WHAT IS A STANDARD GRAVITY?

Abstract

The Scandinavian region is uplifting and the Indian region is a stable area so that a negative gravity anomaly can be found in the former region and no systematic anomaly can be found in the latter. Against these expectations there is no systematic negative anomaly in the Scandinavian region and a negative anomaly in India if the anomalies are calculated on the basis of a normal gravity formula including only (n=2 and 4, m=0) terms in the spherical harmonic expansion. It may be, however, that processes responsible for the Scandinavian uplift and Indian geology occur in shallower depths (probably in the B layer, asthenosphere) and that mass anomaly responsible for the anomaly of rather long wavelengths (or smaller wave numbers n and m) exists deep (probably in the C and D layers, mesosphere) within the earth. If such is the case, in order to get gravity anomaly closely related to geology, we must take as the standard gravity not only (n=2 and 4, m=0) terms but also higher order terms in the spherical harmonic expansion. In short we must choose the standard gravity formula so as to get gravity anomaly which is agreeable with other geophysical and geological observations.Studies are made on the basis of the above point of view. Gaposchkin and Lambeck's data on the spherical harmonic expansion of geoidal undulations are used. It is our tentative conclusion that by including terms up to (n=8, m=8) we get anomalies which are more agreeable with geology than the normal gravity formula.