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.

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.

Details and results are given of a seismic refraction survey made at the request of the Hydro-Electric Commission of Tasmania, to investigate the site of the western portal of the proposed Mossy Marsh Tunnel. The tunnel is part of the No. 2 Tarraleah Canal project to transport water from Lake King William to Tarraleah Power Station. The primary object of the survey was to determine the thickness of till overlying the dolerite bedrock, and hence contours of the bedrock surface.

Presentation delivered at the Tasman Frontier Workshop, 89- March 2012

A seismic reflection survey was made to the south-west, south and east of the Poole Range Structure, Kimberley Division, W.A. as an extension of a previous seismic survey. The results obtained confirm the surface information and show the existence of an anticlinal structure l the axis of which plunges to the east and whose axial plane probably dips to the south. A total thickness of sediments of the order of 20,000 feet is shown. Some slight evidence of deep faulting down to 10,000 feet was recorded, but is not conclusive. Insufficient evidence was obtained to select a site for a test bore, but further seismic work is not recommended at present.

A 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 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 not certain. 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 the north-eastern side of the Pinracle Fault, and 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 degree of conformity between shallow and deep reflections on the northern flank of the dome. Further investigatioll 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 rocks on tbe north-eastern side of the Pinnacle fault are shown to have a probable unexposed thickness of about 900 feet.

A seismic survey was made in the.Ooraminna area of the Amadus Basin 9 Northern Territory 9 by a seismic party of the Bureau of Mineral Resources 9 Geology and Geophysics 9 between 6th July and ,29th August 1962. Reflection traverses were recorded between Deep Well and Alice Springs and across the culmination of the Ooraminna Anticline l and a refraction depth probe was recorded on the crest of the Ooraminna Anticline. The purpose of the survey was to obtain information on the structure of the Basin in the eastern part of the Missionary Plain, and on the structure of the Ooraminna. The Basin was shown to be generally synclinal between Deep Well and Alice Springs with a maximum sediment thickness Of 20,000 feet north of the Ooraminna Anticline. Seismic evidence suggested that the uplift at Deep Well and the Ooraminna Anticline have resulted from thickening in a deep formation, which has been tentatively identified as the Bitter Springs Formation. The survey was unsuccessful in relating reflectors in the crosssections with outcropping formations in the MacDonnell Ranges and on the Ooraminna Anticline.

An experimental seismic survey using both refraction and reflection techniques was carried out in April, 1958, near Morwell in the Latrobe Valley at the request of the State Electricity Commission of Victoria. The object of the survey was to find if the method was of value in mapping the structure of the coal measures of the Latrobe Valley and in providing information on the depth to and type of basement underlying the coal measures. Work was concentrated in an area south-west of Morwell on the southern limb of the Latrobe Syncline. The results obtained indicate that the seismic method may be applied successfully to geological problems of the Latrobe Valley and may provide useful control data for the interpretation of surface geological and gravity mapping. Various interpretations of the results are discussed and although some ambiguity exists, it might be overcome when more work is done, particularly if an accurate knowledge of the velocities of the coal easures is obtained. It has been possible by means of refraction work to map the extension of the basalt which crops out on the southern margin of the Latrobe Syncline beneath the coal measures with reasonable certainty.

The refraction seismic tests described in this report were made at the request of the Victorian Railways Department. The work was located within the railway reserve between the outskirts of Wodonga township and the River Murray, and consisted of the shooting of five refraction traverses, set out approximately parallel to the railway line and covering in detail a total length of 4,800 feet. The aim of the seismic work was two-fold. Firstly an investigation was required of the subsurface formations in the vicinity of the bridges which carry the railway across several creeks intersecting the Murray flood plain. The subsurface information is required in connection with the design of the foundations of new bridges which will be constructed to replace the existing ones. The second purpose of the work was to test whether the refraction seismic method would be suitable for adoption by the Railways Department engineers as a standard routine method for investigating foundation conditions. To fulfil this purpose ideally, the method would need to be applicable generally to all the foundation problems encountered by the Department and to completely eliminate the necessity for test drilling of sites. The flood plain of the Murray, on which the seismic measurements were made, is composed of Recent alluvium, probably underlain by Tertiary river deposits and is crossed by several creeks and anabranches of the main stream. The outcrops in the vicinity of Wodonga show the bedrock to be granite and metamorphic sediments. The field work was done between February 26th and March 12th, 1953. The field party comprised two geophysicists and three field assistants provided by the Railways Department.

A hybrid high-resolution seismic survey was undertaken adjacent to the Eurabba State Forest near Quandialla, Western NSW between June 29 and July 3 2003 acquiring both refraction and reflection data. The Australian National Seismic Imaging Resource (ANSIR) carried out the field work in conjunction with University of Canberra and Dryland Salinity Hazard Mitigation Program (DSHMP) research staff and students. Processing of the acquired refraction information was performed with the assistance of ANSIR staff. This survey was designed to collect and compare a shallow, high-resolution seismic dataset against nearby regional scale seismic datasets and other complementary sources of spatial information including NanoTEM, drill hole data, satellite imagery and regolith-landform mapping. The multi-disciplinary approach is designed for imaging shallow sedimentary structures and determining depth to bedrock. The combined aim of utilizing the multidisciplinary approach is to understand shallow fluid flow within the Booberoi-Quandialla Transect area and how this relates to observed outbreaks of dryland salinity. These observations will aid in the development of a shallow fluid flow model for the Bland catchment, especially over the Booberoi- Quandialla Transect area. Raw data for this survey are available on request from clientservices@ga.gov.au