Exploration Geophysics - Special Issue: Lithospheric to deposit scale magnetotellurics advancements, including AusLAMP in Australia, 2024

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.

The Yerrida Basin in the south-eastern Capricorn Orogen, WA is a Proterozoic sedimentary basin that is prospective for base metals. A regional TEMPEST airborne electromagnetics (AEM) survey over the Capricorn Orogen, coinciding magnetotelluric (MT) survey and available downhole electrical property measurements provide an opportunity to compare the electrical conductivity of the rocks, and the electromagnetic (EM) responses of near surface and deeper sedimentary packages of the Yerrida Basin. Of particular interest is assessing whether it is possible to obtain reliable structural and stratigraphic information from, and to what depth, such as is required for exploration of basin terrains. The MT data were modelled using a 3D inversion algorithm, and the AEM data were modelling using a 1D inversion algorithm. An integrated interpretation of the two datasets allows surface geology to be mapped and for a conductivity section to be created to several kilometres depth. Electrical property measurements showed that one of the stratigraphic units, the Johnson Cairn Formation, is anomalously conductive and hence is readily mapped allowing faults, folds, thickness variations etc to be inferred from the inversions. The underlying and resistive Juderina Formation also gives rise to EM responses, albeit less strong. The results suggest significant modification of the published geological map of the area. The study demonstrates the usefulness of EM methods for exploring sedimentary terrains, but this is greatly facilitated by the presence of conductive stratigraphic units. While not always common practice in green fields exploration, it is recommended that electrical petrophysical measurements are made before committing to large scale EM surveys to better constrain interpretations.

Interpretation of the deep crustal architecture, including the nature and extent of the fundamental basement blocks, is hampered in many regions by limited exposure and because geoscience data tend to reflect the signatures of near-surface units, highlighting the need for tools that image deeper. The northern Mount Isa Province as imaged by long-period magnetotelluric AusLAMP data, has a resistive central core flanked to the east and west by more conductive crust. These structures are elongated in the north–south direction to the extent of current AusLAMP coverage. In the south of the province, sparse broadband magnetotelluric (BBMT) data are available to extend the understanding of the crustal-scale electrical structure of the Mount Isa Province. Two new profiles of BBMT data are used to generate two 3D inversions of impedance and tipper data over a bandwidth of 0.001 to 1.000 s. Data from the 2021 BBMT survey were supplemented with historical MT data in the area, for a total of 285 stations at approximately 10 km station spacing. Two overlapping inversion models, one east–west oriented and one north–south oriented, centred on the township of Boulia were run. A suite of inversion parameters was tested for the two models to establish the reliability of final model features. Consistent with the known electrical structures further north, the inversion models show a major longitudinal resistive block which forms the core of the province, flanked on either side by more conductive crust. Results from the preferred inversion for each model were then compared to deep crustal seismic data, gravity forward modelling and seismic interpretation. The eastern conductive feature is associated with the Numil basement terrane and Gidyea Structure which defines the eastern margin of the Mount Isa Province. The central resistive anomaly extends into the upper mantle and is associated with the Pitta Pitta and Altjawarra East basement terranes, representing the Archean-Paleoproterozoic cratonic core of the Mount Isa Province. The westernmost conductor is associated with the Altjawarra basement terrane and is an extension of the structure imaged in the AusLAMP model.

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 () 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.

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 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 (), 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.

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.

Metasomatised mantle is considered a prerequisite for the formation of many major ore deposits. Since magnetotellurics (MT) is strongly sensitive to mantle metasomatism, MT is a useful tool for analysis of mineral systems that incorporate a metasomatised mantle as a key ingredient. In this work, we combine recently reported kimberlite-hosted mantle xenocryst data from the southern Gawler Craton and Adelaide Superbasin with AusLAMP MT models. We find good correlations between mantle MT conductors and geochemical indicators of mantle metasomatism from clinopyroxene xenocrysts, with the most metasomatically enriched lithospheric mantle beneath the Adelaide Superbasin and more depleted mantle beneath the Gawler Craton. There is a significant step in lithospheric thickness across the Adelaide Superbasin, and particularly around the Terowie kimberlite cluster. Combined with the fertile mantle, this helps explain this region’s prospectivity for sediment-hosted copper deposits. High mantle resistivities and depleted geochemical compositions towards the central Gawler Craton on and north of the Eyre Peninsula are consistent with melting of previously metasomatised mantle, including during production of the Gawler Range Volcanics and Hiltaba Suite Large Igneous Province. Most of the major iron oxide copper gold (IOCG) and gold deposits in the region lie on the edge of this zone of mantle melting, consistent with mineral systems models that require melting of previously metasomatised mantle in the generation of these deposits.

The magnetotelluric (MT) method is increasingly being applied to mineral exploration under cover with several case studies showing that mineral systems can be imaged from the lower crust to the near surface. Driven by this success, the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) is delivering long-period data on a 0.5° grid across Australia, and derived continental scale resistivity models that are helping to drive investment in mineral exploration in frontier areas. Part of this investment includes higher-resolution broadband MT surveys to enhance the resolution of features of interest and improve targeting. To help gain best value for this investment it is important to understand the ability and limitations of MT to resolve features on different scales. Here we present synthetic modelling of continuous, narrow, near-vertical faults 500 m to 1500 m wide with a resistivity 100–200 times less than the background rock resistivity using the ModEM software, and show that for such a situation a station spacing of around 14 km across strike is sufficient to resolve these into the upper crust. However, the vertical extent of these features is not well constrained, with near-vertical planar features commonly resolved as two separate features. This highlights the need for careful interpretation of anomalies in MT inversion. In particular, in an exploration scenario, it is important to consider that a lack of interconnectivity between a lower crustal/upper mantle conductor and conductors higher up in the crust and the surface might be apparent only, and may not necessarily reflect reduced mineral prospectivity.

A geomagnetic storm, also known as a geomagnetic disturbance (GMD), is a major disturbance of the Earth’s magnetic field caused by solar activity. A geomagnetic storm induces electric currents in the Earth that feed into power lines through substation neutral earthing, causing instabilities and even blackouts in electricity transmission systems. The intensity of geomagnetically induced currents (GICs) is closely associated with the electrical conductivity of the surrounding geology. In this paper, we analyse one of the most well-known geomagnetic storms, the 1989 “Québec storm” and 688 magnetotelluric (MT) survey sites from the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) to gain insight into the space weather hazard posed for Australia's modern-day power grids. Transmission lines may exhibit local maxima at differing times depending on their spatial orientation and length with respect to the time-varying magnetic field. Localised peak voltages over 100 V can be observed on some individual lines. This assessment identifies the distribution of GICs in south-eastern Australia for the 1989 Québec storm and transmission lines that are more vulnerable to GICs. It is relevant to national strategies and risk assessment procedures to mitigate space weather hazards in the Australian high-voltage power grid and the design of a more resilient power transmission system. We also analyse the 2015 “St Patrick’s Day storm” to study under-estimation of the space weather hazard associated with the band-limited geomagnetic data and MT data sets.

Key points

• The subsurface geology has a great influence on the intensity of geomagnetically induced electric fields, potentially causing up to three orders of magnitude difference between conductive basins and resistive cratonic regions in south-eastern Australia.
• Analysis using the 1989 “Québec geomagnetic storm” and AusLAMP magnetotelluric data shows the intensity of the geoelectric fields in south-eastern Australia could reach up to 5 [V/km].
• Geomagnetically induced voltages in the Australian high-voltage power grid could be in excess of 100 V in some transmission lines for a geomagnetic storm with intensity comparable with the 1989 Québec geomagnetic storm.