Project I1 project development team meeting, Mount Isa, November 2003

Project I1 project development team meeting, Mount Isa, November 2003.

Review of Karst Hazards in the Wanneroo Area, Perth, Western Australia

Within the Perth region, a karst belt lies five kilometres inland from the coast and stretches for approximately 24 kilometres in a north west to south east direction. This area is made up of one main geological unit, the Tamala Limestone which is surrounded by residual sands formed from the erosion of the limestone.

Seismic velocities and deep reflections on the Australian North West Margin: some implications for crustal models

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.

Camden Sound Special