FREQUENCY DOMAIN HELICOPTER ELECTROMAGNETICS - REDUCING THE COSTS OF ACQUISITION FOR SALINITY AND GROUNDWATER MAPPING
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This study uses FDHEM data collected from the Riverland South Australian Salinity Mapping and Management Projects, using the RESOLVE system. The Riverland survey was collected along the southern floodplains near Loxton, South Australia, an area of largest salt export along the Murray River. The objective of the Riverland survey was to map the depth, thickness and conductivity of two layers, the Blanchetown Clay and the saline groundwater. Definition of useful targets for the AEM survey, rather than a simplistic salinity mapping approach, was undertaken to provide the level of detail and understanding required. Survey design based on knowledge of the hydrogeological framework and pre-survey down-hole induction conductivity logging enabled forward modelling of the likely EM response of the target given a particular airborne EM system and survey parameters. Further to this, use of a priori knowledge of the local geomorphic and geological landscapes, density of boreholes and other geophysical information was investigated in the context of survey planning of line and tie line density. Initial line spacings were derived from a model describing the smallest target of interest in the survey, 150 metre diameter holes in the Blanchetown Clays, so that at least two lines would intersect the smallest target.Keywords:
Electromagnetics
Soil survey
Geophysical survey
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The Riverland area, located on the southern bank of the River Murray in South Australia, is a priority area for intervention under the National Action Plan. As part of the South Australian Salinity Mapping and Management Support Project, airborne geophysics was recognised as having potential to provide valuable biophysical data relevent to the management of irrigation development and groundwater recharge reduction in the area. We examine this potential, giving particular regard to the process involved in understanding the target, defining an appropriate geophysical tool and testing whether the desired output could be delivered economically and at an appropriate resolution.The target in the Riverland area is the near surface Blanchetown Clay unit. Forward modelling suggested that a frequency domain helicopter electromagnetic (HEM) system could map spatial variability associated with this unit. A test survey was conducted using the RESOLVE HEM system further demonstrated that potential. CDI's of these data were compared with shallow drilling, EM31, EM34 and broadband ground TEM data. Results confirmed that near surface conductivity variations mapped by the airborne EM system were associated with the clay and that a product which could be incorporated with hyrogeological models to help predict rechage and influence management decisions in the area could be generated for the whole area.
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Data Reduction
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SummaryThe pumping and disposal of saline groundwater from the margins of the River Murray in South Australia is an integral part of the State Government’s salinity management strategy. It is specifically aimed at reducing ground water levels and salt accession to the River Murray. Large volumes of saline water are typically disposed at the land surface in what are referred to as “saline-disposal basins”. Although these disposal basins are now common, surprisingly little is known about their long-term efficacy or environmental effects. This study focuses on the analysis and interpretation of RESOLVE frequency domain helicopter electromagnetic data acquired over the Stockyard Plains saline-water disposal basins located southwest of Waikerie, South Australia, with a view to determining the extent of saline plume migration and informing our current understanding of the hydrodynamics of saline groundwater disposal in the area. The airborne EM data was calibrated using conductivity borehole data and statistical methods prior to modelling. Two sets of conductivity models were generated using smooth layered inversion and constrained layered earth inversion. The constrained inversion model provided information on the depth, thickness and presence or absence of aquitards, specifically the Blanchetown Clay, and map variations in groundwater conductivity in the region around the existing natural disposal basins. The smoothed inversion model defined the extent and condition of the groundwater mound beneath the existing disposal basin. In addition these data can be used to investigate the potential for extending disposal options in the vicinity of the existing basin by identifying areas where aquitards (the Blanchetown Clay) are present or absent.
Saline water
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Water management decisions that impact Everglades restoration efforts require high quality data and reliable hydrologic models. Traditionally these data for hydrologic models have been obtained through observation wells. In the Everglades, this approach is limited by the difficult access due to water which covers most of the area and to the limited number of roads. Airborne geophysical techniques provide a means of accessing large parcels of land and developing three-dimensional resistivity models of the area. The overall objective of this project is the collection of geophysical data that can be used to develop ground-water flow models of the area capable of modeling saltwater intrusion. This objective includes mapping of subsurface electrical properties of the aquifer and correlation of lateral variation in these properties to aspects of aquifer geometry and water quality that are pertinent to hydrologic model development. Completion of combined ground and airborne geophysical surveys in Everglades National Park and Big Cypress National Preserve has shown the utility of these methods to map saltwater intrusion and provide geological information needed to develop ground-water flow models. The strategy that has been used is to interpret the HEM data as layered-earth resistivity models that slowly vary from place to place. Surface geophysical measurements (time-domain electromagnetic soundings) have been used to assist in this interpretation and provide an independent check on the HEM data. Borehole data in the form of formation resistivities and water quality sampling have allowed us to develop relationships for converting the interpreted resistivity-depth models into estimated water quality given as specific conductance (SC) or chloride concentration. This information is of great value to hydrologic modelers. These data will be used to develop a ground-water flow model which is bounded on the north by the Tamiami Trail, on the south by Florida Bay, on the east by the Atlantic coastal ridge, and on the west by the Gulf of Mexico.
Saltwater intrusion
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The integration between advanced techniques for groundwater exploration is necessary to manage and protect the vital resources. Direct current (DC) resistivity geoelectrical technique, Enhanced Thematic Mapper Landsat (ETM+) images and a geographic information system (GIS) are integrated to identify the groundwater potentiality in the study area. The interpretation of the one-dimensional (1-D) inversion of the acquired resistivity data are implemented for mapping the fresh to slightly brackish water aquifer. This number of vertical electric sounding is quite enough for different geologic mapping. The depth to the top of the ground water table (obtained from the existing Water well) and subsurface lithological information are used to calibrate the results of the resistivity data inversion. This research discussed how the integration between the geoelectrical parameters and hydrological data, could be used to determine the appropriate locations of dams construction and recommend the appropriate methods for management and rehabilitation of the aquifer.
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Thematic map
Vertical electrical sounding
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In Perth, Western Australia, there has been both an increasing demand for water and decreasing rainfall over recent years. Managed Aquifer Recharge is a water recycling method identified as having the potential to reduce pressure on Western Australia’s surface and ground resources. For this reason a treated waste water injection trial is planned for the Beenyup Waste Water Treatment plant. The treatment plant is located in the northern Perth suburb of Craigie. The trial will include detailed hydraulic flow and reactive transport modelling of the injected water. Accurate modelling requires precise knowledge of the hydrostratigraphy below the injection site. Consequently a high resolution 3-D seismic reflection survey will be used to assist in building a detailed groundwater flow model. Optimal 3D survey geometry has been designed based on a preliminary 2D survey, VSP data and the resolution required for the target injection zone within the Leederville formation. 3D survey design was faced with various difficulties as it needed to be designed with a number of exclusions zones related to topographic mounds, vegetation and existing infrastructure. Future construction plans at the site were also factored in to allow for future time lapse seismic surveys after long term injection has pressurised the target aquifer. Forward models of the expected seismic response of the injection process are computed using borehole information and velocity-pressure tests from core sample tests.Technical area: Environmental-groundwater
Geological survey
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Abstract. Geophysical techniques are increasingly being used as tools for characterising the subsurface, and they are generally required to develop subsurface models that properly delineate the distribution of aquifers and aquitards, salt/freshwater interfaces, and geological structures that affect groundwater flow. In a study area covering 730 km2 across the border between Germany and Denmark, a combination of an airborne electromagnetic survey (performed with the SkyTEM system), a high-resolution seismic survey and borehole logging has been used in an integrated mapping of important geological, physical and chemical features of the subsurface. The spacing between flight lines is 200–250 m which gives a total of about 3200 line km. About 38 km of seismic lines have been collected. Faults bordering a graben structure, buried tunnel valleys, glaciotectonic thrust complexes, marine clay units, and sand aquifers are all examples of geological structures mapped by the geophysical data that control groundwater flow and to some extent hydrochemistry. Additionally, the data provide an excellent picture of the salinity distribution in the area and thus provide important information on the salt/freshwater boundary and the chemical status of groundwater. Although the westernmost part of the study area along the North Sea coast is saturated with saline water and the TEM data therefore are strongly influenced by the increased electrical conductivity there, buried valleys and other geological elements are still revealed. The mapped salinity distribution indicates preferential flow paths through and along specific geological structures within the area. The effects of a future sea level rise on the groundwater system and groundwater chemistry are discussed with special emphasis on the importance of knowing the existence, distribution and geometry of the mapped geological elements, and their control on the groundwater salinity distribution is assessed.
Water well
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The construction of salt interception schemes (SIS) in the Riverland region of South Australia forms an integral part of a broader strategy to manage saline groundwater intrusion into the River Murray. Results from the inversion of airborne electromagnetic data provided some insight into the distribution and variability of the Loxton sands aquifer. These data indicated regional facies variations associated with the main barrier systems of this prograded strandline sedimentary sequence. The relevance of this information is being followed up in an examination of options for the Bookpurnong highland borefield. A program involving the acquisition and interpretation of neutron, gamma and inductive conductivity borehole logs, NanoTEM ground TEM traverses, and the analysis of HEM data was undertaken to better define the sedimentary and hydrogeological model of the area. This approach has been critical in explaining local scale changes in the sedimentary environment and elucidating reasons for the variable aquifer yield in the Loxton sands aquifer.
Interception
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AbstractTwo airborne electromagnetic (AEM) surveys were undertaken in the Musgrave Province in South Australia in 2016 with the objective to increase knowledge about cover characteristics, thereby helping reduce exploration risks and to gain an understanding of the groundwater resource potential of the area. The Province is highly prospective for magmatic Ni-Cu-PGE and IOCG deposits, where a transported regolith imposes a significant challenge to exploration. Effective exploration through this region requires an understanding of that cover, its character and its spatial variability. This cover is also a source of groundwater that supports community and environment but our understanding of this resource is compromised by the limited information we have about it.Two different systems, TEMPEST and SkyTEM, were used for the survey, each covering around 8000 line km with a line spacing of 2 km. The line spacing was deliberately chosen to provide a spatially coherent picture of the subsurface conductivity structure, particularly the buried palaeovalleys known to be present in the region. The two datasets were processed and inverted and the results assessed against known information from drill holes. Both systems map the palaeovalley systems in the area well and provide information about the location and geometry of these. Furthermore the results indicate that it is possible to map variability within the cover using AEM, as well as structural controls on the orientation of the palaeovalleys. Airborne electromagnetic surveys used in logistically challenging areas can therefore be a useful mapping tool for areas with varied but unknown cover sequence thickness and thereby reducing exploration risks, as well as increasing the information content about groundwater resources.Musgrave Provincepalaeovalleysairborne electromagneticsTEMPESTSkyTEM AcknowledgmentsThe regional AEM program would not have been successfully completed without the persistence, advice and support of DSD's PACE2020 crew, including Miles Davies, Rian Dutch and others, GSSA's Director Steve Hill, and Geoscience Australia's geophysicists including David McIness & Ross. C. Brodie ably led by Murray Richardson. The eastern survey was funded through the SA Government's Department of Environment, Water and Natural resources (DEWNR), and the western survey by PACE Cu, coordinated through the Geological Survey of South Australia. The program is also indebted to the support of colleagues in DEWNR and in particular Neil Power and Dan Wohling. All involved wish to acknowledge the support of the communities of the Anangu Pitjantjatjara Yankunytjatjara Lands without whom this study would not have been possible.
Regolith
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The Lower Platte South Natural Resources District has collected several thousand line kilometres of Airborne Electromagnetic (AEM) data during five surveys beginning in 2007 and continuing through 2016 to develop a hydrogeologic framework for priority groundwater management areas. Frequency domain systems were originally used in 2007 and 2008. A shift to time domain electromagnetics was required to increase the depth of investigation in areas of conductive glacial till beginning in 2013. The AEM surveys were collected as reconnaissance and block flight lines. Careful calibration and diligent inversions were required to maximize resolution of the AEM data. The AEM-improved hydrogeologic framework was the basis for changes to the management area boundaries and the type of management controls for many of the areas. The revised Dwight-Valparaiso-Brainard management area has experienced improvements in ground water levels and recent regulation changes have allowed an increase in groundwater pumping in the eastern region. Based on the AEM, a new recharge area was identified, and management controls were implemented to reduce non-point source pollution over the recharge area. The AEM-derived hydrogeological framework information has been used for the following: to vary management techniques based on the degree of aquifer confinement and in-season water declines; to determine the amount of groundwater in storage; to locate potential recharge areas; to guide the installation of monitoring wells; to locate and install surface water gages to understand groundwater-surface water relationships; to locate areas for vadose zone characterization; and, finally, to assist local public water suppliers with the management of limited aquifers.
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The Western Australia Department of Water and Environmental Regulation (DWER) utilized the Tempest system in a survey over the North Gnangara Mound, Perth Basin in 2013, to image hydrogeology relevant to groundwater resources important to Perth’s public water supply. In 2017, DWER extended the survey to target the Leederville-Parmelia aquifer, by flying an adjacent area covering the Dandaragan Plateau approximately from Gingin to Eneabba to the north, using an updated Tempest system.In total over 10 000 line kilometres have been flown covering a combined area of over 6000 km2. Borehole resistivity, lithological logs and groundwater chemistry from over 300 bores was used to help interpret and constrain the inversion of the acquired AEM data. Recharge zones, regional throughflow directions, faults that act as flow barriers, groundwater discharge zones, and the extent of regionally important aquitards have been able to be inferred and mapped. Estimates of the minimum thicknesses of fresh groundwater (< 500mg/L and < 1000 mg/L TDS) have been made for the Superficial and Leederville-Parmelia aquifers. The surveys have helped clarify hypotheses about faults that act as flow barriers and regional flow directions that are important for groundwater allocation planning.In this paper we present the results of both surveys, and key hydrogeological outcomes. We also compare the data from the two AEM surveys highlighting system developments, how these have led to improved data quality, and improved interpreted geological and hydrogeological outcomes.
Throughflow
Tempest
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