Airborne Gravity 2010 - Geoscience Australia

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Airborne Gravity 2010 In conjunction with instrument development, considerable effort has been expended on preparations for utilising the type of data we expect to acquire. A major contribution to any airborne gravity gradient survey data is likely to be the effects of topography. Since topography may reflect lithology to varying degrees, these effects are not totally unrelated to the geological signals we want to measure, but they do complicate the results and must be addressed. In order to utilise the higher instrument sensitivity, we are striving for better ways to handle topographic effects. Conventional practise is to apply terrain corrections by calculating the response of the topography using an assumed, usually homogeneous, density contrast at each observation point, and there are many tools available to do this. Spatial algorithms vary in the way they discretise the terrain, in their efficiency and in their ease of use. A number of studies have been conducted both internally and through external groups to assist in optimising these terrain corrections (e.g., Grujic, 2010). It is essential, of course, to have a sufficiently detailed terrain model to undertake the terrain corrections. In flat area, such as peneplains, freely available SRTM data may suffice. More typically, given the sensitivity of the VK1 system, “bare-earth” LiDAR may be required to either calculate terrain corrections or otherwise incorporate terrain in the interpretation process. Such topographic data are becoming more readily available from a range of sources but, regardless of the accuracy of the terrain model, the terrain correction calculations will always be limited by the appropriateness of the chosen density inputs. Procedures are available to grid the observed survey data, project them onto a smooth surface or a horizontal plane, and convert the observed quantities into any of the gravity gradient tensor components. Typically these will be Gzz and Gz. A number of studies have been undertaken to evaluate and improve these processing tools and this work is ongoing. In addition, Rio Tinto is developing tools to predict which targets will be visible to the system after allowing for terrain and terrain clearance effects, expected instrument noise, flight line orientation and host density. These tools will assist users to optimise survey planning and hence maximise the chances of success in various terrains and flying conditions (Figure 6). Figure 6. Example of an AGG simulation, (using Pitney Bowes AGG-SIM software), on shaded and contoured topography. Green signifies where the specific target has sufficient response from background to be detectable. Areas where the target is not detectable are shown in red. Detection is debateable in the grey zones. 11
Airborne Gravity 2010 Acknowledgements The VK1 system has been developed entirely within the UWA and is wholly funded and owned by Rio Tinto. It will initially be deployed in Australia on Rio Tinto projects but ultimately will be able to operate anywhere in the world. We would like to thank Rio Tinto for permission to publish the current status of the VK1 project and to thank Richard Lane for his review which greatly improved this paper. References Anstie, J., Aravanis, T., Haederle, M., Mann, A., McIntosh, S., Smith, R., Van Kann, F., Wells, G., and Winterflood, J., 2009, VK-1 - a new generation airborne gravity gradiometer: Extended Abstract, ASEG-PESA 20th International Geophysical Conference and Exhibition. Dransfield, M. H., 1994, Airborne Gravity Gradiometry: Unpub. PhD thesis, University of Western Australia. Drinkwater, M. R., Haagmans, R., Muzi, D., Popescu, A., Floberghagen, R., Kern, M., and Fehringer, M., 2006, The GOCE Gravity Mission: ESA’s First Core Earth Explorer: 3rd International GOCE User Workshop, Frascati, Italy, 2006, ESA SP-627, 1-8. FALCON, n.d., Fugro Gravity & Magnetic Services, Fugro, http://www.fugrogravmag.com/FALCON/index.php, accessed 7 July 2010. Grujic, M., 2010, Optimisation of terrain corrections for practical Airborne Gravity gradiometer surveys: Unpub. Bachelor Of Science Honours thesis, Monash University. Howard, D., Gujic, M., and Lane, R., 2010, The Kauring airborne gravity and airborne gravity gradiometer test site, Western Australia: In R. J. L. Lane (editor), Airborne Gravity 2010 Workshop - Abstracts from the ASEG-PESA Airborne Gravity 2010 Workshop: Published jointly by Geoscience Australia and the Geological Survey of New South Wales, Geoscience Australia Record 2010/23 and GSNSW File GS2010/0457. Lane, R., Grujic, M., Aravanis, T., Tracey, R., Dransfield, M., Howard, D., and Smith, B., 2009, The Kauring Airborne Gravity Test Site, Western Australia: Eos Trans. AGU Fall Meeting Supplement, v. 90(52), Abstract G51A-0656. Van Kann. F, 2004, Requirements and general principles of airborne gravity gradiometers for mineral exploration: In R.J.L. Lane (editor), Airborne Gravity 2004 - Abstracts from the ASEG-PESA Airborne Gravity 2004 Workshop: Geoscience Australia Record 2004/18, 1-5. 12
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