New Insights Into the Basement and Crustal Structure of the Australian NW Margin Along Newly Analyzed Seismic Refraction Lines
Around 20 years ago the Australian Geological Survey Organization (AGSO) recorded high-quality refraction and wide-angle reflection seismic data on the NW Margin of Australia. They used ocean-bottom seismographs (OBS) with a maximum offset of 300 km along five transects, which coincide with previously acquired deep-crustal seismic reflection profiles. The profiles run across five major basins, from South to North: Northern Carnarvon, Canning, Browse, Vulcan and Petrel. Four of the newly acquired Ion WestraliaSPAN 2D reflection lines, which have long offset and long record length (18 sec), were designed to image down to the base crust along these older refraction lines. Data from 83 OBS locations were recently provided by Geoscience Australia for a study to re-evaluate and constrain the velocities and densities of the deeper section and gain a better understanding of its rock composition and crustal architecture. Observations from selected lines will be presented, as each profile was modelled as an individual 2D line. Modelling was performed using JIVE3D (Joint Interface & Velocity Estimation) tomographic inversion, which uses a combination of both wide-angle OBS data and normal incidence reflection horizons as input. Horizons picked from reflection profiles helped to constrain the shallow structure, in particular when the OBS spacing was quite wide. The deeper section was modeled without horizon constraint. The inclusion of both normal incidence and wide-angle reflections allowed first-order velocity discontinuities to be included in the model. This overcame the shortcomings of more traditional tomographic inversion techniques, which produced a smooth model throughout. Modelling and interpretation of the OBS data has revealed that the sediment and top basement structure is relatively well imaged on the coincident reflection profiles. Crustal refracted arrivals and Moho reflections are observed on the OBS data on all profiles, with particularly good data quality along the Petrel and Vulcan lines. The tomographic velocity models provide good additional constraints on the depth to top basement, particularly in areas of thicker sediment cover, and the Moho. The model results are integrated with gravity, magnetics and seismic reflection data to help constrain the velocity model along the 2D lines used for depth conversion and to better constrain the sediment thickness, crustal thickness and rock composition.
AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015