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
SummaryAirborne time domain electromagnetic and high resolution 2D seismic data were acquired in 2018 to map structures of hydrogeological significance in the Peel region; Western Australia. Interpretation of the airborne electromagnetic (AEM) survey was complicated by the presence of major utilities including power lines. We consider methods for removing the impact of these sources of EM noise within processing prior to interpretation. Generally, we found this to be counterproductive as information was unnecessary lost. Imaging from high resolution seismic reflection data is unaffected by EM noise. We show how a strategically located high quality seismic imaging was instrumental in providing an interpretational framework that could be extended to the full AEM survey area. We provide examples of AEM interpretation for many hydrogeological features including: major faults, 3D hydrostratigraphic surfaces, geological dip, saline water interfaces and zones with potential hydraulic connection between shallow and deeper aquifer systems. This work facilitated significant revision of groundwater systems conceptualisation in the Peel region.
Geophysical Imaging
Reflection
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INTRODUCTION An interpretation of airborne geophysical datasets for Honeysuckle Creek catchment in north central Victoria (Figure 1a and b), has been carried out in conjunction with the application of conventional hydrogeological techniques. Integration of existing bore data and substantial fieldwork with airborne geophysical coverages has been directed towards understanding groundwater and salinity processes in the region.
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A number of helicopter AEM systems are discussed including AeroTEM, HeliGeotem, RepTEM, SkyTEM and VTEM. Field data are shown from a range of exploration environments, including AeroTEM data across an oil sands prospect in Alberta, Canada, HeliGEOTEM mineral exploration data from Labrador, Canada, RepTEM uranium exploration data from South Australia, SkyTEM salinity mapping data from South Australia and VTEM data across a massive sulphide in Western Australia. The data were modelled with layered-earth inversions and selected anomalies were modelled successfully with conductive plates in free-space.
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Subsurface resistivity is a key component of many mineralization models including unconformity-related uranium, palaeochannel hosted uranium and nickel sulphides. Other key applications of subsurface resistivity involve environmental aspects of the subsurface such as groundwater detection, and civil engineering applications including detection of buried pipes and cables. Measuring subsurface resistivity is the aim of electromagnetic (EM) geophysical techniques. It involves transmitting an electromagnetic field into the Earth, and then recording this field - and the Earth response - on a receiver. The transmitted field signal can be removed from the received field to determine the Earth response.Airborne EM (AEM) is a geophysical technique that allows this process to be undertaken from an airborne (aeroplane or helicopter) platform. AEM exploration first commenced in South Australia with AFMAG (Audio- Frequency Magnetic technique) surveys in the 1960s, and VLF (Very Low Frequency) surveys from 1971. Numerous platforms including RepTEM, GeoTEM, Input, QUESTEM, HoistEM, TEMPEST and VTEM are now routinely used within South Australia. Each technique provides a different view of the subsurface dependant on the system parameters and the processing undertaken on the data.Given the increase in AEM surveying within South Australia and the wide applications available for this data a review of this technique within the State has been undertaken. This poster presents a summary of AEM within South Australia, focussing on a number of significant surveys and their outcomes. Surveys in the Cariewerloo Basin and Fowler Domain in particular have been used to model uranium prospectivity and help define nickel deposits. All data reviewed is now downloadable online via SARIG.
Prospectivity mapping
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Electromagnetics
Characterization
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We present a large-scale study of the relationship between dense airborne SkyTEM resistivity data and sparse lithological borehole data.Airborne electromagnetic (AEM) data contains information about subsurface geology and hydrologic properties; however extracting this information is not trivial. Today, geophysical data is used in combination with borehole data to create detailed geological models of the subsurface. The overall statistical relationship is, however, not widely known. The objective of this study is to develop a method for understanding the relationship between petrophysical properties and lithology, and apply this to get a better understanding of large-scale petrophysical structures of the subsurface.The data sampling is carried out in a scheme where data is interpolated onto the position of the boreholes. This allows for a lithological categorization of the interpolated resistivity values, revealing different distribution functions for lithological categories.A very large and extensive dataset is available in Denmark through the national geophysical and borehole databases. These databases contain all geophysical and borehole data in Denmark and covers a large part of its surface. By applying the proposed algorithm to all available airborne electromagnetic data, detailed maps of the large-scale resistivity-lihology structures on a National scale in Denmark are constructed.
Petrophysics
Lithology
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The Geological Survey of Namibia hosts a world leading airborne geophysical data coverage library that has amassed from years of large scale regional airborne magnetic and radiometric survey acquisition with detailed high resolution survey follow up to promote the exploration and mineral wealth of the country. In 2005 the Geological Survey of Namibia acquired a strategically planned Airborne Electromagnetic Survey in Northeast Namibia. Following completion of this initial phase one reconnaissance survey (532 line km), two areas were selected for detailed high resolution follow up, namely Elandspan and Eiseb. In total just over 5,000 line km of AEM was acquired. Initial qualitative interpretation indicated that the Tempest AEM system could detect conductors beneath the Kalahari sediments. Recently a more detailed interpretation has been undertaken with many 2D and 3D mapping products generated. Conductivity Depth Images along with early, mid and late time EM products were created illustrating the surficial effects of a river system and a number of deep basement conductors. The 2D and 3D AEM products led to an integrated geological interpretation of the area and significantly added value to an aeromagnetic interpretation of the area. The CDI?s generated allowed the thickness of the Kalahari sands to be quantified, far shallower than anticipated, increasing the exploration prospectivity of the region and a geological map of the area, based on these results, has been created in an area with very little data coverage and no outcrop.
Prospectivity mapping
Geologic map
Outcrop
Aeromagnetic survey
Basement
Geophysical survey
Geological survey
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Neogene fault systems are increasingly recognised as an important control on hydraulic connectivity in some of Australia’s energy rich basins. However, accurate delineation of these faults systems is challenging and expensive. In this context, the main objective of the Exploring for the Future (EFTF) Surat-Galilee Basin (Phase 1) Project is to test novel methods for more cost-effective mapping of Neogene fault systems in the Coal Seam Gas (CSG) basins of eastern Australia. Methods assessed in this project include morphotectonic mapping using temporal remote sensing data and high-resolution terrain mapping techniques, airborne electromagnetics (AEM), and the use of earthquake databases to inform active tectonic and geomechanical analysis.The project is funded by Geoscience Australia (GA) as part of its EFTF Programme, and is focussed on exemplar areas in the Surat and Galilee Basins where Neogene fault activity has been interpreted on high-resolution 2D and 3D seismic reflection surveys. This paper reports on the use of airborne electromagnetics (AEM) for detecting near-surface (<50-150m) Neogene faults in both basins. Approximately 4,500 line km of AEM data were acquired in a number of smaller acquisition blocks where Neogene faults had previously been identified. The AEM inversion results are compared with interpretation of seismic reflection data, morphotectonic mapping, and other hydrogeological and tectonic/geomechanical data. The utility of AEM to map the broader hydrogeological system in these basins, including groundwater-surface water connectivity (springs and rivers), is also assessed.
Neogene
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Exploration for potable groundwater to supply settlements in the southern Kalahari Desert of southwestern Botswana is currently in progress. A Dighem survey was flown over outcrops of Precambrian rocks after target areas were selected in an initial desktop study. The study included some preliminary ground geophysics, reviewed all available information in the area and completed an interpretation of remotely sensed (Landsat TM and SPOT) and aeromagnetic data. Two target areas were recommended for Dighem airborne electromagnetic (AEM) surveying near outcropping basement. The Dighem survey was designed to map faults and fractures intersecting resistive quartzite of the Olifantshoek Sequence, the most productive groundwater host in the area. Outcrop geological mapping, ground geophysical traverses to test previous successful boreholes and to locate new ones, exploration drilling and borehole logging were then used to follow up and refine targets generated from the AEM survey.Airborne and ground geophysical survey data were carefully integrated with outcrop geology and borehole information. An improved understanding of the geology and structure in Precambrian Olifantshoek quartzite was based on interpretation of the Dighem and aeromagnetic data. Exploration drilling success is attributed to an integrated multidisciplinary approach, which is strongly recommended for other potential groundwater projects in the Kalahari Desert.
Outcrop
Aeromagnetic survey
Basement
Geologic map
Geological survey
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Summary2.5D (2D geology, 3D source) inversion of airborne electromagnetic (AEM) data has evolved into a routine and established practice on datasets from an array of applications. Large datasets may be inverted in days using conventional PC’s, or cloud computing for faster results.The 2.5D inversions in this study were carried out using a highly modified adaptation of the ArjunAir program originally developed by the CSIRO and subsequently by AMIRA project P223F. The new program is called Moksha.Results are presented from a continental scale AEM regional mapping survey carried out by Geoscience Australia. 2.5D inversions performed in a study area in the Mammoth Mines mineral district of Queensland defined discrete conductivity anomalies on a line over the Mount Gordon Fault Zone, and imaged a series of steeply-dipping conductors on a nearby regional traverse.The study demonstrated the ability of 2.5D inversions to image steeply-dipping and folded geology, and present possible exploration targets, in a mineralised deformed terrane.
Traverse
Mammoth
Mineral exploration
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