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 northern Perth Basin is an elongate sedimentary basin, located off the southwestern margin of Australia. The basin is prospective for petroleum resources, but is relatively under-explored, and the nature of the sediment-basement contact is relatively unknown due to a high degree of structuring and deep basement depth inhibiting seismic imagining. Accurate depth conversion of seismic interpretation is vital for use as constraints in gravity modelling and in other basin modelling tasks, but depth conversion requires good quality seismic velocity information. The number and distribution of wells with velocity information in the northern Perth Basin is poor, but there exists a large amount of seismic stacking velocities. Seismic stacking velocities are an outcome of seismic processing and are thus not a direct measurement of the speed of sound in rocks. To improve the quality of stacking velocities we propose a methodology to calibrate stacking velocities against well velocities, which is as follows: 1. Check each velocity dataset for errors 2. Modify the datum of each dataset to the sea floor 3. Convert all datasets to TWT and depth domain 4. Resample all velocity datasets to the same depth intervals 5. Cross plot stacking velocity depths near a well site with corresponding well depths 6. Fit a linear polynomial to this cross-plot (higher order polynomials were tried also), and determine calibration coefficient from the gradient of the polynomial. 7. Grid calibration coefficients 8. Multiply depths derived from stacking velocities by calibration coefficient grid An assessment of depth conversion errors relative to wells shows that this methodology improves depth conversion results to within ±50m; this depth uncertainty translates into a gravity anomaly error of about ±20 gu, which is acceptable for regional scale gravity modelling.

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.

Despite long history of studies the Wallaby Plateau offshore Western Australia remains a controversial feature. Analysis of interval seismic velocities from Geoscience Australia's 2008/09 seismic survey 310 in conjunction with seismic reflection interpretation provides new insights into the geology of the Plateau. Seismically distinctive divergent dipping reflector (DDR) packages have been identified. The seismic character of the DDR packages is similar to seaward dipping reflector (SDR) packages of inferred volcanic composition. Initial analysis of seismic velocity profiles indicated affinities between the DDR packages and known sedimentary strata in the Houtman Sub-basin. Effect of water loading on seismic velocities is commonly ignored in offshore studies. However, direct comparative analysis of interval velocity patterns between areas of significantly different water depth requires various water pressure related changes in velocity to be accounted for. There are controversies in methodology and application of water depth adjustment to seismic velocities, and presentation of velocity models as function of pressure rather than two-way time, or depth emerges as a possible solution. Water depth adjustment of seismic velocities analysed in our study reduces distinction between SDRs, DDRs and sedimentary strata such that discrimination between volcanic and sedimentary strata in DDR or SDR packages is equivocal. A major uncertainty of this interpretation is due to a lack of the reference velocity model of SDRs and DDRs investigated globally.

A method for calibrating seismic stacking velocities against velocities from well measurements has been developed to quantitatively assess the validity of stacking velocities in the vicinity of boreholes and to improve quality of stacking velocities for use in regional depth conversion of interpreted seismic horizons. Accurate depth conversion of seismic interpretation is vital for use as constraints in gravity modelling and in other basin modelling tasks. Examples of this methodology are given for the northern Perth Basin, Australia. The suggested workflow for calibrating seismic stacking velocities against well velocities in a simplified form is as follows: 1. Check each velocity dataset for errors 2. Modify the datum of each dataset to the sea floor 3. Convert all datasets to two-way time and depth domain 4. Resample all velocity datasets to the same two-way time intervals 5. Cross plot stacking velocity depths near a well site with corresponding well depths for equal two-way times 6. Fit a linear polynomial to this cross-plot (higher order polynomials were tried also), and determine calibration coefficient from the gradient of the polynomial. 7. Grid calibration coefficients 8. Multiply depths derived from stacking velocities by calibration coefficient grid An assessment of depth conversion errors relative to wells shows that this methodology improves depth conversion results to within ±50 m down to the maximum well depth analysed (3.5 km below sea floor); this depth uncertainty translates into a modelled gravity anomaly error of about ±20 gu, which is acceptable for regional scale gravity modelling.

<p><b> Please note: This record has been made available internally for reference only</b> <p>PIMS is a search tool for discovery of survey and well data assets and physical samples held by the Geoscience Australia Repository. Data can be ordered and viewing of samples can be arranged via an online form. Although this is public data, fees are charged to cover the cost of transcription and delivery.

Accurate seismic velocity model is essential for depth conversion and rock property determination in the context of fluid flow modelling to support site selection for secure storage of carbon dioxide. The Bonaparte CO2 Storage project funded by the Australian Government will assess the carbon dioxide geological storage potential of two blocks in the Petrel Sub-basin on the Australian NW Margin. These blocks were offered as part of the 2009 release of offshore areas for greenhouse gas (GHG) storage assessment. The Petrel Sub-basin is a northwest-trending Paleozoic rift within the southern Bonaparte Basin. The geological reservoirs of interest include the Jurassic Plover Formation and the Early Cretaceous Sandpiper Sandstone. Primary and secondary seals of interest include the Late Jurassic Frigate Formation and the Cretaceous Bathurst Island Group (regional seal). Trapping mechanisms for injected CO2 may include faulted anticlines, stratigraphic traps, salt diapirs and/or migration dissolution and residual trapping. Water depths are generally less than 100m and depths to reservoir/seal pairs range between 800-2500m below the sea surface. All three main types of seismic velocity measurements are available within the area of our study: velocities derived from stacking of multi-channel reflection seismic data; velocities determined in the process of ray tracing modelling of large offset refraction data acquired by the ocean bottom seismographs (OBS) along the coincident reflection/refraction transect, and velocities from well log (sonic, vertical seismic profiling and check shot) measurements.

Geoscience Australia acquired the Canning Coastal Deep Crustal Seismic Survey in 2014. The survey involved the acquisition of seismic reflection and gravity data along two traverses, 14GA-CC1 (562km) and 14GA-CC2 (143km) between Port Hedland and Derby, WA. The purpose of the survey was to image the crustal architecture of the geology underlying the Canning Basin and its relationship to the boundaries between the crystalline hard rock areas of the North (Kimberley) and West Australian (Pilbara) cratons. As well as establishing the subsurface extent of the Canning Basin and the extent and nature of its sub-basins and troughs. The project was collaboration between the Geological Survey of Western Australia and Geoscience Australia with funding from the Western Australian Royalty for Regions Scheme. Raw data for this survey are available on request from clientservices@ga.gov.au

During 1969, the Mundaring Geophysical Observatory collected seismic refraction data from explosions used by the Bureau of Mineral Resources No. 2 seismic party in the southwest of Western Australia. The seismic party exploded 37 charges up to 4,500 kilograms on a traverse from Balladonia through Kalgoorlie to Perth. Two mobile Willmore seismographs and permanent seismographs at Mundaring and Kalgoorlie recorded the resultant seismic waves. Raw data for this survey are available on request from clientservices@ga.gov.au - Quote eCat# 76503

This report outlines the field geophysical work carried out in the vicinity of Wilkes Base (Lat. 660 15' 8, Long. 1100 31' E), Australian Antarctic Territory, during the Australian National Antarctic Research Expedition of 1965-66. The work can be broadly divided into four parts. 1. An autumn traverse where seismic reflection stations were established every 10 miles around a triangle with corners at Cape Folger, the Dome Centre and Cape Poinsett. 2. A mid winter traverse to the inland glaciology station S-2 for a programme of seismic ice velocity studies. 3. A spring traverse where a rectangular grid network of seismic, gravity, and elevation stations were installed in a region between S-2 and 80 miles south of S-2. 4. Two attempts at recording reflections off the Mohorovicic discontinuity shot in the vicinity of Wilkes.