The main part of this map is a Hue-Saturation-Intensity (HSI) image of De-trended Global Isostatic Residual Gravity data (DGIR) based on the B Series of the 2019 Australian National Gravity Grids (Lane et al., 2020a). This series of grids represent the combination of 1.4 million ground gravity observations stored in the Australian National Gravity Database (ANGD) as of September 2019; 345,000 line km of Airborne Gravity and 106,000 line km of gravity gradiometry data in the National Australian Geophysical Database (NAGD), and the Global Gravity Grid developed at Scripps Institution of Oceanography, University of California at San Diego using data from the United States SIO, NOAA and NGA (Sandwell et al., 2014; Scripps Institution of Oceanography, 2020). The ground and airborne gravity data have been acquired by the Commonwealth, State and Territory Governments, the mining and exploration industry, universities and research organisations from the 1940’s to the present day.

The shading of the image is from the northwest and the colour scale is linear from -500 µm.s-2 (blue) to +500 µm.s-2 (red). A comprehensive description of the input datasets and processing used to generate the grid is given in Lane et al. (2020b).

DGIR data were obtained by subtracting 3 quantities (i.e., the near-field isostatic correction, the far-field isostatic correction, and a first order trend correction) from Complete Bouguer Anomaly data (CBA).

The CBA values were obtained using the methodology given in Hinze et al. (2005). The horizontal and vertical datum was GDA94 (GRS80 ellipsoid) and the gravity datum was AAGD07 (Tracey et al., 2007).

The near-field isostatic response is the gravity response of a density contrast across an isostatic root surface for a flat Earth out to a radius of 166.7 km. The isostatic root surface was derived from the topographic and bathymetric dataset compiled by Whiteway (2009), supplemented by ETOPO1 data (Amante and Eakins, 2009). The calculations were performed with software based on the AIRYROOT program (Simpson et al., 1983; Simpson et al., 1986) which uses a one-dimensional Airy-Heiskanen model of isostatic balance (Airy, 1855; Heiskanen and Vening Meinesz, 1958). The density values for the topography and sea water were 2670 kg.m-3 and 1030 kg.m-3, respectively. A value of 37 km was used for the depth to the root surface at sea level whilst a value of 400 kg.m-3 was used for the density contrast across the root. These values are the same as those used in previous isostatic residual gravity products from Geoscience Australia (Nakamura et al., 2010).

The far-field isostatic response was the combined topographic adjustment-isostatic gravity response for a spherical Earth for a distance of 166.7 km from the observation point to 180° as published by Karki et al. (1961).

To assist with isolation of the anomalies due to sources in the mid- to upper crust, a strong southwest to northeast gradient was removed by fitting and applying a first order trend correction.

References:

Airy, G. B., 1855, On the computation of the effect of the attraction of mountain-masses, as disturbing the apparent astronomical latitude of stations in geodetic surveys: Phil. Trans. R. Soc., 145, 101-104, http://doi.org/10.1098/rstl.1855.0003

Amante, C., and B. W. Eakins, 2009. ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis: NOAA Technical Memorandum NESDIS NGDC-24, National Geophysical Data Center, NOAA, doi:10.7289/V5C8276M

Heiskanen, W. A., and F. A. Vening Meinesz, 1958, The Earth and its gravity field: McGraw Hill Book Co., Ltd., New York, 470 pp.

Karki, P., L. Kivioja, and W. A. Heiskanen, 1961, Topographic-Isostatic reduction maps for the world to the Hayford zones 18-1, Airy-Heiskanen system, T = 30 km: Isostatic Institute of the International Association of Geodesy, 35.

Lane, R. J. L., Wynne, P. E., Poudjom Djomani, Y. H., Stratford, W. R., Barretto, J. A., and Caratori Tontini, F., 2020a, 2019 Australian National Gravity Grids: Geoscience Australia, eCat Reference Number 133023, https://pid.geoscience.gov.au/dataset/ga/133023

Lane, R. J. L., Wynne, P. E., Poudjom Djomani, Y. H., Stratford, W. R., Barretto, J. A. and Caratori Tontini, F., 2020b, 2019 Australian national gravity grids explanatory notes: Record 2020/22, Geoscience Australia, Canberra, http://dx.doi.org/10.11636/Record.2020.022

Nakamura, A., Bacchin, M., and Tracey, R., 2010, Isostatic residual gravity anomaly grid of onshore Australia: Extended Abstracts, ASEG 21st Geophysical Conference, 2010, 1-4.

Sandwell, D. T., R. D. Müller, W. H. F. Smith, E. Garcia, and R. Francis, 2014, New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure: Science, 346 (6205), 65-67, doi: 10.1126/science.1258213

Simpson, R. W., R. C. Jachens, and R. J. Blakely, 1983, Airyroot: A Fortran Program for Calculating the Gravitational Attraction of an Airy Isostatic Root Out to 166.7 KM: U.S.G.S. Open-File Report 83-883, 66 p.

Scripps Institution of Oceanography, 2020, Archive of marine gravity from satellite altimetry - File grav_28.1.img: Satellite Geodesy Research Group, Scripps Institution of Oceanography, University of California San Diego,

ftp://topex.ucsd.edu/pub/archive/grav/ (Accessed 25 October 2020)

Simpson, R. W., R. C. Jachens, R. J. Blakely, and R. W. Saltus, 1986, A new isostatic residual gravity map of the conterminous United States with a discussion on the significance of isostatic residual anomalies: J. Geophys. Res., 91(B8), 8348– 8372, doi:10.1029/JB091iB08p08348.

Tracey, R., M. Bacchin, and P. Wynne, 2007, AAGD07: A new absolute gravity datum for Australian gravity and new standards for the Australian National Gravity Database: Expanded Abstract, 19th ASEG/PESA International Geophysical Conference & Exhibition, Perth, Western Australia, 1-3, https://www.tandfonline.com/doi/abs/10.1071/ASEG2007ab149

Whiteway, T., 2009, Australian Bathymetry and Topography Grid, June 2009: Record 2009/021, Geoscience Australia, Canberra, https://pid.geoscience.gov.au/dataset/ga/67703

Acknowledgements: The authors would like to thank Anandaroop Ray for the peer review of the map.