Preamble

Gravity observations are referred to an equipotential surface, termed the geoid, for which sea level is a close approximation. We can picture the geoidal surface in continental areas as following the water level in hypothetical narrow canals connected to the oceans. For a non-rotating planet in hydrostatic equilibrium the geoid would be a sphere but rotation deforms it to an oblate ellipsoid. For several reasons discussed in Section 6.4 the Earth is slightly more elliptical than equilibrium theory would suggest. One reason is the depression of polar regions by former glaciation, from which recovery is incomplete and the continuing rebound gives a clue to the viscosity of the mantle. There are heterogeneities at all levels, the most obvious of which are seen at the surface as continents and oceans, but the effect of the continent–ocean structure on the geoid is barely discernible and very much less than if the continents were superimposed on an otherwise uniform Earth. On a continental scale the surface features are very nearly in hydrostatic balance. This is the principle of isostasy.

Features of the gravity field on a scale larger than 1000 km are discerned more effectively by studying perturbations of satellite orbits than would be possible from surface observations. With progressive improvements, satellite techniques have been used to distinguish increasingly fine features, although for exploration of local anomalies surface observations on land must still be used, sometimes in combination with satellite data.