How to computationally include regional interpretations into the seismic imaging process

Rashidifard, Mahtab1,3, Giraud, Dr Jérémie1,3, Ogarko, Dr Vitaliy1,2, Lindsay, A/Prof Mark1,3, Jessell, Prof Mark1,3

1Centre for Exploration Targeting, University Of Western Australia, 35 Stirling Highway, WA Crawley 6009, Perth,, Australia, 2International Centre for Radio Astronomy Research (ICRAR), University Of Western Australia, 35 Stirling Highway, WA Crawley 6009, Perth,, Australia, 3Mineral Exploration Cooperative Research Centre, School of Earth Sciences, University of Western Australia, 35 Stirling Highway, WA Crawley 6009, Perth,, Australia

Understanding the regional evolution of the Earth and subsurface processes is a key for mineral exploration. Crucial to this understanding is accounting for geoscience knowledge obtained from
integrated interpretation of geological and geophysical data. Reflection seismic data, although sparsely distributed due to the high cost of acquisition, is the only data that gives a high resolution image of the crust to reveal deep subsurface structures and the architectural complexity that may vector attention to minerally prospective regions. Without an iterative depth migration and a depth conversion step, seismic images remain in time domain and are totally data-driven. However, for reconstructing the architecture and history of the area it is necessary to have these images in depth. To obtain the depth of seismic events, depth correlations need to be applied to them from other sources of information. The limitation here is that existing depth conversion methods rely on borehole information which is rarely available for deep crust studies.

We introduce a new methodology which allows inclusion of deep regional interpretations from potential field and geological sources of information in depth migration of seismic data. This fast algorithm aims at reducing the ambiguity in time-to-depth conversion and reduce the time spent on depth migration of seismic data by numerically including geologists’ interpretations.
A modelling-ray approach, not dependent on borehole information and accounting for lateral variations in velocity models, is used for switching from a time-migrated to depth-converted domain. An imageray approach is used for the reverse process to move from a depth-migrated to a time-converted domain. On the other hand, primary geological models are directly used in potential field inversion algorithms without re-parametrization of the model using an existing generalized level-set algorithm. Integrating solutions of the Eikonal equation in the form of Hamilton-Jacobi for both potential-field levelset inversion and seismic migration leads to a simultaneous modelling through which seismic, potential field, and geological data mitigate each other’s limitations. An investigation of the proposed methodology and a proof-of-concept using a fairly advanced realistic synthetic dataset are presented.

The proposed workflow is a novel approach for questioning the geological meaning of the sparsely distributed seismic sections and to integrate them in a 3D volume following the regional geological data and potential field results. The primary outcome of this study is a step toward adding equal weight to geological data not only as primary models but also as additional quantitative information within geophysical modelling and seismic imaging algorithms.

Our results lead to a significant improvement in the final model consistency with available sparsely distributed data sets. As a result, seismic migration is influenced by complementary information from potential field inversion results while simultaneously respecting sparse seismic sections in the 3D model obtained from regional interpretation and potential field inversion.

We acknowledge the support of the MinEx CRC and the Loop: Enabling Stochastic 3D Geological Modelling (LP170100985) consortia. The work has been supported by the Mineral Exploration
Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Programme. This is MinEx CRC Document 2020/44.


Biography

I completed my undergraduate and masters in petroleum exploration engineering. I started my Ph.D. at UWA (CET) as a minexCRC student focusing on data fusion methodologies for integrated inversion of geological and geophysical data.  This presentation is part of my PhD project for integrating seismic and gravity with different coverage.  

Mapping the near surface architecture of the Amadeus Basin using magnetic data: Petrophysical properties and geophysics pitfalls

Austin, James1, Schmid, Susanne 2 and Foss, Clive1

1Potential Fields Geophysics, CSIRO Mineral Resources, Lindfield, NSW 2070, 2Multidimensional Geoscience, CSIRO Mineral Resources, Kensington, WA 6151

The Amadeus Basin in central Australia is prospective for stratiform base metal deposits and hydrocarbons. The Basin displays subtle magnetic anomalies that trace strata for considerable distance, highlighting complex folding patterns. Magnetic modelling techniques can be utilised on these stratiform anomalies to extrapolate the near-surface structure of the basin. Generally magnetic anomalies are assumed to have predominantly induced magnetisation, and with this assumption dip can be reasonably estimated using magnetic data alone. However, where the magnetisation is not purely induced (i.e., includes remanent magnetisation) the mathematical trade-off between the dip and magnetisation of bodies means that the dip of a body cannot be known unless the magnetisation is also known. Normally it would be optimal to measure the magnetisation, but this is not always possible or feasible, e.g., due to land access issues. In this study, we investigate the relationships between dip and magnetisation using an approach that would generally be considered a little backward. Rather than constrain structure using petrophysics, we use structural geology to constrain petrophysics. Three study areas were chosen to investigate numerous stratigraphic horizons in three major study areas, the Waterhouse Range, Glen Helen Station and Ross River areas. Modelling results suggest that a paucity of layers retain predominantly induced magnetisation, remanence is dominant in some, but both induced and remanent magnetisation are typically present. Remanence is mainly associated with relatively oxidised units that contain only hematite (e.g. Arumbera Sandstone), and comparisons with known apparent polar wander paths suggest that these magnetisations pre-date major folding in the basin. In some cases, magnetic anomalism reflects redox zonation within units, e.g. the Pertatataka Formation near Glen Helen, where discrete magnetic layers coincide with thin grey (reduced, magnetite-rich) horizons interbedded with more prevalent red (oxidised, hematite-rich) horizons, which are only very weakly magnetised. We also found that where magnetised units are relatively thin and occur near the surface, their magnetic response is sharp. However, in coincident aeromagnetic data, adjacent anomalies commonly overlap to form a single anomaly, thus misrepresenting the magnetic field, and mis-mapping the dip of the magnetic horizons. This study highlights some major pitfalls in attempting to map structure using magnetics. Near surface sedimentary units tend to be variably oxidised, and their petrophysical properties are inconsistent along strike. Their total magnetisation is commonly comprised of a significant component of remanent magnetisation, and therefore due to the mathematical trade-off between dip and magnetisation direction, industry standard inversions will commonly mis-map surface structure. Remanent magnetisation pre-dates major folding in many cases therefore, opposite limbs of the same fold can have completely different magnetic signatures. Our ability to target mineral systems in sedimentary systems is contingent on our ability to map the structure of such systems. This study demonstrates that petrophysical knowledge is a pre-requisite constraint for successfully informing structural and tectonic studies using geophysics.


Biography

James Austin is specialised in structural geology and potential fields geophysics, but he dabbles in many aspects of geology. His research is  focused on understanding  relationships between crustal processes and geophysical fields. His recent work is focused on the development of integrated technologies for mapping mineral systems.

Reconstructing the Soldiers Cap Group – Kuridala Group basin: Implications for BHT and IOCG mineralisation

Connors, Karen1

1Sustainable Minerals Institute, The University of Queensland, Brisbane, Australia

The vast basin hosting the 1700-1650 Ma Soldiers Cap and Kuridala groups (SCG-KG), eastern Mount Isa Province, NW Queensland, extends >300 km east to include the ca 1700-1610 Ma Etheridge Group, Georgetown Inlier. The present-day extent and thickness represents only part of the original depocentre following inversion, uplift and erosion (1610-1500 Ma). Whilst the importance of extension has long been recognised, pervasive compression, voluminous 1535-1490 Ma granites, and limited seismic integration, has prevented elucidation of the extensional architecture and its influence on inversion and mineral systems. Integrated interpretation of seismic and potential field data, and review of geochronology has provided a new understanding of the extensional architecture, potential age range and thickness of the basin, and the tectonic evolution.

The preserved thickness, extent, age range, erosion estimates, and the crustal architecture provide first-order constraints on basin reconstruction. The SCG-KG basin overlies several crustal-scale boundaries, including the Gidyea Suture where the thinned eastern margin of the poorly reflective, Mount Isa crust is thrust over the thinner Numil crust. The Numil comprises a series of moderate to low-angle fault blocks, many only 5-15 km thick, and typically has pervasive, dipping reflections.

While outcrop mapping suggests the SCG group is 2-5 km thick, seismic data indicates 3-5 sec TWT, implying 12-15 km preserved thickness. Mapping indicates localised isoclinal folding, nappes and structural repetition within some highly-deformed zones. Although regional structural repetition can’t be ruled out, the seismic data suggests many areas are dominated by limited repetition and thickening on inverted, normal faults.

The minimum age for the SCG-KG is generally accepted as 1650 Ma. However, the 1650-1610 Ma units of the Tommy Creek Domain and Etheridge Group are likely to have been widespread across the basin. In addition, zircon populations from drainages along the eastern outcrop margin and SCG show peaks in juvenile mafic magmatism at 1630-1625 Ma, as well as 1667 Ma. Although the upper SCG-KG unit (Toole Creek Volcanics (TCV)) is attributed to thermal relaxation from ca 1670 Ma, coeval felsic and mafic magmatism at 1655 and 1625 Ma, and the large volume (20-30%) of high-Fe mafic sills within the TCV suggest extension continued or was episodic.

Prior to inversion and erosion, the 12-15km SCG-KG basin was thicker as well as wider than the present ~350 km. While the stretching factor and total extension are unknown, the thin low-angle fault blocks of the Numil, are consistent with highly-thinned to hyperextended crust (i.e. 10 km thickness or less), and exhumation of lower crust or mantle may have occurred. The resulting high geothermal gradient has implications for BHT mineralisation, and raises questions regarding controls on metal deposition.

The extensional fault system and preliminary reconstruction provide insights into the extensional evolution and controls on later inversion. The structural framework and its links to the underlying basement blocks and crustal-scale structures that form the first-order conduits of the plumbing system provide insights for both syn-sedimentary BHT mineral systems and later IOCG deposits.


Biography

Karen has had a varied career in mineral and petroleum exploration. She specialises in integrated interpretation of seismic with potential field data to understand 3D crustal architecture, structural inheritance and influence of basement on basin evolution, 3D modelling, and controls on mineral systems.

Review of Australian Mesoproterozoic basins: Geology and resource potential

Anderson, Jade1, Carr, Lidena1, Henson, Paul1, Carson, Chris1

1Geoscience Australia, Canberra, Australia

Australian cratons underwent substantial tectonism and cratonic reorganisation during the Mesoproterozoic, coinciding globally with the transition from Nuna to Rodinia (e.g. Li et al., 2008; Pisarevsky et al., 2014). The full extent and nature of this tectonism remains contentious (e.g. Bagas, 2004; Betts and Giles, 2006; Cawood and Korsch, 2008; Maidment, 2017).

During the Mesoproterozoic several sedimentary basin systems were deposited, and are now variably preserved, in the Northern Territory, Queensland, Western Australia, South Australia and Tasmania; providing an invaluable indirect record of the evolving Australian lithosphere and tectonic processes. Most of these basins were deposited on or at the margins of Archean to Paleoproterozoic cratons (North Australian Craton, West Australian Craton and South Australian Craton; e.g. see Myers et al. 1996; Cawood and Korsch, 2008 for spatial geography and constituents of these cratons). The remnants of these basins vary from weakly-deformed, relatively continuous units, such as the Roper Group of the McArthur Basin in the Northern Territory, to basins that were subsequently deformed and metamorphosed under high grade conditions, such as the Arid Basin of the Albany Fraser Orogen in Western Australia.

Individual basins are typically studied in isolation or in subsets, for which available geological datasets are commonly disparate with markedly different levels of knowledge. Mineral and energy resources have been identified in some of these basins; including oil and gas resources hosted in the Roper Group in the Beetaloo Sub-basin; manganese deposits in the Collier Basin and Manganese Group (Western Australia); and polymetallic, stratabound, hydrothermal mineralisation in the late Paleoproterozoic to early Mesoproterozoic Edmund Basin (Western Australia). Typically, these more overtly prospective basins, or groups, have been studied in greater detail than other Mesoproterozoic basins or groups.

This study provides a holistic overview of Australian Mesoproterozoic sedimentary basin systems, integrating geological, geochronological, and publically available resource data. As part of this collated approach, we also discuss potential inter-basin correlations for Mesoproterozoic-aged successions in Australia. This study aims to assist future work targeted at improving the geological understanding of these Mesoproterozoic sedimentary provinces and their resource prospectivity.

References

Bagas, L., 2004. Proterozoic evolution and tectonic setting of the northwest Paterson Orogen, Western Australia. Precambrian Research 128(3-4), 475-496.

Betts, P. G. and Giles, D., 2006. The 1800-1100 Ma tectonic evolution of Australia. Precambrian Research 144(1), 92-125.

Cawood, P. A. and Korsch, R. J., 2008. Assembling Australia: Proterozoic building of a continent. Precambrian Research 166(1-4), 1-38.

Li, Z. X., Bogdanova, S. V., Collins, A. S., Davidson, A., De Waele, B., Ernst, R. E., Fitzsimons, I. C. W., Fuck, R. A., Gladkochub, D. P., Jacobs, J., Karlstrom, K. E., Lu, S., Natapov, L. M., Pease, V., Pisarevsky, S. A., Thrane, K. and Vernikovsky, V., 2008. Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Research 160(1-2), 179-210.

Maidment, D. W., 2017. Geochronology from the Rudall Province, Western Australia: implications for the amalgamation of the West and North Australian Cratons. Geological Survey of Western Australia, Perth, 95 pp.

Myers, J. S., Shaw, R. D. and Tyler, I. M., 1996. Tectonic evolution of Proterozoic Australia. Tectonics 15(6), 1431-1446.

Pisarevsky, S. A., Elming, S.-Å., Pesonen, L. J. and Li, Z.-X., 2014. Mesoproterozoic paleogeography: Supercontinent and beyond. Precambrian Research 244, 207-225.


Biography

Jade Anderson completed a PhD at the University of Adelaide in the areas of metamorphic geology, geochronology and Proterozoic Australia tectonics. She currently works as a Geoscientist in Basin Systems at Geoscience Australia.

The tectonostratigraphic evolution of the South Nicholson region, Northern Territory and Queensland: key discoveries from the Exploring for the Future and implications for resource exploration

Carson, Chris1, Henson, Paul1, Lidena, Carr1, Southby, Chris1 and Anderson, Jade1.

1Geoscience Australia, Canberra, Australia

Proterozoic rocks of the South Nicholson region, which straddle the NT and QLD border, are juxtaposed between the Proterozoic Mount Isa Province to the east and the southern McArthur Basin to the northwest. The McArthur Basin and Mount Isa Province are comparatively well-studied and prospective for energy and mineral resources. In contrast, rocks of the South Nicholson region are mostly undercover and, as such, there is incomplete understanding of their geological evolution, relationship with adjacent geological provinces and resource potential. To address this gap, two deep crustal seismic reflection surveys, the South Nicholson and Barkly surveys (completed in 2017 and 2019, respectively), were conducted across the South Nicholson region by Geoscience Australia, under the federally funded Exploring for the Future (EFTF) initiative, in collaboration with the Northern Territory Geological Survey, the Geological Survey of Queensland and AuScope (e.g. Carr et al., 2019, 2020). While the Barkly seismic data are still being interpreted, these seismic datasets, together with other complementary regional studies, provides an improved understanding of the geological evolution and resource potential across this poorly understood region.

Both seismic surveys targeted both suspected undercover sedimentary basins and known crustal structures to resolve regional subsurface fault geometry. A key finding from the South Nicholson seismic survey is the discovery of a large concealed sedimentary sag basin that is up to 8 km deep, around 120 km wide and 190 km from north to south, called the Carrara Sub-basin (e.g. Carr et al., 2019). The sub-basin is interpreted to contain Mesoproterozoic to late Paleoproterozoic rocks equivalent to those outcropping in the Lawn Hill Platform and Mount Isa Province. The eastern end of the one of the lines (17GA-SN1), connects with a legacy seismic line that intersects the world class Pb-Zn Century deposit on the Lawn Hill Platform, the late Paleoproterozoic host rocks of which can be traced into the Carrara Sub-basin.

The South Nicholson profiles also reveal a series of ENE–trending, north-dipping half grabens which evolved during two episodes of crustal extension, at ca. 1725 Ma and ca. 1640 Ma, broadly coinciding with structural and basin forming events identified from the Lawn Hill Platform and the Mount Isa Province. Inversion of the half-graben bounding faults, resulting in south–verging thrusts, probably commenced during N-S crustal contraction characteristic of the early Isan Orogeny at ca. 1600-1580 Ma to at least the Paleozoic Alice Springs Orogeny (ca. 400-300 Ma).

Furthermore, our comprehensive regional geochronology program proposes extensive revision of regional stratigraphic relationships. Some successions, previously mapped as Mesoproterozoic South Nicholson Group may instead represent late Paleoproterozoic successions, that form part of the highly prospective Isa Superbasin (and the broadly stratigraphic equivalent McArthur Group in the McArthur Basin), which hosts numerous viable base metal deposits and is prospective for energy commodities (e.g. Jarrett et al., 2020; MacFarlane et al., 2020). Our findings significantly expand the extent of highly prospective late Paleoproterozoic stratigraphy across the South Nicholson region, which, possibly, extends an as yet unknown distance west beneath the Georgina and Carpentaria basins.

Carr, L.K., et al., 2019. Exploring for the Future: South Nicholson Basin Geological summary and seismic data interpretation. Record 2019/21. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2019.021

Carr, L.K., et al., 2020. South Nicholson seismic interpretation. In: Czarnota, K., et al., (eds.) Exploring for the Future Extended Abstracts, Geoscience Australia, http://dx.doi.org/10.11636/132029

Jarrett A.J.M., et al., 2020. A multidisciplinary approach to improving energy prospectivity in the South Nicholson region. In: Czarnota, K., et al., (eds.) Exploring for the Future Extended Abstracts, Geoscience Australia, http://dx.doi.org/10.11636/134164

MacFarlane, S. et al., 2020. A regional perspective of the Paleo- and Mesoproterozoic petroleum systems of northern Australia In: Czarnota, K., et al., (eds.) Exploring for the Future Extended Abstracts, Geoscience Australia, http://dx.doi.org/10.11636/133716


Biography

Chris has worked in Antarctica, Canadian Arctic, Alaska, New Caledonia and northern and central Australia, specialising in metamorphic petrology, geochronology and structural geology. Joining Geoscience Australia in 2006 he dabbled in SHRIMP geochronology and, in 2017, joined the Onshore Energy program, working in the South Nicholson region of the NT.

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