Presentation slides and speaking notes are provided for a presentation that was given online on Wednesday 7th October 2020, 11:00 to 12:00 AEDT time (UTC +11). The presentation coincided with the release of two products; (1) a new web page for the Australian Fundamental Gravity Network (AFGN), and (2) the 2019 Australian National Gravity Grids (eCat Record 133023). Not mentioned as a separate item, the presentation drew heavily on material in the Explanatory Notes for the gravity grids (eCat Record 144233) which was also released on this day. The presentation was pitched at the level of a general audience. It commenced with an introduction to gravity, and how it changes from one place to another in step with different geological units. The subjects of 2-dimensional digital grids and how coloured images are derived from them were then covered as a prelude to later material. The speakers then described first of the two main topics - the Australian Fundamental Gravity Network (AFGN) and its importance when producing the 2019 Australian National Gravity Grids. The AFGN is a series of gravity benchmarks that allow gravity surveys to be linked to the Australian Absolute Gravity Datum 2007 (AAGD07). This makes it possible for the many separate gravity data sets that have been acquired in Australia to be combined into a seamless whole. Gravity data from 1308 ground surveys and 14 blocks of airborne gravity and airborne gravity gradiometry have been combined with offshore gravity data from satellite altimetry to form the 2019 Australian National Gravity Grids. This marks the first time that airborne data have been incorporated into the national gravity grids. It is also the first time that the offshore data have been fully processed alongside the onshore data. Grids of three types of gravity anomalies were produced; Free Air Anomaly (FAA), Complete Bouguer Anomaly (CBA), and De-trended Global Isostatic Residual (DGIR). During the presentation, various comparisons were made illustrating the improvements made with the 2019 grids in comparison with the previous 2016 grids and the benefits of incorporating airborne data into the grids. The gravity grids were produced to assist those involved in geological mapping and exploration, and it is hoped that the new grids will inspire users to revisit their geological interpretations and to aid explorers to identify new opportunities and to more efficiently focus their efforts on prospective ground. The presentation was recorded, and the recording of the presentation is available on demand on the Geoscience Australia YouTube Channel at https://youtu.be/3CyqrqBM0xg. Introductions were made by Marina Costelloe. The event was controlled by Chris Nelson, and the recording was edited by Douglas Warouw. Note that there are some minor differences between the presentation material given here and the presentation seen in the video recording. These changes were made in the interest of clarity and include the removal of “animation” effects and the provision of some additional text. Speaker Biography for Richard Lane; Richard joined Geoscience Australia in 2001 after a career as a mineral and petroleum geophysicist with CRA Exploration / Rio Tinto and as the Program Leader responsible for the development of the TEMPEST AEM system in CRC AMET. As a Senior Geophysicist in the Geophysical Acquisition and Processing Section, he has been evaluating the role of airborne gravity and airborne gravity gradiometry on a national scale. He is an ASEG Gold Medal recipient, a Society of Exploration Geophysicists Honorary Lecturer, and a Distinguished Geoscience Australia Lecturer. Speaker Biography for Phillip Wynne; Phillip has been with GA for over twenty years. In that time, he has been involved in all aspects of regional gravity surveys. He currently oversees gravity surveys conducted by GA and Australian States and Territories and manages the Australian Fundamental Gravity Network.

This image is a greyscale image of the Total Magnetic Intensity (TMI) Anomaly Image of Australia with Variable Reduction to Pole (VRTP). Total magnetic intensity (TMI) data measures variations in the intensity of the Earth magnetic filed caused by the contrasting content of rock-forming minerals in the Earth crust. Magnetic anomalies can be either positive (field stronger than normal) or negative (field weaker) depending on the susceptibility of the rock. A variable reduction to Pole is aimed at locating magnetic anomalies exactly above their source bodies and without any distortion. The image is created from the 2019 variable reduction to Pole of the TMI grid with a grid cell size of ~3 seconds of arc (approximately 80 m). This image only includes airborne-derived TMI data for onshore and near-offshore continental areas. The image provides a better interpretation of the magnetic data by giving an accurate location of magnetic source bodies.

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.