The dominant north–south strike of the Palaeozoic outcrop of central Victoria has been well documented, but to the north, these rocks are covered by the Cainozoic sedimentary deposits of the Murray Basin. Two magnetotelluric surveys were completed to assist in extrapolation of the known structure and to identify possible new targets for mineral discovery. Supporting the results from previous seismic interpretations for the region, the 2D MT inversion models substantiate an intrazone thrust fault system of listric geometries in the Bendigo Zone connected in the mid-crust. With the zone boundary clearly defined the electrical resistivity structure is distinct between the major subdivisions, indicating a different tectonic evolution for the Bendigo and Melbourne Zones. However, the conductive overburden in the region poses complications for the generation of the 2D resistivity models. Static shifts and electrical anisotropy were identified as distortions in the dataset, with further processing needed to attain a complete picture of the underlying geology. The difficulties caused by galvanic distortion were allayed by using the phase tensor response in place of the distorted amplitude response. Phase tensor analysis of MT data has been completed subsequently, the results of which we present here, along with the original 2D inversion models, confirming that electrical anisotropy persists into the mantle.
SummaryNew ways of energy production through the use of coal seam gas plays and geothermal hot dry rock and hot sedimentary aquifer systems pose challenges in identifying and monitoring fluid in the subsurface. We propose the use of the magnetotelluric (MT) method to image static and dynamic fluid distributions in the subsurface exhausting the contrast in electrical conductivity between resistive host rock and conductive fluid-filled, porous rock. Base line MT measurements provide reference transfer functions and inverse models to characterise the electrical conductivity distribute on which is linked with bore hole and other geophysical data to obtain knowledge about fluid distribution at depth. The reference models are used to accurately forward model fluid injection or extraction temporally and spatially. This work shows results from fluid injections at a hot dry rock system at Paralana, South Australia, and its applicability to other geothermal and coal seam gas systems.
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.au
A long period magnetotelluric (MT) survey, comprising 39 sites over an area of 270 by 150 km, has identified partial melt within the thinned lithosphere of Quaternary Newer Volcanics Province (NVP) in southeast Australia. MT inversion models reveal several important tectonic features and unravel critical information about the tectonics of the area. The models have imaged a conductive anomaly beneath the NVP at -40-80 km depth, which is consistent with the presence of 1.5-4% partial-melt in the lithosphere. The conductive zone is located within thin juvenile oceanic lithospheric mantle, which was accreted onto thicker Proterozoic continental lithospheric mantle, suggesting that the NVP origin is due to decompression melting within the asthenosphere, promoted by lithospheric thickness variations in conjunction with rapid shear. In addition, inversion modelling shows that there is a conductivity contrast across the Moyston Fault that suggests the transition from Proterozoic continental lithospheric mantle under the Delamerian Orogen to the Phanerozoic lithospheric mantle under the Lachlan Orogen.
Realization of enhanced geothermal systems (EGS) prescribes the need for novel methods to monitor fluid inclusion and connectivity at depth. Magnetotellurics (MT) is a passive electromagnetic method sensitive to electrical conductivity contrasts as a function of depth. The goal here is to use MT as a monitoring tool to estimate areal extent of an EGS reservoir by collecting measurements before, during and after fluids are injected. 3D forward modeling suggests changes in the MT response will be small, on the order of a few percent. Repeatability of the MT response is important and it is shown that most stations are within a few percent. Results from a test case at Paralana, South Australia are presented supporting the idea that MT can be used as a monitoring tool by showing changes due to fluids input into the system.
Electrical anisotropy, defined as the directional dependence of electrical conductivity within a medium, is an important property to consider when interpreting magnetotelluric (MT) data. We propose the use of anisotropic forward modelling to model fluid flow within a geothermal setting.Forward models provide synthetic MT responses for hypothetical structures which are compared with measured data to obtain knowledge about the subsurface geology of a region.Comparisons between synthetic and measured data shows anisotropic fluid volumes are acceptable approximations of fluid injected into the crust. As a result, we support the use of anisotropic forward modelling as a means of modelling fluid motion at depth within a fractured geothermal system.
Magnetotellurics is a geophysics technique for characterisation of geothermal reservoirs, mineral exploration, and other geoscience endeavours that need to sound deeply into the earth -- many kilometres or tens of kilometres. Central to its data processing is an inversion problem which currently takes several weeks on a desktop machine. In our new eScience lab, enabled by cloud computing, we parallelised an existing FORTAN program and embedded the parallel version in a cloud-based web application to improve its usability. A factor-of-five speedup has taken the time for some inversions from weeks down to days and is in use in a pre-fracturing and post-fracturing study of a new geothermal site in South Australia, an area with a high occurrence of hot dry rocks. We report on our experience with Amazon Web Services cloud services and our migration to Microsoft Azure, the collaboration between computer scientists and geophysicists, and the foundation it has laid for future work exploiting cloud data-parallel programming models.
Abstract The role of lithospheric architecture and the mantle in the genesis of iron oxide copper‐gold (IOCG) deposits is controversial. Using the example of the Precambrian Gawler Craton (South Australia), which hosts the giant Olympic Dam IOCG deposit, we integrate geophysical data (seismic tomography and magnetotellurics) with geological and geochemical data to develop a new interpretation of the lithospheric setting of these deposits. Spatially, IOCG deposits are located above the margin of a mantle lithospheric zone with anomalously high electrical conductivities (resistivity <10 ohm.m, top at ~100–150 km depth), low seismic shear‐wave velocities (horizontal component, Vsh < 4.6 km/s), and unusually high ratios of compressional‐ to shear‐wave velocities (Vp/Vsh > 1.80). The high conductivity cannot be explained by water‐bearing olivine‐rich rock alone. Relatively fertile and metasomatized peridotitic mantle with additional high‐Vp/Vs phases, for example, clinohumite, hydrous garnet, and/or phlogopite, could explain the anomalous velocity and conductivity. The top of this high‐Vp/Vsh zone marks a midlithospheric discontinuity at ~100–130 km depth that is interpreted to reflect locally orthopyroxene‐rich mantle. A sub‐Moho zone with high Vp/Vsh at ~40–80 km depth correlates spatially with primitive Nd isotope signatures and arc‐related ~1,620–1,610 Ma magmatism and is interpreted as the eclogitic root of a magmatic arc. Mafic volcanics contemporaneous with ~1,590 Ma IOCG mineralization have geochemistry suggesting derivation from subduction‐modified lithospheric mantle. We suggest that Olympic Dam formed inboard of a continental margin in a postsubduction setting, related to foundering of previously refertilized and metasomatized lithospheric mantle. Deposits formed during the switch from compression to extension, following delamination‐related uplift and exhumation.
The thermochemical structure of the lithosphere exerts control on melting mechanisms in the mantle as well as the location of volcanism and ore deposits. Imaging the complex interactions between the lithosphere and asthenospheric mantle requires the joint inversion of multiple data sets and their uncertainties.In particular, the combination of seismic velocity and electrical conductivity with data proxies for bulk composition and elusive minor phases is a crucial step towards fully understanding large-scale lithospheric structure and melting.We apply a novel probabilistic approach for joint inversions of 3D magnetotelluric and seismic data to image the lithosphere beneath southeast Australia. Results show a highly heterogeneous lithospheric structure with deep conductivity anomalies that correlate with the location of Cenozoic volcanism. In regions where the conductivities have been at odds with sub-lithospheric temperatures and seismic velocities, we observe that the joint inversion provides conductivity values consistent with other observations. The results reveal a strong relationship between metasomatized regions in the mantle and i) the limits of geological provinces in the crust, which elucidates the subduction-accretion process in the region; ii) distribution of leucitite and basaltic magmatism; iii) independent geochemical data, and iv) a series of lithospheric steps which constitute areas prone to generating small-scale instabilities in the asthenosphere. This scenario suggests that shear-driven upwelling and edge-driven convection are the dominant melting mechanisms in eastern Australia rather than mantle plume activity, as conventionally conceived. Our study offers an integrated lithospheric model for southeastern Australia and provides insights into the feedback mechanism driving surface processes.
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