Seismic inversion for acoustic impedance and porosity of Cenozoic cool-water carbonates on the upper continental slope of the Great Australian Bight

Seismic inversion is used to estimate detailed seismic and rock properties, such as acoustic impedance and porosity, from seismic data. The method is widely used and proven successful by the petroleum industry, but has hitherto not been widely adopted for academic studies. This paper outlines a workflow and reports the application of model-based seismic inversion to a Cenozoic cool-water carbonate succession on the upper continental slope of the Great Australian Bight that was cored during Ocean Drilling Program Leg 182. Acoustic impedance data and the derived porosity distribution facilitate detailed studies of lithology, compaction and fluid flow in the shallow subsurface (0–500 m). A comparison of reflection and impedance data support the notion that seismic reflection events arise from bed boundaries rather than from lateral changes in impedance, even where these are significant. The uppermost continental slope of SW Australia is swept by a strong (>0.5 m/s) geostrophic current, the Leeuwin Current, and seismic profiles across the upper slope show geometrical similarities with contourite drifts. Cores taken through a conspicuous mounded seismic facies at ODP Site 1131 suggest that bryozoan build-ups nucleated on top of contourite mounds on the uppermost slope. Core recovery at three sites on a transect across the uppermost continental slope systematically decreased with increasing acoustic impedance and depth of the drilled section regardless of age. Because of the enhanced interpretability afforded by acoustic impedance and porosity data, and the possibility of predicting core recovery, the workflow outlined here should be of use in a broad spectrum of continental margin studies.