Tim Munday

Discrete phreatophytic vegetation associated with organic mound springs is present in several places in the semi-arid Walyarta Conservation Park (Park) in northern Western Australia. The mound springs are heritage listed, having significant cultural and environmental significance. Increased industrial (mining and agriculture) development in the region, coupled with a growing demand for groundwater to support these developments, requires an enhanced understanding of how the springs operate and the source of water that sustains their presence. The springs are broadly believed to be situated on geological faults and receive groundwater from artesian sources. However, their association with deeper geological structures and aquifer systems, the focus of this study, is not well understood. This study employed regional- and finer-scale airborne geophysical data, including electromagnetics (AEM) and magnetics, to constrain the sub-basin-scale hydrogeology of the West Canning Basin in Western Australia and to detail tectonic deformation, sedimentological and hydrological processes. The AEM data were inverted using 1- and 2D methods to better define structural discontinuities in the Park, and the results identified the location of faults and other geological structures that were coincident with spring locations. A complementary analysis of spatiotemporal patterns of green vegetation was undertaken using remote sensing data. A model for the extent of green vegetation (in percent), calculated using a constrained linear spectral unmixing algorithm and applied to a select Landsat Thematic Mapper ™ image archive, showed the persistence of green vegetation aligned with interpreted fault systems through extended dry periods. These geophysical and remotely sensed datasets demonstrate that in the Park, the sedimentary aquifers and landscapes are highly compartmentalized and that this constrains aquifer distribution, groundwater quality and the location of wetlands and phreatophytic vegetation. Integrating key information from these datasets allows for the construction of a three-dimensional model that predicts the nature and extent of the critical zone which sustains perennial groundwater discharge within mound springs, drainages and wetlands and provides a framework to assess discharge rates, mixing and, ultimately, sensitivity to changed water availability.

The electrical conductivity distribution within wide palaeochannels are usually well mapped from airborne electromagnetic data using stitched 1D algorithms. Such stitched 1D solutions are however inappropriate for narrow valleys. An alternative option is to consider 2D or 3D models to allow for finite lateral extent of conductors. In airborne electromagnetic data within the Musgrave block near the well-studied Valen conductor, strong induced polarization (IP) and superparamagnetic (SPM) effects make physical property and structure estimation even more uncertain for deep channel clays, particularly those whose channel widths are comparable to their depth of burial. We developed a recursive data fitting algorithm based on dispersive thin sheet responses. The separate IP and SPM components of the fit provide near-surface chargeability and SPM distributions, and the associated EM fit provides stripped data with monotonic decays compatible with a simple non-dispersive conductivity model. The validity of this stripped data prediction was tested through a comparison of 1D conductivity-depth imaging and 3D inversion applied to the original and the stripped data. Due to the forked geometry of the deep conductivity structure in the region we investigated, we successfully used 3D inversion to predict the conductivity distribution related to the EM data. We recovered from the stripped data a continuous conductivity structure consistent with a branching, clay-filled palaeovalley under cover.

A major constraint on development in the Anangu Pitjantjatjara Yankunytjatjara (APY) Lands is the lack of reliable water supplies. Sandstone strata found within palaeovalleys has long been recognised as potentially productive aquifers in outback South Australia. Although the APY Lands are known to contain such geologic features, their development as a viable water supply has been hindered by an acute lack of knowledge. An extensive airborne electromagnetic (AEM) survey was acquired as part of the Goyder Institute for Water Research’s Facilitating Long-term Outback Water Solutions Stage 3 (G-FLOWS S3) project in order to improve the understanding of groundwater resource potential in this remote region of South Australia. The AEM data revealed an intricate drainage pattern below the modern-day land surface that represents palaeovalleys. This facilitated the spatial refinement of these palaeodrainage features and provided a better understanding of their overall thickness. A targeted drilling campaign followed the AEM survey. Whereas the AEM data covers a large area of the APY Lands, drilling focused on two areas near the community of Kaltjiti (Fregon) in the eastern part of the APY Lands. Two hydrogeological test sites helped to understand and monitor the groundwater system associated with the Lindsay East Palaeovalley. Furthermore, drilling determined the true depth of the palaeovalley and provided a stratigraphic record. This enabled the validation of the AEM hydrogeophysical model, which included the identification of groundwater bearing zones within the palaeovalley. In total, 11 groundwater wells were constructed and continuous drill core was collected at the two sites. Drilling near the centre of the Lindsay East Palaeovalley suggests there are at least three groundwater-bearing zones. The basal coarse-grained sandstone unit of the younger palaeovalley fill shows promise as a productive aquifer, with development yields varying between 5 and 20 L/sec and salinities of <1000 mg/L TDS.

Noise is a fundamentally limiting factor for what aspects of the subsurface can be imaged using an AEM system. This paper compares the use of two inversion techniques and two different methods of estimating data noise, to determine how the choice of noise handling impacts inversion results. The use of processing to remove noisy data from an AEM dataset is shown to provide an improved result over using more traditional survey wide system noise level estimates. This result highlights the importance of critically assessing noise present within AEM data and provides a foundation for further development of techniques which can identify and remove noisy data.

In a collaboration between Department of Agriculture and Food, Western Australia (DAFWA) and CSIRO, funded by the Western Australia Government's Royalties for Regions Program and the Gascoyne Foodbowl Project, the Gascoyne River AEM Aquifer and Groundwater Characterization Project was established with the aim of determining whether airborne electromagnetic (AEM) data can be employed to better map attributes of the unconfined alluvial aquifer beneath and adjacent to the ephemeral Gascoyne River. One major aspect of the project was to produce drilling targets, based on interpretation of AEM data, for groundwater production. In this presentation, we briefly recapitulate our method and discuss the result of the drilling campaign that ensued. We show that our exploration targets have resulted in overwhelming success in the conversion of exploration wells to production bores; and that the production wells produce greater yields of better quality groundwater than previous campaigns that were conducted through step-out drilling. We show that interpretation of the AEM inversions allowed us to map the aquitard layers that define the bottom of the Gascoyne River Old Alluvium aquifer system, determine the extent of the saltwater intrusion from the nearby Indian Ocean, and to calculate the overall volume of the aquifer system. These calculations allow us to provide estimates of total groundwater volume contained in the aquifer for sustainable production.

The southern McArthur Basin in Australia’s Northern Territory is host to some Tier-1 sediment-hosted base metal mineral deposits including the McArthur River Zn-Pb-Ag mine. Airborne electromagnetic (AEM) data sets have been employed as a key exploration technology in the search for these mineral systems. A geological interpretation of results arising from the use of different inversion techniques, including a 1, 2.5 and 3D methods, was undertaken on a helicopter EM data set acquired over a structurally complex sediment package in the Batten Fault Zone north of the McArthur River Mine. The exploration targets were conductive, mineralised units (HYC pyritic shale member) associated with the Barney Creek Formation. Results from this study suggested that although the model fits were good, the derived conductivity models for the 2.5D and 3D inversions appeared to be smooth representations of geological reality, particularly when compared with data from drilling and surface geological mapping. Superficially, the 1D smooth model layered Earth inversions appear to map geological variability and structural complexity in greater detail even though the structures are more 3D in nature. IP effects are observed in the data and influence the modelled structure, but can be accounted for and complement the non IP 1D inversion results. The outcome of this study also indicates that when employing higher order inversion methods in the interpretation of AEM data sets, there may be significant benefit in asking a contractor/consultant for 1D inversion results as well. In the resulting interpretations if conductors appear in one but not the other, it is worth asking the question why?

Saline aquifers in the Murray River or SE Australia are traversed by freshwater rivers, with adjoining riparian and floodplain regions containing freshwater lenses. Bore data and more recent Airborne electromagnetic (AEM) surveys have determined that these lenses are spatially extensive, but have widely varying geometries. The maintenance of these lens systems is important as they support ecologically significant riparian vegetation communities such as Red Gum and Black Box. A more complete understanding of their hydrogeology is required to ascertain how they develop and degrade. Limited ground investigations including 14C geochemistry have determined that the lens systems contain recent water, indicating that they are dynamic systems with their development defined by the relative rates of recharge from the river and mixing with groundwater. Changes in groundwater gradients and depth, floodplain extent, and topography are believed to control their initial location. The same controls also govern their stability. The potential of AEM systems for defining the geometry of these lens systems in 3D is considered along with an assessment of their value for monitoring variations associated with these ecosystems. The advent of “calibrated” AEM systems and robust inversion tools have given added impetus to their use for environmental monitoring. Spatio-temporal variations are observed in the near surface (top 20m) from a multi-temporal assessment of Clark’s Floodplain, adjacent to the Bookpurnong irrigation area, with co-incident AEM surveys acquired between 2008 and 2015. Spatial changes in ground conductivity, attributed to changing groundwater quality have been observed. The freshwater lens systems appear to have contracted significantly over the last decade. This is attributed, in part, to land use patterns and the development of an irrigation-related groundwater mound on the highlands adjacent to the floodplain, and an increased hydraulic gradient towards the river. The results indicate the geometry of the hyporheic zone may have also changed along the river.

The Pareto principle, named after the Italian engineer, economist, and sociologist Vilfredo Pareto, (also known as the 80:20 rule) states that, for many events, roughly 80% of the effects come from 20% of the causes. A variation of that principle might be to suggest that to achieve 80% of what is needed, only 20% of effort is required, whereas to deliver the final 20% requires 80% of the effort. I’d like to argue that in the application of geophysical methods to hydrogeology, particularly where airborne electromagnetic techniques are employed, it is perhaps worth remembering that principle when considering their application for groundwater resource assessments in data poor areas. Practitioners of hydrogeophysical investigations, myself included, are sometimes guilty of making the case for analysing and interpreting geophysical data without being necessarily cognisant as to how, at what cost, and to what extent groundwater resource assessments can be improved through their incorporation. Most applications of AEM for groundwater resource assessment in remote parts of Australia only require limited effort on the processing and analysis of the data once acquired. Examples abound on the use of AEM in defining palaeovalleys across the Australian outback to aid the location of groundwater resources for mining purposes. More often than not, their relatively simple transformation using CDI’s, rather than their full inversion, is adequate for determining where to drill, and to then progress conventional hydrogeological investigations that might result in a bore field. Often the products supplied by the contractors are adequate for such needs. Our collective challenge is to understand at what point there is value in going beyond this step. Of course there are also examples which demonstrate the relevance of a more considered approach to geophysical data analysis and interpretation, including studies linked to managed aquifer recharge, to drill targeting which requires water of a certain quality in a particular aquifer, or where the accurate definition of an aquifer bound or salt water interface may be required in the absence of drill hole data. We are well placed to deliver into these areas benefiting from the emergence of practical and robust data inversion techniques. However in many of these studies even the extra effort employed is sometimes not without fault. For example, while the geophysics community regularly deal with measurement and model uncertainty, arising from issues such as equivalence, we seem to gloss over these matters and rarely transfer of them into hydrogeological domain. Similarly, we sometimes appear to ignore the limits of groundwater models. We may expend considerable effort in using geophysical data to help conceptualise an aquifer and its variability, and then argue till we are blue in the face that the detail in the geophysics needs to be incorporated into a “groundwater model”, conveniently forgetting that most models have low absolute accuracy, but good resolution and some predictive capability with the latter being determined by (limited) data other than that from geophysics. The limited data tends to lead to a generalized representation of the aquifer system which is then calibrated to measured data, usually water levels, to enhance model utility. In summary, while acknowledging that geophysics can offer much to improve groundwater resource assessment, it’s appropriate to question the worth of data during the survey design stage, and to consider whether, if and when, the Pareto principle might be enacted.

Access to water is identified a key infrastructure need for mining, energy and industry development. In Western Australia, the scale of planned developments linked to current mineral exploration and mining is set to generate significant economic value for the State, but its realisation is dependent on ensuring access to groundwater. To address these issues, The WA Government Department of Water (DoW) has embarked on a series of groundwater investigations to identify and establish long-term water resources in regional areas where agriculture and mining opportunities have the potential for development. The Murchison in northern WA was identified as one of six key priority areas for this initiative. With numerous known mineral deposits having potential for development, locating and securing an adequate, sustainable long-term water supply is a critical consideration if these mineral resources are to be developed further. While it is known that there are significant groundwater resources in the region, at present these are generally poorly understood. Of particular importance are the palaeovalley aquifers which are known (locally) to contain a significant resource, but which are relatively poorly characterised. To aid an understanding of their extent an airborne electromagnetic (AEM) survey was commissioned and flown in the Murchison extending over an area in excess of ~106 000 km2. Prior studies at a local scale had indicated that airborne EM would be very effective at defining the location and thickness of palaeovalleys in the region. Pilot investigations also determined the most appropriate AEM system to use for acquisition. These studies suggested that the buried palaeovalleys were most likely to be near-coincident with contemporary valley systems developed in a granite/gneiss-greenstone basement. Covering such a large region required a novel approach to survey design to maximize the information relating to their expected spatial variability. Therefore a terrain index (MrVBF) was used with the SRTM 1sec DEM to define the extent of contemporary valleys, and the extent of the AEM survey area. This approach allowed survey acquisition costs to be kept to less than half that of flying a more “traditional” survey over the entire area. It also allowed for the acquisition of data with a closer line spacing than would have been possible otherwise, therefore capturing more of the spatial variability associated with the palaeovalley systems. The results have demonstrated the validity of the strategy adopted and have shown that in the absence of conventional hydrogeological information, geophysical methods are demonstrably a cost and time effective approach to upscaling local hydrogeological information, thereby fast tracking groundwater resource assessments that would otherwise take decades to complete.

We describe interpretation of an AEM survey around the La Grange allocation area, WA. This survey was designed to map aquifer bounds and the sea water intrusion, and then to assess groundwater in the region, and to facilitate planning water use. The simple, stratified nature of sediments of the western onshore Canning Basin allowed us to use blocky layered earth models and we found that five-layer models were the most parsimonious. After deriving surfaces representing the top of the Jarlemai siltstone and the top of the sea water ingress, we were able to effectively characterise the spatial characteristics of the sea water intrusion. We found that in places, sea water intruded 40 km inland, and could be found at a depth of over 250 m.