Magnetotelluric Survey Across the Central Eyre Peninsula
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SummaryMagnetotelluric (MT) measurements were undertaken at 40 broadband (0.01s – 500s) and 12 long-period (10s-10000s) stations across the central-eastern Eyre Peninsula, South Australia, using Auscope MT equipment. Typical site spacing is of the order of 15km in between sites for the long-period stations and and 3-10km for the broadband stations. This ensures sufficient coverage to map the upper crustal to upper mantle structures underneath central Eyre Peninsula.The profile extends south of the Gawler Range Volcanics and crosses the Archaean Sleaford Complex, the Hutchinson Group and the Donington Suite from west to east. The 2D MT profile also crosses the location of the postulated Eyre Peninsula Anomaly (White and Milligan, 1984, Kusi et al., 1998, Thiel et al., 2005) as well as the Kalinjala Shear Zone in the eastern part of the profile (Vassallo and Wilson, 2002, Thiel et al., 2005).Dimensionality analysis and strike determination has been carried out using phase tensor analysis (Caldwell et al., 2004). Subsequently, data influenced by 3D effects could be discarded and the remaining data were inverted for 2D structure using a code by Rodi and Mackie (2001). 2D inverse modelling of the broadband and long-period data indicates a resistive crust in the western part of the profile representing the Archaean Sleaford Complex. Along its eastern margin extends a major conductive crustal boundary in the central part of the profile.Technical Area: Minerals exploration, magnetotelluricsPRESENTER PROFILE (100 words in sentence format):Stephan Thiel is a IMER Research Fellow at the Center of Tectonics, Resources and Exploration (TraX) at University of Adelaide. Stephan completed his Masters at the Freiberg University of Mining and Technology, Germany and obtained his PhD at the University of Adelaide. Stephan’s speciality is the analysis and modelling of electromagnetic data to define large-scale lithospheric structures, mineral systems and geothermal areas. Email: stephan.thiel@adelaide.edu.auKeywords:
Peninsula
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Hadean
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A method is described for finding a resistivity model that fits given magnetotelluric data in the one-dimensional case. The procedure is automatic and objective in that no a priori model structure is imposed. Starting with a uniform half space derived directly from the data, the procedure gradually transforms the half space to one with a continuous and smooth resistivity distribution whose response fits the measured data. The method is illustrated by application to two magnetotelluric data sets.
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Gold mineralisation in the Eastern Goldfields Province of the Yilgarn Block, Western Australia, is associated with shear zones within the greenstone supracrustal succession. Regional shear zones are imaged in seismic reflection sections as bands of strong reflections. Although individual wavelets within the bands of reflections can only be correlated over small distances, the bands of reflections can be correlated over tens of kilometres. The Bardoc Shear, adjacent to which considerable mineralisation has been found, dips west and links with the Ida Fault, which forms the boundary between the Eastern Goldfields Province and the Southern Cross Province farther west. The Ida Fault dips east at about 40°, and can be traced from the surface to about 27 km depth. Bands of reflections within the upper and middle crust have a similar seismic signature to the Ida Fault, Bardoc Shear Zone and the basal detachment of the greenstones, and are therefore Interpreted as shear zones. Interpreted shear zones in the upper crust under the greenstones mostly dip west. Shear zones in the lower crust dip east. The upper crust east of the Ida Fault and below the Bardoc Shear is an exception. There, east-dipping shear zones, including the Ida Fault, are interpreted to extend from the lower crust into the upper crust, thereby providing a simple plumbing system for mineralising fluids migrating from the lower crust, into the Bardoc Shear, and then to high levels in the greenstones.
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Magnetotellurics
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Tensor apparent resistivity values obtained from the magnetotelluric field measurements, in the plains of Alberta indicate a major regional anisotropy. The direction of the major axis of the anisotropy is parallel to the strike of the Rocky Mountains and the sedimentary basin of Western Canada. Results obtained by numerical methods for a two-dimensional geoelectric model support an interpretation of the observed magnetotelluric results, depending on regional structures. The observed polarization parameters of the magnetotelluric fields are also in good agreement with those computed for the two-dimensional model. This study shows that the conclusions based on some earlier magnetotelluric results, which postulated an upper mantle conductivity layer must be treated with strong reservations.
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This paper describes the present situation of magnetotelluric researches abroad, emphatically sums up the situation of 2D magnetotelluric inversion studies, and enumerates the inversion methods currently commonly used in foreign countries. The development trend of magnetotelluric inversion interpretation is also discussed in this paper.
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In this paper, we report a preliminary result of the Magnetotelluric investigation of the Mogod area. The Mogod region is one of the most prominent fields for geophysical study since the region includes young and active faults and geothermal activities. We conducted magnetotelluric measurements at 20 sites during geophysical field seasons in 2018-2021 as a pilot survey to understand data property and the electromagnetic noise level for the detailed electromagnetic studies. During the fieldwork, we used Lemi Magnetotelluric instruments and measured all three orthogonal components of the magnetic field and the horizontal components of the electric field. For the data processing, we used Matlab code by using the M-estimate regression method, and estimated the magnetotelluric transfer function with a lownoise level. The electrical resistivity model of the subsurface of survey layout shows us the existing resistivity anomalies at shallow-depth, and thickness of the upper crust approximately 11-17 km. Here, suggest that the thickness of the upper crust is 17 km and crust is 40 km with local magnetotelluric measurements. Additionally, the electric conductor appears in the southwest of Mogod region, we interpret that conductor play as a source of geological activity of Mogod region, and it might be the signature of a remanent fluid.
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