceleration in the vehicle’s frame, which differs from that of a stationary observer on Earth. Earth’s rotation already contributes a centrifugal acceleration to apparent gravity. When the vehicle moves east, its eastward speed adds to its rotation about the planet’s axis, increasing the outward centrifugal contribution and reducing apparent vertical gravity. Westward motion reduces that contribution and raises the measured value relative to eastward travel. The effect is strongest near the equator, where the east–west component of rotational speed is greatest, and decreases toward the poles. North–south motion has a smaller, distinct curvature-related contribution in full correction formulae.

The Eötvös correction adjusts gravity readings for the platform’s motion relative to Earth. Its computation uses the vehicle’s eastward and northward velocity components, latitude, and—in more exact treatments—height and Earth’s ellipsoidal shape. A ship or aircraft therefore needs reliable position, heading and speed data alongside its gravimeter readings. The correction can reach tens of milligals in moving-platform surveys, large enough to obscure geophysical gravity variations if neglected. Different published correction formulae make slightly different approximations; survey software may offer exact and approximate variants.

The effect is named for Hungarian physicist Loránd Eötvös, who recognized a directional difference in gravity readings taken from moving ships. Historical accounts describe measurements in which eastbound readings were lower and westbound readings higher, prompting correction for motion relative to Earth’s rotation. The effect became an important part of reducing marine and airborne gravimetric data as moving-platform gravity measurement developed.

Gravity surveys aboard ships and aircraft map variations associated with seafloor structure, geology and the distribution of mass. Because the measured signal is small and the carrier moves, processing must distinguish gravity from vehicle acceleration and other instrumental effects. Repeated survey lines in opposite directions can help calibrate moving-base gravimeters: the direction-dependent Eötvös signal provides a known change against which instrument response can be checked. The correction accounts for a predictable rotational effect; it does not remove unrelated platform motion or instrument errors.