DISTRIBUTION OF LATERAL DENSITY ANOMALIES IN THE LOWER MANTLE OBTAINED THROUGH LOW ORDER GEOPOTENTIAL COEFFICIENTS

In this paper, we attempt to use satellite gravity data and a new inversion method to study the lateral density anomalies distribution in the mantle.First, the density difference p(r, , (?)) is expanded in terms of a three-dimensional orthogonal function system, the coefficients of the expansion are to be determined. From the relationship between density anomalies and disturbing geopotential, then, a set of observation equations is established, in which the unknown vector contains the coefficients of density anomaly expansion, and the observational vector is obtained by computing geopotential perturbations using the potential coefficients of GEM10B. By suitably selecting the harmonic orders of geopotential, a filtering process is conducted for the observational values. Finally, the lateral density variations in the lower mantle (670 km to the core-mentle boundary) are investigated. In this case, the orders of disturbing geopotential are selected as l = 2-11, the truncated orders of density anomaly expansion are taken as L = 6 and K - 4, and the damping least squares method is used to solve the observation equations.The resulting model shows the high level of density perturbations at the 670 km discontinuity and core-mantle boundary, a high-density zone circumscribing the Pacific and a lower-density region under the center of the Pacific. These features are in agreement with the features of three-dimensional seismic velocity variation obtained by Dziewonski (1984). In the Antarctic region and some parts of the Atlantic and Indian Oceans, however, the resulting density anomalies are negatively correlated with the seismic velocity anomalies. The cause resulting in these phenomena is preliminarily analysed in this paper.