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

Brief description

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).

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Source lineage datasets:1. 2019 National gravity compilation. Geophysical Acquisition & Processing Section 2020. National Gravity Compilation 2019 includes airborne (CSCBA). Geoscience Australia, Canberra. https://pid.geoscience.gov.au/dataset/ga/144783. 2. 2019 National gravity compilation and calculation of the isostatic residual (Geophysical Acquisition & Processing Section 2020. National Gravity Compilation 2019 includes airborne DGIR grid. Geoscience Australia, Canberra. https://pid.geoscience.gov.au/dataset/ga/144775.3. 2016 National gravity compilation and calculation of the isostatic residual. 4. The Australian National Gravity Database (Murray, A.S. 1997. The Australian National Gravity Database. Record 17:1:145-155. Geoscience Australia, Canberra. https://pid.geoscience.gov.au/dataset/ga/81486).As part of the 2019 Australian National Gravity Grid Project, Richard Lane of Geoscience Australia produced several Isostatic Regional grids using different methods from different software packages. Following the recent work to incorporate Richard Lane’s request for features within the Intrepid. Isostatic tool, Intrepid Geophysics has produced a set of new isostatic residual gravity anomaly grids of Australia utilising the new features. The starting point for Intrepid's work has been the national gravity compilation of 2019. This integrates terrestrial gravity with airborne, marine and satellite; the highest resolution of which is used for the final compilation. Starting with free-air and Bouguer reductions, the compilation includes all intermediary grids leading to the fully integrated coverage with isostatic correction. Intrepid's work replicates Richard's completed reduction to an isostatic corrected grid but via a number of pathways as explained in the accompanying technical report.

Notes

Purpose
Output (Intrepid) products include the following: (1) Grids derived from the bathymetry and topography correction process: Depth to Moho - Isostatic Residual Flat Earth - Isostatic Residual Truncated (to the coastline) - Isostatic Correction Flat Earth Model - Isostatic Correction Flat Earth Model Truncated (to the coastline)(2) Grids derived from the 'Ausmoho' depth correction process: Depth to Moho - Isostatic Residual (truncated to the coastline) - Isostatic Correction (truncated to the coastline)(3) Also included for comparative purposes is GA's and Richard Lane's Isostatic Correction using a Flat Earth model. The products are delivered for and on behalf of Geoscience Australia as an independent test of internally generated national isostatic models. They are provided to interested parties under contract copyright: CC BY 4.0 as additional products that may assist in continental research, regional mapping and pre-competitive resources analyses without any guarantee of the modelling integrity, fundamentals employed or spatial accuracy of features. Any questions pertaining to the methodologies described or presented should be taken up directly with Intrepid Geophysics.