SummaryThis study is part of the groundwater investigations of the Ord Bonaparte plains in the East Kimberley region of Western Australia. A key project aim is to establish a spatial hydrostratigraphic framework to better understand the hydrogeology.To achieve this, AEM data, inverted using 1D SELMA model, were produced as conductivity sections and elevation grids. Interpretation of the AEM data, in conjunction with lithostratigraphic information from three petroleum wells and seven project bores, aided the mapping of hydrostratigraphic units of the Devonian to Permian sequence of the Bonaparte Basin. Mapping results show that the Carboniferous Weaber and Kushill Groups are dipping to the east-northeast and contain laterally continuous stacked aquifers. Within the strata, resistive signatures are associated with sandstone aquifers, slight to moderate conductors are mapped as fine textured aquitard, or as interbedded fine to coarse textured sediment forming semi-confining layers.A water table elevation map was constructed using surface NMR water content profile and machine learning approach to extrapolate across the study area. Using Archie’s Law, groundwater conductivity was predicted from AEM conductivity and porosity derived from borehole NMR measurements.
The BRMC dips towards the north, and at the prospect, the Proterozoic bedrock and saprolite are overlain by 40 to 70 m of Cainozoic sediments. The saprolite is locally overlain by a palaeo-valley filled with light grey clay and sand, informally named as the Portia Unit (Tan 2001). Olive grey clay and dark grey clay of the lacustrine Namba Formation (Callen 1990) overlies the Portia Unit, and where the latter is absent, directly overlies the saprolite. Quaternary degraded dune and alluvium (Gibson 1999, Gibson & Wilford 1999) overlie the Namba Formation (Figure 2).
In the Tintinara area, located south-east South Australia, airborne geophysics was recognized as<br>having potential to provide valuable biophysical data relevant to the management of irrigation<br>development and groundwater recharge reduction in the area. The groundwater of the area sustains<br>irrigation and other dryland agriculture. However the lifetime of this resource is limited by the leaching<br>of salt that has accumulated in the soil prior to land clearing and agricultural development. For some<br>areas, the groundwater may be saline and unusable for irrigation within ten to twenty years. The<br>presence or absence of a near surface clay unit can have an important influence on the rate and timing of<br>this deterioration by slowing recharge. Forward modelling suggested that a frequency domain helicopter<br>electromagnetic (HEM) system could map spatial variability associated with this unit. A survey was<br>conducted using the RESOLVE® HEM system and demonstrated that, through the use of a constrained<br>inversion approach, this unit could be mapped. This was confirmed with shallow drilling. We have<br>generated a product, namely clay thickness, which is now being used as an input into a hydrogeological<br>model to help predict recharge rates and influence management decisions in the area.
The Ord Valley Airborne Electromagnetics (AEM) Interpretation Project was co-funded by the Australian Government and the Western Australian Government to provide information in relation to salinity and groundwater management in the Ord River Irrigation Area (ORIA). The project area covers the existing ORIA Stage 1, and the ORIA Stage 2 areas earmarked for irrigation extension. The project included the acquisition of 5,936 line km of AEM data acquired using the SKYTEM time domain system.
The Ord Valley Airborne Electromagnetics (AEM) Interpretation Project was co-funded by the Australian Government and the Western Australian Government to provide information in relation to salinity and groundwater management in the Ord River Irrigation Area (ORIA). The project area covers the existing ORIA Stage 1, and the ORIA Stage 2 areas earmarked for irrigation extension. The project included the acquisition of 5,936 line km of AEM data acquired using the SKYTEM time domain system.The SkyTEM AEM system successfully mapped key elements of the hydrogeological system over most of the project area. In general terms, the modelled conductivity structure defined from the SkyTEM smooth model Layered Constrained Inversion (LCI) matches that defined from available bore data exceptionally well, with an adjusted R2 = 0.843 determined.Overall, the AEM survey has provided enhanced spatial delineation of key elements of the hydrostratigraphy in 3D, including sand- and gravel-filled palaeochannels, and clay and silt distribution, as well as salt stores and groundwater quality. The study found significant areas of high salinity hazard in several of the Stage 2 areas earmarked for irrigation development, with salt stores and groundwater salinity often higher than in the Stage 1 areas.This study has demonstrated the effective role that AEM methods can play as part of a ‘hydrogeological systems’ approach to the management of groundwater in existing and future irrigation developments in Northern Australia. The study has also demonstrated the potential for ‘calibrated’ AEM systems and Fast Approximate Inversion software to significantly shorten AEM project timelines.Engineering and Community• Geophysics role in increasing innovative engineering opportunities• Better delineating groundwater resources• Case histories in environmental geophysics37C11LUI7
The Ord Bonaparte Plains area is a priority area for irrigated agriculture development as part of the Ord Stage 3 development in the East Kimberley region of Western Australia. Irrigated agriculture in this area will depend on access to groundwater resources in underlying bedrock aquifers. A program of airborne electromagnetics (AEM), drilling, ground and borehole geophysics and hydrogeological investigations is being undertaken to confirm the presence of suitable groundwater resources, map the connectivity between surface and groundwater systems, and identify potential risks to agriculture and water infrastructure including salt stores, groundwater salinity and seawater intrusion.Preliminary analysis shows that the AEM survey has successfully mapped key elements of the groundwater system, including aquifer and aquitard extent, groundwater quality (salinity) distribution, hydraulic properties, compartmentalisation and inter-connectivity, the seawater intrusion (SWI) interface in coastal zones, and key tectonic elements of regional hydrogeological significance. The survey has mapped significant faulting within the Cockatoo Sandstone and Point Springs Sandstone aquifers, while conductivity distributions suggest that faults within and bounding major stratigraphic units display both fault barrier and conduit behaviour. The survey has also found that fresh groundwater in the aquifer system continues offshore as discontinuous lenses.Initial inversions have been used to target drilling, hydrochemical investigations, and a program of ground geophysics (including Surface Nuclear Magnetic Resonance (SNMR)). Further analysis and groundwater modelling is required to determine appropriate development and management of any groundwater resource and the potential risks to agricultural development.
The use of airborne electromagnetics (AEM) for hydrogeological investigations often requires high resolution data. Optimisation of AEM data therefore requires careful consideration of AEM system suitability, calibration, validation and inversion methods. In the Broken Hill Managed Aquifer Recharge (BHMAR) project, the SkyTEM transient EM system was used to map MAR and groundwater targets within the top 100m of unconsolidated sediments of the River Darling Floodplain. Initially, Fast Approximate Inversions (FAI) provided within 48 hours of acquisition were used to target 100 drillholes for calibration and validation. A number of different (Laterally and Spatially Constrained) inversions of the AEM data were carried out, with refinements made as additional information on vertical and lateral constraints became available. Finally, a Wave Number Domain Approximate Inversion procedure with a 1D multi-layer model and constraints in 3D, was used to produce a 3D conductivity model. This inversion procedure only takes days to run, enabling the rapid trialing to select the most appropriate vertical and horizontal constraints. Comparison of borehole induction logs with adjacent AEM fiduciary points confirms high confidence levels in the final inversion. Using this approach has produced quantitative estimates of the 3D conductivity structure, enabling rapid identification and assessment of new groundwater resources and MAR targets. Integration of the AEM data with borehole lithology, textural, mineralogical, groundwater and pore fluid hydrochemical and borehole NMR data has enabled key elements of the hydrogeological system to be mapped including hydrostratigraphy, groundwater salinity, aquifer hydraulic conductivity and transmissivity, and neotectonics. The AEM data have been pivotal in developing new geological and hydrogeological conceptual models. The hydrogeological complexity revealed by AEM mapping greatly improves the parameterisation of groundwater models, and provides a framework for understanding complex hydrogeological and hydrogeochemical processes. This will aid groundwater management and the assessment of a range of MAR and groundwater extraction options.
Salinisation of the River Murray in South Australia has been a constant threat to this vital water resource. River salinity, a consequence of discharging saline groundwater, is being limited through the implementation of salt interception schemes, where numerous water bores were installed along the river bank to lower groundwater levels and reduce discharge. The efficient removal of this water relies on our ability to locate areas of salt accession to the river. This study shows the use of in-river NanoTEM to map the resistivity of the river sediments. Following a ground validation exercise, which included the collection of saturated sediment cores and subsequent laboratory analyses, it was concluded that variations in the observed resistivity response were associated with changes in salinity. Together with hydrological information, conductive areas surveyed along the river were interpreted as zones of salt accession whereas the resistive areas were zones where fresh river water recharged the underlying sediments.
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