Using airborne electromagnetic data for regional stratigraphic and landscape evolution studies, Murray Basin, South Australia
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Introduction The $2.67 m Frome airborne electromagnetic (AEM) survey was flown between May and November 2010 as a joint survey between Geoscience Australia (GA), the Geological Survey of South Australia (PIRSA, now DMITRE) and industry partners. The survey includes 2.5 km and 5 km spaced east-west flight lines across the Murray Basin, Frome Embayment and Strzelecki Desert areas of South Australia (Figure 1) using the Fugro TEMPEST TM AEM system. The survey data were inverted using the Geoscience Australia Layered Earth Inversion (GA LEI; Brodie and Sambridge 2006) and a series of data products including grids and sections were produced. The wide line spacing allows data to be gridded with 500 m cell sizes, providing a regional view of electrical conductivity of the ground, but also provides along-line detail in conductivity sections, allowing detailed interpretations of the near-surface to a depth of up to 400 m where conductivity conditions are ideal. A detailed volume of interpretations and implications from the survey will be published in 2012 (Roach, in prep.). The survey was flown with the intention of reducing risk for mineral explorers by showing areas where uranium systems could be interpreted and where detailed AEM and ground EM surveys could be performed, but the data are also capable of being interpreted for groundwater, structural and landscape evolution studies, as discussed here.Keywords:
Geological survey
Geophysical survey
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The Bureau of Mineral Resources has been routinely acquiring airborne magnetic surveys over the land area of Australia since 1951 to record and map anomalies in the earth’s magnetic field attributable to geological structures and lithologies. In forty years, over four million line kilometres of survey data have been flown, while the technology of survey practice has passed through various stages of development.About 83 per cent of the land area has now been covered with so-called reconnaissance surveys flown 150 m above terrain at line spacing between 1.5 and 3.2 km. Located profile data for these surveys have been gridded using a minimum curvature technique, to 15 second of arc (approximately 400 m) and, where necessary, micro-levelled. Data for much of the remaining areas — particularly the inland sedimentary basins covered by surveys of lower specifications — were obtained from digital data on an approximately 2-km grid (72 seconds of arc) published in 1976; these have also been interpolated to 15 second of arc.The data were first assembled for each of over five hundred 1:250 000 map sheets. The 1:250 000 sheets were linked by minimizing the discrepancies along their common boundaries (which were often also survey boundaries) and reducing remaining mis-ties through Laplacian smoothing to minimize the visibility of boundaries between surveys acquired separately.While the data quality varies with instrumentation and survey parameters, it is almost everywhere good enough to provide a useful synoptic view of magnetic anomaly patterns, which can be expected to give important new insights into geology and tectonics at a continental scale, and to provide a regional framework within which to interpret more local magnetic anomalies.The purpose of this short paper is to report the latest progress on compilation of the Magnetic Anomaly Map of Australia, which is scheduled for publication late in 1992.
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Magnetic survey
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Summary A SkyTEM airborne survey has been flown over 1820 km2 of an area of intense agriculture in Hawkes Bay region of New Zealand. The survey is designed to map the characteristics of a coastal fluvial aquifer and support the management of groundwater resources. Processing the data from a 590 km2 subset of the survey, adjacent three urban centres, has been helped by using ground-based geophysical data. These data include ground-TEM, DC resistivity soundings, and borehole geophysical logs. Seismic reflection data across the region provide constraints on the deeper sections of the aquifer system (200 – 500 m). One of the key aims of the study is to map the variability of the surface geological strata that in places are a recharge zone and in other places a confining layer for the aquifer. The SkyTEM data have a spatial coverage (170 m line spacing and 20–30 m station spacing) that enables these units to be mapped in more detail than is possible with the current boreholes and ground geophysical data. In places the ground-based data provide valuable support for constraining the shallow SkyTEM models where data can be missing and deep parts of the SkyTEM model where the resolution is low.
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Airborne electromagnetic (AEM) surveys were flown during fiscal year (FY) 2008 within the 600 Area in an attempt to characterize the underlying subsurface and to aid in the closure and remediation design study goals for the 200-PO-1 Groundwater Operable Unit (OU). The rationale for using the AEM surveys was that airborne surveys can cover large areas rapidly at relatively low costs with minimal cultural impact, and observed geo-electrical anomalies could be correlated with important subsurface geologic and hydrogeologic features. Initial interpretation of the AEM surveys indicated a tenuous correlation with the underlying geology, from which several anomalous zones likely associated with channels/erosional features incised into the Ringold units were identified near the River Corridor. Preliminary modeling resulted in a slightly improved correlation but revealed that more information was required to constrain the modeling (SGW-39674, Airborne Electromagnetic Survey Report, 200-PO-1 Groundwater Operable Unit, 600 Area, Hanford Site). Both time-and frequency domain AEM surveys were collected with the densest coverage occurring adjacent to the Columbia River Corridor. Time domain surveys targeted deeper subsurface features (e.g., top-of-basalt) and were acquired using the HeliGEOTEM{reg_sign} system along north-south flight lines with a nominal 400 m (1,312 ft) spacing. The frequency domain RESOLVE system acquired electromagnetic (EM) data along tighter spaced (100 m [328 ft] and 200 m [656 ft]) north-south profiles in the eastern fifth of the 200-PO-1 Groundwater OU (immediately adjacent to the River Corridor). The overall goal of this study is to provide further quantification of the AEM survey results, using ground based geophysical methods, and to link results to the underlying geology and/or hydrogeology. Specific goals of this project are as follows: (1) Test ground based geophysical techniques for the efficacy in delineating underlying geology; (2) Use ground measurements to refine interpretations of AEM data; and (3) Improve the calibration and correlation of AEM information. The potential benefits of this project are as follows: (1) Develop a tool to map subsurface units at the Hanford Site in a rapid and cost effective manner; (2) Map groundwater pathways within the River Corridor; and (3) Aid development of the conceptual site model. If anomalies observed in the AEM data can be correlated with subsurface geology, then the rapid scanning and non-intrusive capabilities provided by the airborne surveys can be used at the Hanford Site to screen for areas that warrant further investigation.
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The Australian Bureau of Mineral Resources (BMR) is responsible for the National Airborne Magnetic Database. This data base consists of results from approximately 3 500 000 line‐km of regional survey flying carried out over 35 years, recording total magnetic intensity. The magnetic data base is one of the most important geophysical data bases for Australia and is used extensively by the minerals and petroleum exploration industries. First‐pass coverage of onshore Australia is aimed for completion in 1992. This coverage contains data from surveys with a wide range of specifications, resulting in a wide range of data quality; some of the areas covered by poorer quality data may be reflown later. For the most part, the intention has been to acquire data at a continuous ground clearance of 150 m and with a line spacing of 1500 m. However, over some sedimentary basins, the line spacing is in excess of 3200 m. New color and grey‐scale (image processed type) digital magnetic maps (pixel maps) are in preparation; these will supersede the 1976 digital magnetic map of Australia, which was gridded on a 1.2 minute mesh (2000 m) mostly by digitizing contours on maps. The new map, produced from flight‐line data, will have a grid size of 0.25 minutes. Initially, a series of maps will be produced with each one covering a block of 4 degrees latitude by 6 degrees longitude, coinciding with standard 1 : 1 000 000 map sheets. An example included for the Adelaide 1 : 1 000 000 map sheet in Southern Australia shows a dramatic increase in the number of anomalies over those that were evident in earlier contour presentations.
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Electrical conductivity of is one of the fundamental geophysical properties of rock formations and can be measured at field and laboratory scales. A recent airborne geophysical survey of the Isle of Wight and part of the adjacent mainland has provided an assessment of the near-surface (close to outcrop) electrical conductivities associated with Palaeogene and Cretaceous formations. This study examines the degree to which the high-resolution survey data contain distinctive geological and lithological signatures. The geostatistical nature of the conductivity distributions is examined in relation to two existing sedimentary bedrock schemes involving lithostratigraphical and simpler lithological descriptions. A close association between conductivity and bedrock geology is evident. It is then demonstrated how the central moments and dispersion statistics of the distributions may be used to predict the continuous, bedrock conductivity distribution across a large area of southern England, containing, as it does, a high population density and extensive infrastructure. The effectiveness of the method is demonstrated using the survey data obtained on the mainland and an existing database of vertical electric soundings.
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The Inhaminga hydrocarbon exploration licence in central Mozambique sets the location for a multi-method airborne geophysical survey. The size of the Inhaminga block, spanning some 16 500 km2 from Beira to the Zambezi, limited available data and a tight exploration schedule made an airborne survey attractive for the exploration portfolio. The aim of the survey was to map hydrocarbon seepage zones based on the evidence that seepage may create resistivity, radiometric and sometimes magnetic anomalies. The survey involved a helicopter-borne time domain electromagnetic induction system (AEM) and a fixed wing magnetic gradiometer and radiometer.Our data analysis highlights an anomaly extending some tens of kilometres through the survey area along the eastern margin of the Urema Graben. The area is imaged by AEM as a shallow resistive unit below a strong surface conductor and shows high Uranium and low Potassium concentrations (normalised to mean Thorium ratios). A seismic dimming zone on a 2D seismic line crossing the area coincides with the resistivity and radiometric anomaly. The geological exploration model expects seepage to be linked to the graben fault systems and an active seep has been sampled close to the anomaly. We thus interpret this anomaly to be associated with a gas seepage zone. Further geological ground work and seismic investigations are planned to assess this lead.Airborne data has further improved the general understanding of the regional geology allowing spatial mapping of faults and other features from 2D seismic lines crossing the survey area.
Geophysical survey
Anomaly (physics)
Geologic map
Hydrocarbon exploration
Geological survey
Radiometric dating
Prospecting
Exploration geophysics
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Coastal areas such as the German Bight are at risk from storms and rising sea level that may affect the hydrogeologic setting. Because knowledge of the distribution of clayey sediments is important for understanding the current status of this dynamic setting, the German Federal Institute for Geosciences and Natural Resources (BGR) has focused on geophysical research projects on the North Sea coast applying airborne and ground geophysics. The airborne system operated by BGR was used to survey a 20- by 31-km large coastal area in Eastern Friesland, Germany, including the islands of Langeoog and Spiekeroog. Helicopter-borne electromagnetic (HEM) data were collected at six frequencies. In addition, the Leibniz Institute for Applied Geophysics commissioned a SkyTEM survey covering a 2-km-wide north–south strip. On the island of Langeoog, ground geophysical methods, such as transient electromagnetics (TEM) and magnetic resonance soundings (MRS) were used to investigate the hydrogeologic setting in greater detail. Onshore, the airborne electromagnetic results clearly outlined a complex electrically conductive pattern occurring at a shallow depth. Comparison with borehole results provided by the State Authority for Mining, Energy and Geology confirmed that these conductors were caused by clayey material, particularly down to an approximate 20-m depth. This pattern continues offshore, and it is likely that the saltwater is linked to stripes of clayey sediments and the fresh groundwater flows out to the Wadden Sea in between. On the islands, the HEM results revealed the freshwater lenses and showed some indications for clay layers within these freshwater lenses. The application of TEM and MRS helped to distinguish lithology from salinity and confirmed the existence of these clay layers. We demonstrated the usefulness of combining the spatial airborne data with geophysical and borehole data available at sparsely distributed sites on the ground to investigate hydrogeologic settings.
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An airborne electromagnetic (AEM) survey was flown with the TEMPEST AEM System over the Kamarooka study area north of Bendigo.Conductivity values derived from AEM measurements were compared with borehole conductivity values obtained with an EM39 induction probe, indicating an approximately 1:1 relationship between the two quantities when taking into account the vertical (several metres) and horizontal (100 m) resolution of the AEM values.The thickness of Tertiary and Quaternary cover overlying Paleozoic basement was mapped by resolving the transition from moderately conductive cover to resistive basement. Palaeodrainage lines were mapped in the lower parts of the survey area using the topography of this interface, and in the upper portions of these buried valleys by tracing low amplitude magnetic trends associated with the valley infill material.The lateral extent and depth to a shallow (0 to 15 m), highly conductive layer was mapped. This layer was interpreted to correspond to high salt concentrations at the groundwater table. Areas where this layer intersects the land surface correlate with mapped areas of saline discharge.The conductivity values derived from TEMPEST AEM data provided a regularly sampled 3D framework into which surface observations and sparsely distributed sub-surface observations were incorporated as part of an ongoing integrated hydrogeological investigation.
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Next No AccessProceedings of the 9th SEGJ International Symposium, Sapporo, Japan, 12-14 October 2009A combined airborne and ground geophysical survey to investigate the subsurface structure of Socorro Island, MexicoAuthors: Robert SupperKlaus MotschkaAndreas AhlDavid OttowitzGonzalo Merediz-AlonsoRobert SupperGeological Survey of Austria (),Search for more papers by this authorEmail the author at [email protected], Klaus MotschkaGeological Survey of Austria (),Search for more papers by this authorEmail the author at [email protected], Andreas AhlGeological Survey of Austria (),Search for more papers by this authorEmail the author at [email protected], David OttowitzGeological Survey of Austria (),Search for more papers by this authorEmail the author at [email protected], and Gonzalo Merediz-AlonsoAmigos de Sian Ka'an.Search for more papers by this authorhttps://doi.org/10.1190/segj092009-001.1 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract In early 2009 a complex airborne geophysical survey, comprising magnetic, electromagnetic and gamma ray measurements, was conducted to investigate the shallow geological structure at the Island of Socorro, Mexico, in order to derive indications on possible groundwater resources. For the airborne survey was complemented by ground geoelectrical measurements to further detail the airborne results at selected profiles. A total area of 200 square kilometers was covered in early 2009 by complex airborne measurements using an average line spacing of 100 m. Since on-site pre-processing of the electromagnetic data already showed clear indication on groundwater-relevant structures, the layout of the geoelectric profiles could be optimized according to these findings. As a result advanced knowledge on the shallow subsurface structure of the island could be derived. This outcome allowed setting up a research plan for further groundwater exploration measures of the island. However, apart from search for groundwater, the data provides a valuable data base to be interpreted within further investigation in the area of volcanic research and natural protection. Permalink: https://doi.org/10.1190/segj092009-001.1FiguresReferencesRelatedDetails Proceedings of the 9th SEGJ International Symposium, Sapporo, Japan, 12-14 October 2009ISBN (print):978-4-938493-06-6ISSN (online):2159-6832Copyright: 2009 Pages: publication data© 2009 Published in electronic format with permission by the Society of Exploration Geophysicists of JapanPublisher:Society of Exploration Geophysicists HistoryPublished Online: 29 Oct 2012 CITATION INFORMATION Robert Supper, Klaus Motschka, Andreas Ahl, David Ottowitz, and Gonzalo Merediz-Alonso, (2009), "A combined airborne and ground geophysical survey to investigate the subsurface structure of Socorro Island, Mexico," SEG Global Meeting Abstracts : 1-4. https://doi.org/10.1190/segj092009-001.1 Plain-Language Summary PDF DownloadLoading ...
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AbstractGround magnetic surveys for gold exploration in the Archaean Yilgarn Craton of Western Australia are rapidly being replaced by low-level, high-resolution, fixed-wing and helicopter aeromagnetic surveys for detailed geological mapping of large areas. However, for exploration of areas that are either small, or where the buried magnetic sources are located near the surface, or where the magnetic response of the regolith is of interest, ground magnetic surveys are required in order to measure the high-frequency magnetic responses associated with these environments. Surface and subsurface geological features are often of interest when exploring for near-surface gold mineralisation.Earlier work, by S. Mudge, has shown that bipole plots of second horizontal derivatives of aeromagnetic data are effective in resolving detail in aeromagnetic data. These have been applied to ungridded ground magnetic data and are effective in resolving high-frequency detail in the magnetic responses of subsurface rocks and the regolith. The large high-frequency component of ground magnetic data acquired from maghemite-rich regolith areas presents different and often difficult data processing and presentation problems compared with data acquired from higher-level aeromagnetic surveys. Despite this, second horizontal derivatives of ground magnetic data resolve important detail of magnetic subsurface rocks and the regolith that would otherwise be lost in images and contours of the gridded data.For an area in the Kalgoorlie district of Western Australia, data from a ground magnetic survey and a low-level aeromagnetic survey were transformed with the second horizontal derivative filter to reveal different degrees of resolution of magnetic sources located in the regolith and the subsurface. Line profiles, and images and contours of the gridded data, from both surveys resolved subsurface sources which have assisted with the identification of drill targets. Importantly, however, magnetic features in the regolith were only resolved in the enhanced ground survey data which has assisted with geological mapping of the regolith.Keywordsaeromagnetic databipole mapgoldground magnetic datahorizontal derivativemaghemiteregolithYilgam
Yilgarn Craton
Regolith
Aeromagnetic survey
Magnetic survey
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