Airborne Electromagnetic (AEM) data are one form of the geophysical data acquired by Geoscience Australia. The data are gathered by transmitting an electromagnetic signal from a system attached to a plane or helicopter. The signal induces eddy currents in the ground which are detected by receiver coils towed below and behind the aircraft in a device called a bird. Depending on the system used and the subsurface conditions, AEM techniques can detect variations in the conductivity of the ground to a depth of several hundred metres. The conductivity response in the ground is commonly caused by the presence of electrically conductive materials such as salt or saline water, graphite, clays and sulfide minerals.
Depending on the system used and the subsurface conditions, AEM techniques can detect variations in the electrical conductivity of the ground to a depth of several hundred metres, sometimes up to 2000 metres in particularly favourable conditions.
Applications of AEM
AEM surveys are more expensive to acquire than other airborne geophysical methods like magnetic and radiometrics: government organisations, the minerals industry and groundwater researchers generally acquire AEM data in only relatively small and isolated locations at narrow line spacing (~ 200 metres) for specific purposes across Australia. Since 2006, Geoscience Australia and its State and Territory partners have been collecting AEM data over large areas at broad line spacing (1000-6000 metres) to more fully survey Australia. AEM surveys also require complex processing to allow interpretation and, therefore, are usually designed to detect particular subsurface targets which are based on a perceived conductivity contrast, for example:
- the spatial extent of geological features, such as a clay-rich unit in a sedimentary sequence or a graphite-bearing unit in a metamorphic complex
- the depth of an unconformity between sedimentary cover and the underlying basement rock
- the location of groundwater resources, such as fresh or saline aquifers.
AEM acquisition for Minerals and Energy Resources
Geoscience Australia first acquired large area, broad line spacing AEM surveys for minerals and energy purposes as part of Geoscience Australia's Onshore Energy Security Program (OESP). These surveys were designed to reveal new information about regions that are considered prospective for energy and mineral resources. The survey data were acquired at broad line spacing, between 1000 metres and 6000 metres, to map relatively large areas at low resolution. Mineral exploration companies have benefitted through an improved understanding of the regional geology and having the opportunity to collaborate by paying for more detailed data acquisition over a specific area of interest.
Since the completion of the OESP in 2011, Geoscience Australia's State and Territory partners have commenced their own broad line-spacing, large-area AEM surveys, managed by Geoscience Australia. Geoscience Australia has also continued with targeted broad line-spacing AEM surveys.
AEM acquisition for Natural Resource Management
Geoscience Australia has also been involved in the acquisition and processing of AEM surveys for Natural Resource Management (NRM), particularly in relation to salinity or groundwater issues in the Murray-Darling Basin. These surveys, such as the Lower Macquarie survey in central west New South Wales, were acquired and processed by Geoscience Australia as part of large multi-agency projects managed by the then Bureau of Rural Sciences. The surveys have revealed important new information relevant to NRM, such as the distribution of freshwater aquifers in the sediments of the Murray Basin. AEM data is also a key dataset used to asses groundwater aquifers as part of the Broken Hill Managed Aquifer Recharge Project. Geoscience Australia continues to acquire AEM data in support of NRM.
Using AEM data
There are more than 14 different AEM systems currently operating in Australia, including time domain (TEM) and frequency domain (FEM) methods. The accuracy, integrity and usability of AEM data are reliant on many factors including topography, system geometry (the positions of emitter and receiver coils), noise and geographical location. AEM data funded by Commonwealth, State and Territory government agencies should be fit-for-purpose as well as fit for re-use in the future.
To fulfil the requirements of making AEM data fit-for-purpose and suitable for modelling, interpretation and archiving, a range of data and interpretation products are made available by Geoscience Australia, including:
- Point-located final processed aircraft data
- Point-located high altitude and repeat line data for estimating noise levels
- Point-located final conductivity data calculated from the final processed data
- Gridded data of depth slices and other survey data
- Multiplots (displaying all deliverable data fields)
- Final Report, including relevant processing and calibration information.
Information in the point-located data includes, but is not restricted to: flight number, line number, line bearing, terrain clearance, ground elevation, transmitter position, receiver position, noise monitors (aircraft and powerline electromagnetic noise), "sferics" monitor (lightning and space weather electromagnetic noise), window amplitude data (electromagnetic signal persistence), total magnetic intensity (TMI), location (easting/northing and longitude/latitude), projection, datum, time (fiducial and GPS time), date and the unique project code.
The final report should include: operations and logistics, survey specifications, aircraft equipment and specifications, equipment calibrations, system monitoring, electromagnetic data processing (calibrations, stacking, primary field estimation), product creation details and other relevant information.
The transformation of AEM data to conductivity depth sections is used to assess the AEM data for consistency and to ensure it is consistent with plausible geological models in the survey area. Geoscience Australia uses forward modelling and inversions to check that data can be quantitatively fitted to within estimated noise levels. Geoscience Australia uses the Geoscience Australia-Layered Earth Inversion (GA-LEI) code (Brodie and Fisher, 2008; Brodie and Sambridge, 2009) for these purposes.
All survey data are publicly available through the Geoscience Australia Airborne Electromagnetics Project webpage.
Brodie, R., and Fisher, A., 2008, Inversion of TEMPEST AEM survey data, Honeysuckle Creek, Victoria: Geoscience Australia for the Bureau of Rural Sciences. Available at http://data.daff.gov.au/data/warehouse/pe_ga99010666/HSK_Inversion_Report.pdf.
Brodie, R., and Sambridge, M., 2009, Holistic inversion of frequency-domain airborne electromagnetic data with minimal prior information: Exploration Geophysics,40 , 765-78.