Airborne Gravity 2010 - Geoscience Australia

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Airborne Gravity 2010 Sensitivity to the near-surface is a mixed blessing. On the one hand, it accentuates the influence of topography, at the expense of geology, as illustrated at Red Dog, Alaska. Consequently, “terrain correction” is normally applied prior to interpretation of gravity gradient data. Sensitivity to the nearsurface can be equated to a loss of sensitivity at depth. This was illustrated by examining the detectability of a hypothetical 50 Mt orebody as its depth was increased from 0 to 400 m. When superimposed on real FALCON data from Broken Hill, Australia, the synthetic orebody response was unrecognisable as a target of interest when buried to 200 m. Sensitivity to the near-surface is advantageous for definition of outcropping or sub-cropping targets. This has been illustrated in the context of kimberlite exploration in the vicinity of Ekati, Canada. The lakes in the area can complicate the Gzz response. The apparent bathymetry of lakes was derived via geometry inversion. Several of the known kimberlites coincide with depth anomalies. This provided a means for ranking other Gzz anomalies as potential kimberlite targets. One consequence of the extremely small magnitude of gravity gradients is that the data are lowpassed filtered along flight lines. Therefore for quantitative modelling, it is important to apply an equivalent low-pass filter to calculated gravity gradients, and thereby avoid introducing short wavelength contributions which are absent from the measured data. This was illustrated using synthetic data in the context of terrain correction at Red Dog. Depending on the acquisition system, the interpreter may have a choice between a number of different “gradient quantities” for interpretation. For quantitative interpretation, our experience suggests that there appears to be little benefit in looking beyond Gzz or perhaps the full tensor amplitude, ||G ~ ||, if this is available. Acknowledgments Geological data from Red Dog were compiled during the cited CAMIRO project, and are included by kind permission of Teck Corporation. FALCON data from Ekati are published by kind permission of BHP Billiton Canada. We are grateful to Jon Carlson and Greg Walker of BHP Billiton for their support and technical input in relation to the Ekati example. The Broken Hill Airborne Gravity Gradiometry Survey was a joint project between the NSW Department of Mineral Resources, pmd*CRC, Geoscience Australia, Gravity Capital and BHP Billiton. The partners in this project have kindly made the data available to the public. References Fitzgerald, D., Argast, D., and Holstein, H., 2009, Further developments with full tensor gradiometer data sets: Expanded Abstract, ASEG 20th International Geophysical Conference and Exhibition, Adelaide. Fugro Airborne Surveys, 2003, Acquisition and Processing Report, Job 1572, Broken Hill, NSW, Airborne Gravity Gradiometer and Magnetic Geophysical Survey for BHPBilliton. Fullagar, P. K., Hughes, N., and Paine, J., 2000, Drilling-constrained 3D gravity interpretation: Exploration Geophysics, 31, 17-23. Fullagar, P. K., Pears, G. A., Hutton, D., and Thompson, A, 2004, 3D gravity and aeromagnetic inversion, Pillara region, W.A.: Exploration Geophysics, 35, 142-146. Fullagar, P. K. and Pears, G. A., 2007, Towards geologically realistic inversion: In B. Milkereit, ed., Exploration in the new millenium: Proceedings of 5th Decennial International Conference on Mineral Exploration, 444-460. Fullagar, P. K., Pears, G. A., and McMonnies, B., 2008, Constrained inversion of geological surfaces - pushing the boundaries: The Leading Edge, 27(1), 98-105. Hensley, C., 2003, Data Processing Report, Airborne Gravity Gradiometer Survey, Broken Hill, NSW, Australia: BHPBilliton FALCON Operations Report CR 10657 for Survey USN 142911122002. Jackson, J., Pears, G., and Fullagar, P., 2004, Minimisation of the gravity response from mine infrastructure – an example from Sons of Gwalia mine, WA: Expanded Abstract, ASEG 17th International Geophysical Conference and Exhibition, Sydney. Kass, M. A., and Li, Y., 2007, Practical aspects of terrain correction in airborne gravity gradiometry: SEG 77th International Meeting & Exhibition, San Antonio, Expanded Abstracts, 755-759. 85
Airborne Gravity 2010 Lane, R., and Peljo, M., 2004, Estimating the pre-mining gravity and gravity gradient response of the Broken Hill Ag-Pb-Zn Deposit: Expanded Abstract, ASEG 17th International Geophysical Conference and Exhibition, Sydney. Mataragio, J., and Kieley, J., 2009, Application of full tensor gradient invariants in detection of intrusion-hosted sulphide mineralisation – implications for deposition mechanisms: First Break, 27, 95-98. Mira Geoscience, 2001, Project Report – CAMIRO Project 2001E01: Detectability of mineral deposits with airborne gravity gradiometry: Released as Miscellaneous Release - Data 134 by the Ontario Geological Survey. Pedersen, L. B., and Rasmussen, T. M., 1990, The gradient tensor of potential field anomalies: Some implications on data collection and data processing of maps: Geophysics, 55, 1558-1566. 86
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