Combining Ground, Marine and Airborne Gravity Data into the 2019 Australian National Gravity Grids

Abstract:

In 2019, two sets of Free Air Anomaly (FAA), Complete Bouguer Anomaly (CBA), and De-trended Global Isostatic Residual (DGIR) gravity grids were produced for the Australian region; (1) the “A” Series of grids based on ground and marine satellite-derived gravity data, and (2) the “B” Series of grids which also included airborne gravity survey data. The A Series FAA and CBA grids, produced using a standard procedures, were used as the base grids for their B Series counterparts. These base grids were upward continued to a smooth drape surface, AusDrape2019, that was defined across the whole region at the general terrain clearance of the majority of the airborne surveys acquired to date. The upward continued FAA and CBA data were then low pass filtered to better match the airborne survey data. The data from airborne surveys were added one by one to the base grids, using point located FAA data as the input. These data were gridded and vertically continued to AusDrape2019. Spherical cap Bouguer corrections and terrain adjustments appropriate for AusDrape2019 terrain clearances were calculated for the airborne survey locations. These corrections were then filtered to match the airborne survey FAA data, and used to calculate airborne survey CBA data. From this point, the process to combine the airborne data with the relevant base grid was the same for the FAA and CBA data. A conforming process was applied first, and the resultant grid was then merged with the base grid using a weighted combination of the two grids in a buffer zone around the margins of each airborne survey. Finally, the DGIR grid was created by subtracting 3 contributions from the CBA grid; an isostatic regional response out to 166.7 km, a global isostatic and terrain response from 166.7 km to 180°, and a first order trend surface. The results have provided an unprecedented view of the gravity field of the region, one that will have revolutionary effects on our understanding of the geology.