Gravity anomalies

Gravity is not constant over Earth’s surface. Seamounts on the ocean floor, dense oil deposits surrounded by lighter limestone or clay, tectonic features, and other local variations in elevation and mass distribution result in deviations in the local definition of “vertical,” as measured by a plumb line, and in the local gravitational acceleration.

To a first approximation, Earth can be modeled mathematically as an ellipsoid, and that surface forms the basis for mapping and coordinates. A better reference is the geoid, the surface describing what mean sea level would be in the absence of winds, currents, and tides; it also encompasses what mean sea level would be under the continents. The geoid is an equipotential surface, and the local gravity is everywhere perpendicular to it.

But the actual mean sea level, being subject to dynamic processes such as currents, is more complicated still and doesn’t exactly follow the geoid. So determining local gravity requires that the dynamic surface topography be separated from geoidal undulations. The map shown here does just that. Pulling together geoid models, ocean data from numerous satellites that have measured sea surface heights everywhere, and land data from the US National Geospatial-Intelligence Agency, Ole Andersen and colleagues at the Technical University of Denmark produced a global map, with 1° (2 km) resolution, of local gravity. The so-called free-air gravity anomalies—the deviations of the actual gravity from what it would be if Earth were perfectly ellipsoidal—are plotted here in milligals, or 0.001 cm²/s; 3 mGal corresponds to a 10-m difference in the height of the geoid with respect to a reference ellipsoid. The map improves on previous ones especially at short length scales and in polar and coastal regions.

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