Earth Imaging and Sounding
AuScope’s investment in Geophysical Earth imaging infrastructure and instrument deployment enables the geoscience community to build an increasingly clear and rich picture of the subsurface. This component builds on the resources of the ANSIR Research Facilities in Earth Sounding. Based at the ANU, ANSIR has a pool of state-of-the-art seismic equipment suitable for experiments on a wide variety of scales, from the investigation of geological structures on environmental and mine scales through to studies at the scale of the entire lithosphere. Electromagnetic instrumentation is maintained at Adelaide and has provided new insights into the structure of the Australian continent.
AuScope’s investment in Earth Imaging infrastructure had two objectives.
- To renew capacity for broad-band seismic recording by replacing the ANSIR equipment that was obsolete and no longer supported by the manufacturer.
- To build up a national capacity for MT studies.
Both goals were achieved.
AuScope investment for this component fell into three main categories:
- An upgrade of existing passive seismic and magnetotelluric (MT) equipment to present state-of-the-art. This included the purchase of broadband seismic recorder systems based at the ANU, MT broadband systems based at University of Adelaide and the ANU, and MT low frequency systems based at the University of Adelaide.
- The acquisition of ~250 line-km per annum of seismic (reflection and portable instrument) and MT imaging. The AuScope investment enables the university-based research community to have a seat at the proposed National GeoTransect planning table, and to use its 250 line-km contribution to leverage access to a much larger Earth Imaging dataset.
- Operating and maintaining the instrumentation, and for coordinating and managing the AuScope National GeoTransect Program. These tasks include ensuring that the data / information from transects is processed, stored and managed appropriately, and in liaising with the AuScope Grid manager to ensure broad access and interoperability.
The Australian Seismological Reference Model (AuSREM) was designed to bring together existing information on the Australian continent of many types, and provide a synthesis in the form of a 3-D model that can form the basis for future refinement from more detailed studies.
Seismological models of the Earths crust and upper mantle structure are critical for many tasks, such as the calculation of earthquake source parameters (location, magnitude, geometry), mapping the distribution of shaking after earthquakes, and the imaging of lithospheric dynamic processes.
AuSREM is designed to bring together the diverse sources of information about the structure under the Australian region. The emphasis is on the Australian continent, but with mantle coverage to the local boundaries of the Australian plate. For more information visit the AuSREM page.
The Earth Sounding program at ANU involved the acquisition of Earth Data Recorders which became field ready in December 2012 and are now in heavy use both in Australia and New Zealand and the construction of 200 new generation Seismic Data Loggers, with very low power consumption. From October 2014 these new instruments formed the backbone of two arrays in northern New South Wales and Southern Queensland and another in Western Australia. The third strand involved the acquisition of 50 Trillium Compact Seismometers that have been delivered and made field ready. These instruments are being used in one of the Queensland arrays and in the Western Australian array.
The establishment of an Australian National Ocean Bottom Seismometer (OBS) fleet was a new initiative of AuScope. 20 OBS units were delivered, tested and accepted. Eight units were tested in deep water and the remainder in moderate depths. In January 2015 a very successful OBS survey was completed on the Australian North West Margin during which further testing of the capability of the instruments, to record meaningful geophysical data from airgun seismic source to large offsets, was completed and far exceeded results from previous trials. The survey was sponsored by Shell Australia and conducted by geophysical contractor CCG. The Shell/CGG OBS survey has been the largest OBS recording campaign so far using the new Australian National OBS Fleet. The campaign lasted from 30 November 2014 to 17 January 2015 and included 20 OBS deployments and recoveries; no instruments were lost during the survey. During the campaign at least two major milestones were achieved:
- Instruments were deployed and recovered in record water depth exceeding 2000 m (2425 m maximum at site BART 1).
- Some instruments stayed in the water continuously and recorded data for almost 47 days thus achieving the record duration of deployment of the Australian National OBS Fleet so far.
- The Australian National OBS Pool is fully operational and ready for use on a variety of projects by academia, government organisations and our industry partners. The units are stored by Geoscience Australia.
Based on every available indicator, it appears that there will be sustained demand for the use of the AuScope Earth Imaging infrastructure.
Access to the portable seismic and MT equipment is through the submission of a short scientific proposal assessed by an Access Committee for scientific merit. A regular stream of proposals for use of the component infrastructure is being received and requests frequently exceed the available capacity. In particular the number of requests for deployments overseas is on the increase. The large pool of short-period recorders reached the end of their working life and further investment through AuScope’s Australian Geophysical Observing System (AGOS) replaced this equipment. Seismometers purchased with ANU co-investment will be able to be used with the new low-power systems.
The international trend in deployments of portable seismic instrumentation is to use large numbers of instruments with many cooperating institutions. The Australian Earth Imaging community is small but very active, and will be challenged by this trend. The typical quantum used in current experiments supported by AuScope is 25 broad-band recorders with solar panels or 50 short-period recorders with internal batteries. These numbers represent what a small group can reasonably deploy. Cooperation with overseas institutions with additional personnel may help, particularly since the instrument pools of many other countries are fully subscribed for several years ahead. As well as supporting core seismic imaging work in Australia, the portable seismic instruments will become increasingly used in the surrounding regions (e.g. Indonesia) to help address fundamental issues such as seismic and tsunamogenic hazard. With these new endeavours, a more international and collaborative approach will be required.
For more information on current and potential projects or accessing AuScope’s Earth Imaging and Sounding infrastructure component please contact the Program Director, Professor Brian Kennett, ANU.