Utilising Airborne Electromagnetics (AEM) to Map Key Elements of the Hydrogeological System and Salinity Hazard in the Ord Valley, Western Australia
K.C. LawrieJonathan ClarkeKok Piang TanTim MundayAnthony J. FitzpatrickRoss BrodieC. F. PainHeike AppsK. CullenLarysa HalasTehani KuskeKevin CahillAaron Davis
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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.Keywords:
Electromagnetics
AND HYDROGEOLOGICAL MAPPING OF THE SPIRITWOOD VALLEY AQUIFER, MANITOBA, CANADA V. Sapia, G. Oldenborger and A. Viezzoli 1 Istituto Nazionale di Geofisica e Vulcanologia, Rome,Italy 2 Geological Survey of Canada, Ottawa, Canada 3 Aarhus Geophysics Aps, Aarhus, Denmark Introduction. Buried valleys are important hydrogeological structures in Canada and other glaciated terrains, providing sources of groundwater for drinking, agriculture and industrial applications. Hydrgeological exploration methods such as pumping tests, boreholes coring or ground-based geophysical methods (seismic and electrical resistivity tomography) provide limited spatial information and are inadequate to efficiently predict the sustainability of these aquifers at the regional scale. Airborne geophysics can be used to significantly improve geological and hydrogeological knowledge on a regional scale. There has been demonstrated success at using airborne electromagnetics for mapping and characterization of buried valleys in different geological contexts (Auken et al., 2008; Jorgensen et al., 2003; Jorgensen et al., 2009; Steuer et al., 2009). Despite the fact that both electromagnetic surveys and reflection seismic profiling are used extensively in hydrogeological mapping, integration of the methods is a relatively unexplored discipline (Hoyer et al., 2011). The Spiritwood Valley is a Canada-USA trans-border buried valley aquifer that runs approximately NW – SE and extends 500 km from Manitoba, across North Dakota and into South Dakota (Winter et al., 1984). The Spiritwood aquifer system consists of glacially deposited silt and clay with sand and gravel bodies, infilling a broad north-south trending valley that has been identified primarily based on water wells information (Wiecek, 2009). The valley is incised into bedrock consisting of fractured siliceous shale. As part of its Groundwater Geoscience Program, the Geological Survey of Canada (GSC) has been investigating buried valley aquifers in Canada using airborne and ground-based geophysical techniques. To obtain a regional three-dimensional assessment of complex aquifer geometries for the Spiritwood, both geophysical and geological investigations were performed with the aim to develop an integrated conceptual model for a quantitative description of the aquifer system. In 2010, the Geological Survey of Canada conducted an airborne electromagnetic (AeroTEM III) survey over a 1062 km area along the Spiritwood Valley, north of the US border (Oldenborger 2010a, 2010b). AEM inversion results show multiple resistive valley features inside a wider, more conductive valley structure within the conductive bedrock (Fig. 1). Furthermore, the complexity of the geometries, spatial distribution and size of the channels is evident. Other ground based data collected in the survey area make it possible to provide some constraints on the AEM resistivity model. Downhole resistivity logs were collected that provide information on the electrical model relative to the geological layers (Crow et al., 2012). In addition, over 10 line-km of electrical resistivity data and 42 km of high resolution landstreamer seismic reflection data (Figs. 2a, 2b) were collected at selected sites (Oldenborger et al., 2012). In this short paper we present results obtained from the data inversion and an example of integration of ancillary seismic data into the AEM inversion. In particular, the elevation to a layer (shale bedrock elevation) as interpreted from seismic is added to the inversion to constrain the resistivity model. Data acquisition: AeroTEM III airborne electromagnetic system. The AeroTEM system is based on a rigid, concentric-loop geometry with the receiver coils placed in the centre of the transmitter loop (Balch et al. 2003). Time varying current flow around a transmitter loop produces a time varying primary magnetic field which gives rise to eddy currents in the earth (Fig. 1a). The induced currents generate a secondary magnetic field detected by a receiver coil sensor. The transmitter waveform is a triangular current pulse of 1.75 ms duration operating at a base frequency of 90 Hz. The transmitter loop has an area of 78.5 m, with a maximum current of 480 A. The
Coring
Electrical Resistivity Tomography
Geologic map
Geological survey
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SummaryAirborne electromagnetic (AEM) surveys provide densely sampled data over large areas (typically several hundred sq. km) that cannot be covered effectively using ground-based methods. AEM data are inverted to estimate the three-dimensional distribution of electrical resistivity structures from shallow depths to several hundred meters. These models convey unparalleled details that are used to make inferences about hydrogeologic properties and processes at the watershed and local scale. This information is being used in groundwater models that are critical to water management decisions, to better understand geologic frameworks, and to improve climate change models. The U.S. Geological Survey (USGS) has been engaged in the application of AEM to many watershed and local scale groundwater projects within United States. We present the results of several frequency- and time-domain AEM surveys acquired by the USGS that have been used for mapping alluvial valleys, buried glacial aquifers, fault-bounded basins, and understanding permafrost distributions.
Geological survey
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An airborne electromagnetic (AEM) survey was carried out over the Dampier Peninsula, North of Broome, WA during September-October, 2012. The key objectives of this geophysical survey funded by the Department of Water was (i) to obtain a better understanding of the nature of the contact between the base of the Broome Sandstone and the underlying siltstone; (ii) to identify areas of water retentive clay layers in the near surface, (iii) to create a map of the water table; (iv) to study the detailed geometry of the near shore saline intrusion; and thus (v) assist the conceptualisation of the hydrogeology and determine the quantity and quality of available groundwater resources for the benefit of local communities, government and industry. The survey was conducted using SkyTEM, a helicopter-borne time domain AEM system.The processed AEM data for each of the survey lines were examined and inverted using the industry standard inversion techniques. The results were then compared with available bore-hole geophysical logging as well as the regional geophysical, geological and hydrogeological data. Apart from successfully mapping the depth to water table for the whole project area, this survey has clearly delineated the thickness of Broome Sandstone, shallow impermeable layers within the Broome Sandstone and areas of possible saline sea water intrusions. The survey has also successfully identified a WNW-ESE trending lineament (a basement high) and couple of NW-SE trending structural features (such as fault structures) from the central part of the survey region. The regional geophysical data images obtained from Department of Mines & Petroleum supports this finding.
Geophysical survey
Peninsula
Geological survey
Magnetic survey
Exploration geophysics
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In 1998/99 the Department of Natural Resources (DNR) in Bundaberg, Queensland,<br>Australia contracted Geo-Eng Australia (now GHD Pty Ltd), to undertake a major joint study of the<br>coastal groundwater system of the Bundaberg Irrigation Area (BIA) which was experiencing<br>problems with the unregulated use of much of the aquifer and seawater intrusion problems. The<br>Bundaberg Groundwater Project is arguably the largest integrated geophysical, drilling and<br>hydrogeological project for water resources assessment undertaken in Australia.<br>The Project was a pre-cursor for a proposed new BIA groundwater model. One primary<br>project objective was to acquire sufficient data on the extent and properties of the groundwater<br>system to redefine the conceptual hydrogeological model. Another primary objective was to<br>systematically apply integrated geophysical, geological and hydrogeological techniques and gather<br>permeability data, directly and empirically, for identification of permeability trends.<br>The project provides a case study, showing the value of a strong commitment to the largescale<br>use of routine and innovative geophysics throughout a major groundwater investigation.<br>Firstly, the project budget allowed use of ground resistivity (of 702 kms of traversing and<br>273 soundings), seismic reflection soundings and multi-parameter geophysical logging (270<br>new/existing holes) on a scale generally not contemplated in groundwater studies. The extensive<br>geophysics guided a major drilling program of 130 new holes including 106 new piezometers<br>optimised in position and depth.<br>Secondly, it showed the value for groundwater management decision-making of an integrated<br>analysis of disparate datasets (airborne and ground geophysics, hydrogeological, topographic,<br>hydrochemistry and geophysical logs).<br>Thirdly the application of sequence stratigraphic analysis techniques, to geophysical log data<br>defined a new conceptual hydrogeological model and understanding of the hydrogeological context<br>and information value of each existing and new piezometer.<br>Finally, the geophysical logs assisted by the surface geophysics, proved useful in providing<br>an assessment of permeability trends for groundwater model building.
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Summary Airborne electromagnetics (AEM) has successfully mapped and characterised groundwater systems in a range of landscapes and geological settings in the East Kimberley Region of north-western Australia. The AEM data enabled rapid imaging of key elements of hydrogeological systems in near-surface Cenozoic paleovalley, alluvial fan and colluvial sediments, and in underlying tectonically-inverted sedimentary basins. Rapid mapping and assessment of groundwater systems, MAR targets and salinity hazards involved the integration of AEM data with Ground Magnetic Resonance (GMR), seismic reflection, drilling and pump tests, borehole geophysics, soils, regolith, geological and structural mapping, and hydrogeological and hydrochemical investigations. AEM survey design was aided by the use of spatio-temporal analysis of Landsat data to identify areas of potential surface-groundwater interaction. A suite of equivalent 1D AEM inversion models produced comparable images of the sub-surface hydrostratigraphy and faults. However, 2.5D inversions produced different solutions in key locations. 3D inversions were subsequently performed, and drilling and tectonic analysis was used to assess all AEM inversion models. Recognising zones of structural complexity was important in the successful development of appropriate AEM inversion strategies and models. Overall, the success of groundwater system mapping has been due to the use of AEM within a broader, inter-disciplinary, multi-physics project framework.
Geologic map
Regolith
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