Demonstrates the application of modelling gamma-ray spectrometry and DEM for mapping regolith materials and in predicting salt stores.

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This study reports the findings of salt store and salinity hazard mapping for a 20-km wide swath of the Lindsay - Wallpolla reach of the River Murray floodplain in SE Australia. The study integrated remote sensing data, an airborne electromagnetics (AEM) survey (RESOLVE frequency domain system), and lithological and hydrogeochemical data obtained from a field mapping and drilling program. Maps of surface salinity, and surface salinity hazard identified Lindsay and Wallpolla Islands, and the lower Darling Floodplain as areas of high to extreme surface salinity hazard. In the sub-surface, salt stores were found in general to increase away from drainage lines in both the unsaturated and saturated zones. Beneath the Murray River floodplain, salt stores in both unsaturated and saturated zones are high to very high (100 to 300t/ha/m) across most of the floodplain. Sub-surface salinity hazard maps (incorporating mapped salt stores and lithologies, depth to water table and the hydraulic connectivity between the aquifers), identify Lindsay and Wallpolla Islands; the northern floodplain between Lock 8 and Lock 7; and northern bank of Frenchman's Creek as areas of greatest hazard. Overall, the new data and knowledge obtained in this study has filled important knowledge gaps particularly with respect to the distribution of key elements of the hydrostratigraphy and salinity extent across the Murray River and lower Darling floodplain. These data are being used to parameterise groundwater models for salinity risk predictions, to recalculate estimates of evapotranspiration for salt load predictions, address specific salinity management questions, and refine monitoring and management strategies.

Salinity of groundwater directly affects its suitability for different uses, including human consumption, stock water, agricultural use, and mineral or energy extraction. Traditionally, direct measurements of groundwater salinity at monitoring bores that intersect an aquifer have been used to map the spatial distribution of groundwater salinity. However, drilling is a logistically and economically challenging task, and we are usually left with a sparse set of measurements from which to infer groundwater salinity over large spatial extents. Airborne electromagnetic (AEM) sounding provides a solution to this problem. This is because AEM can be flown rapidly and cost-effectively over large swathes of land, and high subsurface bulk conductivities inferred from the AEM are well correlated with groundwater salinity in porous aquifers. We present here a methodology and case study from the Keep River Plains in the Northern Territory that provides information for land and watershed managers about the confidence with which salinity can be mapped over large areas using AEM. Extensive pore fluid sampling of the saturated zone, which lies beneath the watertable, enables this workflow to be used effectively. The results provided by our method can feed into decision making while accounting for uncertainty, enabling remote communities to manage their land and water resources effectively. <b>Citation:</b> Symington, N.,Ray, A., Harris-Pascal, C., Tan, K.P., Ley-Cooper, A.Y., and Brodie, R.C., 2020. Groundwater salinity estimation using borehole and AEM data: a framework for uncertainty analysis. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.

Airborne electromagnetic (AEM) systems are increasingly being used for mapping conductivity in areas susceptible to secondary salinity, with particular attention on near-surface predictions (ie those in the top 5 or 10 metres). Since measured AEM response is strongly dependent on the height of both the transmitter loop and receiver coil above conductive material, errors in measurements of terrain clearance translate directly into significant errors in predicted near-surface conductivity. Radar altimetry has been the standard in airborne geophysical systems for measuring terrain clearance. In areas of agricultural activity significant artifacts up to five metres in magnitude can be present. One class of error, related to surface roughness and soil moisture levels in ploughed paddocks and hence termed the ?paddock effect?, results in overestimation of terrain clearance. A second class of error, related to dense vegetation and hence termed the ?canopy effect?, results in underestimation of terrain clearance. A survey example where terrain clearance was measured using both a radar and a laser altimeter illustrates the consequences of the paddock and canopy effects on shallow conductivity predictions. The survey example shows that the combination of the dependence of AEM response on terrain clearance and systematic radar altimeter artefacts spatially coincident with areas of differing land-use may falsely imply that land-use practices are the controlling influence on conductivity variations in the near surface. A laser altimeter is recommended for AEM applications since this device is immune to the paddock effect. Careful processing is still required to minimise canopy effects.

This study grew out of the GILMORE project, which is a pilot study designed to test methodologies and technologies for assessing mineral prospectivity and dryland salinity in areas of complex regolith cover (Lawrie, 1999; Apps et al., this volume). The project area is in central NSW, towards the eastern margin of the Murray-Darling Basin, straddling the catchments of the Lachlan and Murrumbidgee. The area can be divided into two parts, with creeks (eg. Bland creek) in the northern catchment flowing towrds the Lachlan River, and creeks in the southern catchments flow towards the Murrumbidgee River. The GILMORE project utilised a multi-disciplinary approach to generate a coherent picture of the various factors controlling the distribution of sediments and the saline groundwater. Thus, geomorphology and landscape processes (past and present), sedimentary environments, pedology, weathering and regolith formation, geophysical attributes of regolith materials and groundwater characteristics were all considered (Lawrie et al., 2000). The project has acquired a wide range of data, including airbone geophysics such as high-resolution magnetics, radiometrics and electromagnetics (AEM, TEMPEST system), as well as ground based (down-hole) geophysics, and information from 6600 drill holes of which 4000 have lithological and geological data (Apps et al., this volume). In addition, there is groundwater chemistry data of over 100 bore holes (Lawrie et al., 2000). Specifically, AEM surveys have been flown across areas 1 and 2 and AEM depth slice images have been utilised to establish the spatial pattern of conductivity and inferred distribution of saline groundwater and preferential groundwater pathways. This study aims to establish the association between mineral composition, lithology units and groundwater chemistry.

<div>This data package provides petrophysical interpretations by Geoscience Australian and the South Australia Department for Energy and Mining (SADEM) for 23 wells generated in support of the energy resource assessments under the Australia’s Future Energy Resources (AFER) project in the Pedirka and western Eromanga basins. Interpreted petrophysical data in this data package include [BB1]&nbsp;[MB2]&nbsp;volume of clay/shale, porosity (total and effective), relative permeability, formation water salinity (NaCl equivalent), and apparent resistivity of water.</div><div>&nbsp;</div><div>The AFER project is part of Geoscience Australia’s Exploring for the Future (EFTF) Program—an eight year, $225 million Australian Government funded geoscience data and precompetitive information acquisition program to inform decision-making by government, community and industry on the sustainable development of Australia's mineral, energy and groundwater resources. By gathering, analysing and interpreting new and existing precompetitive geoscience data and knowledge, Geoscience Australia is building a national picture of Australia’s geology and resource potential. This will help support a strong economy, resilient society and sustainable environment for the benefit of all Australians. The EFTF program is supporting Australia’s transition to a low emissions economy, industry and agriculture sectors, as well as economic opportunities and social benefits for Australia’s regional and remote communities. Further details are available at http://www.ga.gov.au/eftf.This new data package consists of composite logs and supporting data which includes interpreted volume of clay/ shale, porosity, permeability and salinity.</div><div>&nbsp;</div><div>The data package includes the following datasets: </div><div>1)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Composite logs (PDF)</div><div>2)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Well logs (ASCII LAS)</div><div>3)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Well header information (Microsoft Excel™)[BB3]&nbsp;[MB4]&nbsp;</div><div>&nbsp;</div><div>These petrophysical interpretations are being used to support the AFER Project’s play-based energy resource assessments in the Pedirka and western Eromanga basins by building 3D geological models that include derived rock property maps.

The first large scale projects for geological storage of carbon dioxide on the Australian mainland are likely to occur within sedimentary sequences that underlie or are within the Triassic Cretaceous Great Artesian Basin aquifer sequence. Recent national and state assessments have concluded that certain deep formations within the Great Artesian Basin show considerable geological suitability for the storage of greenhouse gases. These same formations contain trapped methane and naturally generated carbon dioxide stored for millions of years. In July 2010, the Queensland Government released exploration permits for Greenhouse Gas Storage in the Surat and Galilee basins. An important consideration in assessing the potential economic, environmental, health and safety risks of such projects is the potential impact carbon dioxide migrating out of storage reservoirs could have on overlying groundwater resources. The risk and impact of carbon dioxide migrating from a greenhouse gas storage reservoir into groundwater cannot be objectively assessed without an adequate knowledge of the natural baseline characteristics of the groundwater within these systems. Due to the phase behaviour of carbon dioxide, geological storage of carbon dioxide in the supercritical state requires depths greater than 800m, but there are few hydrogeochemical studies of these deeper aquifers in the prospective storage areas. Historical hydrogeochemical data were compiled from various State and Federal Government agencies. In addition, hydrogeochemical information has been compiled from thousands of petroleum well completion reports in order to obtain more information on the deeper aquifers, not typically used for agriculture or human consumption. The data were passed through a quality checking procedure to check for mud contamination and ascertain whether a representative sample had been collected. The large majority of the samples proved to be contaminated but a small selection passed the quality checking criteria.