Geophysical implications of satellite determinations of the earth's gravitational field

The variations of the gravitational field have been determined from satellite orbit perturbations with fair reliability up to at least spherical harmonic degree 8.The largest departure from hydrostatic equilibrium by a factor of 2 is the oblateness, which, together with the observed rate of deceleration of the earth's rotation, leads to an estimate of about 10²⁶ poises for the viscosity of the lower mantle.The remaining variations in the field are too large to be associated solely with the crust; their origin must be mainly in the mantle. The positive correlation with topography for degrees l 6 and the rate of decrease of the variations (proportionate to l ^-2 in potential coefficients, or to l ^-1 in gravity anomaly power spectrum) are such that their source must in part be in the upper mantle, less than 400 km deep. However, the lack of obvious correlation with other indicators of upper-mantle activity such as sea-floor spreading and heat flow suggests that the density variations are the consequence of relatively small imbalances between dynamic disturbing effects and compensating restorative effects. At least part of the variations, particularly for degrees l 5, probably have their source in the stiff lower mantle.Publication No. 634, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif., U.S.A., 90024.