Groundwater from the Great Artesian Basin (GAB) is a vital resource for pastoral, agricultural and extractive industries, as well as for town water supplies, supporting at least $12.8 billion in economic activity annually (Frontier Economics, 2016). It is an essential resource that supports Indigenous cultural values and sustains a range of groundwater-dependent ecosystems. The complex nature and large size of the GAB, in conjunction with increasing and competing demands for water to support new or expanding industries, communities and the environment, present complex challenges for the long-term management of the basin’s groundwater resources. Although considerable research has previously been undertaken to improve our understanding of the GAB groundwater systems, in large part by the individual jurisdictions, current knowledge varies across the basin, and there remain major hydrogeological knowledge gaps that limit management of groundwater resources in the GAB. A key challenge is to manage the groundwater resource in a way that protects existing values, accounting for inflows and outflows for the basin and ensures long-term access to artesian groundwater. The project ‘Assessing the Status of Groundwater in the Great Artesian Basin’ (referred to as the ‘Project’), was funded by the Australian Government through offsets from the former National Water Infrastructure Development Fund. Work under the Project is informing the Science Program of the National Water Grid Authority. The Project assessed existing and new geoscientific data and technologies, including satellite data, to improve our understanding of the groundwater system and water balance in the GAB, with focus areas in the northern Surat Basin (Queensland) and western Eromanga Basin (South Australia). This Project has revised and updated fundamental aspects of the GAB hydrogeological system understanding to underpin ongoing groundwater assessments and to guide water policy and resource planning in the basin. - Hydrogeological framework An updated classification of GAB aquifers and aquitards was produced, linking the hydrostratigraphic classification used in Queensland (Surat Basin) with that used in South Australia (western Eromanga Basin). This updated hydrogeological framework was produced at the whole-of-GAB scale, through the development and application of an integrated basin analysis workflow, resulting in an updated whole-of-GAB stratigraphic interpretation, consistent across jurisdictional boundaries. A total of 19 updated geological and hydrogeological surfaces were generated and used to produce a new three-dimensional hydrogeological model of the GAB. - Groundwater recharge estimation Regional groundwater recharge volumes were revised for aquifer outcrop areas along the entire eastern GAB recharge area using an improved groundwater recharge rate mapping method, integrating chloride concentration in groundwater, rainfall, soil clay content, vegetation type and surficial geology. The modelled 50th percentile (‘median’) of new groundwater recharge rate estimates, for the eastern GAB intake beds, range spatially from 0.79 mm/yr to 458.64 mm/yr, with a mean of 15 mm/yr. The modelled groundwater recharge rate output maps are subject to uncertainty. Using 1000 model replicates, the 5th percentile groundwater recharge rate map ranges from 0.06 mm/yr to 349.73 mm/yr (mean of 9 mm/yr) and the 95th percentile groundwater recharge rate map ranges from 1.19 mm/yr to 678.48 mm/yr (mean of 26 mm/yr). Groundwater recharge flow pathways into the main GAB aquifers were assessed using groundwater sample hydrochemical and environmental tracer analyses. Significant revisions to the mapped geometry and heterogeneity of the groundwater recharge beds were made using regional scale airborne electromagnetic (AEM) geophysical data, which identified the geometry of and potential connectivity between aquifers, possible structural controls on groundwater flow paths and plausible groundwater sources of spring discharge. - Groundwater system conceptualisation Revised groundwater system conceptual models of groundwater recharge processes and groundwater flow were developed. These revised groundwater system conceptualisations illustrate aquifer architecture and potential stratigraphic and/or structural variability which has the potential to affect groundwater flow paths. - Water balance estimates The water balance presented here has been undertaken to test incorporating and communicating uncertainty estimates as part of a whole-of-GAB water balance. The water balance incorporates new work, where available, from the jurisdictions and from this project (specifically groundwater recharge estimates). This project has produced quantified uncertainty for a single component of the water balance - groundwater recharge, the largest input component of the water balance. The water balance, as presented in this report, is not intended to represent a comprehensive critical appraisal of the techniques used by previous workers to estimate each element of the water balance, nor was it intended to develop new techniques for the estimation of the water balance elements other than groundwater recharge. The incorporation of a component of the uncertainty in the water balance of the GAB is new. The uncertainty in the water balance has always been there, in all past iterations, though it has not been quantified. This estimation of uncertainty is important in the communication of the water balance, as it highlights key issues such as: 1) a whole-of-basin water balance for the GAB using current information is not sufficiently detailed to be of use to water managers; and 2) local monitoring of groundwater levels and pressures remains the primary management tool for monitoring groundwater resources in the GAB. The Project produced a point-in-time assessment of the water balance of the GAB, comparing inflows (including long-term average groundwater recharge) and outflows to the main regional aquifers, for the year 2019 (The year 2019 was the latest year for which data was available at the start of the Project). The whole-of-GAB water balance, calculated using the 5th, 50th and 95th percentiles of modelled groundwater recharge rates, estimates a range of storage change volumes of -859 GL, -29 GL and +1,212 GL respectively, in 2019. The large variation in estimated storage volumes, ranging from a decreasing to increasing groundwater storage change, highlights the large uncertainty associated with the water balance when considering the groundwater recharge rate uncertainty. GAB sub-basin water balances also show a range of groundwater storage volume changes based on modelled groundwater recharge rates for the Eromanga Basin (-229 GL 5th percentile; 51 GL 50th percentile and 424 GL 95th percentile), Carpentaria Basin (-413 GL 5th percentile; -72 GL 50th percentile and 511 GL 95th percentile) and Surat Basin (-217 GL 5th percentile; -9 GL 50th percentile and 277GL 95th percentile). Using 50th percentile modelled groundwater recharge rates for major aquifer groups across the basin, water balance estimates for the Cadna-owie Aquifer Group and Precipice Aquifer Group suggest negative change in storage volumes, while water balance estimates for the Hutton - Injune Creek Aquifer Group and the Rolling Downs Aquifer Group suggest an increasing change in storage volumes. While the whole-of-GAB, sub-basin and major aquifer water balances provide basin-wide perspectives of the groundwater resource components, they also highlight the uncertainties associated with estimating groundwater recharge at a regional scale. The large range in groundwater storage values calculated for the water balance presented here, are too great to confidently provide a whole-of-GAB scale assessment of groundwater resources. - Assessment of new techniques for whole-of-GAB groundwater evaluation Assessments of new techniques, including spatial and temporal satellite data, show promising results for remote monitoring of groundwater levels at a whole-of-GAB scale. The new monitoring techniques are not currently operationalised and require further work to allow them to be integrated into current monitoring programs. The new techniques rely on ongoing groundwater level and pressure data, making existing on-ground groundwater monitoring networks essential for managing groundwater resources in the GAB. Gravity Recovery and Climate Experiment (GRACE) Gravity Recovery and Climate Experiment (GRACE) satellite derived groundwater storage change estimates were found to be largely consistent with calculated water balance groundwater storage change estimates, for GAB aquifers at the whole-of-GAB and sub-basin scale. The accuracy of GAB GRACE groundwater storage estimates is largely dependent on the accuracy of supplementary datasets required to account for gravity signals not associated with the GAB groundwater (e.g. surface water, soil moisture and shallow groundwater). The assessments undertaken for the Project indicate GRACE satellite observations are a powerful tool to remotely monitor confined groundwater storage change trends, at the whole-of-GAB and sub-basin scale over monthly to decadal time-scales. Interferometric Synthetic Aperture Radar (InSAR) Based on Interferometric Synthetic Aperture Radar (InSAR) satellite data, downward ground surface motion (subsidence) was shown to be associated with decreases in groundwater levels (drawdown) in aquifers of the northern Surat Basin focus area. In the western Eromanga Basin focus area, InSAR derived ground surface measurements were stable, consistent with stable groundwater levels over time. The Project has delivered the largest consistent mosaic of InSAR derived ground surface movement in Australia, with InSAR data processed for the majority of the GAB (~90% coverage). This assessment indicates that, where appropriate datasets for local scale corrections and interpretation are available, InSAR is a useful tool to remotely sense groundwater level changes over time. Groundwater flow model An assessment of whole-of-GAB scale groundwater flow modelling was undertaken using newly developed open-source geodynamic modelling code ‘Underworld’. The ‘Underworld’ code utilises high-performance computing capabilities and has the potential to produce groundwater flow simulations at unprecedented scale and resolution. A Bayesian-approach was applied to model simulations to characterise uncertainties with model results. The model outputs fit the hydraulic head input data acceptably and the results were consistent with the current groundwater system conceptualisation of the basin. The model has been developed as a proof-of-concept steady state model that currently has limitations. However, with further development the technique has potential to reconstruct past GAB groundwater flow regimes, testing revisions to GAB-scale hydrogeological conceptualisations and could be a useful tool to simulate basin-scale groundwater movement to complement and provide a broader context for local-scale groundwater flow models within the basin. - Data and knowledge gaps and recommendations for future work Remaining data and knowledge gaps were identified through the Project. Recommendations for future work are listed by theme below and include data acquisition, data integration and data processing at local, regional and whole-of-GAB scales to better constrain key groundwater system processes in order to improve sustainable resource management. Hydrogeological framework • Further updates to the geological framework, in particular New South Wales and Northern Territory, may be necessary to reduce lithostratigraphic interpretation uncertainty in areas where scarce palynological data combined with the presence of sandy units makes it difficult to interpret and to maintain consistency across jurisdiction boundaries. • Expand mapping of aquifer sand/shale ratios to quantify variability and connectivity within and between aquifers in targeted areas, particularly across jurisdiction boundaries. • Refine three-dimensional hydrogeological model in areas identified as having high uncertainty, such as the Carpentaria Basin. Groundwater recharge evaluation • Quantify the effects that aquifer geometry, lithological heterogeneity and structural influences have on groundwater recharge rates and pathways within the main GAB aquifers through acquisition of targeted complimentary geophysical, geological and hydrogeological data. • Rainwater sample acquisition and chemical analysis to reduce uncertainty of calculated regional groundwater recharge rates. • Acquisition of groundwater hydrochemistry and environmental tracer data to quantify key groundwater processes, including groundwater recharge rates and connectivity within and between aquifers. Groundwater system conceptualisation • Improved conceptualisation of saprolite, including hydraulic properties, extent, thickness, erosional variation, and structural disruption, to quantify the effect of saprolite on groundwater recharge processes. • Measure vertical pressure gradients and groundwater pressure-elevation profiles between GAB aquifers to conceptualise groundwater leakage, and in which direction. Water balance estimates • Standardise the methods used to estimate water balance components across the GAB and report uncertainty to increase confidence in the water balance values. • Estimation of uncertainty for all of the inflow and outflow parameters. • More exploration of how to incorporate uncertainty in the water balance calculations. Assessment of new techniques for whole-of-GAB groundwater evaluation • Develop large scale whole-of-GAB scale datasets to effectively remove non-groundwater components of GRACE signal and independently assess the GRACE estimated groundwater storage change. • Integrate InSAR with complementary datasets to correct for non-groundwater effects and identify groundwater signals at local and regional scales. • Apply the large scale groundwater flow model ensemble to estimate groundwater residence times and fluxes between aquifers as well as assess the role of hydraulic conductivity variations on groundwater flow paths with GAB aquifers. Science knowledge sharing and communication activities • Dedicated workshops with each State and Territory to share findings and identify major changes related to the new work that may have implications for water bore aquifer attribution, water level and pressure monitoring and water management. Groundwater science data and knowledge supporting the Great Artesian Basin Strategic Management Plan Australian and GAB state/territory governments have developed the Great Artesian Basin Strategic Management Plan (GAB SMP) in 2020 (DAWE, 2020). This Strategic Management Plan takes a principles-based approach to guiding governments, industry and the community in managing this important resource together to achieve economic, environmental, cultural and social outcomes for the GAB and its users. The outputs of the current project will contribute to the Great Artesian Basin Strategic Management Plan Principle 6 - Information and knowledge generation ensures that accurate, timely and readily accessible information supports good management of the Great Artesian Basin. This summary report is one of a number of products released as part of the GAB groundwater project. In addition to the key findings and outcomes presented here, companion technical reports, datasets and associated value-added products for the project are available for public use.

<b>This data package is superseded by a second iteration presenting updates on 3D geological and hydrogeological surfaces across eastern Australia that can be accessed through </b><a href="https://dx.doi.org/10.26186/148552">https://dx.doi.org/10.26186/148552</a> The Australian Government, through the National Water Infrastructure Fund – Expansion, commissioned Geoscience Australia to undertake the project ‘Assessing the Status of Groundwater in the Great Artesian Basin’ (GAB). The project commenced in July 2019 and will finish in June 2022, with an aim to develop and evaluate new tools and techniques to assess the status of GAB groundwater systems in support of responsible management of basin water resources. While our hydrogeological conceptual understanding of the GAB continues to grow, in many places we are still reliant on legacy data and knowledge from the 1970s. Additional information provided by recent studies in various parts of the GAB highlights the level of complexity and spatial variability in hydrostratigraphic units across the basin. We now recognise the need to link these regional studies to map such geological complexity in a consistent, basin-wide hydrostratigraphic framework that can support effective long-term management of GAB water resources. Geological unit markers have been compiled and geological surfaces associated with lithostratigraphic units have been correlated across the GAB to update and refine the associated hydrogeological surfaces. Recent studies in the Surat Basin in Queensland and the Eromanga Basin in South Australia are integrated with investigations from other regions within the GAB. These bodies of work present an opportunity to link regional studies and develop a revised, internally consistent geological framework to map geological complexity across the GAB. Legacy borehole data from various sources, seismic and airborne electromagnetic (AEM) data were compiled, then combined and analysed in a common 3D domain. Correlation of interpreted geological units and stratigraphic markers from these various data sets are classified using a consistent nomenclature. This nomenclature uses geological unit subdivisions applied in the Surat Cumulative Management Area (OGIA (Office of Groundwater Impact Assessment), 2019) to correlate time equivalent regional hydrogeological units. Herein we provide an update of the surface extents and thicknesses for key hydrogeological units, reconciling geology across borders and providing the basis for a consistent hydrogeological framework at a basin-wide scale. The new surfaces can be used for facilitating an integrated basin systems assessment to improve our understanding of potential impacts from exploitation of sub-surface resources (e.g., extractive industries, agriculture and injection of large volumes of CO2 into the sub-surface) in the GAB and providing a basis for more robust water balance estimates. This report is associated with a data package including (Appendix A – Supplementary material): • Nineteen geological and hydrogeological surfaces from the Base Permo-Carboniferous, Top Permian, Base Jurassic, Base Cenozoic to the surface (Table 2.1), • Twenty-one geological and hydrogeological unit thickness maps from the top crystalline basement to the surface (Figure 3.7 to Figure 3.27), • The formation picks and constraining data points (i.e., from boreholes, seismic, AEM and outcrops) compiled and used for gridding each surface (Table 3.8).

<div>The project ‘Assessing the Status of Groundwater in the Great Artesian Basin’ assessed existing and new geoscientific data and technologies, including satellite data, to improve our understanding of the groundwater system and water balance in the GAB. An updated classification of GAB aquifers and aquitards was produced, linking the hydrostratigraphic classification used in Queensland (Surat Basin) with that used in South Australia (western Eromanga Basin). This revised hydrogeological framework was produced at the whole-of-GAB scale, through the development and application of an integrated basin analysis workflow, producing an updated whole-of-GAB stratigraphic interpretation that is consistent across jurisdictional boundaries. Groundwater recharge rates were estimated across eastern GAB recharge area using environmental tracers and an improved method that integrates chloride concentration in bores, rainfall, soil clay content, vegetation type and surficial geology. Significant revisions were made to the geometry and heterogeneity of the groundwater recharge beds, by acquiring, inverting and interpreting regional scale airborne electromagnetic (AEM) geophysical data, identifying potential connectivity between aquifers, possible structural controls on groundwater flow paths and plausible groundwater sources of spring discharge. A whole-of-GAB water balance was developed to compare inflows and outflows to the main regional aquifer groups. While the whole-of-GAB and sub-basin water balances provide basin-wide perspectives of the groundwater resources, they also highlight the high uncertainties in the estimates of key water balance components that need to be considered for groundwater resource management. Assessment of satellite monitoring data from Gravity Recovery and Climate Experiment (GRACE) and Interferometric Synthetic Aperture Radar (InSAR) shows promise for remote monitoring of groundwater levels at a whole-of-GAB scale in the future to augment existing monitoring networks. This presentation was given at the 2022 Australasian Groundwater Conference 21-23 November (https://www.aig.org.au/events/australasian-groundwater-conference-2022/)

This web service provides access to geological, hydrogeological and hydrochemical digital datasets that have been published by Geoscience Australia for the Great Artesian Basin (GAB).

This web service provides access to geological, hydrogeological and hydrochemical digital datasets that have been published by Geoscience Australia for the Great Artesian Basin (GAB).

<div>Understanding groundwater flow dynamics within the Great Artesian Basin (GAB) is critical for responsible management of groundwater from an environmental, economic and cultural perspective. Numerical groundwater flow modelling involves generating a simplified representation of a groundwater system and using Darcy’s Law to simulate groundwater flow rates and the distribution of hydraulic heads throughout the system. This is a pilot study aimed at developing a workflow for groundwater flow modelling of the Great Artesian Basin using Bayesian methods. In this report, we present our initial results from building and running a steady-state groundwater flow model of the entire GAB. We demonstrate a Bayesian inference framework to generate an ensemble of groundwater flow models allowing an assessment of the uncertainty of model parameters and flow velocities.&nbsp;</div><div>Several models have been built to simulate groundwater flow across various areas and layers of the GAB. Most of these models aimed to predict the likely impacts on the groundwater system of some future scenario, generally climate change or groundwater extraction relating to mining activities. While these models are well-suited to their purpose, their focus on particular regions or aquifers makes them unsuitable for investigating large-scale groundwater flow throughout the GAB. In contrast, the model built as part of this study captures the entire GAB and aims to simulate large-scale flow. Although not in scope for this pilot study, the questions a model at this scale is capable of addressing include characterising 3D flow within hydrogeological layers, computing groundwater flux between aquifers and between sub-basins, inferring hydraulic properties and identifying poor quality data. As this model is steady-state and uses hydraulic head data from before the year 2000, it provides a baseline estimate of groundwater flow without considering recent anthropogenic forcing or transient system stresses.&nbsp;</div><div>The GAB is represented as a 14 hydrogeological layer model including basement, Permo-Carboniferous basins, Mesozoic sedimentary aquifers and aquitards and Cenozoic layers. This includes updated hydrogeological surfaces from the GAB project. The input data consisted of 8,065 hydraulic head measurements and 6,151 estimates of recharge rate while the model parameters were a single hydraulic conductivity value for each of the 14 layers. The modelling domain was discretised using 10 x 10 km cells in the horizontal plane and the mesh was deformed vertically to fit between the topography and the basement surface, with the resulting mesh having a vertical discretisation of no coarser than 50 metres. The top boundary condition was a constant head boundary that was a smoothed version of topography. The sides and bottom of the model have no flux boundary conditions and a buffer zone around the GAB was included to minimise boundary effects.&nbsp;&nbsp;</div><div>In total 2500 groundwater flow simulations were run using a Bayesian inversion framework. The inversion sampled various combinations of input parameters to find models with a relatively low misfit, which was calculated by squaring the difference between the observed and simulated values of hydraulic head and recharge. Rather than searching for a global minima, the Metropolis Hastings Markov Chain Monte Carlo sampling algorithm was used to explore a range of possible models and estimate the posterior distribution of each layer’s hydraulic conductivity.&nbsp;</div><div>The model performed adequately and the model parameters were generally consistent with the prior probability distributions based on previous modelling studies. However, the posterior distribution of model parameters were very broad indicating the model was not particularly informative in its current form.&nbsp;&nbsp;</div><div>Groundwater flow velocity vectors from the maximum likelihood model were used to investigate groundwater trends within the Cadna-owie-Hooray aquifer. Uncertainty of model predictions were investigated by calculating the groundwater flow velocity variance across the ensemble. This study demonstrates that it is technically feasible to use Bayesian inference to probabilistically mode groundwater flow across the entire GAB. However, for this approach to yield useful results, more work is required to understand the impacts of simplifying assumptions about layer properties, the quality of the input data and model structure on the resulting flow model.&nbsp;</div><div><br></div>

The Australian Government, through the National Water Infrastructure Development Fund, commissioned Geoscience Australia to undertake a 3-year project ‘Assessing the Status of Groundwater in the Great Artesian Basin’. The overall aim of the project was to analyse existing and new geoscientific data acquired by the project to improve understanding of the hydrogeological system and water balance in the GAB. In conjunction, the project assessed satellite based technologies for monitoring groundwater storage and level change. This talk will discuss some of the key results of the project. These include: an updated hydrogeological framework for the GAB, mapping aquifer and aquitard properties, geometry and extent; revised groundwater recharge rate estimates in the eastern GAB groundwater intake beds; new groundwater system conceptual models of groundwater recharge processes and groundwater flow; an assessment of the Gravity Recovery and Climate Experiment (GRACE) satellite derived groundwater storage change estimates for the GAB; and Interferometric Synthetic Aperture Radar (InSAR) satellite data, for detecting changes in groundwater levels.

The National Groundwater Systems (NGS) project, is part of the Australian Government’s Exploring for the Future (EFTF) program, led by Geoscience Australia (https://www.eftf.ga.gov.au/national-groundwater-systems), to improve understanding of Australia’s groundwater resources to better support responsible groundwater management and secure groundwater resources into the future. The project is developing new national data coverages .to further delineate groundwater systems and improve data standards and workflows of groundwater assessment. While our conceptual understanding of the hydrogeology of the Great Artesian Basin (GAB, Figure 1) continues to grow, in many parts of the Eromanga, Surat and Carpentaria basins that form the GAB we are still reliant on legacy data and knowledge from the 1970s of variable quality. Additional information provided by recent studies in various parts of the GAB highlights the level of architectural complexity and spatial variability in stratigraphic and hydrostratigraphic units across the basin. We now recognise the need to standardise these regional studies to map such geological complexity in a consistent, basin-wide hydrostratigraphic framework that can support effective long-term management of GAB water resources. The recent iteration of revision of GAB geological and hydrogeological surfaces (Vizy & Rollet, 2022) provides a framework to interpret various data sets consistently (e.g., boreholes, airborne electromagnetic, seismic data) and in a 3D domain, to improve the aquifer geometry, and the lateral variation and connectivity in hydrostratigraphic units across the GAB (Rollet et al., 2022). Vizy and Rollet (2022) highlighted some areas with low confidence in the interpretation of the GAB where further data acquisition or interpretation may reduce uncertainty in the mapping. One of these areas was in the Carpentaria Basin, particularly the transition from the offshore to onshore across the Gulf of Carpentaria. This data compilation provides open file SEGY, cultural data and value added seismic interpretation in the form of seismic horizons and grids for two key surfaces, these enable improved correlation to existing studies. This data also aim to provide users an efficient mean to rapidly access core data from numerous sources in a consistent and cleaned format, all in a single package. This dataset provides: 1) Seismic data compilation in a digital format with publically accessible information, including scanned seismic sections converted to SEGY format where digital data was not available; 2) Base Mesozoic and Near Base Cenozoic seismic interpretation in two-way-time; 3) Depth converted regional surfaces for the Base Mesozoic and Near Base Cenozoic unconformities generated using additional constraints such as AEM interpretation and borehole constraints previously compiled in Vizy & Rollet (2022). This new interpretation will be used to refine the GAB geological and hydrogeological surfaces in this region.

As part of the Great Artesian Basin (GAB) Project a pilot study was conducted in the northern Surat Basin, Queensland, to test the ability of existing and new geoscientific data and technologies to further improve our understanding of hydrogeological systems within the GAB, in order to support responsible management of basin water resources. This report presents selected examples from the preliminary interpretation of modelled airborne electromagnetic (AEM) data acquired as part of this pilot study. The examples are selected to highlight key observations from the AEM with potential relevance to groundwater recharge and connectivity. Previous investigations in the northern Surat Basin have suggested that diffuse groundwater recharge rates are generally low (in the order of only a few millimetres per year) across large areas of the GAB intake beds—outcropping geological units which represent a pathway for rainfall to enter the aquifers—and that, within key aquifer units, recharge rates and volumes can be heterogeneous. Spatial variability in AEM conductivity responses is identified across different parts of the northern Surat Basin, including within the key Hutton Sandstone aquifer. Consistent with findings from other studies, this variability is interpreted as potential lithological heterogeneity, which may contribute to reduced volumes of groundwater entering the deeper aquifer. The influence of geological structure on aquifer geometry is also examined. Larger structural zones are seen to influence both pre- and post-depositional architecture, including the presence, thickness and dip of hydrogeological units (or parts thereof). Folds and faults within the Surat Basin sequences are, in places, seen as potential groundwater divides which may contribute to compartmentalisation of aquifers. Discrete faults have the potential to influence inter-aquifer connectivity. The examples presented here demonstrate the utility of AEM models, in conjunction with other appropriate geophysical and geological data, for characterising potential recharge areas and pathways within the main GAB aquifer units, by helping to better define aquifer geometry, lithological heterogeneity and possible structural controls. Such assessments have the potential to further improve our understanding of groundwater recharge and flow path variability at local to regional scales. Acquisition of broader AEM data coverage across groundwater recharge areas, along with complementary geophysical, geological and hydrogeological data, would further assist in quantifying recharge variability, facilitating revised water balance estimates for the basin and thereby supporting GAB water resource management and policy decision-making.

<div>This Geoscience Australia Record reports on Interferometric Synthetic Aperture Radar (InSAR) processing over the Great Artesian Basin (GAB) to support an improved understanding of the groundwater system and water balance across the region. InSAR is a geodetic technique that can identify ground surface movement from satellite data at a regional scale and is therefore a valuable and widely used technique for measuring patterns in surface movement over time; including the movement of fluids (i.e. water or gas) beneath the surface.</div><div><br></div><div>This Record is the one of two Geoscience Australia Records that describe ground surface movement monitoring Geoscience Australia have undertaken in the GAB in recent years. Namely;</div><div>1.&nbsp;&nbsp;&nbsp;&nbsp;Ground surface movement in the northern Surat Basin derived from campaign GPS measurements. (Garthwaite et al., 2022).</div><div>2.&nbsp;&nbsp;&nbsp;&nbsp;InSAR processing over the Great Artesian Basin and analysis over the western Eromanga Basin and northern Surat Basin (this Record).</div><div><br></div><div>We have produced ground surface motion data products, which cover about 90% of the GAB for the period of time between January 2016 and August 2020. The data products were created using Sentinel-1 Synthetic Aperture Radar (SAR) data and an InSAR processing workflow designed for large spatial scale processing. The large spatial scale InSAR processing workflow includes using GAMMA software to (i) pre-process SAR images to align the pixels, (ii) generate interferograms and short temporal baseline surface displacement maps and PyRate software to (iii) combine these outputs in an inversion to form pixel-wise time series ground surface displacement data and fit ground surface velocities to the displacement data. The raw SAR data and these subsequent data products of the workflow are partitioned into overlapping frames; the final stage of the large scale processing workflow is to combine the partitioned data into a single map using a mosaicking algorithm. The results of this processing chain demonstrate the feasibility of developing a regional scale ground surface movement reconnaissance tool (i.e. subsidence and uplift). </div><div><br></div><div>We provide a summary of the processing chain and data products and a focused assessment for two case study areas in the western Eromanga Basin (South Australia) and northern Surat Basin (Queensland). Over these case study areas we examine the relationship between the InSAR derived ground surface movement and available groundwater level data. We also assess how land use types may influence the InSAR derived ground surface motion data by comparing the InSAR data to the “land types” over the region which we classify using a machine learning algorithm with Sentinel-2 optical imagery data. </div><div><br></div><div>From our analysis we observe little ground surface motion over the western Eromanga Basin. The surface movement rate over the entire area is estimated to be mostly within ±10 mm/yr. Groundwater level time series data from well monitoring sites in the area did not appear to have any significant trends either. However, large and broad scale ground surface motion (both uplift and subsidence) was observed in the InSAR processing results over the northern Surat Basin. A 75 km x 150 km scale uplift signal, with rates of up to 20 mm/yr, was located over an area we classified as cultivated land, where InSAR signals are likely to be influenced by near-surface cultivation activities (such as irrigation) rather than subsurface groundwater level changes. Furthermore, two approximately 75km x 75 km areas were identified which had subsidence signals of up to -20 mm/yr. Over the same area, groundwater level time series data show long-term negative trends in the water head level. For a more direct comparison between the InSAR results and the well data, we fitted a first order poroelastic model to transform the InSAR derived ground surface motion rates into modelled pore-pressure decline/groundwater drawdown rates. We compared the model to the groundwater time series data in the Walloon Coal Measures, Surat Basin, and found good agreement, which indicates that the observed subsidence signals could be attributable to pore-pressure decline due to the falling water head level.</div><div><br></div><div>We finally provide some preliminary analysis comparing our InSAR results to the results from an Office of Groundwater Impact Assessment (OGIA) InSAR study and a Geoscience Australia GPS land movement study to assist in validating the Geoscience Australia InSAR results. Overall, the comparisons are encouraging, showing a high correlation against the OGIA InSAR results and GPS results. Further work, is required to further validate our results and reduce uncertainty in our analysis process.</div>