Secondary indirect effects in gravity anomaly data inversion or interpretation

The application of topographic corrections to gravity anomalies and disturbances, and their use in formulating and solving the gravimetric inverse problem are reinvestigated. The gravity anomaly, whose definition is based on the disturbing potential by means of the fundamental gravimetric equation, rather than by the vertical derivative of the disturbing potential, differs from the gravity disturbance, which also has implications to the application of the topographic correction. We demonstrate that the application of the topographic correction to the gravity anomaly gives origin to the secondary indirect topographic effect (SITE) and that the formulation of a rigorous relation between the attraction of anomalous subsurface mass density distribution and the gravity anomaly gives rise to a secondary indirect effect of the anomalous mass density distribution (SIEAM). The SITE is shown to be numerically significant in mountainous areas, where it can reach 100 mGal. Because of secondary indirect effects, the gravity anomaly in its rigorous sense is not well suited for the gravimetric inversion. Instead, the topo-corrected gravity disturbance best fits the needs of gravity data inversion or interpretation, as it exactly matches the attraction of the Earth's subsurface anomalous density distribution. It is pointed out that, in geophysics, the gravity data used for inversion or interpretation, although called the "Bouguer gravity anomaly," even if preceded by the adjective "ellipsoidal" as by the newly proposed standards for the North American database, are by the standards of rigor the "topographically corrected gravity disturbance."