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  • In 2010 and 2011, the Australian Government released exploration acreage in the Perth, Mentelle and Southern Carnarvon basins off the southwest margin of Australia. This release was underpinned by two new marine geophysical surveys (GA-310 and GA-2476) that were conducted by Geoscience Australia in late 2008 and early 2009 as part of the Australian Government's Offshore Energy Security Program. These surveys acquired a range of pre-competitive geological and geophysical data that included seismic reflection, gravity, magnetic and swath bathymetry measurements, as well as seafloor dredge samples. The new surveys provided a total of about 26 000 line km of new gravity and magnetic data that add to existing data from around 150 previous marine surveys conducted off the southwest margin since 1960. This Record describes the integration and levelling of the new gravity and magnetic data with existing data, both offshore and onshore, to produce a unified gravity and magnetic dataset for use in constraining regional tectonics, basin structure and petroleum prospectivity. Levelling is a key step in processing ship-borne gravity and magnetic data. This process minimises the mistie errors at ship-track cross-overs that arise from factors such as positioning errors, instrument drift and lack of diurnal corrections to magnetic data. Without accounting for these cross-over errors, gridded data can be rendered un-interpretable by artefacts and distortions at line cross-overs.

  • A six-week geological cruise of the West German research vesselSonne, under the auspices of the Australian-German Science Agreement, hasyielded a considerable amount of information about the Exmouth Plateau, theWallaby Plateau, and the intervening Cuvier Abyssal Plain. The main aim ofthe cruise was to sample pre-Quaternary strata cropping out on the plateaumargins; subsidiary aims were to sample the Quaternary sequence from conti-nental shelf to abyssal plain on two representative profiles, to obtainQuaternary cores for gas analysis, and to search for manganese modules. Allfour aims Were successfully accomplished. During the cruise 31 single-channel seismic profiles were run tohelp select sampling targets. On the Exmouth Plateau and adjacent areas 102stations were successfully occupied, 31 yielding pre-Quaternary rocks. On the Wallaby Plateau and adjacent areas 18 stations were successfully occupied,13 yielding pre-Quaternary rocks. Samples were obtained from depths rangingfrom 100 m to 5200 m. The sampling of pre-Quaternary strata has shed new light on thearea, On the northern Exmouth Plateau thick sequences of Early Jurassicshelf carbonates and Middle Jurassic coal measures were found beneath themain (?Late Jurassic) unconformity. Cretaceous shelf and pelagic sedimentswere also shown to exist. Above the main unconformity there is a condensedsequence of Cainozoic pelagic carbonates. On the northern margin of theWombat Plateau there is a volcanic sequence at least 300 m thick beneaththe main unconformity. On the northwestern Exmouth Plateau Albian andMiocene carbonates were sampled. On the southern margin results were disa-ppointing, but Mesozoic sandstone and shale and Tertiary pelagic carbonateswere sampled. The layered sequence below the main (?Neocomian) unconformity,both on the eastern Wallaby Plateau and on the "Sonne Ridge", which extends northward from the plateau into the Cuvier Abyssal Plain, was shown to consistof interbedded weathered "basalts", tuffs, breccias, and volcaniclasticsediments. This suggests that a thick volcanic pile of Early Cretaceous ageforms much of the Wallaby Plateau. A variety of Quaternary cores, almost all of biogenic carbonates,were obtained on profiles from Rowley Shoals to the Argo Abyssal Plain, andfrom southwest of Barrow Island to the Cuvier Abyssal Plain. Quaternarycores in the central Exmouth Plateau were sampled for gas analysis; pre-liminary results indicate that methane is generally present, but in verylow amounts. Manganese nodules were obtained from the southern and easternmargins of the Wallaby Plateau, and preliminary determinations of contentsof nickel and copper suggest that they are above Indian Ocean averages, butwell below a grade of economic interest.

  • Project 121.30 - "Lord Howe Rise and Norfolk Ridge 'Law of Sea' Study" -was initiatedto improve our understanding of the geological framework of the southern part of theLord Howe Rise region, in the vicinity of the Australia/New Zealand seabed boundary zone. The major objectives of the project are: To investigate the structure, stratigraphy and basin development of the southern LordHowe Rise, southern New Caledonia Basin and the West Norfolk Ridge. To assess the resource potential of the major structural features within the region. To determine the tectonic framework, crustal characteristics and evolution of the region, and attempt to understand the processes that have produced narrow strips of thinned and extended continental lithosphere ("ribbon continents") separated bynarrow ocean basins. To acquire data to assist with the definition of Australia's 'legal' Continental Shelf on the southwestern margin of the Lord Howe Rise. Because of the presence of Lord Howe Island and Norfolk Island, Australia has laid claim to a large part of the seabed in the region, enveloped by a 200 nautical mile Exclusive Economic Zone (EEZ) around each island, plus areas of 'legal' ContinentalShelf beyond the EEZ. The total area of Australia's 'legal' Continental Shelf in the Lord Howe Rise region would be about 1.65 million km2. While the seabed boundary in the northern part of the region has been negotiated between Australia and France, there is not, as yet, a negotiated boundary between Australia and New Zealand in the southernLord Howe Rise/West Norfolk Ridge region. The proposal for Survey 114 was to acquire approximately 3250 km of multichannelseismic and associated geophysical data. The data was acquired mainly on longregional lines, two of which extended from the Tasman Basin to the Norfolk Basin, using a 3000 m streamer (120 x 25 m groups), 49 litre sleeve gun arrays, 16 secondrecords, and 30-fold coverage. The ship departed Sydney at 2000 hrs on Friday, 6 November 1992, and, after transitingthrough rough seas, arrived in the study area on the evening of 9 November. The cablewas deployed and balanced by midnight, 10 November. The next twenty one days wasspent shooting deep crustal (16 sec.) seismic . During this period a total of 3191 km ofseismic was shot, over 70% with two gun arrays operating. This was achieved in spiteof 3.5 days loss as a result of bad weather and 20 hours loss for cable repairs as a resultof fish bites. Seismic acquisition ceased on the afternoon of 1 December. The shipreturned to Sydney at 1730 hrs on Thursday, 3 December. Overall, the quality of the seismic data was very good. When shooting the two verylong lines, the weather remained reasonably calm, and so noise levels were withinspecifications.

  • Two significant offshore data acquisition surveys along Western Australia's continental margin (AusGeo News 92) were recently completed by Geoscience Australia. They form part of the agency's ongoing collection of fundamental pre-competitive data and information to understand Australia's offshore frontier basins, and assist with planning and management of Australia's marine environments.

  • Geoscience Australia has recently completed a survey searching for evidence of natural hydrocarbon seepage in the offshore northern Perth Basin, off Western Australia. The survey formed part of a regional assessment of the basin's petroleum prospectivity in support of ~17,000 sq km of frontier exploration acreage release in the region in 2011. Multibeam bathymetry, sub-bottom profiler, sidescan sonar and echosounder data were acquired to map seafloor and water column features and characterise the shallow sub-surface sediments. A remotely operated vehicle (ROV) was used to observe and record evidence of seepage on the seafloor. 71 sediment grabs and 28 gravity cores were collected and are currently being analysed for headspace gas, high molecular weight biomarkers and infaunal content. Survey data identified an area of high 'seepage' potential in the northernmost part of the study area. Recent fault reactivation and amplitude anomalies in the shallow strata correlate with raised, high-backscatter regions and pockmarks on the seafloor. A series of hydroacoustic flares identified with the sidescan sonar may represent gas bubbles rising through the water column. The ROV underwater video footage identified a dark-coloured fluid in 500 metres water depth proximal to the sidescan flares which may be oil that naturally seeped from the seafloor. The integration of the datasets acquired during the marine survey is indicative of natural oil seepage and provides additional support for the presence of an active petroleum system on this part of the continental margin.