In 2008, as part of the Australian Government's Onshore Energy Security Program, Geoscience Australia, acquired deep seismic reflection, wide-angle refraction, magnetotelluric (MT) and gravity data along a 250 km east-west transect that crosses several tectonic domain boundaries in the Gawler Craton and also the western boundary of the South Australian Heat Flow Anomaly (SAHFA). Geophysical datasets provide information on the crustal architecture and evolution of this part of the Archean-Proterozoic Gawler Craton. The wide-angle refraction and MT surveys were designed to supplement deep seismic reflection data, with velocity information for the upper crust, and electrical conductivity distribution from surface to the upper mantle. The seismic image of the crust from reflection data shows variable reflectivity along the line. The upper 2 s of data imaged nonreflective crust; the middle to lower part of the crust is more reflective, with strong, east-dipping reflections in the central part of the section.The 2D velocity model derived from wide-angle data shows velocity variations in the upper crust and can be constrained down to a depth of 12 km. The model consists of three layers overlying basement. The mid-crustal basement interpreted from the reflection data, at 6 km in depth in the western part of the transect and shallowing to 1 km depth in the east, is consistent with the velocity model derived from wide-angle and gravity data. MT modelling shows a relatively resistive deep crust across most of the transect, with more conductive crust at the western end, and near the centre. The enhanced conductivity in the central part of the profile is associated with a zone of high reflectivity in the seismic image. Joined interpretation of seismic data supplemented by MT, gravity and geological data improve geological understanding of this region.

Various aspects of isostasy concept are intimately linked to estimation of the elastic thickness of lithosphere, amplitude of mantle-driven vertical surface motions, basin uplift and subsidence. Common assumptions about isostasy are not always justified by existing data. For example, refraction seismic data provide essential constraints to estimation of isostasy, but are rarely analysed in that respect. Average seismic velocity, which is an integral characteristic of the crust to any given depth, can be calculated from initial refraction velocity models of the crust. Geoscience Australia has 566 full crust models derived from the interpretation of such data in its database as of January 2012. Average velocity through velocity/density regression translates into average density of the crust, and then into crustal column weight to any given depth. If average velocity isolines become horizontal at some depth, this may be an indication of balanced mass distribution (i.e., isostasy) in the crust to that depth. For example, average velocity distribution calculated for a very deep Petrel sedimentary basin on the Australian NW Margin shows no sign of velocity isolines flattening with depth all the way down to at least 15 km below the deepest Moho. Similar estimates for the Mount Isa region lead to opposite conclusions with balancing of average seismic velocities achieved above the Moho. Here, we investigate average seismic velocity distribution for the whole Australian continent and its margins, uncertainties of its translation into estimates of isostasy, and the possible explanations for misbalances in isostatic equilibrium of the Australian crust.

The Oaklands-Coorabin Coalfield in the Riverina Division of New South Wales has been known for many years. Seismic refraction tests were carried out on a number of sections to assist in the interpretation of the gravity results during July and Sepetember, 1949.

A reconnaissance seismic reflection and refraction survey in the East Otway Basin, Victoria, was carried out by the Bereau of Mineral Resources from mid-February to mid-June 1967. The objective of the survey was to determine whether the gravity low areas of the Torquay Embayment and Port Phillip Sub-Basin in the eastern part of the Otway Basin contain thick Cretaceous sediments like those which has shown potential hydrocarbon source and reservoir characteristics in the western part of the Otway Basin. Nine reflection and five refraction traverses were recorded in the gravity low areas of the Barwon Trough and Port Phillip Sub-basin. Single-coverage reflection results of variable quality were obtained. Evidence for the presence of Tertiary section is provided by shallow reflections of good to fair quality, but the evidence for Cretaceous sediments is tenuous because of the poor quality of the deeper reflections, some of which may be multiples. The presence of several faults, onlappings and pinch-outs is also indicated. The refraction results are considered unreliable because of the difficulty of interpreting the discontinuous profiles and because of the mapped and suspected faults and pinch-outs in the sections.

The seismic survey extending over the Poole Range and Price's Creek areas and the Pinnacle Fault, near the north-eastern boundary of the Fitzroy Basin. The survey was corducted during the winter of 1953. The Poole Range Dome has been mapped in outcropping rocks of Permian age, but its western closure is notcertain. It is at the south-eastern end of a line of anticlinal folding which includes the St. George Range Dome and Nerrima Dome. The target beds for an oil test bore would be the Devonian and/or Ordovician rocks, which crop out on tbe north-eastern side of the Pinracle Fault, ard over which the Permian rocks of the Poole Range are believed to lie unconformably. The seismic results indicate a thick section of sediments on the south-western side of the Pinnacle Fault and show a fair defree of conformity between shallow and deep reflections on the northern flank of the dome. Further investigation was made in 1954 around the flanks of the dome, to determine whether or not the domal structure persists at depth, but the interpretation of the results of the 1954 survey is not yet complete. The Ordovician roeks on tbe northeastern side of the Pinnacle fault are shown to have a probable unexposed thickness of about 900 feet.

A seismic velocity survey was carried out in Associated Freney Oilfields Nerrima No. 1 Bore by the Bureau of Mineral Resources on the 10th August 1955. The well is situated on the Nerrima Dome in the Fitzroy Basin, W.A. Some trouble was experienced with cable breaks for the shallow part of the hole, but in general it was possible to recognise the true formation break. Average measured velocities ranged from 8000 ft/sec near the top to 12,200 ft/sec for the total depth of the bore.

A seismic reflection traverse was surveyed across the Perth Basin, Uestern Australia, between the townships of Rockingham and Mundijong. It was planned in order to give information regarding the depth of the Basin and its structure adjacent to the Darling Scarp. Seismic refraction traverses were surveyed to give the longitudinal velocities in the near surface granitic gneisses on the Precambrian Shield, and in the Cardup Series (Proterozoic) abutting the Darling Scarp. At least 14,000 ft of sediments are indicated in the deepest part of the Basin but there is no clear seismic evidence of what a maximum thickness might be. Seismic reflection results indicate that the sediments on the west of the Darling Scarp abut the older rocks on a plane that dips at about 60 degrees to the west and that cuts the surface some distance in front of the present position of the scarp. This suggests that the Darling Scarp at Eundijong is the surface expression of a normal fault. However, the presence of reflection alignments east of this postulated fault plane, and thus apparently arising within the granitic gneisses, is contrary to the fault hypothesis. The true nature of the tectonic features is thus unresolved. Seismic results indicate that faulting occurred within the Basin and such faulting may have completed closure of possible oil traps. Further seismic investigation of the faults and associated structures is recommended.

An experimental seismograph survey was carried out near Heywood in the Western District Basin, south-western Victoria, during November and December, 1956 by the Bureau of Mineral Resources, Geology and Geophysics. The work was requested by Frome-Broken Hill Pty. Ltd. and was intended primarily to ascertain if reflections from deoper sediments could be recorded through a surface layer of basalt which covers considerable areas in the Western District of Victoria. Several short traverses were shot during the survey at places where a variety of surface conditions for seismic exploration could be tested. Pattern and air-shooting techniques were tried as well as the conventional single shot-hole technique. Good reflections were recorded from depths down to eleven thousand feet in areas where there was no basalt. Some apparent reflections of poor quality were recorded at times as great as 5 seconds after the shot was fired. An attempt has been made to correlate the reflections with stratigraphic horizons. Reflections were obtained from strata beneath a basalt cover in some places when explosive charges were fired in single shot holes; reflection quality was improved when pattern and air-shooting techniques were used.It was not possible to record reflections through a cover of tuff containing basalt bands on the slopes of Mt. Clay. Pattern and air-shooting were tried unsuccessfully. Sub-surface information in the Heywood area is obtainable by seismic exploration and techniques for gaining the best information from the seismic method are discussed.

In July and August 1957 an experimental seismic survey was done in the Oodnadatta area of the Great Artesian Basin. The purposes of the survey were to find whether reflections could be recorded from beneath duricrust, a siliceous surface deposit, and whether structures mapped by surface geological methods persist with depth. Reflections were recorded from beneath the duricrust using shallow pattern holes and six geophones per trace; the sub-surface structure was mapped with reasonable accuracy. In areas where the duricrust is eroded, reflections of fair quality were obtained using a single shot-hole and six geophones per trace. A seismic reflection traverse across the Oodnadatta anticline indicated that the structure was present in a horizon which corresponds to the top of the artesian aquifer at a depth of about 1000 ft below datum (400 ft above MSL). The seismic results indicated that the anticline was of smaller relief than had been e stimated from surface mapping. There was a change from fair-quality persistent reflections at shallow depths to poor-quality less numerous reflections with sporadic dips at greater depths; this probably represents the base of the Cretaceous. The greatest depth from which Cretaceous sediments were recorded was about 2350 ft below datum. Reflection depths computed by seismic methods correspond closely with lithological boundaries, and in particular the base of the Cretaceous sediments, encountered in the Santos No. 1 bore. The results of a refraction traverse on the crest of the Oudnadatta anticline show the presence of a 'basement' refractor with a velocity of 13,900 ft/sec at a depth of about 1245 ft below datum. There is slight evidence of a refractor with a substantially higher velocity at about twice this depth. The 'basement' velocity of 13,900 ft/sec is consistent with the assumption that there is a pre-Cretaceous layer between the Cretaceous sediments and the Precambrian basement complex.

A reconnaissance seismic survey was made in the area of Quilpie and Et.omanga in south-western Queensland. Traverses crossed the Harkaway, Pinkilla, and Tallyabra Domes. Reflection horizons were correlated with horizons within the Mesozoic sediments, and one persistent reflection was correlated with a horizon near the top of the Palaeozoic sediments. A thickness of sediments of up to 15,000 ft, including up to 11,000 ft of Palaeozoic rocks, was indicated on the flanks of the Harkaway and Pinkilla Domes. Results were compared with existing gravity data. Suggestions of faulting are based on seismic and gravity evidence taken together and also on gravity evidence alone in locations not covered by the seismic work.