The Musgrave province, located in South Australia's far north‐west, is experiencing accelerated minerals exploration partly in response to new State Government initiatives. Access to water may become a fundamental infrastructure requirement to support developments emerging from these activities. Our current knowledge of the local and regional groundwater systems, the architecture of aquifers, groundwater quality, availability and sustainability is very limited. Current understanding for the area suggests that fractured rock aquifer systems predominate with local palaeovalleys also present. Groundwater is the primary source of water for the indigenous communities and for pastoral purposes. Given its importance for these purposes and given the potential for increased demand, it has been recognised that a more thorough assessment is needed to inform the development of appropriate management policies. In this paper we examine AEM data from the TEMPEST and VTEM systems and consider their value in contributing to our knowledge of groundwater systems in the area. Specifically we examine results from their full inversion and compare the hydrogeological information content contained in the derived models for the two systems. Both data sets indicate the presence of a litho‐structurally controlled palaeovalley system, and local drilling indicates the presence of a fresh ‐ brackish groundwater. The AEM conductivity models indicate that the valleys have a step‐like geometry with a wide valley margin filled with clays and sands, and then a V‐shaped central valley up to 100m+ deep. Regional vacuum drilling confirms the pattern defined in the AEM data. Local (facies related?) variations are defined in the central parts of the palaeovalleys and the observed conductivity structure suggests these may be higher yielding (sandy) units with a clay rich valley fill. Further drilling is required to confirm this.
Inverted RESOLVE FDEM data for the Bookpurnong stretch of the River Murray, were compared with the Insteam NanoTEM data and available river-bed core data. The AEM data were not collected along the river itself, but extracted from gridded profile data flown in a WNW-ESE direction. The HEM data show very similar trends in conductivity variation identified in the corresponding NanoTEM data as shown in Figures 1 and 2. There are some minor differences between the two images shown in Figure 1, but this is attributed to the NanoTEM data representing the conductivity of riverbed sediments, whereas the RESOLVE data represents the 1.5-3 metre depth from the waterlevel surface of the river. The inverted conductivity depth sections shown in Figure 2 provide a better means of comparing the two techniques. For this reach of the river the RESOLVE HEM and NanoTEM data effectively map gaining and losing stream conditions and provide significant insight into the interplay between an irrigation induced groundwater mound, the regional groundwater system and river salinity. Regolith 2006 - Consolidation and Dispersion of Ideas
Saltwater intrusion is the greatest risk to coastal community water supplies where they are dependent on fresh groundwater as the main source of supply. For small, fractured bedrock island aquifers, the fresh groundwater lens dynamics and transition zone geometry are complex. This study investigated the impacts of projected increases in groundwater pumping on a fresh groundwater lens to evaluate changes to the lens geometry and localized up-coning from the deeper, more saline aquifers beneath a small bedrock island in the tropics. A comprehensive conceptual model was developed using traditional hydrogeological datasets and complemented with an airborne electromagnetic survey, which were used to construct a three-dimensional density-dependent groundwater flow and solute transport model using the SEAWAT code. The model was calibrated using observed groundwater hydraulic heads and chloride concentration data, and calculated chloride values based on bulk conductivity measurements determined from inverted geophysical data. A staged calibration approach was adopted, firstly assessing the time-average lens extent and geometry, and secondly considering the seasonal groundwater level response. In the calibration, the geophysical data helped constrain the lens geometry in the absence of hydraulic head and chloride data. The calibrated model was used to test scenarios where groundwater pumping rates were increased above the current demand of 452 m3 d-1, showing that the lens is likely to be stable, i.e., its available storage is not expected to contract excessively, for extraction rates of up to 3,000 m3 d-1. The combined use of geophysical data and a numerical modeling approach was advantageous in investigating the lens characteristics. It also demonstrated how these techniques can be used together to evaluate coastal water resources and to manage water supply risks for coastal communities. The study demonstrated that the freshwater lens can likely support the freshwater demands of the remote community, and is a preferred option compared with high-cost and more complicated options such as seawater desalination and managed aquifer recharge.
Although the notion of spatio-temporal monitoring of natural landscapes and phenomena using multi-date airborne electromagnetic (AEM) surveys has been around for some time, examples are very limited in scope, particularly when defining vertical and lateral changes with time. We demonstrate an effective procedure for defining spatio-temporal variations in ground conductivity across a salinised floodplain in South Australia, using multi-date FDHEM data. Lateral and vertical changes in the conductivity of the floodplain have been resolved. We believe the advent of improved calibration procedures, geometry correction, calibrated broad band AEM systems and advanced inversion procedures that obviate the necessity of system calibration – recalibration, such as the holistic inversion, provide for the realistic proposition of using AEM data for the semi-quantitative and quantitative monitoring of landscape change in the subsurface. However, we emphasize the need for caution when considering observed spatial variations, stressing the importance of accounting for system investigation depth and the potential for artifacts that might be introduced from noise, system geometry and/or data interpretation procedures, when comparing data and derived conductivity models from different dates.
SALTMAP airborne electromagnetic (AEM) data for the Lawlers District, W.A., show that areas of complex regolith cover are characterised by marked variations in electrical conductivity. Their interpretation against ground electromagnetic, petrophysical and drill hole data indicates that conductive zones lie within the regolith, usually between a thin, relatively resistive, surface layer and a resistive basement. The conductive layer is commonly associated with the saprolite and to a lesser extent with alluvium in palaeovalleys and drainage sumps. Interpretation of the AEM with aeromagnetic and field data indicates that there is a strong lithodependence in the observed conductivity structure. Structure is also important. One-dimensional layered earth inversions and conductivity depth sections showed that the conductivity and thickness of regolith materials (predominantly saprolite) varied with lithology. For example, the felsic volcanics located in the NE margin of the survey area are characterised by a thick, poorly conductive saprolite. This contrasts with a thinner, more conductive saprolite developed over adjacent mafic lithologies. Somewhat surprising were the electrical characteristics of the ultramafic units, which appeared to be similar to those of the felsic volcanic units, suggesting thick, relatively resistive, materials. This behaviour is at odds with that reported in other studies concerning the electrical properties of weathered ultramafics in other environments. The Lawlers study suggests that differences in the electrical properties of in-situ regolith materials can be attributed to the complex interplay between the manner in which particular lithologies weather, the character of the resulting regolith in terms of porosity and permeability, and to variations in the soluble salt content and quantity of the saturant waters. The "EM response map" of the Lawlers area is further complicated by transported cover which appears to impose a cross cutting conductivity structure over that of the underlying saprolites.
The Aboriginal population of the Aṉangu Pitjantjatjara Yankunytjatjara (APY) lands in South Australia is dependent on groundwater for nearly all water needs. In that region, placement of wells in productive aquifers of appropriate water quality is challenging because of lack of hydrologic data and variable aquifer properties. It is desirable to have an improved ability to identify and evaluate groundwater resources in this remote region with cost-effective methods that make minimal impact on the environment. A project supported by the Society of Exploration Geophysicists program Geoscientists Without Borders tested a combined geophysical approach with airborne and ground-based data sets to locate a potential aquifer, confirm water content, and estimate the subsurface extent of the water-bearing zone. This hydrogeophysical approach was an effective means for exploration and evaluation of groundwater resources in APY lands generally, and it characterized a specific aquifer as a case study.
AbstractAccess 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.Survey designPalaeovalley aquifersairborne EMAEM systems AcknowledgmentsThe authors acknowledge the funding support of the WA Governments' Royalties for Regions Program linked to DoW's regional water availability activities. The work was also support by CSIRO Mineral Resources, as a contribution to its UNCOVER agenda, given that the work provide insight into the cover and groundwater characteristics of a large part of WA.
Abstract A small‐diameter nuclear magnetic resonance ( NMR ) logging tool has been developed and field tested at various sites in the United States and Australia. A novel design approach has produced relatively inexpensive, small‐diameter probes that can be run in open or PVC ‐cased boreholes as small as 2 inches in diameter. The complete system, including surface electronics and various downhole probes, has been successfully tested in small‐diameter monitoring wells in a range of hydrogeological settings. A variant of the probe that can be deployed by a direct‐push machine has also been developed and tested in the field. The new NMR logging tool provides reliable, direct, and high‐resolution information that is of importance for groundwater studies. Specifically, the technology provides direct measurement of total water content (total porosity in the saturated zone or moisture content in the unsaturated zone), and estimates of relative pore‐size distribution (bound vs. mobile water content) and hydraulic conductivity. The NMR measurements show good agreement with ancillary data from lithologic logs, geophysical logs, and hydrogeologic measurements, and provide valuable information for groundwater investigations.
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