8 results on '"Brodie, Ross C."'
Search Results
2. Application of audio-frequency magnetotelluric data to cover characterisation – validation against borehole petrophysics in the East Tennant region, Northern Australia.
- Author
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Jiang, Wenping, Roach, Ian C., Doublier, Michael P., Duan, Jingming, Schofield, Anthony, Clark, Andrew, and Brodie, Ross C.
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MARKOV chain Monte Carlo ,PROSPECTING ,MINERALOGY ,GEOLOGY ,PETROPHYSICS - Abstract
The characterisation of the thickness and geology of cover sequences significantly improves targeting for mineral exploration in buried terrains. Audio-frequency Magnetotelluric (AMT) data is applicable to characterise cover sequences, where their conductivity (inverse resistivity) can be differentiated. We present a regional study from the under-cover East Tennant region in the Northern Territory (Australia) where we have applied deterministic and probabilistic inversion methods to derive 2D and 1D resistivity models. We integrated these models with information of co-located basement penetrating boreholes (lithological and geophysical logs) to ground-truth and validate the models and to improve geophysical interpretations. In the East Tennant region, borehole lithology and wireline logging demonstrate that the modelled AMT response is largely controlled by the mineralogy of the cover and basement rocks. The bulk conductivity is due primarily to bulk mineralogy and the success of using the AMT models to predict cover thickness is shown to be dependent on whether the bulk mineralogy of cover and basement rocks are sufficiently different to provide a detectable conductivity contrast. Our investigation of a range of geological scenarios that differ in thickness, complexity and geology of the cover and basement rocks suggests that in areas where there is sufficient difference in bulk mineralogy and where the stratigraphy is relatively simple, AMT models predict the cover thickness with high certainty. In more complex scenarios interpretation of AMT models may be more ambiguous and requires integration with other data (e.g. drilling, wireline logging, potential field modelling). Overall, we conclude that the application of the method has been validated and the results compare favourably with borehole stratigraphy logs once geological (i.e. bulk mineralogical) complexity is understood. This demonstrates that the method is capable of identifying major litho-stratigraphic units with resistivity contrasts. Our results have assisted with the planning of regional drilling programs and have helped to reduce the uncertainty and risk associated with intersecting targeted stratigraphic units in covered terrains. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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3. An information theoretic Bayesian uncertainty analysis of AEM systems over Menindee Lake, Australia.
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Ray, Anandaroop, Ley-Cooper, Yusen, Brodie, Ross C, Taylor, Richard, Symington, Neil, and Moghaddam, Negin F
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BAYESIAN analysis ,AERODYNAMIC stability ,INVERSE problems ,GEOLOGICAL time scales ,FLIGHT testing ,UNCERTAINTY (Information theory) ,ESTIMATES - Abstract
Long-range, active-source airborne electromagnetic (AEM) systems for near-surface conductivity imaging fall into two categories: helicopter (rotary-wing) borne or fixed-wing aircraft borne. A multitude of factors such as flying height, transmitter loop area and current, source waveforms, aerodynamic stability and data stacking times contribute to the geological resolvability of the subsurface. A comprehensive comparison of the relative merits of each system considering all such factors is difficult, but test flights over well-constrained subsurface geology with downhole induction logs are extremely useful for resolution studies. However, given the non-linear nature of the electromagnetic inverse problem, handling transmitter–receiver geometries in fixed-wing aircraft is especially challenging. As a consequence of this non-linearity, inspecting the closeness of downhole conductivities to deterministic inversion results is not sufficient for studying resolvability. A more comprehensive picture is provided by examining the variation in probability mass of the depth-wise Bayesian posterior conductivity distributions for each kind of AEM system within an information theoretic framework. For this purpose, probabilistic inversions of data must be carried out. Each acquiring system should fly over the same geology, survey noise levels must be measured and the same prior probabilities on conductivity must be used. With both synthetic models as well as real data from over the Menindee calibration range in New South Wales, Australia, we shed new light on the matter of AEM inverse model uncertainty. We do this using two information theoretic attributes derived from different Kullback–Leibler divergences—Bayesian information gain, and a strictly proper scoring rule, to assess posterior probabilities estimated by a novel Bayesian inversion scheme. The inversion marginalizes fixed-wing geometry attributes as generic nuisance parameters during Markov chain sampling. This is the first time-domain AEM study we know of, that compares nuisance marginalized subsurface posterior conductivities from a fixed-wing system, with rotary-wing derived posterior conductivities. We also compare field results with induction log data where available. Finally, we estimate the information gain in each case via a covariate shift adaptation technique that has not been used before in geophysical work. Our findings have useful implications in AEM system selection, as well as in the design of better deterministic AEM inversion algorithms. [ABSTRACT FROM AUTHOR]
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- 2023
- Full Text
- View/download PDF
4. Fixed-wing vs helicopter AEM systems: A Bayesian resolution analysis
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Anandaroop Ray, Yusen LeyCooper, Brodie, Ross C., Taylor, Richard, Symington, Neil, and Negin F. Moghaddam
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airborne electromagnetics ,inversion ,resolution ,Bayesian - Abstract
Long range, high-power airborne electromagnetic (AEM) systems fall into two categories: helicopter borne or fixedwing aircraft borne. These platforms have their own advantages and disadvantages in terms of flying height, transmitter loop area and current, aerodynamic stability of transmitter and receiver frames, speed of acquisition, stacking times, cost, etc. With the exception of cost, all of these factors ultimately contribute to the resolvability of subsurface geological features. A comprehensive comparison of the relative merits of all these factors is hard, but flights over known subsurface geology with downhole induction logs are extremely useful for resolution studies. However, given the non-linear nature of the electromagnetic inverse problem, especially when it comes to handling transmitter-receiver geometries in fixed wing aircraft, it is not enough to examine closeness of inverted conductivities to available downhole logs. Instead, the width of the Bayesian posterior credibility intervals (CIs) of conductivity with depth for each kind of system, flown over the same geology with measured survey noise levels, sampled with the same priors, provides a more complete picture of resolution. With the aid of synthetic models and real data examples flown over the Menindee calibration range in New South Wales, we shed new light on the uncertainty with depth for different AEM systems. Specifically, we use a novel Bayesian inversion which handles fixed-wing geometries as nuisance parameters during sampling. Our findings have useful implications in AEM system selection, as well as in the design of better deterministic inversion codes., Open-Access Online Publication: May 29, 2023
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- 2023
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5. AusAEM is the world's largest airborne electromagnetic (AEM) survey undertaken
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A Yusen Ley-Cooper, Brodie, Ross C., and Anandaroop Ray
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Salt diaper ,Airborne Electromagnetics ,Exploration ,Critical Minerals ,Groundwater - Abstract
Airborne electromagnetics surveys are at the forefront of addressing the challenge of exploration undercover. They have been essential in the regional mapping programmes to build Australia's resource potential inventory and provide information about the subsurface. In collaboration with State and Territory geological surveys, Geoscience Australia (GA) leads a national initiative to acquire AEM data across Australia at 20 km line spacing, as a component of the Australian government Exploring for The Future (EFTF) initiative. Regional models of subsurface electrical conductivity show new undercover geological features that could host critical-mineral deposits and groundwater resources. The models enable us to map potential alteration and structural zones and support environmental and land management studies. Several features observed in the AEM models have also provided insights into possible salt distribution analysed for its hydrogen storage potential. The AusAEM programme is rapidly covering areas with regional AEM transects at a scale never previously attempted. The programme's success leans on the high-resolution, non-invasive nature of the method and its ability to derive subsurface electrical conductivity in three dimensions - made possible by GA's implementation of modern high-performance computing algorithms. The programme is increasingly acquiring more AEM data, processing it, and working towards full national coverage., Open-Access Online Publication: May 29, 2023
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- 2023
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6. AusAEM: Australia's airborne electromagnetic continental-scale acquisition program.
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Ley-Cooper, Alan Yusen, Brodie, Ross C., and Richardson, Murray
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CARTOGRAPHY software , *TARGET acquisition , *ACQUISITION of data , *DATA mapping , *AREA studies , *PROSPECTING , *GEOLOGICAL statistics - Abstract
The AusAEM airborne electromagnetic (AEM) survey is one of the main components of the Exploring for the Future program. This Australian Government initiative is aimed at enhancing the geoscientific information available, to support resource exploration and to showcase Australia as a destination for investment opportunities. Regional geophysical mapping programs are the basis for informed mineral exploration and have enabled the building of a continental-scale inventory of Australia's potential resource endowment. They also provide elements of the scientific backing for decision-making and answering a range of different geoscientific questions. The latest of these airborne initiatives AusAEM is composed of a series of wide line-spaced studies targeting acquisition of data for regional mapping in area of new frontiers; it is gathering data and unveiling information on extents never previously attempted by AEM surveys. [ABSTRACT FROM AUTHOR]
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- 2020
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7. Trans-dimensional Bayesian inversion of airborne electromagnetic data for 2D conductivity profiles.
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Hawkins, Rhys, Brodie, Ross C., and Sambridge, Malcolm
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ELECTROMAGNETIC fields , *ELECTROMAGNETIC induction , *ELECTROMAGNETIC theory - Abstract
This paper presents the application of a novel trans-dimensional sampling approach to a time domain airborne electromagnetic (AEM) inverse problem to solve for plausible conductivities of the subsurface. Geophysical inverse field problems, such as time domain AEM, are well known to have a large degree of non-uniqueness. Common least-squares optimisation approaches fail to take this into account and provide a single solution with linearised estimates of uncertainty that can result in overly optimistic appraisal of the conductivity of the subsurface. In this new non-linear approach, the spatial complexity of a 2D profile is controlled directly by the data. By examining an ensemble of proposed conductivity profiles it accommodates non-uniqueness and provides more robust estimates of uncertainties. [ABSTRACT FROM AUTHOR]
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- 2018
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8. Application of multiscale magnetotelluric data to mineral exploration: an example from the east Tennant region, Northern Australia.
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Jiang W, Duan J, Doublier M, Clark A, Schofield A, Brodie RC, and Goodwin J
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The footprint of a mineral system is potentially detectable at a range of scales and lithospheric depths, reflecting the size and distribution of its components. Magnetotellurics is one of a few techniques that can provide multiscale data sets to image and understand mineral systems. We have used long-period data from the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) as a first-order reconnaissance survey to resolve large-scale lithospheric architecture for mapping areas of mineral potential in northern Australia. The 3-D resistivity model reveals a broad conductivity anomaly extending from the Tennant Creek district to the Murphy Province in the lower crust and upper mantle, representing a potential fertile source region for mineral systems. Results from a higher-resolution infill magnetotelluric survey reveal two prominent conductors in an otherwise resistive host whose combined responses result in the lithospheric-scale conductivity anomaly mapped in the AusLAMP model. Integration of the conductivity structure with deep seismic reflection data reveals a favourable crustal architecture linking the lower, fertile source regions with potential depositional sites in the upper crust. The enhanced conductivity likely resulted from the remnant (metallic) material deposited when fluids were present during the 'ancient' tectonic events. This observation strongly suggests that the deep-penetrating major faults potentially acted as pathways for transporting metalliferous fluids to the upper crust where they could form mineral deposits. This result and its integration with other geophysical and geochronological data sets suggest high prospectivity for major mineral deposits in the vicinity of these major faults, that is, Gulunguru Fault and Lamb Fault. In addition to these insights, interpretation of high-frequency magnetotelluric data acquired during the infill survey helps to characterize cover and assist with selecting targets for stratigraphic drilling which, in turn, can validate the models and improve our understanding of basement geology, cover sequences and mineral potential. This study demonstrates that integration of geophysical data from multiscale surveys is an effective approach to scale reduction during mineral exploration in covered terranes with limited geological knowledge., (© The Author(s) 2022. Published by Oxford University Press on behalf of The Royal Astronomical Society.)
- Published
- 2022
- Full Text
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