The Layered Geology of Australia web map service is a seamless national coverage of Australia’s surface and subsurface geology. Geology concealed under younger cover units are mapped by effectively removing the overlying stratigraphy (Liu et al., 2015). This dataset is a layered product and comprises five chronostratigraphic time slices: Cenozoic, Mesozoic, Paleozoic, Neoproterozoic, and Pre-Neoproterozoic. As an example, the Mesozoic time slice (or layer) shows Mesozoic age geology that would be present if all Cenozoic units were removed. The Pre-Neoproterozoic time slice shows what would be visible if all Neoproterozoic, Paleozoic, Mesozoic, and Cenozoic units were removed. The Cenozoic time slice layer for the national dataset was extracted from Raymond et al., 2012. Surface Geology of Australia, 1:1 000 000 scale, 2012 edition. Geoscience Australia, Canberra.

Experience over the past 15 years has demonstrated that the use of airborne electromagnetics (AEM) for near-surface hydrogeological investigations in the Australian landscape context often requires high resolution data to map key functional elements of the hydrogeological system. Optimisation of AEM data therefore requires careful consideration of AEM system suitability, calibration, validation and inversion methods. The choice of an appropriate AEM system for a given task should be based on a comparative analysis of candidate systems, consisting of both theoretical considerations and field studies including test lines over representative hydrostratigraphic targets. In the Broken Hill Managed Aquifer Recharge (BHMAR) project, the SkyTEM AEM system was chosen, after a rigorous selection process, to map a multi-layered stratigraphy in unconsolidated sediments in the top 100 m of the River Darling Floodplain. The AEM acquisition strategy was governed by the need to rapidly identify and assess potential managed aquifer recharge (MAR) and groundwater resource targets over a large area (>7,500 km2), with a high degree of confidence. A flight line spacing of 200-300 m successfully mapped the key elements of the hydrostratigraphy, important neotectonics features, and 14 potential MAR and groundwater targets. Subsequent to successful completion of the project, the AEM data were re-inverted to assess optimal line spacings for the different mapping objectives. Data for the central project area were re-inverted, corresponding to a line spacing of 200 m, 600 m, 1 km, 2 km and 5 km. Analysis of these data show that a number of key features of the hydrogeological system required for MAR target mapping and evaluation are only mapped with high resolution (200m) line spacings. In contrast, the larger groundwater resource targets can be identified at coarser line spacings (even at km spacings). For many groundwater mapping objectives, recconaisance surveys at wide line spacings can be used to identify broad-scale features, with higher resolution data acquired subsequently to address specific questions. This strategy is not always possible in project timelines, and, in the BHMAR project, it was fortunate that a large number of targets were mapped at high resolution simultaneously due to a high failure rate in MAR evaluations.

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Sonic drilling is a relatively new technology that was used successfully to obtain relatively uncontaminated and undisturbed continuous core samples with excellent (>99%) recovery rates to depths of 206m in unconsolidated fluvio-lacustrine sediments of the Darling River floodplain. However, there are limitations with the standard sonic coring method. Sands, in particular, are disturbed when they are vibrated out of the core barrel into the flexible plastic sampling tube. There can be changes to moisture content, pore fluid chemistry and sediment mineralogy on exposure to the atmosphere, even when the samples are processed and analysed soon after collection. The option exists during sonic drilling to encapsulate the core in rigid polycarbonate lexan tubes. Although this increases costs and reduces drilling rates, atmospheric exposure of the core during drilling is reduced to the ends of the lexan tubes before being capped. In addition, the tubes can be purged with an inert gas such as argon. Lexan coring is best carried out below the watertable as the heat from drilling dry clays can cause the polycarbonate to melt. In the study, 60 sonic holes (4.5 km) and 40 rotary mud holes (2 km) were obtained as part of a program to map and assess potential groundwater resources and managed aquifer recharge (MAR) targets over a large area (7,500 km2) of the Darling River floodplain. Two of the sonic bores were drilled to depths of 60 metres to obtain lexan-encapsulated core samples. These cores were used to obtain less perturbed samples for pore fluid analysis (salinity, major ions, trace metals, stable isotopes), textural analysis, and analysis of mineral phases to help assess aquifer clogging potential (using XRD, XRF, SEM). An additional advantage of the lexan coring was the recovery of encapsulated and intact sediment intervals for determining porosities, effective porosities, hydraulic conductivities, and other geophysical and petrophysical measurements. By painting some tubes black, sand samples were also successfully obtained for optically stimulated luminescence (OSL) dating. Alternatively, opaque black lexan can be made to order by the supplier. Overall, the superior sample integrity obtained from lexan coring enables a greater range of hydrogeological and hydrochemical parameters to be assessed.

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