EARTH IMAGING - Transect Program

Background

The AuScope initiative funded under the National Collaborative Infrastructure Strategy (NCRIS) includes funding for Earth Imaging, including the establishment of data infrastructure through transects across regions of major scientific interest.

The 2003 National Strategic Plan for the Earth Sciences advocated that:
That the nation invest in a major geotransect study to gain fundamental information about the Australian plate, from its basic structure and evolution through to its mineral and petroleum systems and surficial processes

In 2003 the necessary resources needed to undertake even a fraction of such a Geotransect program were not available, but subsequent major investment by the Australian Government means that, with suitable coordination, we can make significant progress. The Onshore Energy Security Program at Geoscience Australia has made a significant investment in seismic reflection profiling, of the order of 2500 line km, with the aim of improved understanding of energy resources in less explored areas. The AuScope Earth Imaging funding was sufficient for three segments of 200 km of seismic reflection work, with associated collection of other classes of geophysical information.

Through the National Committee for Earth Sciences a Geotransect Working Group has been established with Prof. Brian Kennett (ANU) as the Chair, with the aim of developing a unified national concept and taking best advantage of the available opportunities. This working group has endorsed the concept of designating a suite of corridors across the continent that cross major geological features, as a basis for planning (Figure 1). The aim is that, collectively, the results from such corridors provide major insight into both the scientific issues associated with the structure and evolution of the continent, but also the associated resources. Within each corridor a full range of geophysical, geological, geochemical and geochronological information should be assembled to provide a genuine GEOtransect in each of the corridors.

The transect corridors are of the order of a couple of hundred kilometres across, but certainly nothing should exclude taking in nearby information. It is convenient to be able to identify the corridors and so names have been attached, as used in the discussion of the Working Group. It is hoped that the identification of the corridors will help to focus activity, in particular the framing of important geological questions and the coordination of existing information of all types.

Figure 1: Configuration of geotransect corridors across the Australian continent

Transect Planning

A trial AuScope transect of 200 km of reflection profiling was carried out in July 2007 in conjunction with a major investment by Geoscience Australia and the Geological Survey of Queensland who conducted 1250 km of reflection work on three lines from Cloncurry to near Croydon, across the Georgetown Inlier and from Mt Surprise through Charters Towers. The AuScope component from just south of Mareeba to Mt. Surprise adds a further crossing of the expected location of the Tasman Line and hence improves three-dimensional coverage in this area.

In July 2007 the Earth Imaging component of AuScope issued a call for proposals for data infrastructure in the form of acquisition of data along transect segments during the next three years. The proposals were to be directed to major issues that should help to elucidate the structure and evolution of the continent. A total of six detailed proposals were submitted and all were carefully reviewed for both scientific and technical merit. A number of priority projects were identified (Figure 2)

Figure 2. Endorsed transect projects from AuScope Earth Imaging. Most data infrastructure projects were larger than can be funded directly by AuScope and so co-investment was required.

The highest priority was given to a profile across the northern edge of the Gawler Craton into the Musgrave block crossing from South Australia into the Northern Territory (Gawler-Musgrave in Fig 2), and to a profile from the Pilbara to the northern edge of the Yilgarn Craton in Western Australia (Capricorn in Fig 2). At the next level is a profile across the Newer Volcanic Province and the Delamerian Orogen connecting from near Stawell in Victoria to Mt Gambier in South Australia (NVP in Figure 2). Other endorsed projects are the installation of passive seismic recording around the recent reflection profiles in Northern Queensland (Isa-Coast SP in Figure 2), a profile across the southern edge of the Mt. Isa block into the Thompson Orogen (Diamantina in Figure 2), and a profile through the Halls Creek fold belt in the Kimberley Block (Halls Creek in Figure 2).

Transect Implementation

As AuScope was developing its transect plans additional funding from the Australian Government became available under the Onshore Energy Security Program (OESP). AuScope Earth Imaging joined with the OESP and state funding from South Australia to create a 590 km reflection line in the Stuart corridor. This Gawler-Officer-Musgrave-Amadeus (GOMA) line completed in 2008 followed the line of the railway towards Alice Springs and crossed the border into the Northern Territory. The cooperative venture minimised the mobilisation costs for the survey and enabled a longer profile to be collected. The GOMA line provided good control on crustal structure and the configuration of crustal components, including a large upward step in the Moho beneath the Musgrave block, consistent with the large gravity anomaly.

A further set of profiles in western Victoria and adjacent South Australia was largely funded by AuScope. Logistic considerations prevented a direct link to the recent volcanics, instead the line of the earlier profiles in Victoria was extended to the west and a cross-line included to link to prior work with explosive sources. Geoscience Victoria provided considerable help in the planning and made their main investment in an additional line in the Ararat area. The Southern Delamarian lines shed considerable light on the nature of transition from the Delamerian to the Lachlan Orogen, and revealed a remarkable feature at the base of the crust in the west that may well represent an imbricated Moho. Fortunately this feature was crossed by two lines so there is full 3-D control.

Unseasonably wet weather in central Australia freed up a seismic crew and so AuScope cooperated with the Geological Survey of Western Australia (GSWA) to carry out a major transect from the Pilbara across the Capricorn Orogen into the northern Yilgarn. The seismic work was accompanied with magnetotelluric sounding supported by AuScope. Subsequently GSWA continued with a further 600km+ of reflection profiling in the Younami domain, and later in the Albany-Fraser area. Additional investment through the OESP program has seen a dramatic increase in coverage in the west and centre as can be seen in Figure 3 that shows the full set of over 12,000 km of full crustal reflection profiles that have been carried out across the continent.

AuScope was also able to make investment in passive seismic deployments in South Australia in the Gawler and Curnamona areas to link with reflection work undertaken with the OESP program, and extend coverage from ANU funded work in New South Wales. Further passive seismic stations were deployed to cover much of the Mt. Isa block and the adjacent basins to the east to link to the earlier reflection profiles in northern Queensland. The long timescale for passive seismic data collection, and the considerable data processing effort mean that this data is only just beginning to reveal its full potential.

The net result of all the sources of investment is that there is a dramatic improvement in continental coverage and many of the transect corridors are now sampled quite well with some types of seismological data.

A challenge for each of the transects is how a full range of complementary studies (geophysical, geological, geochemical and geochronological) can be organised for each Geotransect, and the necessary co-investment required to make this happen.

Figure 3. Seismological coverage of Australia by all classes of data at the end of 2012.

Last updated 2016 November 7