2019 Isostatic residual grids of Australia using elevation derived and seismic Moho grids

The Geoscience Australia 2016 and 2019 Australian National Gravity Grid Projects produced Isostatic Regional grids as part of the comprehensive data reduction process. The grids, including a de-trended global isostatic residual (DGIR) can be found on the Geoscience Australia's electronic catalogue - eCat, with the latest version at: Geophysical Acquisition & Processing Section 2020, National Gravity Compilation 2019 DGIR grid, Geoscience Australia, Canberra. http://pid.geoscience.gov.au/ dataset/ga/144778. On behalf of Geoscience Australia, third party provider Intrepid Geophysics has produced an additional set of isostatic residual gravity anomaly grids of Australia. This follows a collaborative effort between Richard Lane (Geoscience Australia) and Intrepid to further develop Intrepid’s Isostatic Correction Tool. Starting with the 2019 GA national Complete Bouguer Anomaly (CBA) grid, Intrepid undertook the project with the primary objectives of: 1. Generating a regional isostatic residual gravity anomaly grid utilising new elevation models and seismic derived Moho grids, and; 2. To produce a new benchmark isostatic residual gravity anomaly grid and other derivative and supplementary products. Intrepid's modelling utilised the Airy Isostatic compensation method described as follows: "The Airy model of isostatic compensation operates on the assumption that all vertical columns from the surface to a given depth have an equal mass. The vertical columns consist of a low-density crustal section and a high-density mantle section. The interface between the crust and mantle is known as the Mohorovicic Discontinuity or Moho. Topographic highs are supported by an extended crust whereas ocean basins have a reduced crustal depth. Using a digital terrain model, it is a relatively straightforward process to calculate the depth to the Moho and the gravitational effect of the vertical columns". Methodology employed for the derivation of the digital elevation model and the depth to Moho are supplied in Intrepid's report. Both an Airy-derived and seismic-derived Moho depth have been calculated to produce two flow streams for regional isostatic products. These are tested against Geoscience Australia's 2016 and 2019 isostatic models. Comparisons of the various derived products show good similarities between GA's isostatic model and Intrepid's newly derived bathymetric and topographically corrected isostatic model, with differences typically less than a few um/s2. Larger differences were observed for the seismic-derived Moho depth-based model and despite a number of density tests, the approach appears to be sub-optimal. Further analyses are required, including the implementation of variable densities to calculate the mass between surface to Moho (see Intrepid’s report for references).