2019 Australian National Gravity Grids: Free Air Anomaly, Complete Bouguer Anomaly, De-trended Global Isostatic Residual, 400 m cell size (includes Point Located Data)

Brief description

N.B., The DGIR grids, A7 and B7, were updated on 20 October 2020 after low amplitude, short wavelength artefacts were found in the original grids. If you have previously downloaded either of the DGIR grids, Geoscience Australia recommends you download the new versions of these grids (i.e., A7_2019_DGIR_v2 and B7_2019_GDIR_v2). The other grids were not updated since the artefacts are not present in these grids.

This series of grids represent the combination of gravity data stored in the Australian National Gravity Database (ANGD) as of September 2019, Airborne Gravity/gravity gradiometry data in the National Australian Geophysical Database (NAGD), and the Global Gravity Grid from National Oceanic and Atmospheric Administration (NOAA). All the grids have a cell size of 400 metres, an improvement from 800 metres.

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. 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 ANGD contains over 1.8 million observations, of which nearly 1.4 million were considered suitable for inclusion in the calculation of this grid. The 2016 versions of the Australian National Gravity Grids were masked to the coastline. In contrast, in 2019 we chose to supplement the onshore data with offshore data that were sourced from v28.1 of the Global Gravity grid developed using data from the Scripps Institution of Oceanography, the National Oceanic and Atmospheric Administration (NOAA), and National Geospatial-Intelligence Agency (NGA) at Scripps Institution of Oceanography, University of California San Diego. This provides valuable context to the onshore ground gravity data. Airborne Gravity Gradiometry surveys totalling 345,000 line km and 106,000 line km of Airborne Gravity Gradiometry were included to provide better resolution to areas where ground gravity data was not of a suitable quality.
These grids were produced by Geoscience Australia and the Institute of Geological and Nuclear Sciences Limited (GNS Science).
The grids are organised into 2 series of 3 gravity and 2 supplementary grids. The gravity grids provide Free Air Anomaly (FAA), Complete Bouguer Anomaly (CBA), and De-trended Global Isostatic Residual (DGIR) values. The supplementary grids provide the elevation values that define the observation surface for the relevant set of gravity grids, referenced to the geoid and GRS80 ellipsoid for the GDA94 datum. A more complete description of the grids and their details are located in the explanatory notes.

The “A” Series grids were produced from a combination of ground gravity data for Australian onshore locations, gravity data derived from satellite altimetry for offshore locations, and gravity data from the EGM2008 global gravity model for the onshore locations in countries other than Australia.

The “B” Series grids includes the gravity data used in the A Series grids, with the addition of data from a number of airborne gravity and airborne gravity gradiometer surveys. All of the data in the “B” series grids (i.e., ground, sea surface and airborne) were vertically continued to a smooth drape surface with a minimum terrain clearance of 250m.

The grids are available to download as a complete package, which includes all grids, the explanatory notes, and associated documentation. Each grid is also available as a discrete dataset for download, including the explanatory notes and other documentation. The explanatory notes are also available as a separate download via its own publication entry.

This series of grids represent the combination of gravity data stored in the Australian National Gravity Database (ANGD) as of September 2019, Airborne Gravity/gravity gradiometry data in the National Australian Geophysical Database (NAGD), and the Global Gravity Grid from National Oceanic and Atmospheric Administration (NOAA). N.B., The DGIR grids, A7 and B7, were updated on 20 October 2020 after low amplitude, short wavelength artefacts were found in the original grids. If you have downloaded either of the DGIR grids prior to 20 October 2020, Geoscience Australia recommends you download the new versions of these grids (i.e., A7_2019_DGIR_v2 and B7_ 2019_GDIR_v2). The other grids were not updated since the artefacts are not present in these grids.

In addition, a copy of the Point Located Data is provided for public download. This dataset contains all publicly available ground gravity data as of September 2019. Clients wishing to use only the data included in the national grids can filter the dataset using the “gridding_flag” field where it equals one (“1”). The flag has been updated to reflect the changes resulting from creating the grids.

Lineage

Maintenance and Update Frequency: asNeeded

Statement: This series of gravity grids is derived from three main sources. For onshore, observations stored in the Australian National Gravity Database (ANGD) as of September 2019 and airborne Gravity/Gravity Gradiometry surveys stored in the National Airborne Geophysics Database, and for off shore from the Sandwell and Smith v28p1 data (1997) for offshore areas. The grid has a cell size of 15 arc seconds, or approximately 400 metres. The ground and airborne gravity data are referenced to the Australian Absolute Gravity Datum 2007 (AAGD07) (Tracey et al., 2007). Nearly 1.4 million ground gravity observations were used to generate the “A” series grids. For the “B” series 451,000 line km of airborne gravity/gravity gradiometry data was included. The grids show Free Air anomalies, Complete Bouguer anomalies, and De-trended Global Isostatic Residual (DGIR) over onshore continental and offshore Australia. The ground gravity data used in this grid has 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. Continental Australia has a base coverage of 11 kilometres, with South Australia, Tasmania and part of New South Wales covered with gravity stations at a spacing of 7 kilometres. Victoria has station coverage of approximately 1.5 kilometres. Federal, State and Territory Government initiatives have systematically infilled at a station spacing of 2 to 4 kilometres to improve coverage in areas of scientific or economic interest. Other areas of detailed coverage have been surveyed by private companies for exploration purposes. Only open file data held in the ANGD at September 2019 were used in the creation of the grid. The fourteen airborne surveys have a line spacing ranging from 0.5 km to 2.5 km (Bates et al, 2011; Howard et al., 2018; Bates et al., 2019a; 2019b; Carter et al., 2019). AUSGeoid09 was used to translate geoid elevations to ellipsoidal elevations onshore and EGM96 was used for this purpose offshore. Theoretical gravity was calculated using the Somigliana closed-form formula (Somigliana, 1930; Hinze et al., 2005) with the GRS80 reference ellipsoid parameters provided by Moritz (2000). The atmospheric corrections were calculated using an equation developed by Wenzel (1985) and reproduced in Hinze et al. (2005). The second order approximate formula from Heiskanen and Moritz (1969) was used for the height corrections, sometimes known as the Free Air correction. The horizontal datum is GDA94. We used ellipsoidal heights for the atmospheric effect and height corrections. Offshore data were removed from a buffer zone extending 10 cells (approximately 4 km) out from the coastline. This was done to remove offshore data that appeared to contain high levels of noise. This buffer zone was filled in during the gridding process. The "surface" gridding function in the Generic Mapping Tools (GMT) software package (Wessel et al., 2013) was used to grid the data. A Tension parameter value of zero was specified. This grid was produced by Geoscience Australia and GNS Science International Limited.

References:
Bates, M., S. Elieff, K. Kaski, D. Howard, J. Brett, and R. Lane, 2019a, Levelling large-scale airborne gravity surveys without control lines in Western Australia: KEGS Symposium, 2 March 2019, Toronto, Canada.
Bates, M., S. Elieff, K. Kaski, D. Howard, J. Brett and R. Lane, 2019b, Regional airborne gravity surveys in Western Australia: Considerations for the end user: ASEG Extended Abstracts, 2019, 1-5, DOI: 10.1080/22020586.2019.12072976
Bates, M., O. Matsetse and S. Elieff, 2011, CarbonNet Project Airborne Gravity Survey, Gippsland Basin, Victoria, Australia 2011: Technical Report for the Victorian State Government, Department of Primary industries, http://earthresources.efirst.com.au/product.asp?pID=1107&cID=13&c=41831Heiskanen, W. A., and H. Moritz, 1969, Physical geodesy: W. H. Freeman Co.
Hinze, W. J., C. Aiken, J. Brozena, B. Coakley, D. Dater, G. Flanagan, R. Forsberg, T. Hildenbrand, R. Kellar, J. Kellogg, R. Kucks, X. Li, A. Mainville, R. Morin, M. Pilkington, D. Plouff, D. Ravat, D. Roman, U. F. Jamie, M. Veronneau, M. Webring, and D. Winester, 2005, New standards for reducing gravity data: the North American gravity database: Geophysics, 70, J25-J32.
Lane, R., P. Wynne, Y. Poudjom Djomani, W. Stratford, F. Caratori Tontini, and J. Barretto, 2019, Australian National Gravity Grid 2019 - Free Air Anomaly Values: Geoscience Australia eCat133023 http:// pid.geoscience.gov.au/dataset/ga/133023
Moritz, H., 2000, Geodetic Reference System 1980: J. Geod., 74 (1), 128-162, doi: 10.1007/S001900050278.
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
Somigliana, C., 1930, Geofisica — Sul campo gravitazionale esterno del geoide ellissoidico: Atti della Accademia nazionale dei Lincei: Rendiconti. Classe di scienze fisiche, matematiche e naturali, 6, 237–243.
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. http://library.seg.org/doi/abs/10.1071/ASEG2007ab149
Wenzel, H., 1985, Hochauflosende Kugelfunktionsmodelle fur des Gravitationspotential der Erde [1]: Wissenschaftliche arbeiten der Fachrichtung Vermessungswesen der Universitat Hannover, 137
Wessel, P., W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe, 2013, Generic Mapping Tools: Improved Version Released: EOS Trans. AGU, 94(45), 409-410, doi:10.1002/2013EO450001