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.

In this study, airborne electromagnetics (AEM), high resolution LiDAR, and drilling (100 bores) were acquired to map and assess groundwater resources and managed aquifer recharge options in the River Darling Floodplain. Neotectonic faulting and uplift has previously been described along the north-western margin of the Murray Basin along the adjacent Darling Lineament, however no evidence of neotectonics had previously been identified in the study area. Initial inversions of the AEM data revealed a multi-layered conductivity structure broadly consistent with the hydrostratigraphy identified in drilling. However, initial laterally and spatially constrained inversions showed only moderate correlations with ground data in the near-surface (~20m). As additional information from drilling and ground and borehole geophysical surveys became available, various horizontal and vertical constraints were trialled using a new Wave Number Domain Approximate Inversion procedure with a 1D multi-layer model and constraints in 3D. The resultant 3D conductivity model revealed that an important Pleistocene aquitard (Blanchetown Clay) confining the main aquifer of interest (Calivil Formation), has an undulating surface, which is locally sharply offset. An interpreted top surface suggests that it has been affected by significant warping and faulting, as well as regional tilting due to basin subsidence or margin uplift. Overall, the top surface of the Blanchetown Clay varies in elevation by 60m. Many of the sharp offsets in the conductivity layers are coincident with lineaments observed in the LiDAR data, and with underlying basement faults mapped from airborne magnetic data. The identification of neotectonics in this area was made possible through the acquisition of high resolution AEM data, and the selection of appropriate horizontal and vertical constraints in inversion procedures. Recognition of faulting in the unconsolidated sedimentary sequence helps explain the rapid recharge of underlying Pliocene aquifers, with neotectonics recognised as a key component of the hydrogeological conceptual model.

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The Broken Hill Managed Aquifer Recharge (BHMAR) project is part of a larger strategic effort aimed at securing Broken Hill's water supply and identifying significant water-saving measures for the Darling River system. In this study, airborne electromagnetics (AEM) mapping validated by drilling, field and laboratory measurements has identified significant volumes of fresh to acceptable quality groundwater stored beneath the Darling Floodplain. These potential resources were identified in 14 discrete targets within Pliocene aquifers (Calivil Formation and Loxton-Parilla Sands) at depths of 25-120m. The Calivil Formation occurs predominantly within structurally-controlled palaeovalleys. Aquifer quality is best where thick (30-50m), high-yielding zones (test flows > 25 L/s) occur in palaeochannels at the confluence of palaeo-river systems. Here, the hydraulic properties make the Calivil Formation aquifer best suited for groundwater extraction (and/or MAR injection), with excellent recovery efficiencies predicted where ambient salinities are low. The aquifer is sandwiched between variably thick clay aquitards, and is confined to semi-confined. Indicative groundwater volumes have been calculated using groundwater salinity and texture mapping derived for the AEM depth slices, combined with porosity statistics derived from laboratory measurements and borehole nuclear magnetic resonance (NMR) logging. In most of the targets, further investigation is required to quantify natural recharge and discharge processes, identify the negative impacts associated with groundwater pumping (particularly the potential for saline groundwater ingress), delineate the more transmissive parts of the formation, and assess the economics and logistics of borefield and water supply design. Calibrated, transient numerical groundwater flow and solute transport models are also needed to determine appropriate groundwater extraction rates. The multi-disciplinary systems-based methodology used in this project has enabled rapid identification and assessment of largely unknown potential groundwater resources and aquifer storage. These have the potential to provide drought security for regional communities and industries, and to assist with regional development.

Produced from a 250 dpi scanned image of the original out-of-print 1967 map Available as a product from NT Geological Survey or as a resource from GA Library

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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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