Interpretation of Global Gravity Anomalies

Abstract

I recently published¹ an interpretation of global gravity anomalies up to eighth degree and order spherical harmonic terms based on varying depth to the olivine-spinel phase transition of the mantle transition zone caused by lateral temperature variation at this depth. Higbie and Stacey² have also now presented similar work using higher harmonics which were not available to me two years ago. They state (page 131) that I “assumed that the form of the surface at depth would be the same as that of the surface geoid instead of accentuating the higher harmonics”. This statement is incorrect as should be clear from the legend of Fig. 3 of my article. I computed the true equivalent layer at 410 km depth in exactly the same way as one assumes that it was done by Higbie and Stacey. They seem to have overlooked the fact that I plotted the gravity anomaly and not the geoid (or potential) in Fig. 1 of my paper. The gravity anomaly accentuates the higher harmonics in relation to the geoid as the corresponding coefficients are connected by a factor of (n-1) where n is the degree of the harmonic.