Exploring for the Future—Remodelling the Oolloo–Jinduckin interface across the Daly Basin, Northern Territory: Interpreting stratigraphic boundaries using probabilistic airborne electromagnetic inversions

This report presents key results from the Daly River groundwater project conducted as part of Exploring for the Future (EFTF), an Australian Government funded geoscience data and information acquisition program. The four-year (2016-20) program focused on better understanding the potential mineral, energy and groundwater resources in northern Australia. In this investigation we use models of sub-surface bulk electrical conductivity within the geological Daly Basin to model the depth of the interface between the Jinduckin Formation and the overlying Oolloo Dolostone. The Olloo dolostone is the most extracted aquifer in the Daly basin, while the Jinduckin Formation is an aquitard separating the Olloo from the lower Tindall Limestone aquifer. Airborne electromagnetic (AEM) data acquired across the basin were inverted with both deterministic and stochastic methods to generate a suite of bulk electrical conductivity models. Comparison with boreholes suggested that the Jinduckin Formation is significantly more conductive than the Oolloo Dolostone and this interface is well resolved in these AEM conductivity models. We developed an interactive plot for visualising the probability distribution of bulk conductivities for AEM points inverted with the stochastic inversion routine. We interpreted 389 AEM points using this approach and used interpolation to derive a new stratigraphic Olloo—Jinduckin surface. The new surface is generally deeper than current models of the interface, which were derived by interpolating stratigraphic picks from boreholes. In the data-sparse south-west of the Daly Basin the new geological surface is up to 390 m deeper than what is currently mapped. This new interface can be used to better constrain aquifer architecture in groundwater flow modelling and support groundwater management of this region. The method developed for interpreting stratigraphy directly from the posterior probability distribution of electrical conductivity is applicable for other geophysical interpretation tasks.