No abstract available

The research project, M346, was divided into two phases. The initial phase involved collection of 70 km of deep seismic reflection data along two sub-parallel traverses (01AGSNY2 and 01AGSNY4) to the south of the regional traverse (Figure 1). The second phase of the project concentrated on the processing of the seismic data, followed by interpretation of these seismic reflection data jointly by the researchers and industry participants. This research project was designed to provide information on the uppermost crustal structure within specific mineralised regions within the Laverton Tectonic Zone of the Eastern Goldfields and image the internal structural and stratigraphic relationships within this zone.

Deep seismic reflection profiles in mineral provinces in Australia are used to identify the main crustal-scale architecture which is the key to determining regional scale controls on mineralisation and possible fluid migration pathways. The results constrain the crustal architectures during the building of 3D geological maps of the regions, and are challenging our current understanding of the geology, ore deposit models and prospectivity of Australia.

The seismic stacking velocity data in the Otway Basin are a useful dataset for calculating depths and sediment thicknesses. This work presents time-depth relationships computed from unsmoothed stacking velocities and compares these with functions obtained from sonobuoy refraction data and exploration well sonic logs. The comparison suggests that a total sediment thickness over-estimate for the Otway Basin of about 15% can be expected from the depths derived from stacking velocities alone. On the other hand, for sediment thickness calculations down to ~3 s two-way travel time below sea floor, stacking velocity data give comparable depths to those obtained from the sonic logs. A piece-wise formula is offered which scales the time-depth function for the Otway Basin in order to compensate for the depth overestimate inherent in using stacking velocities to calculate total sediment thickness.

No abstract available

This report presents results from a pilot study conducted within the northern part of the Great Australian Bight, focusing primarily on the Eyre Sub-basin. The aim of the study was to develop and test a methodology for creating petroleum prospectivity maps based primarily on the extent of sealing lithologies. The Eyre Sub-basin provides a good basis for this study due to the good seismic coverage and lithological data provided from eight ODP holes and an Esso petroleum exploration well (Jerboa-1). Well data are used to determine which sequences contain potential sealing lithologies, while seismic data are used to map out the extent and distribution of potential sealing sequences across the study area. The regional extent of sealing units to their first pinch-out is used as a first-order assessment of petroleum prospectivity. Results from Jerboa-1 show that there is a proven oil source and adequate reservoir facies with seal and trap integrity being the major play risks. Mesozoic sediments provide the primary potential sealing units across this region and have greatest thickness across the Eyre Sub-basin. To the west and north of this sub-basin, the distribution of the Mesozoic sediments is discontinuous, resulting in lower sealing potential. Mesozoic sediments are absent within the Eucla Inlier immediately north of the Eyre Sub-basin, resulting in very low sealing potential. Three categories of petroleum prospectivity have been determined in this study. Good prospectivity has been assigned up to the limits of the thick and continuous Mesozoic sediments in the Eyre Sub-basin. Low prospectivity has been assigned to the western Apollo Shelf, reflecting the reliance on relatively thin and discontinuous Mesozoic seals and the poor seal potential of Tertiary age carbonates. The Eucla Inlier is designated as a non-prospective area due to the absence of Mesozoic sediments and any potential sealing units for Tertiary age reservoirs.

3D visualisation of the Mount Isa Crustal Seismic Survey

A seismic survey on the Capel and Faust basins, 800 km east of Brisbane, was undertaken by Geoscience Australia in November 2006 - January 2007 to improve the understanding of the petroleum prospectivity of the area. The survey was part of the New Oil Program to encourage exploration. A comparison between pre-existing and newly acquired seismic data correlated with the newly acquired gravity and magnetic data provides insights into the deeper structure and the thickness of the various depocentres in the region. Reprocessing of older seismic survey can produce significantly improvements in data quality and interpretation but in difficult data areas it may be limited in its effectiveness due to non-optimum data collection. We will examine the advantages of an 8 kilometre streamer used to collect the current survey over previous survey with streamer lengths of less than 3.3 kilometres. Modern processing techniques although powerful can be limited in their effectiveness to produce an interpretable product and we will examine these limitations. The new long offset data has allowed better imaging of the sedimentary packages so better estimates of economic basement can be determined. Several new depocentres have been seen and for the first time show up to 4 seconds twt thickness. The thickness, shape and size of these depocentres suggest that they may be favourable for hydrocarbon generation. Structure and fabric within the basement allow a better understanding of the basin subsidence history.

The tectonic setting of the giant Olympic Dam iron oxide-copper-gold ore deposit in South Australia has been uncertain to date. Given the economic significance of the Olympic Dam deposit and its influence in defining the mineral deposit class, resolving its tectonic setting is important. To help address this, two orthogonal deep seismic reflection profiles, centred on the ore deposit, were recorded to 18 s TWT. An approximately north-south line, 193 km long, was oriented as near to a regional dip direction ?defined by potential-field data? as land access would allow. It imaged units of the Archaean?Proterozoic Gawler Craton and a possible allochthonous Proterozoic terrane to the north. A shorter east-west cross-line (57 km long) provided information on any out-of-plane structures imaged on the longer profile in the region of the ore deposit. The seismic data show that the Olympic Dam deposit lies between two distinctly different pieces of crust. To the north of the deposit and the craton, the upper crust beneath the cover sequences shows south-dipping reflectors, interpreted as shear zones that cut through an upper crust of moderate sub-horizontal reflections and a lower crust of higher-amplitude reflections. To the south, the upper crust and lower crust have signatures indicating thrusting towards the craton, in a pattern indicative of a fold and thrust belt of crustal scale. However, in the south, the upper crustal deformation is decoupled from lower crustal deformation at a 3-4km thick band of sub-horizontal reflectors centred at about 12km depth (4s TWT). This zone is mostly un-deformed, except where, in places, it is thrust and duplicated. The middle to lower crust in the region between the two distinctly different crustal types in the north and south contains few reflections, and appears anomalous but homogenised compared with the crust to the north and south, and is interpreted as a likely source region for the Burgoyne batholith which hosts and is approximately coeval with the Olympic Dam ore deposit. Four hypotheses are tested for the lower crustal heat source that generated the granite melt. The intrusion of mafic sills into the lower crust is not favoured because no evidence of widespread seismically reflective underplate can be seen in the data. Heating by a plume is not favoured because the region contains no plume-related geological evidence, and the signature in the seismic data would probably be similar to that of underplating. Extension is not favoured, because the data show no evidence of crustal extension accompanied by surface subsidence and sediment deposition; rather they can be interpreted to show crustal shortening at the time. Heating by radioactive decay is worthy of further consideration because of the high concentrations of heat producing elements in the granite.

Geoscience Australia has more than 50 years experience in the acquisition of deep crustal onshore seismic data, beginning with analogue low-fold explosive data, progressing through digital explosive data, and finally, in the last 12 years, moving into the digital vibroseis era. Over the years, shot data in a variety of formats has been recovered from a variety of media, both in-house and by external contractors. Processing through to final stack stage was used as a QC tool for transcription of some of the older analogue surveys, and proved so successful that the reprocessed data was released for interpretation. In other cases, more recent digital explosive surveys have benefited from reprocessing using modern processing algorithms. Key modules in Paradigm Geophysical's Disco/Focus software used by Geoscience Australia for reprocessing old data include refraction statics, spectral equalisation, stacking velocity analysis, surface consistent automatic residual statics and coherency enhancement. Coherency enhancement is commonly carried out on both NMO corrected shots and stack sections, with several iterations of NMO and autostatics. With the irregular offset distribution and low fold of legacy explosive data, dip moveout (DMO) correction is not possible, but due to the shorter spreads is not as critical as for modern high fold vibroseis data. Nevertheless, 'poor man's DMO' has proved successful in the shallow section, by the simple expedient of omitting 25% of the traces with the longest offsets.