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Isotropic three-dimensional (3-D) inversion has become a standard tool in the interpretation of magnetotelluric (MT) data. 3-D anisotropic inversion codes are under development, yet the number of unknowns increases by a factor of 6 rendering the problem extremely ill-posed. The presence of anisotropy is usually inferred from (i) spurious sequences of conductive and resistive bodies or (ii) comparison with two-dimensional anisotropic modelling approaches. Here, we investigate the 3-D structure of the Gawler Craton down to ∼250 km depth using 282 sites of the AusLAMP array located in the southern half of South Australia. Inversions of the MT impedance as phase tensors and real and imaginary parts result in diverging structures at depths > 70 km. We demonstrate that a unifying model that explains all data types similarly well is suggestive of an anisotropic resistivity structure at the base of the Gawler Craton lithosphere at depths of 120–210 km. Depth location and orientation of the anisotropy agree well with results from the analysis of seismic receiver functions. We suggest that electric anisotropy in the Gawler Craton is a result of lattice-preferred orientation of olivine crystals and metasomatic processes with macroscopic preferential orientation. Our results illustrate that inversion of phase tensor data is superior for the direct imaging of anisotropic resistivity contrasts in otherwise isotropic resistivity models; inversion models obtained with impedances may miss such structures. "Comparable" overall RMS misfits are often meaningless when comparing inversion results for various data types since sensitivities differ between data types. Reliable inversion results consistent with the entire data set can only be recovered if data fits are assessed systematically for all data representations. We also discuss the influence of error settings for phase tensors on the inversion. Our study also revealed that, if persistent across large areas, (i) parallel orientation of phase tensor major axes, (ii) constantly high phase tensor maximum phases or (iii) diverging directions of phase tensor major axes and induction arrows are suggestive of anisotropic structures and corresponding hypotheses should be evaluated. [ABSTRACT FROM AUTHOR]

When building 3D models of the subsurface, reconciling several geological and geophysical data of diverse nature, resolutions, coverage, or sensitivity, is challenging, both numerically and petrophysically. In this work, we propose a workflow for mapping selected geological features and characterise their uncertainty using a Bayesian Estimate Fusion algorithm. Different datasets such as 1D probabilistic models derived from geophysical data, drillholes and geological data are combined to produce probabilistic maps of selected geological boundaries, relying on petrophysical and geological assumptions. Leveraging large, high-quality geophysical datasets acquired in the eastern Gawler Craton in South Australia, we demonstrate the applicability of our approach with two examples: (1) we map in 3D the top of a stratigraphic unit in the cover, the Tregolana Shale, using 1D magnetotelluric (MT) and 1D Airborne Electromagnetic (AEM) probabilistic models, drill holes and surface geology; (2) we map the depth to basement using 1D probabilistic MT models, drill holes and interpreted structural information. Our results show that the different resolution, data sampling, depth of investigation and reliability of the utilised datasets can be combined in a complementary fashion, overcoming their respective limitations, to find solutions/models that satisfy all the datasets. We show that probabilistic workflows permit characterisation and reduce uncertainty when mapping the location of features of interest, but also permit the testing of geological hypotheses against other geophysical and geological data. These types of models are valuable to better characterise, interpret, and conceptualise the subsurface, enabling better exploration targeting and supporting efforts to discover new mineral deposits. [ABSTRACT FROM AUTHOR]

West Tasmania has a complex geological history, including a Cambrian tectonic collision and related orogenesis, which formed ore deposits in a volcanic hosted massive sulfide (VHMS) setting. While the topography and dense vegetation covering the area presents challenges for on-ground investigations, the area is relatively well-studied such that 3D geological models are available. This contribution presents results of a broadband magnetotelluric (MT) 2D transect across the area (∼30 stations over a $ \sim 80\,{\rm km} $ ∼ 80 km line, deployed in early 2016) that enables a combined interpretation of regional-scale geoelectric and geological structures. After accounting for noise, distortion and geoelectric strike; we produce 2D resistivity models using OCCAM2D inversion. We infer low resistivity anomalies in the shallow ( $ \lt 3\,{\rm km} $ < 3 km ), mid (3–8 km) and deeper crust. Along this MT transect the shallow and mid crust low resistivity zones correspond to major crustal faults. Deep low resistivity features in the east of the transect suggest fluid pathways associated with metamorphism along the western boundary of the Tyennan block during the Cambrian Tyennan Orogeny, while others potentially represent the metasomatism of lower crustal rocks and fluid pathways converging into the mid crust. [ABSTRACT FROM AUTHOR]

Heat flow data obtained in connection with geothermal resource exploration suggests anomalous upper crustal structure and processes in parts of central east Tasmania. The regional scale crustal geology of the Midlands of Tasmania is, however, mostly obscured at the surface by the Permo-Triassic sedimentary sequences of the Tasmania Basin together with extensive exposures of Jurassic dolerite. We investigate controls on undercover crustal processes in this region by combining long period and broadband magnetotelluric (MT) datasets in 3D inversions for the geoelectric structure; followed by an interpretation that is informed by aspects of the pre-existing 3D regional geological and geophysical model. The new 3D model allows improved resolution of low resistivity anomalies together with a qualitative appraisal of spatially variable model sensitivity. The most robust features ( $ \lt 1\,\Omega {\rm m} $ < 1 Ω m ) in the 2–3 km depth interval occur where N–S and E–W faults intersect with a high point in the topography of the upper surface of a deep seated granite body. Enhancement of conductivity in this zone by clay, graphite or mineralisation, or a combination thereof, is likely. Other low resistivity features suggest that conductive pathways exist where major or multiple faults are present. These interpretations provide support for continued exploration in the Midlands of Tasmania for a variety of resources related to crustal fluids and fracturing. [ABSTRACT FROM AUTHOR]