The seismic stacking-velocity data in the Great Australian Bight are a useful dataset for calculating depths and sediment thicknesses on a regional scale. 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 available the unfiltered stacking velocities using conventional and horizon-consistent formats.

Deep-seismic reflection data have provided information on the crustal architecture of several highly mineralised regions within the Archaean northeastern Yilgarn Craton, Western Australia. These seismic data are characterised by several prominent features and include 1) a change in the thickness of the crust across the northeastern Yilgarn Craton, 2) subdivision of the crust into three broad layers, 3) a prominent east dip to the majority of the reflections, and 4) the identification of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones divide the region into four terranes and are surprisingly similar in geometry. In the hangingwalls of the shears, there is evidence of a marked increase in deformation adjacent to the shear zone. This region is underlain by another low-angle shear zone at depth. Major orogenic Au deposits in the northeastern Yilgarn are spatially associated with major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class Au deposits plus many other smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed as the Laverton structure. We infer that the complex deformed region within the hangingwall and underlain by a low-angle shear zone forms a wedge-shaped trap with upward and/or sub-horizontal moving fluids being focused into the apex of the wedge.

Porosity and saturation are two important petrophysical properties among many others that play a crucial role in the study of reservoir characterization, flow modeling, simulation etc. Well logging techniques supplemented with geostatistical methods could provide a very high resolution estimate of those properties; but it becomes severely constrained due to availability of limited number of wells only at sparse locations. An overall estimate of porosity and saturation over a wide spatial extent (both vertically and laterally) is nonetheless necessary for a detailed study of a reservoir. We demonstrate that full waveform inversion of prestack seismic data can be a useful tool in estimating porosity and saturation of a reservoir. We conduct sensitivity analysis of porosity and saturation on seismic velocities. We use modified Biot-Gasmann equations for sensitivity analysis and forward modeling computation. A gradient-based technique aided with adaptive regularization is used for inverse modeling of full-waveform prestack seismic data. We present the results of numerical experiment on both synthetic and field seismic data.

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A two-dimensional crustal velocity model has been derived from 1997 wide-angle seismic profiling across the Lachlan Transverse Zone (LTZ) in the eastern Lachlan Orogen. The LTZ is considered to be a significant early tectonic feature controlling structural evolution in eastern Australia. The 364 km north-south profile passed from Ordovician volcanic and volcaniclastic rocks (Molong Volcanic Belt of the Macquarie Arc) in the north across the LTZ into Ordovician turbidites and Early Devonian intrusive granitoids in the south. The velocity model highlights significant lateral variations in sub-surface crustal architecture within the upper and middle crust. In particular, there is a higher P-wave velocity (6.24-6.32 km/s) unit identified in the upper crust under the arc at 5 to15 km depth that is not seen south of the LTZ. Near-surface P-wave velocities within the LTZ are markedly less than those along other parts of the profile and these are attributed to mid-Miocene volcanic centres. In the middle and lower crust there are also poorly defined velocity features that we also interpret to be related to the LTZ. The interpreted Moho depth increases from 37 km in the north to 47 km in the south above an underlying upper mantle with a P-wave velocity of 8.19 km/s. The seismic data indicate significant differences in crustal architecture between the northern and southern parts of the profile within the upper and middle crust, with associated strong indications that the LTZ may also have through-going crustal features to Moho depths.

Crustal reflectivity and bulk seismic velocity variation in the crust do not always closely correlate. Even the most prominent reflection horizons do not always follow iso-velocity contours. These are the major conclusions of co-interpretation of refraction/wide-angle reflection data and conventional reflection profiles on the North West Australian Margin (NWAM).

The North West Australian Margin, which has a large hydrocarbon potential, has been studied by combined ocean-bottom seismograph (OBS) and deep reflection profiling along five regional transects which have crossed all major structural elements of this region. Average spacing between the OBSs of 30 km and shot spacing of 100 m with data recording to maximum offsets of 300 km enabled development of accurate crustal-scale seismic velocity models. The thorough wave field analysis, a distinctive feature of our approach to the interpretation of the OBS data, provided realistic starting models for subsequent travel-time inversion by iterative forward modelling. Deep reflection data along the coincident profiles were recorded as part of AGSO's 35,000 km regional grid of seismic lines. Consistent interpretation of several key horizons tied to over 100 petroleum exploration wells through the entire grid created the basis for co-interpretation of the OBS and deep reflection data. Due to the effects of fine seismic stratification of the crust, prominent seismic reflectors and changes in reflectivity patterns imaged by reflection data do not necessarily correspond to significant bulk velocity changes. Velocity variation estimated from the OBS data along the interpreted reflection horizons shows no single simple trend. A number of factors affect this velocity variation and it has to be interpreted individually for individual horizons in different basins. Velocity models improved definition of the basement on the Carnarvon and Canning transects, but it remains problematic in the Vulcan and Petrel sub-basins. Carnarvon Basin is the only one at the NW Australian Margin where the crustal extension is associated with decrease of seismic velocity in the lower crust. Reduction in total crustal thickness beneath the Browse Basin is achieved mostly due to the thinning of the lower crust. Underplating, which is often associated with large-scale extension of the crust, was not a major crustal forming event in the region. It appears to have been restricted only to the offshore Canning Basin and the outer, western part of the Carnarvon Basin. Moho, a difficult target for deep reflection profiling, is imaged by the OBS data quite well. Transition from continental crust to oceanic is accompanied by the non-uniform reduction of crustal thickness from 28-36 km to 8-14 km. The steepest Moho was found next to the continent-ocean boundary in the Canning Basin, where crustal thickness reduces from 34 to 13 km over a distance of ~100 km. On some transects (most of the Canning, Vulcan and Petrel) depth to Moho increases with depth to basement increase, although conventional models of crustal extension suggest otherwise. On the Canning and Petrel transects local Moho highs correspond to the steepest slopes of the basement. These observations have to be accounted for by models of crustal extension. Crust adjacent to the outer boundary of the NWAM studied by the outer parts of the Carnarvon, Browse and, to a lesser degree, Canning transects is not purely oceanic but rather transitional.

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.

Towards the end of 1950, the deep bore which the Shell Development Company was drilling to test for oil on the Morella structure, 35 miles south of Rolleston, entered andesite at approximately 4000 ft. After boring about 200 ft into the andesite, the Company decided to abandon the hole. A new site for a test bore was selected on the Comet anticlinal structure, 60 miles north of Rolleston. Before proceeding with this new test, evidence was needed to ensure that neither a shallow basement nor volcanic rocks existed under this structure. The Bureau was therefore asked to shoot a refraction traverse over the anticline, in order to determine if basement rocks were likely to be present at a shallow depth. As a result the Bureau sent a seismic party to the area during January 1951. Fairly definite evidence was obtained of basement velocities occurring at a depth of less than 3000 ft, and after discussions, a decision was made not to proceed with the drilling of the Comet structure. The results have now been investigated in detail and are presented in this report.