What is a gravity anomaly?

It is the difference between gravity measured at a point and the value expected for a smooth reference Earth after standard corrections; positive or negative anomalies indicate denser or lighter material below, making gravity a tool for probing the subsurface.

Why is the Earth's shape described by a geoid rather than a sphere?

Because the Earth's mass is unevenly distributed and it rotates, its gravity field is irregular; the geoid is the gentle, lumpy equipotential surface that follows mean sea level, providing a physically meaningful reference for heights that a simple sphere cannot.

Gravity and Geodesy

Gravity and geodesy measure the Earth's gravity field, shape, and size, using variations in gravity to probe density structure beneath the surface and to define the geoid against which heights and sea level are referenced.

Definition

Gravity and geodesy together comprise the study and measurement of the Earth's gravity field and geometric shape, using gravimetry, leveling, and space techniques to determine the geoid, reference frames, and the subsurface density variations recorded in gravity anomalies.

Scope

This area covers the Earth's gravity field and its geodetic determination: potential-field theory and the reduction of gravity measurements into anomalies, the geoid and the figure of the Earth as a reference ellipsoid plus undulations, and the principle of isostasy that links topography to compensating density structure. It treats satellite and space geodesy, including precise positioning and dedicated gravity missions, and the interpretation of gravity anomalies for crustal and mantle structure. The emphasis is on the relationship between the gravity field, the Earth's shape, and its internal density distribution.

Sub-topics

Core questions

How are gravity measurements reduced into anomalies that reveal subsurface density?
What is the geoid, and how does it define the figure of the Earth?
How does isostasy relate surface topography to compensating mass at depth?
How do satellite missions and space geodesy measure the gravity field and positions?

Key concepts

Gravitational potential and gravity anomalies
Free-air and Bouguer corrections
The geoid and reference ellipsoid
Isostasy and crustal compensation
Satellite gravimetry and space geodesy

Key theories

Isostasy: Surface topography is largely compensated at depth so that columns of the crust and mantle exert nearly equal pressure at a depth of compensation; the Airy and Pratt models explain mountains by deep crustal roots or by lateral density differences.
The geoid and physical geodesy: The geoid, an equipotential surface of the gravity field approximating mean sea level, defines the figure of the Earth and the datum for heights; physical geodesy relates measured gravity to the geoid through potential theory and boundary-value problems.

Clinical relevance

Gravity and geodesy provide the reference frames and height systems underlying mapping, navigation, and engineering; gravity anomalies guide mineral and petroleum exploration, and satellite gravimetry tracks ice-sheet loss, groundwater storage, and sea-level change.

History

Newton predicted the Earth's flattening, eighteenth-century arc-measurement expeditions confirmed it, Bouguer and Airy developed gravity reductions and isostasy in the eighteenth and nineteenth centuries, and the satellite era, culminating in dedicated missions such as GRACE and GOCE, transformed gravity-field and geoid determination.

Key figures

Isaac Newton
Pierre Bouguer
George Biddell Airy
Friedrich Robert Helmert

Seminal works

fowler2005
hofmannwellenhof2006
turcotte2014

Frequently asked questions

What is a gravity anomaly?: It is the difference between gravity measured at a point and the value expected for a smooth reference Earth after standard corrections; positive or negative anomalies indicate denser or lighter material below, making gravity a tool for probing the subsurface.
Why is the Earth's shape described by a geoid rather than a sphere?: Because the Earth's mass is unevenly distributed and it rotates, its gravity field is irregular; the geoid is the gentle, lumpy equipotential surface that follows mean sea level, providing a physically meaningful reference for heights that a simple sphere cannot.