No abstract available

Deep seismic reflection profiling of the crust often images structures with dips that are less than those predicted from outcrop scale geology. This is often explained in terms of the seismic process being tuned to sub-horizontal reflectors. However, for typical acquisition parameters in Australian continental studies, steep dips can be imaged in the near surface, and dips up to 40? near the base of normal thickness crust. The key processing steps for imaging steep reflectors at shallow levels are spectral equalisation to suppress near surface noise, fine-tuned statics corrections and detailed stacking (NMO) velocity analysis, including application of dip moveout (DMO). Two effects on stacking velocity must be considered: (1) A high stacking velocity gradient exists at shallow levels due to the low velocity regolith which can exceed 100 m thickness in many parts of Australia and (2) stacking velocity for dipping reflectors equals V/cos theta, theta being the dip. Since normal moveout is most sensitive to stacking velocity at small two-way travel times, it is impossible to simultaneously stack shallow horizontal and steeply dipping reflectors unless DMO or pre-stack migration is applied. In areas of rapidly varying bedrock topography, refraction statics alone may not be sufficient. A useful technique for fine-tuning refraction statics with automatic residual statics involves keying on deeper, more continuous horizons with minimal sensitivity to stacking velocity. A combination of these steps for both the Lachlan Fold Belt and the Yilgarn Craton successfully imaged reflectors dipping up to 60? and extending from 1 to 2 seconds TWT to base of regolith. Since theory and practice confirm that steep dips can be imaged, the dichotomy between seismic results and geological prediction would therefore indicate that outcrop geology is a poor predictor of regional dip, at least at the scale of the seismic wavelength.

In 1999, a grid of five deep seismic reflection traverses was acquired within an area approximately 50 km wide by 50 km long in the Kalgoorlie Region, Eastern Goldfields Province, Yilgarn Craton. The grid tied into the existing 1991 deep seismic reflection transect (EGF1) and the 1997 high resolution and regional seismic profiles acquired by the Australian Geodynamics Cooperative Research Centre (AGCRC) and Kalgoorlie Consolidated Gold Mines (KCGM). The data were acquired to examine the geometry of the major structural features of the region, particularly the highly mineralised Bardoc Shear, and to provide three-dimensional information on granites-greenstones relationships. This paper describes the geometry of the crust and, in particular, the geometry of the granite and greenstones above the prominent regional detachment surface that occurs at about 4-6 km depth, though in one place it may extend to a depth of approximately 11 km. From the seismic, the Bardoc Shear is confirmed as west dipping and a non-planar crustal penetrating structure. The gravity modelling suggests that there is no need for the large volumes of mafic or ultramafic material previously assumed to be at depth, apart from those mapped at the surface and projected to depth.

The seismic stacking velocity data in the Great Australian Bight are a useful dataset for calculating depths and sediment thicknesses. This work compares these data with P-wave velocities from sonobuoys and sonic logs from wells, and on this basis a depth over-estimate of at least 15% can be expected from the depths derived from stacking velocities. Megasequence boundary depths are calculated for the Ceduna Terrace to further illustrate data quality. The database makes avaliable the unfiltered stacking velocities using conventional and horizon-consistent formats.

Chapter in Geoscience Australia Record for Northern Yilgarn Seismic Workshop

No abstract available

Labuan Basin lies in deep water adjacent to the eastern Kerguelen Plateau. The basin is about 800 km long and 300 km wide and contains up to 4.5 km of sediment. A general lack of geophysical data and geological samples in this remote basin have inhibited understanding of its stratigraphy and crustal origin. Our new seismic stratigraphic interpretation of the Labuan Basin is based on deep multichennel seimic data collected by Geoscience Australia in 1997 during "Rig Seismic" surveys 179 and 180 intergrated with results of Ocean Drilling Program (ODP) Leg 183 (1998-1999)

No abstract available

A ~400 km long deep crustal reflection seismic survey across central Victoria, Australia, was carried out in 2006 as a collaborative project between the pmd*CRC, Geoscience Australia, the Victorian Government, Ballarat Goldfields NL, Gold Fields Australasia Pty Ltd and Perseverance Corporation Ltd, using the facilities of the National Research Facility for Earth Sounding (ANSIR). The aim was to cross several Neoproterozoic-Palaeozoic basement zones and provide information on the crustal architecture, particularly across the highly prospective Palaeozoic rocks occurring along strike to the north of the major Victorian goldfields, such as Bendigo. In the west, the Moyston Fault is a major east-dipping planar fault near the eastern edge of the Grampians-Stavely Zone, which was probably the eastern margin of continental Australia in the Cambrian. It cuts through the entire crust to the Moho. The Stawell Zone, immediately east of the Moyston Fault, has the geometry of a doubly vergent wedge. The boundary between the Stawell Zone and the Bendigo Zone farther to the east is the Avoca Fault, which appears to be a west-dipping listric fault that links to the Moyston Fault at a depth of about 22 km, forming a Y-shaped geometry. Internal faults in the Stawell and Bendigo zones are almost entirely west-dipping listric faults, which cut deep into the highly reflective lower crust, interpreted to be stacked ? Cambrian oceanic crust. Previous models advocating the presence of a mid-crustal detachment are not supported by these deep crustal scale faults. The boundary between the Bendigo and Melbourne zones, the Heathcote Fault Zone, forms a zone of strong west-dipping reflections about three kilometres wide to a depth of at least 20 km, and possibly to the Moho. The fault zone is complex and contains a boninite-tholeiite association along with blueschists in a serpentinite-matrix melange, and oceanic sedimentary rocks. The Melbourne Zone contains a deformed sedimentary pile up to 15 km thick, and contains previously unrecognised north-dipping listric faults, interpreted to be thrusts. The Governor Fault separates the Melbourne Zone from the Tabberabbera Zone and contains similar rocks to the Heathcote Fault Zone. Near the surface, the Governor Fault dips to the north at about 10°. The seismic character of the lower crust below the Melbourne Zone (the "Selwyn Block") is significantly different to that observed below the Bendigo and Stawell zones, and consists of several very strong subhorizontal reflections about 5-6 km thick starting at about 18 km depth, with a less reflective zone below it. In summary, the deep seismic data across central Victoria has allowed the geometry of the rocks and structures mapped at the surface to be projected through the entire crust, thus providing important constraints to test previous tectonic models.

No abstract available