This National Gravity Compilation 2019 DGIR tilt image is produced from the 2019 Australian National Gravity Grids A series. These gravity data were acquired under the project No. 202008. The national grid has a cell size of 0.00417 degrees (approximately 435m). The data were derived from ground observations stored in the Australian National Gravity Database (ANGD) as at September 2019, supplemented by offshore data sourced from v28.1 of 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). Out of the approximately 1.8 million gravity observations, nearly 1.4 million gravity stations in the ANGD together with marine data were used to generate this grid. The ground gravity data used in the national grid 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. Station spacing varies from approximately 11 km down to less than 1 km, with major parts of the continent having station spacing between 2.5 and 7 km. The image shows a tilt of the de-trended global isostatic residual anomalies (A series) over Australia and its continental margins. The DGIR grid was 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) of the 2019 Australian National Gravity Grids A series. 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 degrees 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. More information about the 2019 national gravity grids and the processing steps can be found in Lane et al. (2020a,b). A tilt filter was calculated by applying a fast Fourier transform (FFT) process to the DGIR grid of the 2019 Australian National Gravity Grids A series to produce this image. This tilt filter was calculated using an algorithm from the INTREPID Geophysics software package. A tilt filter is a ratio of the vertical derivative to the total horizontal derivative and is used for detection of edges of geological units. Details of the specifications of individual surveys held in the Australian National Gravity Database (ANGD) can be found in the Second Edition of the Index of Gravity Surveys (Wynne and Bacchin, 2009).

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.;

Intrepid Geophysics, http://www.intrepid-geophysics.com;

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. Muller, 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. ;

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;

Wynne, P. and Bacchin, M., 2009. Index of Gravity Surveys (Second Edition). Geoscience Australia, Record 2009/07.