By Dr Brian R Spies FTSE
Australia has a proud history of achievement in mineral exploration and in innovation in geophysical exploration technology. The gold rushes of the 1960s highlighted Australia's mineral wealth, but prospectors were limited to searching in the relatively small proportion of Australia where rock was exposed at the surface. The flat weathered expanses of Australia posed significant challenges to mineral exploration; less than 15% of the continent has reasonable outcrop that can be mapped by conventional geological techniques.
The Imperial Geophysical Survey of 1928-1930, funded by the British and Australian governments, was the first systematic test of geophysical exploration techniques suited to Australia, and eventually led to the formation of the Bureau of Mineral Resources, Geology and Geophysics in 1946. The BMR, as it came to be known, carried out systematic regional airborne and ground mapping programs to cover the entire country with geological mapping, and extensive airborne magnetic and radiometric surveying. Today, Australia is one of the few countries in the world with complete airborne geophysical coverage. The mining industry is crucial to Australia - mineral exports were $44 billion in 1999-2000 and have contributed about $500 billion over the past twenty years. New discoveries are needed to maintain the nation's wealth as mines reach the end of their operational life.
Electromagnetics and mineral exploration
The arsenal of geophysical techniques covers a wide range of methods, from gravity, magnetics, electromagnetics, electrical and radiometics used for mineral exploration, to the advanced seismic techniques which produce detailed 3-D images of the earth which have become essential tools in petroleum exploration.
Electromagnetic methods are ideally suited to the search for many types of ore deposits, in particular electrically conductive copper and nickel ore bodies. Electromagnetic, or EM, methods are based on the principle of inducing eddy currents in conductive material by varying the magnetic field in a transmitter wire loop. The eddy currents are detected by measuring their associated secondary magnetic fields in an ultra-sensitive induction coil. The principle is identical to that of the ubiquitous metal detectors used for airport security and treasure hunting.
Electromagnetic prospecting methods were first developed in Scandinavia and Canada in the 1940s and were spectacularly successful in discovering dozens of ore deposits hidden tens of metres under the ground. It was in these countries, too, that the concept of mounting EM systems on aircraft was tested and perfected, initially on all-wooden aircraft and later on conventional aircraft where the sensor and transmitter could be physically separated from the metal airframe by some distance. The golden days of airborne EM were no doubt in the 1950s and 1960s when the method was responsible for over US$10billion of mineral discoveries in Canada. In retrospect, electromagnetic methods were well suited to Scandinavia and Canada because of relatively recent glaciation which had scraped off older weathered rock and soil, leaving a host rock which was highly resistive and easily penetrated by the relatively high frequencies used in the airborne electromagnetic systems.
Airborne electromagnetics comes to Australia
After the spectacular successes in Canada, it was natural that airborne electromagnetics would be brought to Australia. An airborne EM system mounted on a DC-3 was flown in various parts of Australia in the 1960s, but with disappointing results. It was not recognised at the time that the high-frequency systems then in use would be ineffective in penetrating the almost ubiquitous deeply weathered soil and rock (regolith) that covered most of Australia. These systems produced a highly variable response (sometimes known as geological 'noise') which dominated the subtle signals originating from the bedrock.
In the decades since the 1970s, various fixed-wing and helicopter-borne systems were tried in Australia, the most successful being helicopter systems suited to the mountainous terrains of Tasmania (itself recently glaciated and thus relatively resistive) that resulted in the discovery of the Que River deposit in Tasmania in 1972. In the rest of Australia though, airborne EM continued to be plagued by widespread but variable conductive, salty soil and ancient weathering.
In the meantime Australian geophysical airborne contracting companies, through a series of company mergers and takeovers, were taking an increasingly dominant role in the international marketplace. Perth based World Geoscience Corporation successfully took over a number of Canadian and Australian airborne contractors in the late 1980s and soon became the dominate global player. The industry downturn in the mid 1990s, tied to a collapse in commodity prices, resulted in cutthroat competition, and airborne contractors looked to new markets.
Government funding and the salinity link
Australian federal and state governments have played a major role in fostering scientific development, commercialisation and innovation within Australia. World Geoscience Corporation, a subsidiary of Aerodata Holdings, tapped into the GIRD grant scheme in the late 1980s with collaboration from CSIRO to develop the SALTMAP system, the world's first fully digital broadband airborne EM system.
The SALTMAP system was targeted at Australia's increasingly important salinity market, where it was recognised by landcare groups, local and state governments as being an increasingly major problem in rural areas. Airborne EM was seen as the 'holy grail', the high-tech answer to a politically sensitive national issue. Scientific and business entrepreneurs had turned a 'problem' geological noise from salty conductive weathered rock and soil into an opportunity use airborne EM to map the salt directly!
Unfortunately, the promises and expectations exceeded what technology could realistically deliver. Early SALTMAP surveys produced inconclusive, poor quality conductivity maps that sometimes appeared to correlate with known salt stores and sometimes not. While marketing people scrambled to secure additional funding for surveys, in the back room engineers and technicians worked to improve the technology.
In 1992, the Cooperative Research Centre for Australian Mineral Exploration Technologies (CRC AMET) was formed, with the goal of developing airborne EM techniques able to explore to depths of 300 metres through Australia's conductive weathered layers † a key goal identified by the Australian minerals industry. World Geoscience Corporation, CSIRO, The Australian Geological Survey Organisation (AGSO), the Geological Survey of WA, the Australian Minerals Industry Research Association (AMIRA), and Macquarie and Curtin universities were members of this CRC. World Geoscience contributed the prototype SALTMAP system into the CRC where it continued to be improved, and also brought knowledge of the deeper-probing QUESTEM system into the Centre. Other parties contributed expertise in computer modelling, visualisation and data processing, and geological understanding which would all prove to be vital ingredients in getting airborne EM to work effectively in Australia.
Figure 1. Schematic of an airborne electromagnetic prospecting system. The transmitter loop, wrapped around the aircraft, induces eddy currents in subsurface conductors. A sensitive receiver towed behind the aircraft detects their associated magnetic fields.
Figure 2. Conductivity-depth images derived from TEMPEST airborne electromagnetic data, from the Toolibin catchment in Western Australia. The top image is the surface topography and the lower image shows the subterranean water flow 50-70m beneath the surface. The survey found elevated levels of salinity in the vicinity of the underground water courses flowing into Lake Toolibin. (Figures courtesy CRC AMET).
The success story of CRC AMET was the TEMPEST system (Figure 1), much better suited to mineral exploration in Australia than previous technologies. The TEMPEST system featured broadband (25 Hz to 37 kHz) multi-component acquisition, and streaming data recording suitable for post-survey digital processing. Using powerful interpretation and imaging software, users were able to quickly and reliably visualise conductivity variations in the subsurface in three dimensions (Figure 2).
In parallel to CRC AMET, Federal and State governments embarked on trials of airborne geophysics for dryland salinity, funded through the National Airborne Geophysics Program (NAGP) and coordinated by the National Dryland Salinity Program on behalf of State and Federal Governments. The NAGP received $1m in support from the Federal Government with matching funds from the State governments of WA, NSW, Queensland and Victoria. Several airborne contractors and systems were used but with inconclusive results, due in part to the range of expectations raised by the technologists and in part to the sufficiency of ground data for calibration of the datasets. One major issue encountered was that while the data often very accurately described the landscape, they could not routinely be used to better define management options. This significantly limited the benefits of acquiring data. Nevertheless, the NAGP results were sufficiently interesting to prompt further interest in the Gilmore Project and the National Action Plan on Salinity, as described later.
The newly developed TEMPEST system needed well-publicised field trials to ensure its economic viability. Two federal agencies † AGSO, the Bureau of Resource Sciences (BRS, now part of AFFA) teamed up with CRC AMET and CRC LEME to fund an interdisciplinary regional survey in the Temora/Gilmore area of western NSW in 1998, which later became known as Project Gilmore. This survey clearly demonstrated the potential of the TEMPEST system for mineral exploration using a 'geological systems' approach for highlighting prospective areas, as well for regolith-based salinity mapping, and was the impetus for much larger-scale surveys being planned under the National Action Plan for Salinity and Water Quality. An important outcome of Project Gilmore was confirmation that ground and borehole control is crucial in calibrating and making geo-hydrological sense of the airborne data, a point also made in the NAGP reports.
Australia's National Dryland Salinity Program
Australia's National Dryland Salinity Program is a collaborative R&D effort investigating the causes of, and solutions to, the national problem of dryland salinity. The program is now into its second five-year phase, which has received $15m of funding, and will use information on the costs and extent of salinity in Australia to develop policy options for federal, state and local governments to address the most pressing issues. It will also work with scientific and industry sectors to develop strategies for better management responses that can be put into practice by communities and farmers. It is expected that airborne geophysics, originally developed for mineral exploration, could play an important role in environmental management in Australia, but only when the linkages between the geophysical data and land management options are made more explicit.
Government support for science
It is clear from the history of airborne electromagnetics in Australia that government support for R&D played a major role in the development of advanced techniques now being used in Australia for mineral and environmental applications. In particular, collaborative, focused R&D programs involving researchers from government agencies working closely with Australian companies were effective in solving real problems and developing commercially viable technologies. It is also clear that spin-off benefits, unforeseen in the conduct of scientific research, often appear in unexpected places. The airborne EM example demonstrates the crucial role that a strong government-supported science and technology base plays in underpinning the nation's prosperity.
Postscript: More than 2.6 million hectares of Australia are currently affected by dryland salinity at a cost to all Australians of more than $500 million a year in environmental degradation, degraded water supplies, lost agricultural production and damage to infrastructure such as roads, buildings and recreational facilities. Salinisation of water supplies is part of a general concern for water quality being addressed by the National Academies Forum project supported in 2002 by the Australian Research Council.
Selected reading
- Commonwealth of Australia, 2000. National Action Plan for Salinity and Water Quality: http://www.affa.gov.au/actionsalinityandwater
- Commonwealth of Australia, 1998. National Dryland Salinity Program: http://www.ndsp.gov.au/
- Commonwealth of Australia, 1998. National Airborne Geophysical Program: http://www.ndsp.gov.au/NAGP/nagphome.htm
- CRC AMET: Cooperative Research Centre for Australian Mineral Exploration Technologies: http://www.crcamet.mq.edu.au/
- Dent, D., 2001. Environmental geophysics shows its paces: Preview, no 94, 32-34.
- George, R. J., Campbell, C., Woodgate, P.W., Farrell, S. J., and Taylor, P., 2001. Complementary data and cost benefit analysis of utilising airborne geophysics data: Report to Land and Water Resources Research and Development Corporation, RM 17, Canberra.
- George, R.J., Beasley, R., Gordon, I., Heislers, D., Speed., Brodie, R, McConnell, C., and Woodgate, P., 1999. Evaluation of airborne geophysics for catchment management, National Report, pp 81.
- Lawrie, K., 1999. The 'Gilmore' project, Lachlan Fold Belt, western New South Wales. AGSO Research Newsletter, no 50, 14-15
- Lawrie, K., Munday, T.J., Dent, D.L., Gibson, D.L., Brodie, R.C., Reilly, N.S., Chan, R.A., and Baker, P., 2000. A geological systems approach to understanding the processes involved in land and water salinisation: AGSO Research Newsletter, no 52, 13-32.
- Pain, C., 2001. Cover up penetrated to reconstruct landscape history, 2001. AusGEO News, Issue 62, 3-6.
- Spies, B., Fitterman, D., Holladay, S., and Liu, G. [Eds], 1998. Proceedings of the International Conference on Airborne Electromagnetics (AEM 98): Austr. Soc. Explor. Geophys. V29, issue 1&2.
- Spies, B., Macnae, J., Munday, T., Green, A., Lane, R., and Worrall, L, 1999. Exploring under cover in three dimensions with broadband electromagnetics: CSIRO Exploration and Mining Research News, Dec 99, 11, 3-5.
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